**Oscillatory Phenomena as a Probe to Study Pitting Corrosion of Iron in Halide-Containing Sulfuric Acid Solutions**

Dimitra Sazou<sup>∗</sup> , Maria Pavlidou, Aggeliki Diamantopoulou and Michael Pagitsas∗∗ *Department of Chemistry, Aristotle University of Thessaloniki, Thessaloniki Greece* 

### **1. Introduction**

60 Pitting Corrosion

Oltra, R., Chapey B., Renaud L., Abrasion-corrosion studies of passive stainlesssteels in

Prateepasen, A., Kaewtrakulpong, P. and Jirarungsatean, C., 2006a, "Semi-Parametric

Prateepasen, A., Jirarungsatean, C. and Tuengsook, P., 2006b, "Effect of Sulfuric Acid

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Prateepasen, A. (December 2007), Non-destructive testing in welds and researches, Chulalongkorn University Press, ISBN 978-974-456-679-9, Bangkok, Thailand. Prateepasen, A. and Jirarungsatian, C. , May 2011, Implementation of acoustic emission

Rettig, T.W. , and Felsen, M.J., Corrosion-NACE, Vol. 32, No. 4, 1976Acoustic emission for

Ronnie, K.M. and Paul, M.,1987, Nondestructive Testing Handbook volume 5 Acoustic Emission, American Society for Nondestructive Testing,ISBN 0-931403-02-2, USA. Saenkhum, N., Prateepasen A. and Kaewtrakulpong P., "Classification of Corrosion Detected by AE signal", IMECE'2003, Washington, D.C., Nov., 2003 Xidong, F. Ju, C. Lin, Research on the brittle fracture of FRP rods and itsacoustic emission

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Oscillatory phenomena and other nonlinear phenomena such as bistability and spatiotemporal patterns are frequently observed in metal and alloy electrodissolutionpassivation processes (Hudson & Tsotsis, 1994; Koper, 1996a; Krischer, 1999; Krischer, 2003b). Current oscillations during the Fe electrodissolution-passivation in acid solutions were reported as early as 1828 (Fechner, 1828). Since then, metal|electrolyte interfacial systems have received considerable interest over the past three decades for many reasons. Among them, is that progress in the theory of nonlinear dynamical systems, achieved in parallel over last decades, has led to the formulation of new theoretical concepts and tools that could apply to electrochemical oscillators. Therefore, an understanding of the fundamental principles underlying the nonlinear phenomena observed in electrochemical processes has been considerably improved (Berthier et al., 2004; Eiswirth et al., 1992; Karantonis & Pagitsas, 1997; Karantonis et al., 2005; Karantonis et al., 2000; Kiss & Hudson, 2003; Kiss et al., 2003; Kiss et al., 2006; Krischer, 2003b; Parmananda et al., 1999; Parmananda et al., 2000; Sazou et al., 1993a). On the other hand, electrochemical systems can be readily controlled through the variation of the potential (under current-controlled conditions) or the current (under potential-controlled conditions) and have served as experimental model systems to implement and test new theoretical concepts.

Technological applications of nonlinear electrochemical phenomena in materials science are exemplified by their impact on electrodissolution, electrodeposition and electrocatalytic reactions (Ertl, 1998; Ertl, 2008; Nakanishi et al., 2005; Orlik, 2009; Saitou & Fukuoka, 2004). Another promising application might be the preparation of self-organized nanostructures such as TIO2 nanotubes (Taveira et al., 2006). Regarding electrodissolution processes, the progress in defining the conditions for metal stability and dynamical transitions in metal|electrolyte systems is of fundament importance for metal performance and safety in natural environments (Hudson & Basset, 1991; Lev et al., 1988; Sazou et al., 2000b; Sazou et

<sup>∗</sup> Corresponding author

<sup>∗∗</sup> Deceased 26 April 2009.

Oscillatory Phenomena as a Probe to Study

measurements used in all sections is provided.

that is transformed rather to an HN-NDR oscillator.

periodic oscillations are briefly summarized.

oscillations.

**2. Experimental** 

be used in studying initiation of pitting at early stages.

Pitting Corrosion of Iron in Halide-Containing Sulfuric Acid Solutions 63

In this article only few of these aspects will be touched upon. Emphasis is placed on displaying certain features of the oscillatory response of the halide-containing Fe|*n* M H2SO4 system that might be employed in establishing new tools, useful in detecting and

The chapter starts with a short section (section 2) in which basic information about

In section 3, the basics regarding the origin of oscillations in metal|electrolyte systems under corrosion conditions are discussed briefly to show that the halide-free Fe|*n* M H2SO4 system, being an N-NDR oscillator, is unlikely to display either current oscillations within the stable passive state, extended beyond the Flade potential, or potential oscillations under galvanostatic conditions. Thus oscillatory phenomena discussed in this chapter are the result of the interplay between pitting corrosion and basic dynamics of an N-NDR system

Section 4, displays briefly the characteristic current-potential, *I* = f(*E*) or potential-current, *E*  = f(*I*) polarization curves of the halide-free Fe|0.75 M H2SO4 system traced under potentiodynamic and galvanodynamic conditions, respectively. This section aims in demonstrating that in the absence of aggressive ions only single periodic oscillations occur. The fundamental physico-electrochemical processes underlying the mechanism of single

Section 5, focuses on the nonlinear dynamics of the halide-perturbed Fe|0.75 M H2SO4 system at relatively low halide concentrations. Halide-induced changes in *I* = f(*E*) and *E* = f(*I*) polarization curves are pointed out. By choosing appropriate potential and current values from *I* = f(*E*) and *E* = f(*I*) curves, current and potential time-series are traced under potentiostatic and galvanostatic conditions, respectively. Experimental results are analyzed in order to establish appropriate kinetic quantities as a function of either the potential or current and the halide concentration. Emphasis is placed here on how these quantities can

Section 6, provides selected experimental examples displaying the nonlinear response of the halide-containing Fe|0.75 M H2SO4 system at relatively high halide concentrations. It is thus concerned with late stages of pitting corrosion, which are exemplified by a different type of

Section 7, includes an overview of conditions under which the nonlinear response of the halide-containing Fe|0.75 M H2SO4 system appears and a summary of proposed diagnostic

Electrochemical measurements were carried out using a VoltaLab 40 electrochemical system and the VoltaMaster 4 software from Radiometer Analytical. Additionally, a Wenking POS 73 potentioscan from Bank Elektronik was employed. It was interfaced with a computer, which was equipped with an analog-to-digital, and vice versa, converter PCL-812PG enhanced multi-Lab. Card (Advantech Co. Ltd). The maximum sampling rate of the PCL-

criteria, appropriate for characterization of pit initiation and its growth. In section 8, conclusions are presented, while section 9 contains references.

characterizing the extent of pitting corrosion on passive Fe surfaces.

al., 1993a; Sazou & Pagitsas, 2003b). The existence of passivity on metals is well recognized as the most important factor for the metal safe use in our metal-based civilization (Sato, 1990; Schmuki, 2002; Schultze & Lohrenger, 2000). It has been shown that depassivating factors, resulting either in uniform dissolution of passive films (general corrosion) or localized breakdown of an otherwise stable passivity on metals (pitting corrosion) give rise to temporal as well as spatiotemporal instabilities (Green & Hudson, 2001; Otterstedt et al., 1996; Sazou & Pagitsas, 2003b; Sazou et al., 2009; Wang et al., 2004). For Fe, these instabilities are more pronounced in pitting corrosion occurring in different corrosive media, but mostly, in those containing halides such as chlorides, bromides and iodides. This will be the main theme of this brief review.

In practice, chlorides are of a major concern due to their abundance in environments encountered in industry and in domestic, commercial and marine industry. Extensive studies have been carried out over the last century and continue aiming to estimate the conditions leading to local breakdown, gain a deeper understanding of mechanisms and processes underlying pitting and develop effective strategies of metal protection (Bohni, 1987; Frankel, 1998; Kaesche, 1986; Macdonald, 1992; Sato, 1982; Sato, 1989; Sato, 1990; Strehblow, 1995). Exploring the nonlinear dynamical phenomena associated with pitting corrosion of Fe and other metals might provide a new approach in investigating passivity breakdown from both mechanistic and kinetic points of view. Especially, electrochemical measurements and nonlinear dynamics in conjunction with new surface analytical techniques constitute a promising way towards studying pitting corrosion (Maurice & Marcus, 2006; Wang et al., 2004).

The onset of current and potential oscillations is the most common nonlinear behavior of the Fe|electrolyte system in acidic solutions containing halides, X- (X- ≡ Cl-, Br-, I-) (Georgolios & Sazou, 1998; Koutsaftis et al., 2007; Li et al., 2005; Ma et al., 2003; Pagitsas & Sazou, 1999; Sazou et al., 2000a; Sazou et al., 2000b; Sazou et al., 1993b; Sazou et al., 1992). In particular, it was shown that adding small amounts of X- in the Fe|*n* M H2SO4 system induces complex periodic and aperiodic current oscillations under potential-controlled conditions (Georgolios & Sazou, 1998; Koutsaftis et al., 2007; Li et al., 2005; Pagitsas & Sazou, 1999; Sazou et al., 2000a; Sazou et al., 2000b). These oscillations are of large amplitude and represent passive-active events emerged out of an extensive potential region. A gradual increase of halide concentration results in the establishment of a limiting current region out of the Fe passive state. This new state of Fe is accompanied by complex aperiodic current oscillations of small amplitude. The latter oscillations occur under mass-transport controlled conditions, which are established inside pits due to the formation of ferrous salt layers (Sazou & Pagitsas, 2003a; Sazou & Pagitsas, 2006a). Moreover, as was mentioned above, halides induce also potential oscillations under current-controlled conditions, associated with either early (Postlethwaite & Kell, 1972; Rius & Lizarbe, 1962; Sazou et al., 2009) or late stages (Li & Nobe, 1993; Li et al., 1993; Li et al., 1990; Strehblow & Wenners, 1977) of pitting corrosion.

Unambiguously, both current and potential oscillations include valuable information related to the kinetics of the oxide growth and its breakdown (Pagitsas et al., 2001; Pagitsas et al., 2002; Sazou & Pagitsas, 2003a; Sazou & Pagitsas, 2006a). Though, several investigations aiming to reveal and use profitably this information have been brought about some progress in understanding underlying processes, many aspects remain to be revealed and exploited.

al., 1993a; Sazou & Pagitsas, 2003b). The existence of passivity on metals is well recognized as the most important factor for the metal safe use in our metal-based civilization (Sato, 1990; Schmuki, 2002; Schultze & Lohrenger, 2000). It has been shown that depassivating factors, resulting either in uniform dissolution of passive films (general corrosion) or localized breakdown of an otherwise stable passivity on metals (pitting corrosion) give rise to temporal as well as spatiotemporal instabilities (Green & Hudson, 2001; Otterstedt et al., 1996; Sazou & Pagitsas, 2003b; Sazou et al., 2009; Wang et al., 2004). For Fe, these instabilities are more pronounced in pitting corrosion occurring in different corrosive media, but mostly, in those containing halides such as chlorides, bromides and iodides. This will be the main

In practice, chlorides are of a major concern due to their abundance in environments encountered in industry and in domestic, commercial and marine industry. Extensive studies have been carried out over the last century and continue aiming to estimate the conditions leading to local breakdown, gain a deeper understanding of mechanisms and processes underlying pitting and develop effective strategies of metal protection (Bohni, 1987; Frankel, 1998; Kaesche, 1986; Macdonald, 1992; Sato, 1982; Sato, 1989; Sato, 1990; Strehblow, 1995). Exploring the nonlinear dynamical phenomena associated with pitting corrosion of Fe and other metals might provide a new approach in investigating passivity breakdown from both mechanistic and kinetic points of view. Especially, electrochemical measurements and nonlinear dynamics in conjunction with new surface analytical techniques constitute a promising way towards studying pitting corrosion (Maurice &

The onset of current and potential oscillations is the most common nonlinear behavior of the

Sazou, 1998; Koutsaftis et al., 2007; Li et al., 2005; Ma et al., 2003; Pagitsas & Sazou, 1999; Sazou et al., 2000a; Sazou et al., 2000b; Sazou et al., 1993b; Sazou et al., 1992). In particular, it was shown that adding small amounts of X- in the Fe|*n* M H2SO4 system induces complex periodic and aperiodic current oscillations under potential-controlled conditions (Georgolios & Sazou, 1998; Koutsaftis et al., 2007; Li et al., 2005; Pagitsas & Sazou, 1999; Sazou et al., 2000a; Sazou et al., 2000b). These oscillations are of large amplitude and represent passive-active events emerged out of an extensive potential region. A gradual increase of halide concentration results in the establishment of a limiting current region out of the Fe passive state. This new state of Fe is accompanied by complex aperiodic current oscillations of small amplitude. The latter oscillations occur under mass-transport controlled conditions, which are established inside pits due to the formation of ferrous salt layers (Sazou & Pagitsas, 2003a; Sazou & Pagitsas, 2006a). Moreover, as was mentioned above, halides induce also potential oscillations under current-controlled conditions, associated with either early (Postlethwaite & Kell, 1972; Rius & Lizarbe, 1962; Sazou et al., 2009) or late stages (Li & Nobe, 1993; Li et al., 1993; Li et al., 1990; Strehblow & Wenners, 1977) of pitting

Unambiguously, both current and potential oscillations include valuable information related to the kinetics of the oxide growth and its breakdown (Pagitsas et al., 2001; Pagitsas et al., 2002; Sazou & Pagitsas, 2003a; Sazou & Pagitsas, 2006a). Though, several investigations aiming to reveal and use profitably this information have been brought about some progress in understanding underlying processes, many aspects remain to be revealed and exploited.

(X- ≡ Cl-, Br-, I-) (Georgolios &

Fe|electrolyte system in acidic solutions containing halides, X-

theme of this brief review.

Marcus, 2006; Wang et al., 2004).

corrosion.

In this article only few of these aspects will be touched upon. Emphasis is placed on displaying certain features of the oscillatory response of the halide-containing Fe|*n* M H2SO4 system that might be employed in establishing new tools, useful in detecting and characterizing the extent of pitting corrosion on passive Fe surfaces.

The chapter starts with a short section (section 2) in which basic information about measurements used in all sections is provided.

In section 3, the basics regarding the origin of oscillations in metal|electrolyte systems under corrosion conditions are discussed briefly to show that the halide-free Fe|*n* M H2SO4 system, being an N-NDR oscillator, is unlikely to display either current oscillations within the stable passive state, extended beyond the Flade potential, or potential oscillations under galvanostatic conditions. Thus oscillatory phenomena discussed in this chapter are the result of the interplay between pitting corrosion and basic dynamics of an N-NDR system that is transformed rather to an HN-NDR oscillator.

Section 4, displays briefly the characteristic current-potential, *I* = f(*E*) or potential-current, *E*  = f(*I*) polarization curves of the halide-free Fe|0.75 M H2SO4 system traced under potentiodynamic and galvanodynamic conditions, respectively. This section aims in demonstrating that in the absence of aggressive ions only single periodic oscillations occur. The fundamental physico-electrochemical processes underlying the mechanism of single periodic oscillations are briefly summarized.

Section 5, focuses on the nonlinear dynamics of the halide-perturbed Fe|0.75 M H2SO4 system at relatively low halide concentrations. Halide-induced changes in *I* = f(*E*) and *E* = f(*I*) polarization curves are pointed out. By choosing appropriate potential and current values from *I* = f(*E*) and *E* = f(*I*) curves, current and potential time-series are traced under potentiostatic and galvanostatic conditions, respectively. Experimental results are analyzed in order to establish appropriate kinetic quantities as a function of either the potential or current and the halide concentration. Emphasis is placed here on how these quantities can be used in studying initiation of pitting at early stages.

Section 6, provides selected experimental examples displaying the nonlinear response of the halide-containing Fe|0.75 M H2SO4 system at relatively high halide concentrations. It is thus concerned with late stages of pitting corrosion, which are exemplified by a different type of oscillations.

Section 7, includes an overview of conditions under which the nonlinear response of the halide-containing Fe|0.75 M H2SO4 system appears and a summary of proposed diagnostic criteria, appropriate for characterization of pit initiation and its growth.

In section 8, conclusions are presented, while section 9 contains references.

### **2. Experimental**

Electrochemical measurements were carried out using a VoltaLab 40 electrochemical system and the VoltaMaster 4 software from Radiometer Analytical. Additionally, a Wenking POS 73 potentioscan from Bank Elektronik was employed. It was interfaced with a computer, which was equipped with an analog-to-digital, and vice versa, converter PCL-812PG enhanced multi-Lab. Card (Advantech Co. Ltd). The maximum sampling rate of the PCL-

Oscillatory Phenomena as a Probe to Study

1976).

Pitting Corrosion of Iron in Halide-Containing Sulfuric Acid Solutions 65

Co|H3PO4 and Zn|NaOH systems being few of them (Hudson & Tsotsis, 1994). 2. HN-NDR, characterized by a regime of a hidden negative differential resistance in the *I*-*E* polarization curve. Potentiostatic current oscillations around a regime of a positive slope occur when *R*s>(*R*s)crit whereas galvanostatic potential oscillations occur as well. Example of this category is the transpassive electrodissolution of Ni in H2SO4 (Lev et al., 1988). 3. S-NDR, characterized by an S-type polarization curve (Fig. 2). S-NDR systems oscillate under galvanostatic conditions at applied current values located within the NDR regime of the polarization curve. Example of this category may include the complicated dynamics of the electrodissolution of Fe in concentrated nitric acid (Gabrielli et al.,

Fig. 2. Multisteady-state current-potential curves of S-type under potential-controlled conditions with (a) a vanishing *R*s, (b) current oscillations for *R*s>(*R*s)crit. (c) Potential oscillations under current-controlled conditions within the NDR regime, which in practice

As was shown in Fig. 1, the NDR in N-type current-potential curves is destabilized by increasing the ohmic resistance, *R*s. Considering the general equivalent circuit (EC) of an electrochemical cell (Fig. 1e), the *R*s represents the ohmic resistance, which includes un uncompensated cell resistance and a resistor connected in series between the working electrode, WE and ground. The total current *I* through the electrolyte interface consists of the faradaic current, *I*F through the faradaic impedance, *Z*F, and the current, *I*C through the capacitor, *C* of the double layer. Under potential-controlled conditions, the potential, *E*

becomes not accessible and the *I*-*E* curve exhibits bistability.

values of *R*s (Fig. 1b) whereas bistability without oscillations exists for *R*s > (*R*s)crit (Fig. 1c). Galvanostatic potential oscillations do not occur (Fig. 1d). The majority of corroding metal|electrolyte systems that exhibit current oscillations across the active-to-passive transition are related to N-NDR systems. Among them the Fe|H2SO4, Fe|H3PO4

812PG card was equal to 30 kHz. The working electrode (WE) was the cross section of an iron wire with a diameter equal to 3 mm from Johnson Matthey Chemicals (99.9%) embedded in a 1 cm diameter PTFE cylinder (surface area=0.0709 cm2). A volume of 150 ml was maintained in a three-electrode electrochemical cell. A Pt sheet (2.5 cm2) and a saturated calomel electrode (SCE) were used as the counter (CE) and reference electrodes (RE), respectively. The Fe-disc surface was polished by wet sand papers of different grit size (100, 180, 320, 500, 800, 1000, 1200 and superfine) and cleaned with twice-distilled water in an ultrasonic bath. Solutions were prepared with H2SO4 (Merck, pro-analysis 96% w/w) and NaF or NaCl or NaBr or NaI, all from Fluka (puriss p.a.), using twice-distilled water. Measurements were carried out at 298 K, while N2 was passed above the solution during the course of the experiment. A scanning electron microscope (SEM) JEOL JSM-840A was used for the Fe surface observation. Further experimental details can be found in previous studies (Pagitsas et al., 2003; Sazou & Pagitsas, 2003b; Sazou et al., 2009).

### **3. Origin of oscillations in corroding metal|electrolyte interfacial systems**

Spontaneous oscillatory phenomena observed in metal electrodissolution-passivation reactions are often associated with multisteady-state current-potential (*I*-*E*) curves due to the occurrence of a region with negative differential resistance (NDR). NDR appears either in Ntype (the electrode potential acts as activator, positive feedback variable) (Fig. 1) or S-type (the electrode potential acts as inhibitor, negative feedback variable) *I*-*E* curves (Fig. 2).

Fig. 1. Multisteady-state current-potential curves of N-type under potential controlled conditions with (a) a stable NDR-region at vanishing *R*s, (b) current oscillations around NDR for intermediate values of *R*s, (c) bistability without oscillations. (d) N-type curve under current-controlled conditions with bistability but without potential oscillations. (e) A general equivalent circuit of an electrochemical cell where *E* is the applied potential and *V* is the electrode potential.

Three basic categories are suggested to classify homogeneous (the spatial coupling is neglected) electrochemical oscillators. (Koper, 1996b; Krischer, 2003a):

1. N-NDR, characterized by an N-type current-potential curve for a vanishing ohmic resistance, *R*<sup>s</sup> → 0 (Fig. 1a). Potentiostatic current oscillations occur for intermediate

812PG card was equal to 30 kHz. The working electrode (WE) was the cross section of an iron wire with a diameter equal to 3 mm from Johnson Matthey Chemicals (99.9%) embedded in a 1 cm diameter PTFE cylinder (surface area=0.0709 cm2). A volume of 150 ml was maintained in a three-electrode electrochemical cell. A Pt sheet (2.5 cm2) and a saturated calomel electrode (SCE) were used as the counter (CE) and reference electrodes (RE), respectively. The Fe-disc surface was polished by wet sand papers of different grit size (100, 180, 320, 500, 800, 1000, 1200 and superfine) and cleaned with twice-distilled water in an ultrasonic bath. Solutions were prepared with H2SO4 (Merck, pro-analysis 96% w/w) and NaF or NaCl or NaBr or NaI, all from Fluka (puriss p.a.), using twice-distilled water. Measurements were carried out at 298 K, while N2 was passed above the solution during the course of the experiment. A scanning electron microscope (SEM) JEOL JSM-840A was used for the Fe surface observation. Further experimental details can be found in previous studies

**3. Origin of oscillations in corroding metal|electrolyte interfacial systems** 

Fig. 1. Multisteady-state current-potential curves of N-type under potential controlled conditions with (a) a stable NDR-region at vanishing *R*s, (b) current oscillations around NDR for intermediate values of *R*s, (c) bistability without oscillations. (d) N-type curve under current-controlled conditions with bistability but without potential oscillations. (e) A general equivalent circuit of an electrochemical cell where *E* is the applied potential and *V* is

Three basic categories are suggested to classify homogeneous (the spatial coupling is

1. N-NDR, characterized by an N-type current-potential curve for a vanishing ohmic resistance, *R*<sup>s</sup> → 0 (Fig. 1a). Potentiostatic current oscillations occur for intermediate

neglected) electrochemical oscillators. (Koper, 1996b; Krischer, 2003a):

the electrode potential.

Spontaneous oscillatory phenomena observed in metal electrodissolution-passivation reactions are often associated with multisteady-state current-potential (*I*-*E*) curves due to the occurrence of a region with negative differential resistance (NDR). NDR appears either in Ntype (the electrode potential acts as activator, positive feedback variable) (Fig. 1) or S-type (the electrode potential acts as inhibitor, negative feedback variable) *I*-*E* curves (Fig. 2).

(Pagitsas et al., 2003; Sazou & Pagitsas, 2003b; Sazou et al., 2009).

values of *R*s (Fig. 1b) whereas bistability without oscillations exists for *R*s > (*R*s)crit (Fig. 1c). Galvanostatic potential oscillations do not occur (Fig. 1d). The majority of corroding metal|electrolyte systems that exhibit current oscillations across the active-to-passive transition are related to N-NDR systems. Among them the Fe|H2SO4, Fe|H3PO4 Co|H3PO4 and Zn|NaOH systems being few of them (Hudson & Tsotsis, 1994).


Fig. 2. Multisteady-state current-potential curves of S-type under potential-controlled conditions with (a) a vanishing *R*s, (b) current oscillations for *R*s>(*R*s)crit. (c) Potential oscillations under current-controlled conditions within the NDR regime, which in practice becomes not accessible and the *I*-*E* curve exhibits bistability.

As was shown in Fig. 1, the NDR in N-type current-potential curves is destabilized by increasing the ohmic resistance, *R*s. Considering the general equivalent circuit (EC) of an electrochemical cell (Fig. 1e), the *R*s represents the ohmic resistance, which includes un uncompensated cell resistance and a resistor connected in series between the working electrode, WE and ground. The total current *I* through the electrolyte interface consists of the faradaic current, *I*F through the faradaic impedance, *Z*F, and the current, *I*C through the capacitor, *C* of the double layer. Under potential-controlled conditions, the potential, *E*

Oscillatory Phenomena as a Probe to Study

between -0.5 – 2.5 V.

curve:

reaction,

1e).

Fe2+ + <sup>2</sup> *SO*<sup>4</sup>

Pitting Corrosion of Iron in Halide-Containing Sulfuric Acid Solutions 67

electrochemical processes occur upon increasing/decreasing the potential within the region

Fig. 3. (a) Potentiodynamic *I* = f(*E*) curve traced at d*E*/d*t* = 2 mV s-1 and (b) galvanodynamic

As can be seen in Fig. 3a, five typical regions are distinguished in the potentiodynamic *I*-*E*

i. Active electrodissolution region, where Fe dissolution occurs from a film-free Fe surface

ii. Limiting current region (LCR) where, under proper conditions, current oscillations may occur (Geraldo et al., 1998; Kiss et al., 2006; Kleinke, 1995; Sazou et al., 2000b; Sazou et al., 2000c; Sazou & Pagitsas, 2006b) beyond the peak potential, *E*p and before the establishment of a steady transport-controlled LCR within which formation-dissolution of the ferrous salt, FeSO4 ⋅7H2O proceeds at equal rates, according to the overall

<sup>−</sup> + 7H2O ↔ FeSO4.

iii. Active-to-passive transition region, as defined during the forward potential scan (or passive-to-active transition region defined during the backward scan), associated with a hysteresis loop since transition to passivity, during the forward potential scan, occurs at the primary passivation potential, *E*pp whereas reactivation of the Fe, during the backward scan, occurs at the Flade potential, *E*F (Rush & Newman, 1995). The *E*F and not the *E*pp is considered as the passivation potential of the Fe electrode since the ohmic potential drop, *IRs* becomes almost zero at *E*F due to the very low current value established in the passive state traced during the backward scan. Therefore, the *E*<sup>F</sup> determined in *I* = f(*E*) curves is very close to the electrode potential *V*, which is related to the applied potential, *E* via the relationship, E = *V* + *IRs*. In practice, *Rs* includes any series resistance added to the general EC of the electrochemical cell (Fig.

iv. The passive region, located between the *E*F and transpassivation potential, *E*tr.

Transition of Fe to passivity can be represented by the overall reaction ,

Fe + nH2O → [Fe(H2O)]2+ + 2e (I)

Fe + x/2H2O → FeOx/2 + xH+ + xe (III)

7H2O (II)

through multiple stages (Keddam et al., 1984) and an overall reaction ,

*E* = f(*I*) curve traced at d*I*/d*t* = 0.05 mA s-1 of the Fe|0.75 M H2SO4.

between the WE and reference electrode (RE) should be constant and equal to *E*=*V* + *IR*s. Destabilization of the N-type curves might be caused through the variation of the electrode potential, *V*. It introduces a positive feedback loop (activator) in the system by increasing *R*s, which is used as a bifurcation parameter. Bifurcation parameter is a system parameter, which induces changes in the dynamic behavior of the system at a critical value. Dynamical changes occur through bifurcations (Koper, 1996b; Koper & Sluyters, 1993b).

Elucidation of the origin of oscillations includes the effect of the ohmic potential drop, *IR*<sup>s</sup> and the discontinuous variation of the surface coverage ratio with the electrode potential due to the formation-dissolution of anodic surface films or the presence of autocatalytic chemical reactions concurrently occurring with electron transfer reactions. Mechanistically, the faradaic impedance, *Z*F, depicted in the EC of an electrochemical cell (Fig. 1e), is related to the electrochemical processes at the metal (WE)|interface. *Z*F should be derived from a reaction scheme. The basic equations are the mass balance for the reaction intermediates and charge balance equations derived from the general EC shown in Fig. 1e.

$$\frac{\mathbf{d}c\_i}{\mathbf{d}t} = f\_i(c\_i, V) \tag{1}$$

$$\frac{\text{d}\,V}{\text{d}\,t} = \frac{I - I\_{\text{f}}(V)}{CA} \tag{2}$$

where *c*i is the surface concentration of reaction intermediates and *A* is the surface area. At least one intermediate species, which introduces a negative feedback loop (inhibitor), is required for an N-NDR system to exhibit periodic current oscillations. Details on this issue can be found in several comprehensive reviews and related articles (Koper, 1996b; Krischer, 1999; Krischer, 2003b).

As was mentioned above, on the basis of certain essential dynamical features, the Fe|H2SO4 system can be classified in the N-NDR category (Sazou et al., 1993a) where most of the metal|electrolyte systems belong. Therefore, only potentiostatic current oscillations are anticipated within a fixed potential region (Fig. 1b), when the *IR*s exceeds an upper critical value, *IR*s > (*IR*s)crit bistability is expected, without oscillations. Under current-controlled conditions oscillations are not anticipated but only bistable behavior (Fig. 1d). The halideperturbed Fe|H2SO4 system cannot be readily classified in one of the above categories and there is not doubt that it deserves a further investigation within this context. However, its dynamical behavior observed at relatively low chloride concentrations bears resemblance with the essential features of the HN-NDR oscillators (Krischer, 2003b). It seems, that oxide growth causes the NDR, whereas the slower action of chlorides on the oxide film and the gradual increase of passive-state current inhibits the appearance of NDR (Sazou et al., 2009).

### **4. Electrochemical behavior of the Fe|H2SO4 system**

Fig. 3a illustrates the anodic current-potential (*I*-*E)* polarization curve of the Fe|0.75 M H2SO4 system traced under potential-controlled conditions at d*E*/d*t*=2 mV s-1 during both the forward and reverse backward potential scans. It seems that a variety of physico-

between the WE and reference electrode (RE) should be constant and equal to *E*=*V* + *IR*s. Destabilization of the N-type curves might be caused through the variation of the electrode potential, *V*. It introduces a positive feedback loop (activator) in the system by increasing *R*s, which is used as a bifurcation parameter. Bifurcation parameter is a system parameter, which induces changes in the dynamic behavior of the system at a critical value. Dynamical changes occur through bifurcations (Koper, 1996b; Koper & Sluyters,

Elucidation of the origin of oscillations includes the effect of the ohmic potential drop, *IR*<sup>s</sup> and the discontinuous variation of the surface coverage ratio with the electrode potential due to the formation-dissolution of anodic surface films or the presence of autocatalytic chemical reactions concurrently occurring with electron transfer reactions. Mechanistically, the faradaic impedance, *Z*F, depicted in the EC of an electrochemical cell (Fig. 1e), is related to the electrochemical processes at the metal (WE)|interface. *Z*F should be derived from a reaction scheme. The basic equations are the mass balance for the reaction intermediates and

*(ci ,V )* (1)

*CA* (2)

charge balance equations derived from the general EC shown in Fig. 1e.

d*ci* <sup>d</sup>*<sup>t</sup>* <sup>=</sup> *<sup>f</sup> i*

d*V*

<sup>d</sup>*<sup>t</sup>* <sup>=</sup> *<sup>I</sup>* <sup>−</sup> *IF* (*V*)

where *c*i is the surface concentration of reaction intermediates and *A* is the surface area. At least one intermediate species, which introduces a negative feedback loop (inhibitor), is required for an N-NDR system to exhibit periodic current oscillations. Details on this issue can be found in several comprehensive reviews and related articles (Koper, 1996b; Krischer,

As was mentioned above, on the basis of certain essential dynamical features, the Fe|H2SO4 system can be classified in the N-NDR category (Sazou et al., 1993a) where most of the metal|electrolyte systems belong. Therefore, only potentiostatic current oscillations are anticipated within a fixed potential region (Fig. 1b), when the *IR*s exceeds an upper critical value, *IR*s > (*IR*s)crit bistability is expected, without oscillations. Under current-controlled conditions oscillations are not anticipated but only bistable behavior (Fig. 1d). The halideperturbed Fe|H2SO4 system cannot be readily classified in one of the above categories and there is not doubt that it deserves a further investigation within this context. However, its dynamical behavior observed at relatively low chloride concentrations bears resemblance with the essential features of the HN-NDR oscillators (Krischer, 2003b). It seems, that oxide growth causes the NDR, whereas the slower action of chlorides on the oxide film and the gradual increase of passive-state current inhibits the appearance of

Fig. 3a illustrates the anodic current-potential (*I*-*E)* polarization curve of the Fe|0.75 M H2SO4 system traced under potential-controlled conditions at d*E*/d*t*=2 mV s-1 during both the forward and reverse backward potential scans. It seems that a variety of physico-

1993b).

1999; Krischer, 2003b).

NDR (Sazou et al., 2009).

**4. Electrochemical behavior of the Fe|H2SO4 system** 

electrochemical processes occur upon increasing/decreasing the potential within the region between -0.5 – 2.5 V.

Fig. 3. (a) Potentiodynamic *I* = f(*E*) curve traced at d*E*/d*t* = 2 mV s-1 and (b) galvanodynamic *E* = f(*I*) curve traced at d*I*/d*t* = 0.05 mA s-1 of the Fe|0.75 M H2SO4.

As can be seen in Fig. 3a, five typical regions are distinguished in the potentiodynamic *I*-*E* curve:

i. Active electrodissolution region, where Fe dissolution occurs from a film-free Fe surface through multiple stages (Keddam et al., 1984) and an overall reaction ,

$$\text{Fe} + \text{nH}\_2\text{O} \rightarrow [\text{Fe(H}\_2\text{O)}]^{2+} + 2\text{e} \tag{1}$$

ii. Limiting current region (LCR) where, under proper conditions, current oscillations may occur (Geraldo et al., 1998; Kiss et al., 2006; Kleinke, 1995; Sazou et al., 2000b; Sazou et al., 2000c; Sazou & Pagitsas, 2006b) beyond the peak potential, *E*p and before the establishment of a steady transport-controlled LCR within which formation-dissolution of the ferrous salt, FeSO4 ⋅7H2O proceeds at equal rates, according to the overall reaction,

$$\text{Fe}^{2+} + \text{SO}\_4^{2-} + 7\text{H}\_2\text{O} \leftrightarrow \text{FeSO}\_4\text{7H}\_2\text{O} \tag{\text{II}}$$


$$\text{Fe} + \text{x}/2\text{H}\_2\text{O} \rightarrow \text{FeO}\_{\text{x}/2} + \text{xH}^\* + \text{xe} \tag{\text{III}}$$

Oscillatory Phenomena as a Probe to Study

oscillations may occur.

*pH*,

Pitting Corrosion of Iron in Halide-Containing Sulfuric Acid Solutions 69

stability is determined roughly by the ratio *c*Fe3+ / *c*Fe2+. Increasing the c /c 3 2 *Fe Fe* + + ratio within the oxide film results in an increase of the oxide stability in acid media (Engell, 1977). Thus upon increasing the potential at *E* > *E*F, the oxide structure is related rather with the stable γ-Fe2O3 than with the less stable in acidic solutions Fe3O4 prevailing at *E* < *E*F where

The mechanism of spontaneous current oscillations of the Fe|0.75 M H2SO4 system is understood on the basis of the early suggested Franck-FitzHugh (F-F) model (Franck, 1978; Franck & Fitzhugh, 1961) according to which periodic passivation/activation of the Fe occurs due to local *pH* changes (Rush & Newman, 1995; Wang & Chen, 1998) that lead to a shift of the *E*F with respect to the electrode potential, *V* because the *E*F depends on the

 *E*F = 0.58-0.058*pH* vs. NHE at 293 K (3)

Fig. 5. Flow diagram illustrating the principle physico-electrochemcial processes involved in

Furthermore, the formation of the ferrous salt layer and the ohmic potential drop, *IR*s should be also taken into account for a realistic description of the periodic oscillations arisen across

a current oscillatory cycle of the Fe|0.75 M H2SO4 system.

v. The transpassive region, extended beyond the *E*tr, where the oxygen evolution reaction (OER) occurs (Sazou et al., 2009).

The galvanodynamic *I*-*E* curve of the Fe|0.75 M H2SO4 system (Fig. 3b) differs from the corresponding potentiodynamic *I*-*E* curve (Fig. 3a) in that region **ii** is not recorded. A sudden transition to passivity and, in turn, to OER (region **v**) occurs during the forward current scan, while the LCR (region **iii**) is skipped. It seems that the sudden active-topassive transition occurs once the ferrous salt layer is established at the critical current value, *I*pas. The *I*pas corresponds to the peak potential *E*p of the potentiodynamic curve (Fig. 3a). During the backward current scan, Fe sustains passivity (region **iv**) up to the corrosion potential, *E*cor whereas the passive-to-active transition occurs at a very low current, *I*act. A hysteresis loop exists because *I*pas ≠ *I*act. Potential oscillations are never observed under galvanostatic conditions at any applied current value up to 60 mA (Sazou et al., 2011; Sazou et al., 2009), in line with the galvanodynamic curve (Fig. 3b).

On the contrary, periodic current oscillations occur under potentiostatic conditions, immediately after switching on the potential within the oscillatory region, Δ*Ε*osc at *E* < *E*<sup>F</sup> (Δ*Ε*osc = 30-35 mV for the Fe|0.75 M H2SO4 system). Typical potentiostatic current oscillations occurring within the Δ*Ε*osc are illustrated in Figs. 4a-c.

Fig. 4. (a-c) Potentiostatic current oscillations traced at different values of the applied potential, *E* and (d) dependence of the oscillation period, *T* on *E* for the Fe|0.75 M H2SO4 system.

Single periodic relaxation oscillations are revealed with a potential-dependent period, *T*. Fig. 4d shows that *T* increases upon increasing the potential (Podesta et al., 1979; Sazou et al., 1993a; Sazou & Pagitsas, 2003b). This indicates that the stability of the passive oxide film increases upon increasing the potential as a result of the increase of the oxide-film thickness and the concentration of Fe3+ in the oxide lattice (Engell, 1977; Vetter, 1971). The composition of the iron oxide film is related to Fe3O4 and γ-Fe2O3 (Toney et al., 1997) and its

v. The transpassive region, extended beyond the *E*tr, where the oxygen evolution reaction

The galvanodynamic *I*-*E* curve of the Fe|0.75 M H2SO4 system (Fig. 3b) differs from the corresponding potentiodynamic *I*-*E* curve (Fig. 3a) in that region **ii** is not recorded. A sudden transition to passivity and, in turn, to OER (region **v**) occurs during the forward current scan, while the LCR (region **iii**) is skipped. It seems that the sudden active-topassive transition occurs once the ferrous salt layer is established at the critical current value, *I*pas. The *I*pas corresponds to the peak potential *E*p of the potentiodynamic curve (Fig. 3a). During the backward current scan, Fe sustains passivity (region **iv**) up to the corrosion potential, *E*cor whereas the passive-to-active transition occurs at a very low current, *I*act. A hysteresis loop exists because *I*pas ≠ *I*act. Potential oscillations are never observed under galvanostatic conditions at any applied current value up to 60 mA (Sazou et al., 2011; Sazou

On the contrary, periodic current oscillations occur under potentiostatic conditions, immediately after switching on the potential within the oscillatory region, Δ*Ε*osc at *E* < *E*<sup>F</sup> (Δ*Ε*osc = 30-35 mV for the Fe|0.75 M H2SO4 system). Typical potentiostatic current

Fig. 4. (a-c) Potentiostatic current oscillations traced at different values of the applied potential, *E* and (d) dependence of the oscillation period, *T* on *E* for the Fe|0.75 M H2SO4

Single periodic relaxation oscillations are revealed with a potential-dependent period, *T*. Fig. 4d shows that *T* increases upon increasing the potential (Podesta et al., 1979; Sazou et al., 1993a; Sazou & Pagitsas, 2003b). This indicates that the stability of the passive oxide film increases upon increasing the potential as a result of the increase of the oxide-film thickness and the concentration of Fe3+ in the oxide lattice (Engell, 1977; Vetter, 1971). The composition of the iron oxide film is related to Fe3O4 and γ-Fe2O3 (Toney et al., 1997) and its

(OER) occurs (Sazou et al., 2009).

system.

et al., 2009), in line with the galvanodynamic curve (Fig. 3b).

oscillations occurring within the Δ*Ε*osc are illustrated in Figs. 4a-c.

stability is determined roughly by the ratio *c*Fe3+ / *c*Fe2+. Increasing the c /c 3 2 *Fe Fe* + + ratio within the oxide film results in an increase of the oxide stability in acid media (Engell, 1977). Thus upon increasing the potential at *E* > *E*F, the oxide structure is related rather with the stable γ-Fe2O3 than with the less stable in acidic solutions Fe3O4 prevailing at *E* < *E*F where oscillations may occur.

The mechanism of spontaneous current oscillations of the Fe|0.75 M H2SO4 system is understood on the basis of the early suggested Franck-FitzHugh (F-F) model (Franck, 1978; Franck & Fitzhugh, 1961) according to which periodic passivation/activation of the Fe occurs due to local *pH* changes (Rush & Newman, 1995; Wang & Chen, 1998) that lead to a shift of the *E*F with respect to the electrode potential, *V* because the *E*F depends on the *pH*,

Fig. 5. Flow diagram illustrating the principle physico-electrochemcial processes involved in a current oscillatory cycle of the Fe|0.75 M H2SO4 system.

Furthermore, the formation of the ferrous salt layer and the ohmic potential drop, *IR*s should be also taken into account for a realistic description of the periodic oscillations arisen across

Oscillatory Phenomena as a Probe to Study

a greater degree than during the forward scan.

region (Pagitsas et al., 2007; Pagitsas et al., 2008).

H2SO4 system traced at d*E*/d*t* = 2 mV s-1.

The *E*pit is the critical potential for stable pitting to occur.

et al., 2000b).

Pitting Corrosion of Iron in Halide-Containing Sulfuric Acid Solutions 71

2. The lower, *Ε*low and upper, *E*upp potential limits of the oscillatory region shift towards higher values indicating destabilization of the stable passive state existing in the halidefree system. It is found that both *Ε*low and *E*upp vary linearly with the log(*c*Cl-). Appropriate analysis, leads to the critical values of *c*Cl-, beyond which oxide formation becomes unlikely and hence transition to a mass-transport LCR may occur due to the formation-dissolution of ferrous salts signifying a stable pit growth. This value is found to be ~30 mM in agreement with experimental observations (Sazou et al., 2000a; Sazou

3. The current in the passive state increases. It is lower during the forward potential scan (*I*pas,f in Fig. 6b) than during the backward one (*I*pas,b in Fig. 6c). This can be interpreted considering that pitting corrosion is a dynamical process and therefore, the longer time elapsed for the backward scan allows the progress of pit propagation and/or growth to

4. The maximum oscillatory current, (*I*osc)max (Fig. 6b) deviates from the kinetics of the linear segment in region **i**, indicating a larger real surface of the Fe electrode. This is attributed to the increase of the surface roughness due to pitting corrosion as compared with the uniformly corroding Fe surface in the halide-free system (Fig. 6a). The magnitude of the deviation is expressed as the ratio (*I*osc)max/(*I*osc)max, exp, where the (*I*osc)max, exp is the current expected on the basis of the relationship *E* = *V* + *IRs*. The latter relationship is valid in the linear segment of the active region **i** located beyond the Tafel

5. No access to *E*tr is possible whereas the critical pitting potential *E*pit appears (Fig. 6b).

Fig. 6. Chloride-induced changes in the potentiodynamic *I*= f(*E*) curves of the Fe|0.75 M

the passive-to-active transition of Fe in acid solutions (Birzu et al., 2001; Birzu et al., 2000; Koper & Sluyters, 1993a; Pagitsas & Sazou, 1991; Rush & Newman, 1995). As was mentioned above, the electrode potential, *V* coincides with the applied potential, *E* at *E*F. Therefore, *E* - *E*F = ε denotes the difference from the passivation potential. It becomes evident that at ε < 0, the Fe active electrodissolution via the overall reaction (I) occurs. Inversely, at ε > 0, passivation of the Fe via the overall reaction (II) occurs. In between, the LCR is esrablished through the reaction (III) (Pagitsas et al., 2003). These processes, identified in the *I* = f(*E*) of the Fe|0.75 M H2SO4 system (Fig. 3), are in practice the principle physico-electrochemical processes occurring during an oscillation cycle. Identical processes are also taken into account in improved versions of the F-F model (Koper & Sluyters, 1993a; Krischer, 2003a) as can be seen in the flow diagram displayed in Fig. 5.

### **5. Chemical perturbation of the Fe|H2SO4 system by adding small amounts of halides**

In a series of studies, was shown that addition of halides, X ≡Cl-, Br-, I- gives rise to changes in both the potentiodynamic *I*=f(*E*) (Pagitsas & Sazou, 1999; Sazou et al., 2000a; Sazou et al., 2000b; Sazou & Pagitsas, 2003b; Sazou et al., 1993b; Sazou et al., 1992; Sazou et al., 2009) and galvanodynamic *E* = f(*I*) curves (Sazou et al., 2011; Sazou et al., 2009) of the Fe|0.75 M H2SO4 system. All halide-induced changes can be identified approximately within regions **iv** (Fig. 3). This indicates that halides participate in both the electrodissolution and passivation processes of Fe. However, since this article focuses on features that might be exploited to characterize pitting corrosion, special emphasis is placed on the passive and passive-active transition states of Fe.

The halide-induced changes together with nonilinear phenomena are investigated first on the basis of potentiodynamic, *I* = f(*E*) and galvanodynamic, *E* = f(*I*) curves. These curves can be considered as characteristic curves of the nonlinear system under study, in an analogy with the semiconductor "characteristic curve" used in solid state physics, or as a oneparameter "phase diagram" or "bifurcation diagram" in terms of nonlinear dynamics. Characteristic *I* = f(*E*) and *E* = f(*I*) curves exhibit all transitions between different steady and oscillatory states upon varying the applied potential acting as a bifurcation parameter. Then, the different states of the system being known, current or potential time-series are recorded under potentiostatic or galvanostatic conditions, respectively, at potentials or current values located within the corresponding oscillatory regions. A slight deviation is noticed in determining the upper and lower limits of the oscillatory region under static conditions as compared with dynamic *I* = f(*E*) and *E* = f(*I*) curves.

### **5.1 Under potential-controlled conditions**

The effect of an increasing chloride concentration, within a relatively low-concentration range (*c*Cl- < 20 mM), on potentiodynamic *I*= f(*E*) curves is displayed in Fig. 6.

Inspection of the *I*= f(*E*) curves of Fig. 6, reveal that pitting corrosion manifests itself in changes that are summarized as follows:

1. The halide-induced oscillatory region, Δ*Ε*osc,Cl, relative to the halide-free Δ*Ε*osc, is extended towards higher potentials (Δ*Ε*osc, Cl > Δ*Ε*osc).

the passive-to-active transition of Fe in acid solutions (Birzu et al., 2001; Birzu et al., 2000; Koper & Sluyters, 1993a; Pagitsas & Sazou, 1991; Rush & Newman, 1995). As was mentioned above, the electrode potential, *V* coincides with the applied potential, *E* at *E*F. Therefore, *E* -

denotes the difference from the passivation potential. It becomes evident that at

passivation of the Fe via the overall reaction (II) occurs. In between, the LCR is esrablished through the reaction (III) (Pagitsas et al., 2003). These processes, identified in the *I* = f(*E*) of the Fe|0.75 M H2SO4 system (Fig. 3), are in practice the principle physico-electrochemical processes occurring during an oscillation cycle. Identical processes are also taken into account in improved versions of the F-F model (Koper & Sluyters, 1993a; Krischer, 2003a) as

**5. Chemical perturbation of the Fe|H2SO4 system by adding small amounts of** 

In a series of studies, was shown that addition of halides, X ≡Cl-, Br-, I- gives rise to changes in both the potentiodynamic *I*=f(*E*) (Pagitsas & Sazou, 1999; Sazou et al., 2000a; Sazou et al., 2000b; Sazou & Pagitsas, 2003b; Sazou et al., 1993b; Sazou et al., 1992; Sazou et al., 2009) and galvanodynamic *E* = f(*I*) curves (Sazou et al., 2011; Sazou et al., 2009) of the Fe|0.75 M H2SO4 system. All halide-induced changes can be identified approximately within regions **iv** (Fig. 3). This indicates that halides participate in both the electrodissolution and passivation processes of Fe. However, since this article focuses on features that might be exploited to characterize pitting corrosion, special emphasis is placed on the passive and

The halide-induced changes together with nonilinear phenomena are investigated first on the basis of potentiodynamic, *I* = f(*E*) and galvanodynamic, *E* = f(*I*) curves. These curves can be considered as characteristic curves of the nonlinear system under study, in an analogy with the semiconductor "characteristic curve" used in solid state physics, or as a oneparameter "phase diagram" or "bifurcation diagram" in terms of nonlinear dynamics. Characteristic *I* = f(*E*) and *E* = f(*I*) curves exhibit all transitions between different steady and oscillatory states upon varying the applied potential acting as a bifurcation parameter. Then, the different states of the system being known, current or potential time-series are recorded under potentiostatic or galvanostatic conditions, respectively, at potentials or current values located within the corresponding oscillatory regions. A slight deviation is noticed in determining the upper and lower limits of the oscillatory region under static conditions as

The effect of an increasing chloride concentration, within a relatively low-concentration

Inspection of the *I*= f(*E*) curves of Fig. 6, reveal that pitting corrosion manifests itself in

1. The halide-induced oscillatory region, Δ*Ε*osc,Cl, relative to the halide-free Δ*Ε*osc, is

range (*c*Cl- < 20 mM), on potentiodynamic *I*= f(*E*) curves is displayed in Fig. 6.

extended towards higher potentials (Δ*Ε*osc, Cl > Δ*Ε*osc).

the Fe active electrodissolution via the overall reaction (I) occurs. Inversely, at

can be seen in the flow diagram displayed in Fig. 5.

passive-active transition states of Fe.

compared with dynamic *I* = f(*E*) and *E* = f(*I*) curves.

**5.1 Under potential-controlled conditions** 

changes that are summarized as follows:

ε< 0,

ε> 0,

*E*F = ε

**halides** 


Fig. 6. Chloride-induced changes in the potentiodynamic *I*= f(*E*) curves of the Fe|0.75 M H2SO4 system traced at d*E*/d*t* = 2 mV s-1.

Oscillatory Phenomena as a Probe to Study

potential at various *c*Cl-

Pitting Corrosion of Iron in Halide-Containing Sulfuric Acid Solutions 73

Besides changes observed in potentiodynamic *I* = f(*E*) curves, pitting corrosion manifests itself in potentiostatic current oscillations too. An example of halide-induced oscillations is given in Fig. 7a. Fig. 7a displays a transition between single periodic oscillations observed in the halide-free system, immediately after switching on the potential at *E* < *E*F, and the halide-induced complex periodic oscillations appeared after an induction period of time, *t*ind (Fig. 7b). The halide-induced current oscillations occur over a wide potential region (Table 1) and their periodicity was found to follow period doubling, quadrupling and aperiodicity by increasing the applied potential, *E* and *c*X- (Sazou et al., 2000b). Depending on *E* and *c*Xdifferent temporal patterns were recorded such as bursting and beating. Variation of the current waveform was also observed at longer times as it is anticipated for pitting corrosion,

Fig. 7. (a) Transition of the single periodic to a complex periodic current oscillation induced by adding Cl- into the Fe|0.75 M H2SO4 system. Next to each oscillation waveform, SEM micrographs display the corresponding Fe surface morphologies. (b) Induction time, *t*ind occurring prior the onset of chloride-induced oscillations and dependence of *t*ind on the

The complex oscillations induced by Cl- as well as Br- and I- is the result of the aggressive action of halides on the Fe surface (Sazou et al., 2000a; Sazou et al., 2000b). This is confirmed by SEM observations, an example of which is depicted in Fig. 7a. The morphology of the Fedisc surface during the occurrence of single periodic oscillations reflects a general corrosion induced by H+ ions. Hydrogen ions catalyze the uniform dissolution of the passive oxide film consisting primarily of Fe3O4 (Engell, 1977; Sato, 1990). Associated with temporal current patterning, are also spatial phenomena that deserve to be investigated (Hudson et al., 1993). On the other hand, when complex periodic current oscillations occur in the presence of chlorides and other X- ions, the morphology of the Fe surface reveals hemispherical pits as a result of the local breakdown of passivity on Fe. Local active areas

being strongly- dependent on time (Sazou et al., 2000b; Sazou et al., 1993b).

Moreover, Fig. 6 shows that increasing gradually the *c*Cl- the current in the passive state increases (Table 1). At *c*Cl- > 15 mM, both *I*pas,f and *I*pas,b tend to reach a limiting value within the potential region between ~0.3 and 2.5 V whereas new oscillatory states emerge out of the passive state. These current oscillations are associated with the precipitation-dissolution of ferrous salt layers in front of pits grown on the Fe surface, while the OER rate diminishes (Sazou et al., 2000b; Sazou & Pagitsas, 2003a).

Adding Br and I ions leads to similar changes in the corresponding potentiodynamic *I*-*E* curves with those mentioned above. However, comparing quantities such as the (*I*osc)max /(*I*osc)max, exp, Δ*Ε*osc and the current in the passive state allows characterization of the extent of pitting corrosion induced by each halide ion (Sazou et al., 2000a). This becomes obvious from Table 1, which summarizes the values of these quantities for all halides in comparison with those obtained for the halide-free Fe|0.75 M H2SO4 system. At relatively low halide concentrations, these quantities depend on *c*X- and the aggressiveness of halides. It is thus concerned with localized oxide breakdown and repeated activation-repassivation events of the entire Fe-disc surface at early stages of pitting corrosion. However, depending on the halide identity, additional individual differences are noticed in the case of I and fluoride species. In the former case, it is observed that *I*pas,f > *I*pas,b, which is an inverse relationship compared to that anticipated for pitting corrosion induced by Cl- and Br-. This is assigned to the formation of a compact iodine layer over the Fe surface due to iodide electrochemical oxidation. Iodine layer seems to prevent the evolution of pit growth. In he case of fluoride species, though the Δ*Ε*osc is extended towards higher potentials, drastic changes in the *I*osc, max and the current in the passive state are not observed indicating an enhanced general corrosion instead of pitting.


Table 1. Effect of halide ions, X- on the oscillatory potential region Δ*Ε*osc, the current in the passive state observed during the forward, *I*pas,f and backward, *I*pas,b potential scans, the maximum oscillatory current ratio (*I*osc)max/(*I*osc)max, exp and the transpassivation potential, *E*tr or the pitting potential, *E*pit appeared in the presence of X-.

Moreover, Fig. 6 shows that increasing gradually the *c*Cl- the current in the passive state increases (Table 1). At *c*Cl- > 15 mM, both *I*pas,f and *I*pas,b tend to reach a limiting value within the potential region between ~0.3 and 2.5 V whereas new oscillatory states emerge out of the passive state. These current oscillations are associated with the precipitation-dissolution of ferrous salt layers in front of pits grown on the Fe surface, while the OER rate diminishes

curves with those mentioned above. However, comparing quantities such as the (*I*osc)max /(*I*osc)max, exp, Δ*Ε*osc and the current in the passive state allows characterization of the extent of pitting corrosion induced by each halide ion (Sazou et al., 2000a). This becomes obvious from Table 1, which summarizes the values of these quantities for all halides in comparison with those obtained for the halide-free Fe|0.75 M H2SO4 system. At relatively low halide concentrations, these quantities depend on *c*X- and the aggressiveness of halides. It is thus concerned with localized oxide breakdown and repeated activation-repassivation events of the entire Fe-disc surface at early stages of pitting corrosion. However, depending on the

species. In the former case, it is observed that *I*pas,f > *I*pas,b, which is an inverse relationship compared to that anticipated for pitting corrosion induced by Cl- and Br-. This is assigned to the formation of a compact iodine layer over the Fe surface due to iodide electrochemical oxidation. Iodine layer seems to prevent the evolution of pit growth. In he case of fluoride species, though the Δ*Ε*osc is extended towards higher potentials, drastic changes in the *I*osc, max and the current in the passive state are not observed indicating an enhanced general

Addition *c* (mM) Δ*Ε*osc (mV) *I*pas, f(mA) *I*pas, b(mA) (*I*osc)max/(*I*osc)max, exp *E*tr , *E*pit (V)

None - 235-270 0.15 0.15 1.01 1.65 NaF 10 240-290 0.22 0.22 1.05 1.65 20 245-310 0.25 0.25 1.03 1.65 NaCl 10 240-380 2.4 13.7 1.2 1.00 20 290-520 23.7 22.4 1.23 LCR NaBr 10 255-500 3.9 3.7 1.13 1.4 20 280-700 22.5 22.5 1.18 LCR NaI 10 243-440 6.8 1.12 1.01 1.55 20 245-450 5.5 0.6 1.02 1.37

Table 1. Effect of halide ions, X- on the oscillatory potential region Δ*Ε*osc, the current in the passive state observed during the forward, *I*pas,f and backward, *I*pas,b potential scans, the maximum oscillatory current ratio (*I*osc)max/(*I*osc)max, exp and the transpassivation potential, *E*tr

or the pitting potential, *E*pit appeared in the presence of X-.

halide identity, additional individual differences are noticed in the case of I-

and I- ions leads to similar changes in the corresponding potentiodynamic *I*-*E*

and fluoride

(Sazou et al., 2000b; Sazou & Pagitsas, 2003a).

Adding Br-

corrosion instead of pitting.

Besides changes observed in potentiodynamic *I* = f(*E*) curves, pitting corrosion manifests itself in potentiostatic current oscillations too. An example of halide-induced oscillations is given in Fig. 7a. Fig. 7a displays a transition between single periodic oscillations observed in the halide-free system, immediately after switching on the potential at *E* < *E*F, and the halide-induced complex periodic oscillations appeared after an induction period of time, *t*ind (Fig. 7b). The halide-induced current oscillations occur over a wide potential region (Table 1) and their periodicity was found to follow period doubling, quadrupling and aperiodicity by increasing the applied potential, *E* and *c*X- (Sazou et al., 2000b). Depending on *E* and *c*Xdifferent temporal patterns were recorded such as bursting and beating. Variation of the current waveform was also observed at longer times as it is anticipated for pitting corrosion, being strongly- dependent on time (Sazou et al., 2000b; Sazou et al., 1993b).

Fig. 7. (a) Transition of the single periodic to a complex periodic current oscillation induced by adding Cl into the Fe|0.75 M H2SO4 system. Next to each oscillation waveform, SEM micrographs display the corresponding Fe surface morphologies. (b) Induction time, *t*ind occurring prior the onset of chloride-induced oscillations and dependence of *t*ind on the potential at various *c*Cl-

The complex oscillations induced by Cl- as well as Br- and I- is the result of the aggressive action of halides on the Fe surface (Sazou et al., 2000a; Sazou et al., 2000b). This is confirmed by SEM observations, an example of which is depicted in Fig. 7a. The morphology of the Fedisc surface during the occurrence of single periodic oscillations reflects a general corrosion induced by H+ ions. Hydrogen ions catalyze the uniform dissolution of the passive oxide film consisting primarily of Fe3O4 (Engell, 1977; Sato, 1990). Associated with temporal current patterning, are also spatial phenomena that deserve to be investigated (Hudson et al., 1993). On the other hand, when complex periodic current oscillations occur in the presence of chlorides and other X- ions, the morphology of the Fe surface reveals hemispherical pits as a result of the local breakdown of passivity on Fe. Local active areas

Oscillatory Phenomena as a Probe to Study

(Sazou & Pagitsas, 2003).

Besides, Cl-, Br-

*I*pas,b, (*I*osc)max/(*I*osc)max, exp, *E*pit, *t*ind and *T*.

corrosive media (Sazou & Pagitsas, 2002).

**5.2 Under current-controlled conditions** 

1. Potential oscillations at a critical *c*Cl-.

by increasing *c*Cl-.

gradually increasing *c*Cl-. Chlorides seem to induce:

Pitting Corrosion of Iron in Halide-Containing Sulfuric Acid Solutions 75

induced by H+ ions only around the *E*F (Fig. 3) and hence *T* decreases with increasing the *c*H+

Fig. 8. Dependence of the oscillation period, *T* as a function of the applied potential, *E* at (a)

Therefore, distinction between pitting and general corrosion is possible on the basis of quantities obtained from both potentiodynamic *I* = f(*E*) and potentiostatic *I* = f(*t*) curves.

perturbing the Fe|0.75 M H2SO4 system (Pagitsas et al., 2007; Pagitsas et al., 2008). The effect of chlorates and perchlorates on the Fe passive surface, being disputable in the literature, was clarified using all the above-mentioned diagnostic criteria including Δ*Ε*osc, *I*pas,f and

Moreover, these diagnostic criteria were also tested in the presence of nitrates in chloridecontaining sulfuric acid solutions (Sazou & Pagitsas, 2002). Newman and Ajjawi characterized the effect of nitrates on stainless steel as peculiar (Newmann & Ajjawi, 1986). Regarding pitting corrosion, nitrates may act either as activating or inhibiting species (Fujioka et al., 1996). This property of nitrates was also realized in the case of Fe. It is manifested in several features of current oscillations observed over a wide potential region. Appropriate analysis of *I* = f(*E*) and *I* = f(*t*) curves demonstrated that nitrates may stimulate pitting corrosion at lower potentials, while may cause a sudden passivation of Fe at higher potentials. This behavior is interpreted by taking into account the electrochemical and homogeneous chemical reactions of nitrates. Current oscillation seems to be a suitable probe to indicate both qualitatively and quantitatively if a stable passive state is established in

Fig. 9 shows galvanodynamic *E* = f(*I*) curves of the Fe|0.75 M H2SO4 system traced at

2. Occurrence of more potential oscillations during the backward current scan as well as

, I- and fluoride species, chlorates and perchlorates were also used in

various *c*Cl- and (b) various concentrations of fluoride species.

generated by the local action of halides results in an inhomogeneous passive Fe surface and perhaps in new spatiotemporal patterns.

It is noted that besides the potential, the solution resistance, *R*s or equally a variable external series-resistance, *R*ex inserted between the working and reference electrodes, the rotation speed, ω of the rotating Fe-disc electrode, the solution *pH* and temperature, all parameters control the nonlinear behavior of the Fe|0.75 M H2SO4 electrochemical oscillator. However, though these contol parameters influence the onset, the period and amplitude of oscillations, none of them changes the oscillation waveform. Whenever current oscillations appear across the passive-to-active transition region (at *E* < *E*F), they are single periodic of relaxation type. To our knowledge, the type of these oscillations will change only when a halide-induced chemical perturbation of the passive state of the Fe| H2SO4 system is conducted and pitting instead of uniform corrosion occurs. This is a striking indication that new physico-electrochemical processes have been triggered by halides manifested in the variety of complex oscillations.

Inspection of Fig. 7b shows that a fluctuating steady current exist during the induction period of time, *t*ind elapsed before the onset of oscillations. This is indicative of the aggressive action of Cl- , which leads to the nucleation of small pits that in turn are repassivated immediately. It proceeds until a complete destabilization of the passive state occurs (1st activation event). The transition to the active state is followed by repeated passivation-activation events (complex oscillations) that constitute a phenomenon termed as unstable pitting corrosion. Therefore, the *t*ind characterizes the kinetics of the oxide attack by X- ions. It was found that *t*ind depends on both the *c*X- and *E*. An example is given in Fig. 7b for a chloride- perturbed Fe|0.75 M H2SO4 system. As Fig. 7b shows, increasing the *c*Xleads to the decrease of *t*ind indicating promotion of the local breakdown of the oxide film. On the contrary, increasing the applied potential the *t*ind increases. The rate of unstable pitting corrosion diminishes by increasing the potential due to the enhancement of oxide stability.

Another quantity describing quantitatively the competitive process between halide action and enhancement of oxide stability is the oscillation period, *T*. As can be seen in Fig. 8a, *T* decreases with *c*Cl-, while it increases with *E*. It is noticed that the increase of *T* observed at lower *c*Cl- is interpreted in terms of changes in periodicity since period doubling and quadrupling occurs. More accurate empirical relationships are obtained if an average activation rate (number of spikes over a period of time) is employed instead of *T* (Pagitsas et al., 2001; Pagitsas et al., 2002; Sazou et al., 2000a).

In this context, it should be noted that the decrease of *T* with *c*Cl- is also associated with the general corrosion occurring concurrently with unstable pitting corrosion. As can be seen in Fig. 8b, a similar dependence of *T* on the concentration of fluoride species is also obtained. It is well known that fluorides in acid solutions (*pH* ~ 0-0.5) cause general corrosion but not pitting since HF is the largely predominant species, while other fluoride species may coexist at negligible amounts (Pagitsas et al., 2001; Pagitsas et al., 2002; Strehblow, 1995). In agreement with the fluoride effect, is also the effect of *c*H+ on *T* (Sazou & Pagitsas, 2003b). As was mentioned above, the onset of current oscillations in the halide-free Fe|0.75 M H2SO4 system is assigned to destabilization of the oxide film due to *pH* changes occurring uniformly at the Fe surface (Eq. (3)). In this case, general corrosion of the oxide film is

generated by the local action of halides results in an inhomogeneous passive Fe surface and

It is noted that besides the potential, the solution resistance, *R*s or equally a variable external series-resistance, *R*ex inserted between the working and reference electrodes, the rotation speed, ω of the rotating Fe-disc electrode, the solution *pH* and temperature, all parameters control the nonlinear behavior of the Fe|0.75 M H2SO4 electrochemical oscillator. However, though these contol parameters influence the onset, the period and amplitude of oscillations, none of them changes the oscillation waveform. Whenever current oscillations appear across the passive-to-active transition region (at *E* < *E*F), they are single periodic of relaxation type. To our knowledge, the type of these oscillations will change only when a halide-induced chemical perturbation of the passive state of the Fe| H2SO4 system is conducted and pitting instead of uniform corrosion occurs. This is a striking indication that new physico-electrochemical processes have been triggered by halides manifested in the

Inspection of Fig. 7b shows that a fluctuating steady current exist during the induction period of time, *t*ind elapsed before the onset of oscillations. This is indicative of the aggressive action of Cl-, which leads to the nucleation of small pits that in turn are repassivated immediately. It proceeds until a complete destabilization of the passive state occurs (1st activation event). The transition to the active state is followed by repeated passivation-activation events (complex oscillations) that constitute a phenomenon termed as unstable pitting corrosion. Therefore, the *t*ind characterizes the kinetics of the oxide attack by X- ions. It was found that *t*ind depends on both the *c*X- and *E*. An example is given in Fig. 7b for a chloride- perturbed Fe|0.75 M H2SO4 system. As Fig. 7b shows, increasing the *c*Xleads to the decrease of *t*ind indicating promotion of the local breakdown of the oxide film. On the contrary, increasing the applied potential the *t*ind increases. The rate of unstable pitting corrosion diminishes by increasing the potential due to the enhancement of oxide

Another quantity describing quantitatively the competitive process between halide action and enhancement of oxide stability is the oscillation period, *T*. As can be seen in Fig. 8a, *T* decreases with *c*Cl-, while it increases with *E*. It is noticed that the increase of *T* observed at lower *c*Cl- is interpreted in terms of changes in periodicity since period doubling and quadrupling occurs. More accurate empirical relationships are obtained if an average activation rate (number of spikes over a period of time) is employed instead of *T* (Pagitsas et

In this context, it should be noted that the decrease of *T* with *c*Cl- is also associated with the general corrosion occurring concurrently with unstable pitting corrosion. As can be seen in Fig. 8b, a similar dependence of *T* on the concentration of fluoride species is also obtained. It is well known that fluorides in acid solutions (*pH* ~ 0-0.5) cause general corrosion but not pitting since HF is the largely predominant species, while other fluoride species may coexist at negligible amounts (Pagitsas et al., 2001; Pagitsas et al., 2002; Strehblow, 1995). In agreement with the fluoride effect, is also the effect of *c*H+ on *T* (Sazou & Pagitsas, 2003b). As was mentioned above, the onset of current oscillations in the halide-free Fe|0.75 M H2SO4 system is assigned to destabilization of the oxide film due to *pH* changes occurring uniformly at the Fe surface (Eq. (3)). In this case, general corrosion of the oxide film is

perhaps in new spatiotemporal patterns.

variety of complex oscillations.

al., 2001; Pagitsas et al., 2002; Sazou et al., 2000a).

stability.

induced by H+ ions only around the *E*F (Fig. 3) and hence *T* decreases with increasing the *c*H+ (Sazou & Pagitsas, 2003).

Fig. 8. Dependence of the oscillation period, *T* as a function of the applied potential, *E* at (a) various *c*Cl- and (b) various concentrations of fluoride species.

Therefore, distinction between pitting and general corrosion is possible on the basis of quantities obtained from both potentiodynamic *I* = f(*E*) and potentiostatic *I* = f(*t*) curves. Besides, Cl- , Br- , I- and fluoride species, chlorates and perchlorates were also used in perturbing the Fe|0.75 M H2SO4 system (Pagitsas et al., 2007; Pagitsas et al., 2008). The effect of chlorates and perchlorates on the Fe passive surface, being disputable in the literature, was clarified using all the above-mentioned diagnostic criteria including Δ*Ε*osc, *I*pas,f and *I*pas,b, (*I*osc)max/(*I*osc)max, exp, *E*pit, *t*ind and *T*.

Moreover, these diagnostic criteria were also tested in the presence of nitrates in chloridecontaining sulfuric acid solutions (Sazou & Pagitsas, 2002). Newman and Ajjawi characterized the effect of nitrates on stainless steel as peculiar (Newmann & Ajjawi, 1986). Regarding pitting corrosion, nitrates may act either as activating or inhibiting species (Fujioka et al., 1996). This property of nitrates was also realized in the case of Fe. It is manifested in several features of current oscillations observed over a wide potential region. Appropriate analysis of *I* = f(*E*) and *I* = f(*t*) curves demonstrated that nitrates may stimulate pitting corrosion at lower potentials, while may cause a sudden passivation of Fe at higher potentials. This behavior is interpreted by taking into account the electrochemical and homogeneous chemical reactions of nitrates. Current oscillation seems to be a suitable probe to indicate both qualitatively and quantitatively if a stable passive state is established in corrosive media (Sazou & Pagitsas, 2002).

### **5.2 Under current-controlled conditions**

Fig. 9 shows galvanodynamic *E* = f(*I*) curves of the Fe|0.75 M H2SO4 system traced at gradually increasing *c*Cl-. Chlorides seem to induce:


Oscillatory Phenomena as a Probe to Study

occur due to the localized action of I-

Pitting Corrosion of Iron in Halide-Containing Sulfuric Acid Solutions 77

Vitt & Johnson, 1992). This iodine film hinders the activation of the Fe surface expected to

fact, the *I*act remains equal with that of the unperturbed system (Fig. 9a). Instead of large amplitude oscillations, indicative of localized corrosion, a new type of potential oscillation emerges (Fig. 10d) associated with the OER occurring concurrently with the formationdissolution of the iodine layer. The features of these low amplitude oscillations are influenced by the *c*I- and applied current values and become more pronounced at higher *c*I-

action on the passive Fe surface becomes very complicated under current-controlled conditions. Further investigation within a different context deserves to be carried out.

Fig. 10. Comparison of the effect of various halide species on the galvanodynamic *E* = f(*I*)

Table 2 summarizes the values of *I*pas and *I*act obtained for the halide-free Fe|0.75 M H2SO4 system in comparison with the corresponding values evaluated for the halide-perturbed one. The occurrence of potential oscillations and the quantity *I*act are associated with pitting corrosion. The *I*act increases by increasing either the halide concentration or the aggressiveness of halides implying stimulation of pitting corrosion. The higher the *I*act or the lower the width of the hysteresis loop is, Δ*Ι*, the greater is the susceptibility of Fe to pitting

> Br- is found, in agreement with the order found from the nonlinear dynamical response obtained under potential-controlled conditions (Pagitsas et al., 2002; Sazou et al., 2000a), as well as with literature data based on other criteria (Janik-Czachor, 1981; Macdonald, 1992;

The current region within which potential oscillations are expected to occur at a constant applied current, *I*appl can be deduced from the *E*=f(*I*) curves. As was mentioned in the

curves of the Fe|0.75 M H2SO4 system traced at d*I*/d*t* = 0.05 mA s-1.

corrosion. Comparing the aggressiveness of Cl- and Br-

Strehblow, 1995).

(~50 mM). Therefore, the electrochemical and chemical behavior of I-

. Thus any noticeable increase of *I*act is not shown. In

together with their

in terms of the *I*act or Δ*Ι* , the order Cl-

3. Considerable increase of *I*act with a slight decrease of *I*pas by increasing *c*Cl-. Hence the width of the hysteresis loop, Δ*I* = |*I*pas - *I*act| decreases (Fig. 9a). The *I*pas and *I*act are defined as the critical current values where transition to passive and active states occurs during the forward and inversely backward current scans, respectively.

Fig. 9. Chloride-induced changes in the galvanodynamic *E* = f(*I*) curves of the Fe|0.75 M H2SO4 system traced at d*I*/d*t* = 0.05 mA s-1.

Apparently, potential oscillations in galvanodynamic *E* = f(*I*) curves constitute manifestation of pitting corrosion since, as was mentioned in sections 3 & 4, no oscillations should occur for the halide-free Fe|0.75 M H2SO4 system under a current control. Corresponding changes in *E* = f(*I*) curves are also induced by adding other halide species. An example of *E* = f(*I*) curves at 20 mM of fluorides, Cl- , Br- and I ions is illustrated in Fig. 10.

Comparing the *E* = f(*I*) curves illustrated in Fig. 10, it seems that fluorides do not induce potential oscillations, as it should be anticipated, since fluorides cause only general corrosion of the Fe surface. In the present case, general corrosion proceeds concurrently with the OER exemplifying itself by the occurrence of small amplitude potential fluctuations due to the formation and subsequent escape of oxygen bubbles from the Fe surface (Fig. 10a). Regarding Br-, potential oscillation does appear (Fig. 10c), though to a lesser degree than in the presence of Cl- (Fig. 10b), in agreement with the lesser aggressiveness of Br as compared with that of Cl-.

Inspecting Fig. 10d, one may see that no pitting corrosion occurs in the presence of I-. However, it is well known that iodide does cause pitting corrosion (Strehblow, 1995), manifested also in both the *I*=f(*E*) and *I*=f(*t*) curves (Pagitsas et al., 2002; Sazou et al., 2000a). This apparent discrepancy can be interpreted by taking into account oxidation processes of iodides that result in the formation of a solid iodine film on the Fe surface (Ma & Vitt, 1999;

3. Considerable increase of *I*act with a slight decrease of *I*pas by increasing *c*Cl-. Hence the width of the hysteresis loop, Δ*I* = |*I*pas - *I*act| decreases (Fig. 9a). The *I*pas and *I*act are defined as the critical current values where transition to passive and active states occurs

Fig. 9. Chloride-induced changes in the galvanodynamic *E* = f(*I*) curves of the Fe|0.75 M

Apparently, potential oscillations in galvanodynamic *E* = f(*I*) curves constitute manifestation of pitting corrosion since, as was mentioned in sections 3 & 4, no oscillations should occur for the halide-free Fe|0.75 M H2SO4 system under a current control. Corresponding changes in *E* = f(*I*) curves are also induced by adding other halide species. An example of *E* = f(*I*)

Comparing the *E* = f(*I*) curves illustrated in Fig. 10, it seems that fluorides do not induce potential oscillations, as it should be anticipated, since fluorides cause only general corrosion of the Fe surface. In the present case, general corrosion proceeds concurrently with the OER exemplifying itself by the occurrence of small amplitude potential fluctuations due to the formation and subsequent escape of oxygen bubbles from the Fe surface (Fig. 10a). Regarding Br-, potential oscillation does appear (Fig. 10c), though to a lesser degree than in

Inspecting Fig. 10d, one may see that no pitting corrosion occurs in the presence of I-. However, it is well known that iodide does cause pitting corrosion (Strehblow, 1995), manifested also in both the *I*=f(*E*) and *I*=f(*t*) curves (Pagitsas et al., 2002; Sazou et al., 2000a). This apparent discrepancy can be interpreted by taking into account oxidation processes of iodides that result in the formation of a solid iodine film on the Fe surface (Ma & Vitt, 1999;

ions is illustrated in Fig. 10.

(Fig. 10b), in agreement with the lesser aggressiveness of Br- as compared

H2SO4 system traced at d*I*/d*t* = 0.05 mA s-1.

curves at 20 mM of fluorides, Cl- , Br- and I-

the presence of Cl-

with that of Cl-.

during the forward and inversely backward current scans, respectively.

Vitt & Johnson, 1992). This iodine film hinders the activation of the Fe surface expected to occur due to the localized action of I- . Thus any noticeable increase of *I*act is not shown. In fact, the *I*act remains equal with that of the unperturbed system (Fig. 9a). Instead of large amplitude oscillations, indicative of localized corrosion, a new type of potential oscillation emerges (Fig. 10d) associated with the OER occurring concurrently with the formationdissolution of the iodine layer. The features of these low amplitude oscillations are influenced by the *c*I- and applied current values and become more pronounced at higher *c*I- (~50 mM). Therefore, the electrochemical and chemical behavior of I together with their action on the passive Fe surface becomes very complicated under current-controlled conditions. Further investigation within a different context deserves to be carried out.

Fig. 10. Comparison of the effect of various halide species on the galvanodynamic *E* = f(*I*) curves of the Fe|0.75 M H2SO4 system traced at d*I*/d*t* = 0.05 mA s-1.

Table 2 summarizes the values of *I*pas and *I*act obtained for the halide-free Fe|0.75 M H2SO4 system in comparison with the corresponding values evaluated for the halide-perturbed one. The occurrence of potential oscillations and the quantity *I*act are associated with pitting corrosion. The *I*act increases by increasing either the halide concentration or the aggressiveness of halides implying stimulation of pitting corrosion. The higher the *I*act or the lower the width of the hysteresis loop is, Δ*Ι*, the greater is the susceptibility of Fe to pitting corrosion. Comparing the aggressiveness of Cl- and Br in terms of the *I*act or Δ*Ι* , the order Cl- > Br- is found, in agreement with the order found from the nonlinear dynamical response obtained under potential-controlled conditions (Pagitsas et al., 2002; Sazou et al., 2000a), as well as with literature data based on other criteria (Janik-Czachor, 1981; Macdonald, 1992; Strehblow, 1995).

The current region within which potential oscillations are expected to occur at a constant applied current, *I*appl can be deduced from the *E*=f(*I*) curves. As was mentioned in the

Oscillatory Phenomena as a Probe to Study

decreases by increasing the *c*Cl-

nitrates on pitting corrosion.

perhaps never-activated sites.

chloride-perturbed Fe|0.75 M H2SO4 system.

critical value (>5 mM) and only if *I*appl > *I*act.

on Fe and activation of the entire Fe surface.

average frequency that increases with increasing *c*Cl- and *I*appl.

and *I*appl.

presence of Br-

Pitting Corrosion of Iron in Halide-Containing Sulfuric Acid Solutions 79

Similar potential oscillations, with those illustrated in Fig. 11, were also observed in the

1. Onset of potential oscillations of large amplitude (~2 V) when *c*Cl- is higher than a

2. The potential oscillates between the two steady states, namely the passive and active states. This indicates that initiation of pitting results in the destabilization of passivity

3. Different waveforms of potential oscillations depending on *c*Cl- (or *c*Br-) and *I*appl with an

4. Occurrence of certain induction period of time, *ti*nd before oscillations start, which

The dependence of *ti*nd and average oscillation frequency with *c*Cl- and *I*appl is displayed in Figs. 12a, b. The oscillation frequency is expressed as the average firing rate, <*r*> defined by the ratio, <*r*> = *N*/Δ*τ*, where *N* is the number of spikes (passive-active events) appeared during a fixed duration, Δ*τ* of the experiment (Dayan & Abbott, 2001). The *N* is measured at *t* > *t*ind (Sazou et al., 2009). It becomes clear that *ti*nd reflects the kinetics of pit initiation on the passive Fe surface whereas <*r*> is rather related to the pit growth and propagation. Therefore, both *ti*nd and <*r*> can be used to describe quantitatively pitting corrosion on passive Fe. The quantities *ti*nd and <*r*> are currently used to estimate the inhibiting effect of

When Fe is in the passive state (high-potential state) and chlorides start their action generating local active areas on the Fe surface, the oxide becomes gradually dark brown due to the conversion of its outer layer into ferrous oxo-chloride complexes. At the moment of Fe activation, all anodic layers, being separated from the Fe substrate, seem streaming away from the electrode. Due to the high current, the active Fe surface abruptly passivates and correspondingly the potential increases to its highest value. SEM images reveal an inhomogeneous growth of the passive oxide since it covers both localized activated and

Fig. 12. Dependence of the (a) induction time, *t*ind required for potential oscillations to start and (b) average firing rate, <*r*> as a function of the applied current, *I*appl and *c*Cl- for the

curves at constant *I*appl for various *c*Cl- and at constant *c*Cl- for various *I*appl include:

. In summary, chloride- and bromide-induced changes in galvanostatic *E*=f(*t*)



Table 2. Effect of halides, X on the current, *I*pas at which transition to passivity occurs during the forward current scan, the current *I*act, where reactivation occurs during the backward current scan and the width of the hysteresis loop, Δ*Ι* defined from galvanodynamic curves (d*I*/d*t*=0.05 mA s-1) of the Fe|0.75 M H2SO4 system.

Fig. 11. Chloride-induced potential oscillations of the Fe|0.75 M H2SO4 system traced under galvanostatic conditions at *I*appl=30 mA.

beginning of this section, *E*=f(*I*) curves represent roughly one-parameter bifurcation diagrams. It seems that, for oscillations to occur, the *I*appl should be approximately higher than *I*act. Fig. 11 shows examples of galvanostatic *E*=f(*t*) curves traced for 20 min at *I*appl = 30 mA for the halide-free and chloride-perturbed Fe|0.75 M H2SO4 system at different *c*Cl-.

Addition *c* (mM) *I*pas (mA) *I*act (mA) Δ*I*= *I*pas- *I*act

on the current, *I*pas at which transition to passivity occurs during

None - 33 0.15 32.85 NaF 10 33 0.22 32.78 20 32 0.25 31.75 NaCl 10 30 20 10 20 29 25 4 NaBr 10 32 18 14 20 29 22.6 6.4 NaI 10 24 0.15 23.85 20 28.9 0.15 28.75

the forward current scan, the current *I*act, where reactivation occurs during the backward current scan and the width of the hysteresis loop, Δ*Ι* defined from galvanodynamic curves

Fig. 11. Chloride-induced potential oscillations of the Fe|0.75 M H2SO4 system traced under

Table 2. Effect of halides, X-

(d*I*/d*t*=0.05 mA s-1) of the Fe|0.75 M H2SO4 system.

galvanostatic conditions at *I*appl=30 mA.

Similar potential oscillations, with those illustrated in Fig. 11, were also observed in the presence of Br- . In summary, chloride- and bromide-induced changes in galvanostatic *E*=f(*t*) curves at constant *I*appl for various *c*Cl- and at constant *c*Cl- for various *I*appl include:


The dependence of *ti*nd and average oscillation frequency with *c*Cl- and *I*appl is displayed in Figs. 12a, b. The oscillation frequency is expressed as the average firing rate, <*r*> defined by the ratio, <*r*> = *N*/Δ*τ*, where *N* is the number of spikes (passive-active events) appeared during a fixed duration, Δ*τ* of the experiment (Dayan & Abbott, 2001). The *N* is measured at *t* > *t*ind (Sazou et al., 2009). It becomes clear that *ti*nd reflects the kinetics of pit initiation on the passive Fe surface whereas <*r*> is rather related to the pit growth and propagation. Therefore, both *ti*nd and <*r*> can be used to describe quantitatively pitting corrosion on passive Fe. The quantities *ti*nd and <*r*> are currently used to estimate the inhibiting effect of nitrates on pitting corrosion.

When Fe is in the passive state (high-potential state) and chlorides start their action generating local active areas on the Fe surface, the oxide becomes gradually dark brown due to the conversion of its outer layer into ferrous oxo-chloride complexes. At the moment of Fe activation, all anodic layers, being separated from the Fe substrate, seem streaming away from the electrode. Due to the high current, the active Fe surface abruptly passivates and correspondingly the potential increases to its highest value. SEM images reveal an inhomogeneous growth of the passive oxide since it covers both localized activated and perhaps never-activated sites.

Fig. 12. Dependence of the (a) induction time, *t*ind required for potential oscillations to start and (b) average firing rate, <*r*> as a function of the applied current, *I*appl and *c*Cl- for the chloride-perturbed Fe|0.75 M H2SO4 system.

Oscillatory Phenomena as a Probe to Study

(Sazou & Pagitsas, 2003a).

Pitting Corrosion of Iron in Halide-Containing Sulfuric Acid Solutions 81

used in pitting corrosion studies under steady state conditions (Sato, 1987; Sato, 1989). *E*R it is the critical potential at which a transition from a polishing state dissolution (bright pits) to active state dissolution (etching pits) occurs. Critical conditions for the onset of different types of oscillations may be defined in terms of the critical pit solution composition (critical *c*Cl- and *c*H+) at which Fe cannot sustain passivity and, thereby, pit stabilization is possible

Fig. 13. (a) Non-linear dynamical response of the Fe|0.75 M H2SO4 system in the presence of relatively high *c*Cl-. It exhibits a transition from a situation where oscillations of type I and II appear in the *I*-*E* curves to one where oscillations of type II dominate the whole LCR at *E* > 0.3 V. Polarization curves were traced at d*E*/d*t* = 2 mV s-1. (b) Representative examples of type I and type II current oscillations corresponding to the *I*=f(*E*) curves shown in (a).

An example of a potential-induced transition between potentiostatic current oscillations of type I (aperiodic bursting) to those of type II (low amplitude chaotic oscillations) is illustrated in Fig. 14 for the Fe|0.75 M H2SO4 + 30 mM Cl- system. It seems that this transition occurs around *E*bif = 0.55 V which coincides with the *E*R (Sazou & Pagitsas, 2003a). Oscillations of type II occurring at either high potentials of the oscillatory region at relatively low *c*Cl- or within the entire oscillatory region at sufficiently high *c*Cl- originate from processes similar to those responsible for chaotic oscillations observed at the beginning of the LCR (*E* < 0.3 V) shown in the *I* = f(*E*) curve of the halide-free Fe|0.75 M H2SO4 system (Fig. 3a) (Sazou & Pagitsas, 2006b). Supersaturation conditions of the ferrous salts established inside pits results in a density gradient Δ*d* between the solution in the interfacial regime in front of the Fe electrode and the bulk solution. When Δ*d* exceeds a critical value the steady limiting current becomes unstable. This condition is fulfilled in

The mechanism of passive-active oscillations associated with unstable pitting corrosion includes the formation and detachment of the oxide film that can be sufficiently explained in terms of the point defect model (PDM) (Macdonald, 1992; Pagitsas et al., 2001; Pagitsas et al., 2002; Pagitsas et al., 2003; Sazou et al., 2009). PDM is a realistic quantitative model that includes many of the oxide properties and explains many of the experimental observations during oxide growth and its breakdown. The processes leading to pitting corrosion are associated with the occupation of oxygen vacancies by halides, X- . This reaction results in perturbation of a Schottky-pair equilibrium and autocatalytic generation of cation vacancies. Cation vacancies accumulate at the Fe|oxide interface leading to the formation of void and separation of the oxide from the Fe substrate. Simultaneously, the thickness of the oxide film decreases due to general corrosion through the formation of surface complexes between iron lattice-cations and halides. When the void exceeds a critical size and the oxide film over the void thins below a critical thickness film breakdown occurs at this particular site (Macdonald, 1992; Sazou et al., 2009).

At sufficiently high *c*Cl- or *c*Br-, dissolution rates are enhanced whereas oxide formation becomes unlikely. Instead, formation of ferrous salt layers is facilitated leading to the electropolishing dissolution state (Li et al., 1993; Li et al., 1990). This situation is discussed briefly below only in the case of potential-controlled conditions.

### **6. Non-linear dynamical response of the Fe|H2SO4 system at relatively high concentrations of halides**

Fig. 13a shows that at relatively high halide concentrations (i.e. *c*Cl- > 20 mM), Fe cannot sustain passivity and a limiting current region (LCR) is established out of the passive state within 0.3 and 2.7 V (the upper potential limit used in the potentiodynamic measurements). This LCR is due to the precipitation-dissolution of salt layers since the oxide growth is prevented by Cl- and differs from the LCR appeared for *E* < *E*F, where oxide formation is thermodynamically prohibited. Within this "new" LCR, two distinct types of current oscillations are observed.


An induction period of time, *ti*nd is elapsed before current oscillations of type I or II appear. During *ti*nd, the current reaches a steady state value during which precipitation-dissolution of ferrous salts occurs at equal rates. Precipitation of ferrous salt occurs inside pits when a local supersaturation condition for Fe2+ and sulfates/chlorides is reached. There are evidences (Sazou & Pagitsas, 2003a) that the bifurcation potential, *E*bif, for the transition from oscillations of type II to those of type I coincides with the repassivation potential, *E*<sup>R</sup>

The mechanism of passive-active oscillations associated with unstable pitting corrosion includes the formation and detachment of the oxide film that can be sufficiently explained in terms of the point defect model (PDM) (Macdonald, 1992; Pagitsas et al., 2001; Pagitsas et al., 2002; Pagitsas et al., 2003; Sazou et al., 2009). PDM is a realistic quantitative model that includes many of the oxide properties and explains many of the experimental observations during oxide growth and its breakdown. The processes leading to pitting corrosion are

perturbation of a Schottky-pair equilibrium and autocatalytic generation of cation vacancies. Cation vacancies accumulate at the Fe|oxide interface leading to the formation of void and separation of the oxide from the Fe substrate. Simultaneously, the thickness of the oxide film decreases due to general corrosion through the formation of surface complexes between iron lattice-cations and halides. When the void exceeds a critical size and the oxide film over the void thins below a critical thickness film breakdown occurs at this particular site

At sufficiently high *c*Cl- or *c*Br-, dissolution rates are enhanced whereas oxide formation becomes unlikely. Instead, formation of ferrous salt layers is facilitated leading to the electropolishing dissolution state (Li et al., 1993; Li et al., 1990). This situation is discussed

**6. Non-linear dynamical response of the Fe|H2SO4 system at relatively high** 

Fig. 13a shows that at relatively high halide concentrations (i.e. *c*Cl- > 20 mM), Fe cannot sustain passivity and a limiting current region (LCR) is established out of the passive state within 0.3 and 2.7 V (the upper potential limit used in the potentiodynamic measurements). This LCR is due to the precipitation-dissolution of salt layers since the oxide growth is prevented by Cl- and differs from the LCR appeared for *E* < *E*F, where oxide formation is thermodynamically prohibited. Within this "new" LCR, two distinct types of current

• Type I, called also as passive-active oscillations appeared within the lower potential regime (*E* < 0.6 V) either as a continuous spiking (beating) or aperiodic bursting. These oscillations arise out of a limiting current state with a full-developed amplitude and differ from those observed at relatively low *c*Cl- , which arise out of a passive

• Type II, chaotic oscillations of a relatively small amplitude occurring at higher potentials (*E* > 0.6 V). The extent of each oscillatory regime depends on the halide concentration and halide identity. Upon a further increase of *c*Cl-, the regime corresponding to oscillations of type I is restricted gradually. For *c*Cl->40 mM, current

An induction period of time, *ti*nd is elapsed before current oscillations of type I or II appear. During *ti*nd, the current reaches a steady state value during which precipitation-dissolution of ferrous salts occurs at equal rates. Precipitation of ferrous salt occurs inside pits when a local supersaturation condition for Fe2+ and sulfates/chlorides is reached. There are evidences (Sazou & Pagitsas, 2003a) that the bifurcation potential, *E*bif, for the transition from oscillations of type II to those of type I coincides with the repassivation potential, *E*<sup>R</sup>

oscillations of type II dominate the entire LCR for *E* > 0.3 V (Fig. 13a).

. This reaction results in

associated with the occupation of oxygen vacancies by halides, X-

briefly below only in the case of potential-controlled conditions.

(Macdonald, 1992; Sazou et al., 2009).

**concentrations of halides** 

oscillations are observed.

state (Fig. 7).

used in pitting corrosion studies under steady state conditions (Sato, 1987; Sato, 1989). *E*R it is the critical potential at which a transition from a polishing state dissolution (bright pits) to active state dissolution (etching pits) occurs. Critical conditions for the onset of different types of oscillations may be defined in terms of the critical pit solution composition (critical *c*Cl- and *c*H+) at which Fe cannot sustain passivity and, thereby, pit stabilization is possible (Sazou & Pagitsas, 2003a).

Fig. 13. (a) Non-linear dynamical response of the Fe|0.75 M H2SO4 system in the presence of relatively high *c*Cl-. It exhibits a transition from a situation where oscillations of type I and II appear in the *I*-*E* curves to one where oscillations of type II dominate the whole LCR at *E* > 0.3 V. Polarization curves were traced at d*E*/d*t* = 2 mV s-1. (b) Representative examples of type I and type II current oscillations corresponding to the *I*=f(*E*) curves shown in (a).

An example of a potential-induced transition between potentiostatic current oscillations of type I (aperiodic bursting) to those of type II (low amplitude chaotic oscillations) is illustrated in Fig. 14 for the Fe|0.75 M H2SO4 + 30 mM Cl- system. It seems that this transition occurs around *E*bif = 0.55 V which coincides with the *E*R (Sazou & Pagitsas, 2003a).

Oscillations of type II occurring at either high potentials of the oscillatory region at relatively low *c*Cl- or within the entire oscillatory region at sufficiently high *c*Cl- originate from processes similar to those responsible for chaotic oscillations observed at the beginning of the LCR (*E* < 0.3 V) shown in the *I* = f(*E*) curve of the halide-free Fe|0.75 M H2SO4 system (Fig. 3a) (Sazou & Pagitsas, 2006b). Supersaturation conditions of the ferrous salts established inside pits results in a density gradient Δ*d* between the solution in the interfacial regime in front of the Fe electrode and the bulk solution. When Δ*d* exceeds a critical value the steady limiting current becomes unstable. This condition is fulfilled in

Oscillatory Phenomena as a Probe to Study

growth on an otherwise passive Fe surface.

Pitting Corrosion of Iron in Halide-Containing Sulfuric Acid Solutions 83

affect characteristic features of oscillations, which point to pit initiation, propagation, and

Fig. 15. Flow diagram displaying a phenomenological classification of the nonlinear dynamical response of the halide-perturbed Fe|0.75 M H2SO4 system arisen at various

corrosion associated with complex passive-active current oscillations arisen within a fixed potential region. These oscillations may be employed to distinguish between general and

(*c*Cl- <20 mM) leads to unstable pitting

stages of pitting corrosion.

Perturbation with relatively small amounts of Cl-

pitting corrosion and characterize pit initiation and propagation.

the presence of a critical *IR* drop (Georgolios & Sazou, 1998; Pickering, 1989; Pickering & Frankenthal, 1972).

Fig. 14. Sequence of current oscillations at late stages of pitting corrosion of Fe in 0.75 M H2SO4 + 30 mM Cl- displaying a transition from oscillations of type I to those of type II upon increasing the applied potential, *E*.

### **7. Alternate diagnostic criteria to characterize pitting corrosion at early stages of pitting corrosion**

It becomes clear that the non-linear dynamical response of the halide-perturbed Fe|0.75 M H2SO4 system exemplified either under potential- or current-controlled conditions reflects the aggressive action of halide ions, especially of Cl- on the Fe passive oxide film. Steadystate processes leading to passive and active states of Fe in a halide-free sulfuric acid solution are perturbed through a series of physico-electrochemical reactions including autocatalytic steps. In fact, pit nucleation, propagation and growth are autocatalytic processes (Budiansky et al., 2004; Lunt et al., 2002; Macdonald, 1992). Pit repassivation or stable growth can be realized by investigating the system oscillatory states and oscillation waveform. Therefore, oscillations might be used like a "spectroscopic" technique to detect pitting corrosion and moreover to characterize unstable and stable stages during pit evolution. A summary of oscillatory phenomena expected to arise at different stages of pitting corrosion can be seen in the flow diagram displayed in Fig. 15.

Under potential-controlled conditions, the nonlinear dynamical response of the halideperturbed Fe|0.75 M H2SO4 system recorded in *I*=f(*E*) and *I*=f(*t*) curves is characterized by complex current oscillations. The halide concentration, *c*X-, applied potential, *E* and time, all

the presence of a critical *IR* drop (Georgolios & Sazou, 1998; Pickering, 1989; Pickering &

Fig. 14. Sequence of current oscillations at late stages of pitting corrosion of Fe in 0.75 M H2SO4 + 30 mM Cl- displaying a transition from oscillations of type I to those of type II upon

**7. Alternate diagnostic criteria to characterize pitting corrosion at early** 

pitting corrosion can be seen in the flow diagram displayed in Fig. 15.

It becomes clear that the non-linear dynamical response of the halide-perturbed Fe|0.75 M H2SO4 system exemplified either under potential- or current-controlled conditions reflects the aggressive action of halide ions, especially of Cl- on the Fe passive oxide film. Steadystate processes leading to passive and active states of Fe in a halide-free sulfuric acid solution are perturbed through a series of physico-electrochemical reactions including autocatalytic steps. In fact, pit nucleation, propagation and growth are autocatalytic processes (Budiansky et al., 2004; Lunt et al., 2002; Macdonald, 1992). Pit repassivation or stable growth can be realized by investigating the system oscillatory states and oscillation waveform. Therefore, oscillations might be used like a "spectroscopic" technique to detect pitting corrosion and moreover to characterize unstable and stable stages during pit evolution. A summary of oscillatory phenomena expected to arise at different stages of

Under potential-controlled conditions, the nonlinear dynamical response of the halideperturbed Fe|0.75 M H2SO4 system recorded in *I*=f(*E*) and *I*=f(*t*) curves is characterized by complex current oscillations. The halide concentration, *c*X-, applied potential, *E* and time, all

Frankenthal, 1972).

increasing the applied potential, *E*.

**stages of pitting corrosion** 

affect characteristic features of oscillations, which point to pit initiation, propagation, and growth on an otherwise passive Fe surface.

Fig. 15. Flow diagram displaying a phenomenological classification of the nonlinear dynamical response of the halide-perturbed Fe|0.75 M H2SO4 system arisen at various stages of pitting corrosion.

Perturbation with relatively small amounts of Cl- (*c*Cl- <20 mM) leads to unstable pitting corrosion associated with complex passive-active current oscillations arisen within a fixed potential region. These oscillations may be employed to distinguish between general and pitting corrosion and characterize pit initiation and propagation.

Oscillatory Phenomena as a Probe to Study

systems.

**8. Conclusions** 

aspects of pitting corrosion remain unclear.

kinetics of pit initiation on the passive Fe surface.

Pitting Corrosion of Iron in Halide-Containing Sulfuric Acid Solutions 85

3. The decrease of the induction period of time, *t*ind, elapsed before potential oscillations start, upon increasing gradually either *c*Cl- and *c*Br- or *I*appl. The *t*ind characterizes the

4. The decrease of the average firing rate, <*r*> upon increasing gradually either *c*Cl- and *c*Bror *I*appl. The <*r*> characterizes pit growth and is associated with the conversion of the outermost oxide layer on the Fe surface to an unstable porous, nonprotective iron chloride or bromide ferrous salt film related rather to electropolishing state dissolution.

The formation of mutually interacted pits on the passive metal surface is necessary for the appearance of potential oscillations. An autocatalytic process formed by a coupling between the oxide detachment and oxide growth causes the repetitive passivation-activation

Analogous phenomena of current and potential oscillations have been also observed for other metals and certainly for several iron alloys during pitting corrosion (Podesta et al., 1979). Thus an approach within the framework of nonlinear dynamics might be used and further developed to study efficiently localized corrosion phenomena in other corroding

Pitting corrosion is a complex multi-stage phenomenon of a great technological importance. It has been investigated intensively over many decades. Numerous theoretical and experimental contributions brought about considerable progress in understanding critical factors controlling pitting corrosion. Noticeable progress in elucidating pit nucleation processes during last decades might be attributed to the combined application of electrochemical and surface analytical techniques (Winston Revie, 2011). However, many

In this brief review, an alternate route to investigate pitting corrosion is suggested. This includes a closer look on the conditions related to the onset of nonlinear dynamical response of the metal|electrolyte system as well as on characteristics of oscillations related to different stages of pitting. The halide-containing Fe|H2SO4 system was selected as a paradigm using current oscillations observed under potentiostatic conditions as well as potential oscillations observed under galvanostatic conditions. Since oscillatory phenomena is a widespread phenomenon in electrochemical reactions, many other metal|electrolyte systems should certainly respond by an oscillatory current and/or potential to a halide ion perturbation. A wide variety of processes can lead to oscillation in the current and potential. In the case of the halide-perturbed Fe|H2SO4 system these processes can be classified at first in two broad categories, those associated with general corrosion and those associated with pitting corrosion. General corrosion corresponds to either a stable steady-state passivity under current- controlled conditions or single periodic current oscillations under potentialcontrolled conditions. Pitting corrosion corresponds to complex periodic and aperiodic (bursting and continuous spiking) oscillation. Second, processes associated with pitting corrosion can be distinguished to those leading to early stages of pitting and those leading to late stages. At early stages, unstable pitting gives rise to passive-active current and potential oscillation. Both current and potential oscillate between the active state (high

processes resulting in the appearance of the potential oscillation (Sazou et al., 2009).

In summary, the localized breakdown of passivity on Fe and pit initiation are characterized by:


Upon increasing *c*Cl-> 20 mM and *E* the rate of pit growth is accelerated resulting in late stages of pitting corrosion. At late stages of pitting corrosion, formation of the iron oxide film becomes unlikely and precipitation of ferrous salts may occur. When a steady pit growth is established and formation of the oxide film becomes unlikely, the precipitationdissolution of ferrous salt layers results in new oscillatory phenomena related to the following changes:


Under current-controlled conditions, the nonlinear dynamical response of the halideperturbed Fe|0.75 M H2SO4 system recorded in *E*=f(*I*) and *E*=f(*t*) curves is characterized by potential oscillations. It is worth-noting that under a current control the halide free-system exhibits only bistability without oscillations (Fig. 3b). Therefore, potential oscillations can be used alternatively to identify at a first glance pitting corrosion occurring at a critical *c*Cl- or *c*Br- that depends on the applied current. Iodides do not induce potential oscillations of this type due to the formation of a compact iodine surface layer. Potential oscillations recorded at different *c*Cl- or *c*Br- exhibit characteristic properties that correspond either to early or late stages of pits. In summary, identification and characterization of pitting corrosion should be based on the following criteria:


The formation of mutually interacted pits on the passive metal surface is necessary for the appearance of potential oscillations. An autocatalytic process formed by a coupling between the oxide detachment and oxide growth causes the repetitive passivation-activation processes resulting in the appearance of the potential oscillation (Sazou et al., 2009).

Analogous phenomena of current and potential oscillations have been also observed for other metals and certainly for several iron alloys during pitting corrosion (Podesta et al., 1979). Thus an approach within the framework of nonlinear dynamics might be used and further developed to study efficiently localized corrosion phenomena in other corroding systems.

### **8. Conclusions**

84 Pitting Corrosion

In summary, the localized breakdown of passivity on Fe and pit initiation are characterized

1. A gradual decrease of the current in the passive state, *I*pas,f and *I*pas,b upon increasing

2. No access to *E*tr and the onset of *E*pit. At potentials higher than *E*pit a steady pit growth

3. The disappearance of the single periodic relaxation oscillation of the halide-free system and the onset of complex passive-active oscillations that represent early stages of

4. The induction time, *t*ind elapsed before oscillations start. During *t*ind pit nucleation and

5. Deviation of the (*I*osc)max from the kinetics of the linear part of the active region, which is

Upon increasing *c*Cl-> 20 mM and *E* the rate of pit growth is accelerated resulting in late stages of pitting corrosion. At late stages of pitting corrosion, formation of the iron oxide film becomes unlikely and precipitation of ferrous salts may occur. When a steady pit growth is established and formation of the oxide film becomes unlikely, the precipitationdissolution of ferrous salt layers results in new oscillatory phenomena related to the

1. The current in the passive state tends to a limiting current value and a LCR emerges out

3. At lower potentials either aperiodic bursting oscillations or continuous spiking

4. At higher potentials small amplitude chaotic oscillations (type II) arise around the LCR, instead of the large amplitude oscillations of type I. Beyond a halide concentration threshold, oscillations of type II occur within the entire oscillatory potential region. Under current-controlled conditions, the nonlinear dynamical response of the halideperturbed Fe|0.75 M H2SO4 system recorded in *E*=f(*I*) and *E*=f(*t*) curves is characterized by potential oscillations. It is worth-noting that under a current control the halide free-system exhibits only bistability without oscillations (Fig. 3b). Therefore, potential oscillations can be used alternatively to identify at a first glance pitting corrosion occurring at a critical *c*Cl- or *c*Br- that depends on the applied current. Iodides do not induce potential oscillations of this type due to the formation of a compact iodine surface layer. Potential oscillations recorded at different *c*Cl- or *c*Br- exhibit characteristic properties that correspond either to early or late stages of pits. In summary, identification and characterization of pitting corrosion should be

2. The increase of the current *I*act at which activation of Fe occurs during the backward current scan in the galvanodynamic *E* = f(*I*) curves upon increasing gradually either *c*Clor *c*Br-. The *I*act coincides with the current in the passive state of the potentiodynamic *I* = f(*E*) curves and hence quantifies the extent of pitting corrosion and aggressiveness of

assigned to the increase of the Fe active surface due to pitting.

by:

gradually the *c*Cl- .

repassivation occur repeatedly.

occurs.

pitting.

following changes:

of the passive state.

based on the following criteria:

halides.

2. A critical pitting potential *E*pit does not exist.

(beating) of the current (type I) are observed.

1. Onset of potential oscillations in the *E* = f(*I*) and *E* = f(*t*) curves.

Pitting corrosion is a complex multi-stage phenomenon of a great technological importance. It has been investigated intensively over many decades. Numerous theoretical and experimental contributions brought about considerable progress in understanding critical factors controlling pitting corrosion. Noticeable progress in elucidating pit nucleation processes during last decades might be attributed to the combined application of electrochemical and surface analytical techniques (Winston Revie, 2011). However, many aspects of pitting corrosion remain unclear.

In this brief review, an alternate route to investigate pitting corrosion is suggested. This includes a closer look on the conditions related to the onset of nonlinear dynamical response of the metal|electrolyte system as well as on characteristics of oscillations related to different stages of pitting. The halide-containing Fe|H2SO4 system was selected as a paradigm using current oscillations observed under potentiostatic conditions as well as potential oscillations observed under galvanostatic conditions. Since oscillatory phenomena is a widespread phenomenon in electrochemical reactions, many other metal|electrolyte systems should certainly respond by an oscillatory current and/or potential to a halide ion perturbation. A wide variety of processes can lead to oscillation in the current and potential.

In the case of the halide-perturbed Fe|H2SO4 system these processes can be classified at first in two broad categories, those associated with general corrosion and those associated with pitting corrosion. General corrosion corresponds to either a stable steady-state passivity under current- controlled conditions or single periodic current oscillations under potentialcontrolled conditions. Pitting corrosion corresponds to complex periodic and aperiodic (bursting and continuous spiking) oscillation. Second, processes associated with pitting corrosion can be distinguished to those leading to early stages of pitting and those leading to late stages. At early stages, unstable pitting gives rise to passive-active current and potential oscillation. Both current and potential oscillate between the active state (high

Oscillatory Phenomena as a Probe to Study

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current, low potential) and the passive state (low current, high potential). At late stages, the oxide growth becomes unlikely and stable pitting evolves through the precipitationdissolution of ferrous salt layers whereas complex current or potential oscillations arise. Quantities such as the dissolution current, the induction period required for oscillation to occur and the frequency of oscillations can describe the kinetics of different processes.

It is also worth noting, that current transients of a stochastic nature (noise) of the order of μΑ might be also induced by halide ions due to randomly nucleated metastable pits over the passive metal surfaces. They occur at potentials lower than the *E*pit, being the critical potential for pit stabilization. Spatial and temporal interactions among metastable pits leading to clustering and hence high corrosion rates of stainless steel were investigated thoroughly over last decade within the context of nonlinear dynamics and pattern formation (Lunt et al., 2002; Mikhailov et al., 2009; Organ et al., 2005; Punckt et al., 2004). The potential region where these current transients appear is distinctly different from the potential region within which the large-amplitude complex passive-active current oscillations, discussed in this article, arise. Both stochastic noise and deterministic oscillations can be useful in investigating localized corrosion, which is by itself a typical intrinsically complex system (Aogaki, 1999).

This brief review, not necessarily comprehensive, has focused on results from a research project being carried out by our research group over last two decades. It is noticeable that complex and chaotic current or potential oscillations can be further analyzed using numerical diagnostics (i.e. power spectral density, phase portraits, correlation dimension of chaotic attractors, Lyapunov exponents) developed to characterize time series in nonlinear dynamical systems (Corcoran & Sieradzki, 1992; Hudson & Basset, 1991; Kantz & Schreiber, 1997; Karantonis & Pagitsas, 1996; Li et al., 2005; Li et al., 1993). This analysis might provide new diagnostic criteria that can be profitably used in pitting corrosion studies. However, the purpose of this chapter was restricted to point out the rich dynamical response that may arise under appropriate conditions when localized breakdown of the passivity on a metal occurs. It seems that the need for continuing research into the field remains mandatory. It is our belief that the rich nonlinear dynamical response of corrosive systems can be used profitably to gain a further understanding of complex, not-fully understood processes underlying technologically important problems.

### **9. References**


current, low potential) and the passive state (low current, high potential). At late stages, the oxide growth becomes unlikely and stable pitting evolves through the precipitationdissolution of ferrous salt layers whereas complex current or potential oscillations arise. Quantities such as the dissolution current, the induction period required for oscillation to occur and the frequency of oscillations can describe the kinetics of different processes.

It is also worth noting, that current transients of a stochastic nature (noise) of the order of μΑ might be also induced by halide ions due to randomly nucleated metastable pits over the passive metal surfaces. They occur at potentials lower than the *E*pit, being the critical potential for pit stabilization. Spatial and temporal interactions among metastable pits leading to clustering and hence high corrosion rates of stainless steel were investigated thoroughly over last decade within the context of nonlinear dynamics and pattern formation (Lunt et al., 2002; Mikhailov et al., 2009; Organ et al., 2005; Punckt et al., 2004). The potential region where these current transients appear is distinctly different from the potential region within which the large-amplitude complex passive-active current oscillations, discussed in this article, arise. Both stochastic noise and deterministic oscillations can be useful in investigating localized corrosion, which is by itself a typical intrinsically complex system

This brief review, not necessarily comprehensive, has focused on results from a research project being carried out by our research group over last two decades. It is noticeable that complex and chaotic current or potential oscillations can be further analyzed using numerical diagnostics (i.e. power spectral density, phase portraits, correlation dimension of chaotic attractors, Lyapunov exponents) developed to characterize time series in nonlinear dynamical systems (Corcoran & Sieradzki, 1992; Hudson & Basset, 1991; Kantz & Schreiber, 1997; Karantonis & Pagitsas, 1996; Li et al., 2005; Li et al., 1993). This analysis might provide new diagnostic criteria that can be profitably used in pitting corrosion studies. However, the purpose of this chapter was restricted to point out the rich dynamical response that may arise under appropriate conditions when localized breakdown of the passivity on a metal occurs. It seems that the need for continuing research into the field remains mandatory. It is our belief that the rich nonlinear dynamical response of corrosive systems can be used profitably to gain a further understanding of complex, not-fully understood processes

Aogaki, R., Nonequilibrium fluctuations in the corrosion process. In: E. White, et al., (Eds.),

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**5** 

**Systemic and Local** 

*2National Research Council (CONICET) 3National University of General San Martin 4National Atomic Energy Commission* 

*1University of Buenos Aires* 

*Argentina* 

**Tissue Response to Titanium Corrosion** 

The term biomaterials refers to materials that have been designed to be implanted or placed inside a live system with the aims to substitute or regenerate tissue and tissue functions. Williams defines biomaterials as those that are used in devices for biomedical use designed to interact with biological systems (Williams, 1986). Classically, biomaterials are divided into four types: polymers, metals, ceramics and natural materials. Two different types of biomaterials can be combined to obtain a fifth type known as composite biomaterials (Abramson et al., 2004). Biomaterials are widely used in orthopedic, dental, cardiovascular, ophthalmological, and reconstructive surgery, among other applications. The discovery of relatively inert metals and alloys has led to their use in the field of biomedical applications such as orthopedics and dentistry, and their use in increasing due to their physical-chemical properties and compatibility with biological surroundings (Ratner et al., 2004). One of the most frequently employed metallic biomaterials is titanium (Anderson et al., 2004). Though zirconium is not widely used as a clinical material, it is chemically closely related to and has several properties in common with titanium (Thomsen et al., 1997). Although both titanium and zirconium are transition metals, their physicochemical properties such as oxidation velocity, interaction with water, crystalline structure, transport properties, and those of their oxides differ quantitatively (Henrich & Cox, 1994); these differences may have an effect on biological response (Thomsen et al., 1997). Indeed, the use of zirconium and zirconium alloys to manufacture implants for traumatological, orthopedic, and dental applications has

Titanium and zirconium are highly reactive metals and when exposed to fluid media or air, they quickly develop a layer of titanium dioxide (TiO2) or zirconium dioxide (ZrO2). This layer of dioxide forms a boundary at the interface between the biological medium and the metal structure. It produces passivation of the metal, determining the degree of biocompatibility and the biological response to the implant (Kasemo 1983, Kasemo & Lausmaa 1988, Long & Rack, 1998). Titanium dioxide exists naturally, mainly in the form of three crystalline structures: rutile, anatasa, and brookite. In the case of titanium implants,

been reported (Sherepo et al., 2004; Sollazzo et al., 2007).

**1. Introduction** 

Daniel Olmedo1,2, Deborah Tasat1,3, Gustavo Duffó2,3,4,

Rómulo Cabrini1,4 and María Guglielmotti1,2


## **Systemic and Local Tissue Response to Titanium Corrosion**

Daniel Olmedo1,2, Deborah Tasat1,3, Gustavo Duffó2,3,4, Rómulo Cabrini1,4 and María Guglielmotti1,2 *1University of Buenos Aires 2National Research Council (CONICET) 3National University of General San Martin 4National Atomic Energy Commission Argentina* 

### **1. Introduction**

92 Pitting Corrosion

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The term biomaterials refers to materials that have been designed to be implanted or placed inside a live system with the aims to substitute or regenerate tissue and tissue functions. Williams defines biomaterials as those that are used in devices for biomedical use designed to interact with biological systems (Williams, 1986). Classically, biomaterials are divided into four types: polymers, metals, ceramics and natural materials. Two different types of biomaterials can be combined to obtain a fifth type known as composite biomaterials (Abramson et al., 2004). Biomaterials are widely used in orthopedic, dental, cardiovascular, ophthalmological, and reconstructive surgery, among other applications. The discovery of relatively inert metals and alloys has led to their use in the field of biomedical applications such as orthopedics and dentistry, and their use in increasing due to their physical-chemical properties and compatibility with biological surroundings (Ratner et al., 2004). One of the most frequently employed metallic biomaterials is titanium (Anderson et al., 2004). Though zirconium is not widely used as a clinical material, it is chemically closely related to and has several properties in common with titanium (Thomsen et al., 1997). Although both titanium and zirconium are transition metals, their physicochemical properties such as oxidation velocity, interaction with water, crystalline structure, transport properties, and those of their oxides differ quantitatively (Henrich & Cox, 1994); these differences may have an effect on biological response (Thomsen et al., 1997). Indeed, the use of zirconium and zirconium alloys to manufacture implants for traumatological, orthopedic, and dental applications has been reported (Sherepo et al., 2004; Sollazzo et al., 2007).

Titanium and zirconium are highly reactive metals and when exposed to fluid media or air, they quickly develop a layer of titanium dioxide (TiO2) or zirconium dioxide (ZrO2). This layer of dioxide forms a boundary at the interface between the biological medium and the metal structure. It produces passivation of the metal, determining the degree of biocompatibility and the biological response to the implant (Kasemo 1983, Kasemo & Lausmaa 1988, Long & Rack, 1998). Titanium dioxide exists naturally, mainly in the form of three crystalline structures: rutile, anatasa, and brookite. In the case of titanium implants,

Systemic and Local Tissue Response to Titanium Corrosion 95

Moreover, mechanical disruption during insertion, abutment connection, or removal of failing implants has been suggested as a possible cause of the release of particles from metal structures (Flatebø, 2006; Jacobs, 1998). The release of particles/ions from the implant into the surrounding biological compartment, their biodistribution in the body, and their final destination are issues that lie at the center of studies on biocompatibility and biokinetics. The chemical forms of these released elements have not been identified to date. It is unclear whether these products remain as metal ions or metal oxides, or whether they form protein or cell-bound complexes (Brown et al., 1987; Urban et al., 2000). In the particular case of titanium, little is known about the valence with which it exerts its action, the organic or

The potential toxicity and biological risks associated with ions and/or particles released due to corrosion of metallic implants is a public health concern for the community of patients who have a prosthesis (orthopedic and/or dental), since these prostheses remain inside the body over long periods of time. Likewise, the subject of corrosion is of interest to researchers; corrosion studies aim at avoiding the possible corrosion-related health problems that may arise when metallic implants are placed in humans. Controlling corrosion is most relevant for, in order to protect patient health, corrosion should be negligible. Thus, managing and controlling corrosion of a biomedical implant is a paramount issue from a biological, sanitary, metallurgic, economic and social point of view. The current massive use of these metal biomaterials in the biomedical field renders it necessary to have detailed knowledge not only on their early effects (short term failure) but especially on their long term effects, given that these materials remain inside the patients over long periods of time, sometimes throughout their entire life. With the aims to improve biocompatibility and mechanical resistance, manufacturers of biomedical implants seek to develop an adequate design with minimal degradation, corrosion, dissolution, deformation,

The study of corrosion requires an interdisciplinary approach including chemists, biologists, physicists, engineers, metallurgists, and specialists in biomedicine. The Biomaterials Laboratory of the Department of Oral Pathology of the University of Buenos Aires, the National Commission of Atomic Energy and the University of San Martin have been conducting collaborative research on corrosion aimed at evaluating both local tissue response in the peri-implant microenvironment and the systemic effects and possible

As mentioned above, the titanium dioxide layer prevents corrosion. However, this layer is prone to break, releasing ions/ particles into the milieu. The potential risk of corrosion and the detrimental consequences of corrosion byproducts in the surrounding tissue are issues

The Biomaterials Department has a Failed Human Dental Implants Service devoted to the *in situ* evaluation of the interface, which consists of the implant and the peri-implant tissues, using systematic histological studies. Studying the implant-tissue interface allows detecting osseointegration, implant- marrow tissue interface (myelointegration), fibrous tissue

consequences of corrosion, focusing mainly on dental implants (Olmedo et al., 2009).

inorganic nature of its ligands, and its potential toxicity (Jacobs, 1991).

and fracture.

**2. Local effects of corrosion** 

of clinical importance (Kumazawa, 2002).

the passive oxide layer is composed of anatase and rutile or anatase alone (Effah et al., 1995; Olmedo et al., 2008a; Sul et al., 2001). Zirconium, however, does not exist as a free metal in nature; it occurs as the minerals zircon, or zirconium silicate (ZrSiO4), and the rare mineral baddeleyite or zirconium dioxide (ZrO2) which has a monoclinic crystal structure (Zirconium. Mineral Information Institute, 2009). Baddeleyite, also known as zirconia, is the most naturally occurring form and can be transformed into a tetragonal (1100 ºC) or cubic (2370 ºC) crystallographic form depending on temperature (Chowdhury et al., 2007; Manicone et al., 2007).

Titanium is widely used in the manufacture of dental and orthopedic implants due to its excellent biocompatibility. The latter is defined as the ability of a material to perform with an appropriate host response in a specific application (Williams, 1987). The use of titanium dental implants has revolutionized oral implantology. Currently, almost 300,000 patients in the United States have dental implants. In the area of orthopedics, replacement hip joints are implanted in more than 200,000 humans each year (Ratner et al., 2004). Dental implants are surgically inserted into the jaw bone primarily as a prosthetic foundation. The process of integration of titanium with bone was termed "osseointegration" by Brånemark (Brånemark et al., 1977; Chaturvedi, 2009).

No metal or metal alloy is completely inert *in vivo*. Corrosion is the deterioration of a metal due to interaction (electrochemical attack) with its environment, which results in the release of ions into the surrounding microenvironment (Jacobs, 1998). There are "noble" metals such as rhodium (Rd), palladium (Pd), iridium (Ir) and platinum (Pt), whose resistance to corrosion is due to their high thermodynamic stability. Passivating metals, such as titanium (Ti), vanadium (V), zirconium (Zr), niobium (Nb), and tantalum (Ta), however, are thermodynamically unstable and their resistance to corrosion results from the formation of a protective oxide layer on their surface (Lucas et al, 1992). Titanium is available as commercially pure (c.p.) titanium or as Ti-6Al-4V alloy with 6% aluminum and 4% vanadium. The addition of Al and V increases strength and fatigue resistance; however, this may affect the corrosion resistance properties and may result in the release of metal ions (Textor et al., 2001). C.p. titanium and Ti-6Al-4V alloy are the two most common titaniumbased implant biomaterials (Abramson et al., 2004). There are four standard types or grades of c.p. titanium used for the manufacture of surgical implants, which differ in their content of interstitial elements. This content determines the mechanical properties of a material: the higher the content the higher the grade. In other words, grade 1 is the most pure and grade 4 contains the greatest amount of impurities and has the greatest mechanical resistance. C.p. titanium is used to manufacture dental implants, whereas a Ti6Al4V alloy is used mostly in orthopedics.

As previously stated, all the metallic materials employed in surgery as permanent implants are liable, to a certain degree, to corrosion due to variations in the internal electrolyte milieu (Jacobs, 1998). Corrosion, one of the possible causes of implant failure, implies the dissolution of the protective oxide layer. When metal particles/ions are released from the implant surface, they can migrate systemically, remain in the intercellular spaces near the site where they were released, or be taken up by macrophages (Olmedo 2003, 2008b). The presence of metallic particles in peri-implant tissues may not only be due to a process of electrochemical corrosion but also to frictional wear, or a synergistic combination of the two.

the passive oxide layer is composed of anatase and rutile or anatase alone (Effah et al., 1995; Olmedo et al., 2008a; Sul et al., 2001). Zirconium, however, does not exist as a free metal in nature; it occurs as the minerals zircon, or zirconium silicate (ZrSiO4), and the rare mineral baddeleyite or zirconium dioxide (ZrO2) which has a monoclinic crystal structure (Zirconium. Mineral Information Institute, 2009). Baddeleyite, also known as zirconia, is the most naturally occurring form and can be transformed into a tetragonal (1100 ºC) or cubic (2370 ºC) crystallographic form depending on temperature (Chowdhury et al., 2007;

Titanium is widely used in the manufacture of dental and orthopedic implants due to its excellent biocompatibility. The latter is defined as the ability of a material to perform with an appropriate host response in a specific application (Williams, 1987). The use of titanium dental implants has revolutionized oral implantology. Currently, almost 300,000 patients in the United States have dental implants. In the area of orthopedics, replacement hip joints are implanted in more than 200,000 humans each year (Ratner et al., 2004). Dental implants are surgically inserted into the jaw bone primarily as a prosthetic foundation. The process of integration of titanium with bone was termed "osseointegration" by Brånemark (Brånemark

No metal or metal alloy is completely inert *in vivo*. Corrosion is the deterioration of a metal due to interaction (electrochemical attack) with its environment, which results in the release of ions into the surrounding microenvironment (Jacobs, 1998). There are "noble" metals such as rhodium (Rd), palladium (Pd), iridium (Ir) and platinum (Pt), whose resistance to corrosion is due to their high thermodynamic stability. Passivating metals, such as titanium (Ti), vanadium (V), zirconium (Zr), niobium (Nb), and tantalum (Ta), however, are thermodynamically unstable and their resistance to corrosion results from the formation of a protective oxide layer on their surface (Lucas et al, 1992). Titanium is available as commercially pure (c.p.) titanium or as Ti-6Al-4V alloy with 6% aluminum and 4% vanadium. The addition of Al and V increases strength and fatigue resistance; however, this may affect the corrosion resistance properties and may result in the release of metal ions (Textor et al., 2001). C.p. titanium and Ti-6Al-4V alloy are the two most common titaniumbased implant biomaterials (Abramson et al., 2004). There are four standard types or grades of c.p. titanium used for the manufacture of surgical implants, which differ in their content of interstitial elements. This content determines the mechanical properties of a material: the higher the content the higher the grade. In other words, grade 1 is the most pure and grade 4 contains the greatest amount of impurities and has the greatest mechanical resistance. C.p. titanium is used to manufacture dental implants, whereas a Ti6Al4V alloy is used mostly in

As previously stated, all the metallic materials employed in surgery as permanent implants are liable, to a certain degree, to corrosion due to variations in the internal electrolyte milieu (Jacobs, 1998). Corrosion, one of the possible causes of implant failure, implies the dissolution of the protective oxide layer. When metal particles/ions are released from the implant surface, they can migrate systemically, remain in the intercellular spaces near the site where they were released, or be taken up by macrophages (Olmedo 2003, 2008b). The presence of metallic particles in peri-implant tissues may not only be due to a process of electrochemical corrosion but also to frictional wear, or a synergistic combination of the two.

Manicone et al., 2007).

et al., 1977; Chaturvedi, 2009).

orthopedics.

Moreover, mechanical disruption during insertion, abutment connection, or removal of failing implants has been suggested as a possible cause of the release of particles from metal structures (Flatebø, 2006; Jacobs, 1998). The release of particles/ions from the implant into the surrounding biological compartment, their biodistribution in the body, and their final destination are issues that lie at the center of studies on biocompatibility and biokinetics. The chemical forms of these released elements have not been identified to date. It is unclear whether these products remain as metal ions or metal oxides, or whether they form protein or cell-bound complexes (Brown et al., 1987; Urban et al., 2000). In the particular case of titanium, little is known about the valence with which it exerts its action, the organic or inorganic nature of its ligands, and its potential toxicity (Jacobs, 1991).

The potential toxicity and biological risks associated with ions and/or particles released due to corrosion of metallic implants is a public health concern for the community of patients who have a prosthesis (orthopedic and/or dental), since these prostheses remain inside the body over long periods of time. Likewise, the subject of corrosion is of interest to researchers; corrosion studies aim at avoiding the possible corrosion-related health problems that may arise when metallic implants are placed in humans. Controlling corrosion is most relevant for, in order to protect patient health, corrosion should be negligible. Thus, managing and controlling corrosion of a biomedical implant is a paramount issue from a biological, sanitary, metallurgic, economic and social point of view. The current massive use of these metal biomaterials in the biomedical field renders it necessary to have detailed knowledge not only on their early effects (short term failure) but especially on their long term effects, given that these materials remain inside the patients over long periods of time, sometimes throughout their entire life. With the aims to improve biocompatibility and mechanical resistance, manufacturers of biomedical implants seek to develop an adequate design with minimal degradation, corrosion, dissolution, deformation, and fracture.

The study of corrosion requires an interdisciplinary approach including chemists, biologists, physicists, engineers, metallurgists, and specialists in biomedicine. The Biomaterials Laboratory of the Department of Oral Pathology of the University of Buenos Aires, the National Commission of Atomic Energy and the University of San Martin have been conducting collaborative research on corrosion aimed at evaluating both local tissue response in the peri-implant microenvironment and the systemic effects and possible consequences of corrosion, focusing mainly on dental implants (Olmedo et al., 2009).

### **2. Local effects of corrosion**

As mentioned above, the titanium dioxide layer prevents corrosion. However, this layer is prone to break, releasing ions/ particles into the milieu. The potential risk of corrosion and the detrimental consequences of corrosion byproducts in the surrounding tissue are issues of clinical importance (Kumazawa, 2002).

The Biomaterials Department has a Failed Human Dental Implants Service devoted to the *in situ* evaluation of the interface, which consists of the implant and the peri-implant tissues, using systematic histological studies. Studying the implant-tissue interface allows detecting osseointegration, implant- marrow tissue interface (myelointegration), fibrous tissue

Systemic and Local Tissue Response to Titanium Corrosion 97

Fig. 1. A) Failed dental implant that shows tissue fragments obtained by curettage of the surgical bed. B) Photomicrograph of macrophages near the surface of the implant (→). Note

produces local attack, especially on isolated spots of the passivated metal surface, propagating into the metal. The histologic results of our studies showed scarce osseointegration at the bone-implant interface, i.e. the lack of a union between the bone tissue and the surface of the implant; osseointegration was only observed at sites where the metal remained passivated (areas with no pitting and/or surface alterations) (Fig. 2 A,B). The decrease in the percentage of osseointegration in the areas corresponding to the pits would be associated to a change in the chemical composition and/or structure (e.g. crystallography) of the oxide on the pit surface. It is important to point out that the presence of particulate corrosion and wear products in the tissue surrounding the implant may ultimately result in a cascade of events leading to periprosthetic bone loss (Jacobs, 1998; Urban, 1994). The microchemical analysis of corrosion products by energy dispersive x-ray

the presence of particles in their cytoplasm. Ground section. Orig. Mag. X1000

(fibrointegration) and/or inflammatory reactions. According to the experience of our laboratory, the most frequent causes of dental implant failure in humans are mobility, fracture of the metal (fatigue), and early exposure (Guglielmotti & Cabrini, 1997). Interestingly, implants that had failed due to metal fatigue were found to show satisfactory osseointegration, in other words, good integration of titanium with bone. This means the implants were successful from a biological point of view (osseointegration) but a clinical failure from a mechanical viewpoint. Studying the peri-implant tissue at the metal/tissue level allows obtaining relevant data to determine the possible cause/causes of implant failure.

### **2.1 Tissue response at the metal-tissue interface**

Throughout their histologic studies of failed dental implants, Guglielmotti & Cabrini (1997) consistently observed metal particles inside osseointegrated bone tissue and bone marrow of implants that had failed due to metal fatigue, thus finding evidence of corrosion of the metal structure. Similarly, Olmedo et al. (2003a) found macrophages loaded with metal-like particles in peri-implant soft tissues of failed human dental implants indicating the occurrence of corrosion processes (Fig. 1 A-B). Microchemical analysis of the metallic particles inside macrophages using X-ray dispersion (EDX) confirmed the presence of titanium. It is noteworthy that a greater number of macrophages loaded with particles was observed in the vicinity of the metal surface than at more distant sites. Likewise, numerous case reports in the literature describe histological evidence of inflammatory response and the presence of metallic ions/particles in the tissues adjacent to orthopedic prostheses of titanium or titanium based alloys (Jacobs, 1998).

Titanium is widely used in oral and maxillofacial materials such as grids, fixation plates, screws, and distractors. According to a number of studies reported in the literature, the removal of titanium miniplates after bone healing is complete is unnecessary precisely due to the excellent biocompatibility and corrosion resistance properties of titanium. This is beneficial to the patient since a second surgery is avoided (Rosenberg et al., 1993). Moreover, some authors suggest that miniplates must be removed only when they cause patient complaints and in cases of wound dehiscence or infection (Rosenberg et al., 1993). However, as mentioned above, no metal or alloy is completely inert *in vivo*. In this regard, some authors claim that titanium miniplates should be removed to allow for physiologic bony adaptation and avoidance of a foreign body reaction (Ferguson, 1960; Katou et al., 1996; Moran et al., 1991; Rosenberg et al., 1993; Young-Kyun et al., 1997).

Thus, whether titanium miniplates or grids should be removed after bone healing is complete remains controversial to date. Bessho & Iizuka (1993) examined 113 titanium miniplates that had been retrieved after miniplate fixation of mandibular fractures, and identified surface depressions apparently caused by pitting corrosion (Matthew et al., 1996). Zaffe et al. (2003) evaluated the pre and post-implantation surface features and surface alterations of titanium grids and plates in patients and observed, among other alterations, the presence of pitting on the surface of one of the grids. Experimental studies analyzing the biological effect of a type of localized corrosion, pitting corrosion, on the peri-implant environment have been conducted at our laboratory (Olmedo, 2008c). Pitting corrosion

(fibrointegration) and/or inflammatory reactions. According to the experience of our laboratory, the most frequent causes of dental implant failure in humans are mobility, fracture of the metal (fatigue), and early exposure (Guglielmotti & Cabrini, 1997). Interestingly, implants that had failed due to metal fatigue were found to show satisfactory osseointegration, in other words, good integration of titanium with bone. This means the implants were successful from a biological point of view (osseointegration) but a clinical failure from a mechanical viewpoint. Studying the peri-implant tissue at the metal/tissue level allows obtaining relevant data to determine the possible cause/causes of implant

Throughout their histologic studies of failed dental implants, Guglielmotti & Cabrini (1997) consistently observed metal particles inside osseointegrated bone tissue and bone marrow of implants that had failed due to metal fatigue, thus finding evidence of corrosion of the metal structure. Similarly, Olmedo et al. (2003a) found macrophages loaded with metal-like particles in peri-implant soft tissues of failed human dental implants indicating the occurrence of corrosion processes (Fig. 1 A-B). Microchemical analysis of the metallic particles inside macrophages using X-ray dispersion (EDX) confirmed the presence of titanium. It is noteworthy that a greater number of macrophages loaded with particles was observed in the vicinity of the metal surface than at more distant sites. Likewise, numerous case reports in the literature describe histological evidence of inflammatory response and the presence of metallic ions/particles in the tissues adjacent to orthopedic prostheses of

Titanium is widely used in oral and maxillofacial materials such as grids, fixation plates, screws, and distractors. According to a number of studies reported in the literature, the removal of titanium miniplates after bone healing is complete is unnecessary precisely due to the excellent biocompatibility and corrosion resistance properties of titanium. This is beneficial to the patient since a second surgery is avoided (Rosenberg et al., 1993). Moreover, some authors suggest that miniplates must be removed only when they cause patient complaints and in cases of wound dehiscence or infection (Rosenberg et al., 1993). However, as mentioned above, no metal or alloy is completely inert *in vivo*. In this regard, some authors claim that titanium miniplates should be removed to allow for physiologic bony adaptation and avoidance of a foreign body reaction (Ferguson, 1960; Katou et al., 1996;

Thus, whether titanium miniplates or grids should be removed after bone healing is complete remains controversial to date. Bessho & Iizuka (1993) examined 113 titanium miniplates that had been retrieved after miniplate fixation of mandibular fractures, and identified surface depressions apparently caused by pitting corrosion (Matthew et al., 1996). Zaffe et al. (2003) evaluated the pre and post-implantation surface features and surface alterations of titanium grids and plates in patients and observed, among other alterations, the presence of pitting on the surface of one of the grids. Experimental studies analyzing the biological effect of a type of localized corrosion, pitting corrosion, on the peri-implant environment have been conducted at our laboratory (Olmedo, 2008c). Pitting corrosion

failure.

**2.1 Tissue response at the metal-tissue interface** 

titanium or titanium based alloys (Jacobs, 1998).

Moran et al., 1991; Rosenberg et al., 1993; Young-Kyun et al., 1997).

Fig. 1. A) Failed dental implant that shows tissue fragments obtained by curettage of the surgical bed. B) Photomicrograph of macrophages near the surface of the implant (→). Note the presence of particles in their cytoplasm. Ground section. Orig. Mag. X1000

produces local attack, especially on isolated spots of the passivated metal surface, propagating into the metal. The histologic results of our studies showed scarce osseointegration at the bone-implant interface, i.e. the lack of a union between the bone tissue and the surface of the implant; osseointegration was only observed at sites where the metal remained passivated (areas with no pitting and/or surface alterations) (Fig. 2 A,B). The decrease in the percentage of osseointegration in the areas corresponding to the pits would be associated to a change in the chemical composition and/or structure (e.g. crystallography) of the oxide on the pit surface. It is important to point out that the presence of particulate corrosion and wear products in the tissue surrounding the implant may ultimately result in a cascade of events leading to periprosthetic bone loss (Jacobs, 1998; Urban, 1994). The microchemical analysis of corrosion products by energy dispersive x-ray

Systemic and Local Tissue Response to Titanium Corrosion 99

Based on the aforementioned observations, the occurrence of corrosion phenomena at the interface is of paramount importance to the clinical course of both dental and orthopedic implants since such phenomena could be a possible cause of mid-term implant failure.

The gingiva around dental implants is called peri-implant mucosa, and consists of wellkeratinized oral epithelium, sulcular epithelium, and junctional epithelium with underlying connective tissue. Between the implant surface and epithelial cells are hemidesmosomes and

Gingival hyperplasia, mucositis, and peri-implantitis have been described amongst the soft tissue complications associated to dental implants (Adell et al., 1981; Lang et al., 2000). The causes that lead to the development of reactive lesions associated to dental implants have not been fully elucidated to date. In this regard, our research group has reported two clinical cases of reactive lesions in the peri-implant mucosa (inflammatory angiohyperplastic granuloma and peripheral giant cell granuloma) associated to dental implants, in which the presence of metallic particles was detected histologically (Fig. 3 A-B). The presence of metallic particles in the studied tissue suggests that the etiology of the lesions might be

Fig. 3. A) Clinical intraoral photograph showing an exophytic lesion (→) in the area of the first lower right molar. B) Reactive lesion (pyogenic granuloma). Note the significant vascular proliferation and the presence of metal-like particles inside macrophages (→). H-E;

Abraham et al. (2006) demonstrated the presence of titanium in saliva and gingival fluid of patients carrying titanium dental implants. According to the authors, the highest titanium levels corresponded to patients carrying implants over longer periods of time, thus indicating that titanium accumulates in peri-implant gingival tissue. Oral exfoliative cytology is a diagnostic method which involves the study and interpretation of the features of cells exfoliated from the oral mucosa (Diniz-Freitas et al., 2004). Thus, we performed an exploratory work using exfoliative cytology around the peri-implant mucosa of human dental implants (Nalli et al., 2009). The cytological smears of patients carrying dental

attributed to a corrosion process of the metal structure (Olmedo et al., 2010).

**2.2 Peri-implant mucosa response** 

Orig. Mag. X400

the basal lamina (Newman & Flemming, 1988).

analysis (EDX) in the peri-implant milieu revealed the presence of titanium. It is noteworthy that craters, pits, surface cracks, and depressions may appear during the preparation of the sheets that will be used to manufacture miniplates (Matthew et al., 1996) and may be potential sites for the initiation of corrosion. The results obtained in our study by scanning electron microscopy showed initiation of pitting in areas with surface cracks. Titanium exhibits the greatest resistance to generalized corrosion, pitting corrosion, and crevice corrosion compared to other metals or alloys used in oral surgery, such as stainless steel or chromium-cobalt (Matthew et al., 1996). The severity of corrosion and the quantity of corrosion products that are released may depend not only on the susceptibility to corrosion of the implant material but also on the tissue response to the implant and to the surgical procedures used during implantation (Moberg et al., 1989). The histological results of our study showed the presence of corrosion products around the implant, both outside and phagocytosed in macrophages. In various cases the products of corrosion were found around the blood vessels (Fig. 2 C), in keeping with the histological study of soft tissue adjacent to titanium implants reported by Meachim & Williams (1973) and Torgersen et al. (1995). The observation of metal particles located intracellularly or in association with vessels may represent a biologic response aimed at eliminating the foreign material (Meachim & Williams, 1973; Schliephake et al., 1993; Torgersen et al., 1995).

The properties and quality of the implant material, the shape of the implant, and the handling and surgical procedure are of crucial importance for an optimal biologic performance of any implant device. Unstable conditions in the fracture area after osteosynthesis lead to continuous fretting at the screw/plate interface. Removal of the passivating surface oxide and oxygen depletion in the crevices between plate and screws increase the risk of both crevice corrosion and fretting attack (Williams, 1982). It is speculated that an increased mechanical stability during healing may reduce the fretting component, and thereby reduce corrosion. The adverse local effects caused by pitting corrosion suggest that titanium plates and grids should be used with caution as permanent fixation structures.

Fig. 2. A and B) Bone tissue-implant interface. A) Control case showing adequate osseointegration (OI) of the bone tissue (B) with the surface of the titanium implant (I). B) Experimental case (pitting corrosion). Note the irregularities on the implant surface (I) and bone tissue (B) far from the surface (lack of osseointegration). C) Blood vessel in the bone marrow near an implant surface and products of corrosion (→) in the vicinity. Ground sections. Orig. Mag. X1000

Based on the aforementioned observations, the occurrence of corrosion phenomena at the interface is of paramount importance to the clinical course of both dental and orthopedic implants since such phenomena could be a possible cause of mid-term implant failure.

### **2.2 Peri-implant mucosa response**

98 Pitting Corrosion

analysis (EDX) in the peri-implant milieu revealed the presence of titanium. It is noteworthy that craters, pits, surface cracks, and depressions may appear during the preparation of the sheets that will be used to manufacture miniplates (Matthew et al., 1996) and may be potential sites for the initiation of corrosion. The results obtained in our study by scanning electron microscopy showed initiation of pitting in areas with surface cracks. Titanium exhibits the greatest resistance to generalized corrosion, pitting corrosion, and crevice corrosion compared to other metals or alloys used in oral surgery, such as stainless steel or chromium-cobalt (Matthew et al., 1996). The severity of corrosion and the quantity of corrosion products that are released may depend not only on the susceptibility to corrosion of the implant material but also on the tissue response to the implant and to the surgical procedures used during implantation (Moberg et al., 1989). The histological results of our study showed the presence of corrosion products around the implant, both outside and phagocytosed in macrophages. In various cases the products of corrosion were found around the blood vessels (Fig. 2 C), in keeping with the histological study of soft tissue adjacent to titanium implants reported by Meachim & Williams (1973) and Torgersen et al. (1995). The observation of metal particles located intracellularly or in association with vessels may represent a biologic response aimed at eliminating the foreign material

(Meachim & Williams, 1973; Schliephake et al., 1993; Torgersen et al., 1995).

Fig. 2. A and B) Bone tissue-implant interface. A) Control case showing adequate

osseointegration (OI) of the bone tissue (B) with the surface of the titanium implant (I). B) Experimental case (pitting corrosion). Note the irregularities on the implant surface (I) and bone tissue (B) far from the surface (lack of osseointegration). C) Blood vessel in the bone marrow near an implant surface and products of corrosion (→) in the vicinity. Ground

fixation structures.

sections. Orig. Mag. X1000

The properties and quality of the implant material, the shape of the implant, and the handling and surgical procedure are of crucial importance for an optimal biologic performance of any implant device. Unstable conditions in the fracture area after osteosynthesis lead to continuous fretting at the screw/plate interface. Removal of the passivating surface oxide and oxygen depletion in the crevices between plate and screws increase the risk of both crevice corrosion and fretting attack (Williams, 1982). It is speculated that an increased mechanical stability during healing may reduce the fretting component, and thereby reduce corrosion. The adverse local effects caused by pitting corrosion suggest that titanium plates and grids should be used with caution as permanent The gingiva around dental implants is called peri-implant mucosa, and consists of wellkeratinized oral epithelium, sulcular epithelium, and junctional epithelium with underlying connective tissue. Between the implant surface and epithelial cells are hemidesmosomes and the basal lamina (Newman & Flemming, 1988).

Gingival hyperplasia, mucositis, and peri-implantitis have been described amongst the soft tissue complications associated to dental implants (Adell et al., 1981; Lang et al., 2000). The causes that lead to the development of reactive lesions associated to dental implants have not been fully elucidated to date. In this regard, our research group has reported two clinical cases of reactive lesions in the peri-implant mucosa (inflammatory angiohyperplastic granuloma and peripheral giant cell granuloma) associated to dental implants, in which the presence of metallic particles was detected histologically (Fig. 3 A-B). The presence of metallic particles in the studied tissue suggests that the etiology of the lesions might be attributed to a corrosion process of the metal structure (Olmedo et al., 2010).

Fig. 3. A) Clinical intraoral photograph showing an exophytic lesion (→) in the area of the first lower right molar. B) Reactive lesion (pyogenic granuloma). Note the significant vascular proliferation and the presence of metal-like particles inside macrophages (→). H-E; Orig. Mag. X400

Abraham et al. (2006) demonstrated the presence of titanium in saliva and gingival fluid of patients carrying titanium dental implants. According to the authors, the highest titanium levels corresponded to patients carrying implants over longer periods of time, thus indicating that titanium accumulates in peri-implant gingival tissue. Oral exfoliative cytology is a diagnostic method which involves the study and interpretation of the features of cells exfoliated from the oral mucosa (Diniz-Freitas et al., 2004). Thus, we performed an exploratory work using exfoliative cytology around the peri-implant mucosa of human dental implants (Nalli et al., 2009). The cytological smears of patients carrying dental

Systemic and Local Tissue Response to Titanium Corrosion 101

Corrosion is not only a local problem since the particles released during this process can

migrate to distant sites. This issue is of particular interest to biocompatibility studies.

Fig. 4. A) Human oral mucosa covering an implant cover screw. Note the presence of titanium particles (→) inside cells or phagocytosed in macrophages at the epitheliumchorion interface. H-E. Orig. Mag. X1000. B) Scanning Electron Microscopy of an area of mucosa with particles. Note the fine particles (→) among connective tissue elements. Orig.

**3. Studies on the dissemination of titanium towards other biological** 

The local effect of corrosion and subsequent release of ions/metal-like particles into the periimplant biological milieu could compromise other biological compartments. The chemically

Mag. X4000

**compartments** 

implants exhibited metal-like particles varying in quantity, shape, and size. The particles were found both inside and among epithelial cells and macrophages.

The results of the study showed that ions/particles are released from the surface of the implant into the biological milieu. Both epithelial cells and macrophages located in the periimplant area are able to capture these metal-like particles. Thus, exfoliative cytology is a simple, minimally-invasive, well-tolerated technique, which may prove useful to detect metal particles in cells exfoliated from the peri-implant mucosa, and be a valuable method to monitor dental implant corrosion.

The peri-implant milieu consists of bone tissue, soft tissues, and saliva. Biochemical changes in the peri-implant environment may lead to implant corrosion. According to Abraham et al. (2006) the molecular mechanism of interaction between metal ions and biological molecules or cells remains unclear to date.

The release of ions/particles can cause pigmentation of soft tissues adjacent to an implant (metallosis). Metallosis is defined as aseptic fibrosis, local necrosis, or loosening of a device secondary to metal corrosion and release of wear debris (Black et al., 1990; Bullough, 1994). It involves deposition and build-up of metallic debris in the soft tissues of the body. In a previous study we evaluated histologically tissue response in human oral mucosa associated to submerged titanium implants, using biopsies of the supra-implant oral mucosa adjacent to the implant cover screw (Olmedo et al., 2007a). We observed the presence of different sized particles inside cells or phagocytosed in macrophages in epithelial and connective tissue (Fig. 4 A-B). Interestingly, the titanium particles in the superficial layers of the epithelium might have been associated not only with the cover screw surface but also with other exogenous sources. For example, titanium oxide (TiO2) is widely used in food products, toothpastes, prophylaxis pastes and abrading and polishing agents, which have been reported in oral biopsies (Koppang et al., 2007).

Microchemical analysis by EDX revealed the presence of titanium in the particles. Immunohistochemical staining with antibodies anti CD68 and anti CD45RO was positive, confirming the presence of macrophages and T lymphocytes associated with the metal particles. In agreement with other reports, (Evrard et al., 2010; Lalor et al., 1991; Matthew et al., 1996;) the T-lymphocyte infiltrate would seem to suggest the presence of an immune response mediated by cells.

Scanning electron microscopy allowed visualizing depressions and irregularities on the surface of the studied metal cover screws. Both unused cover screws and those removed from patients exhibited alterations on their surface. As mentioned previously, craters, pits, surface cracks and depressions may appear during the preparation of the sheets that will be used to manufacture miniplates (Matthew et al., 1996), and be potential sites for the initiation of corrosion. Based on this observation, it would seem advisable for professionals to handle cover screws with utmost care since the observed scratches were most likely caused during placement or removal of the cover screws and could also be potential sites for the initiation of corrosion.

The potential long-term biological effects of particles on soft tissues adjacent to metallic devices should be further investigated as these effects might affect the clinical outcome of the implant.

implants exhibited metal-like particles varying in quantity, shape, and size. The particles

The results of the study showed that ions/particles are released from the surface of the implant into the biological milieu. Both epithelial cells and macrophages located in the periimplant area are able to capture these metal-like particles. Thus, exfoliative cytology is a simple, minimally-invasive, well-tolerated technique, which may prove useful to detect metal particles in cells exfoliated from the peri-implant mucosa, and be a valuable method

The peri-implant milieu consists of bone tissue, soft tissues, and saliva. Biochemical changes in the peri-implant environment may lead to implant corrosion. According to Abraham et al. (2006) the molecular mechanism of interaction between metal ions and biological molecules

The release of ions/particles can cause pigmentation of soft tissues adjacent to an implant (metallosis). Metallosis is defined as aseptic fibrosis, local necrosis, or loosening of a device secondary to metal corrosion and release of wear debris (Black et al., 1990; Bullough, 1994). It involves deposition and build-up of metallic debris in the soft tissues of the body. In a previous study we evaluated histologically tissue response in human oral mucosa associated to submerged titanium implants, using biopsies of the supra-implant oral mucosa adjacent to the implant cover screw (Olmedo et al., 2007a). We observed the presence of different sized particles inside cells or phagocytosed in macrophages in epithelial and connective tissue (Fig. 4 A-B). Interestingly, the titanium particles in the superficial layers of the epithelium might have been associated not only with the cover screw surface but also with other exogenous sources. For example, titanium oxide (TiO2) is widely used in food products, toothpastes, prophylaxis pastes and abrading and polishing agents, which have

Microchemical analysis by EDX revealed the presence of titanium in the particles. Immunohistochemical staining with antibodies anti CD68 and anti CD45RO was positive, confirming the presence of macrophages and T lymphocytes associated with the metal particles. In agreement with other reports, (Evrard et al., 2010; Lalor et al., 1991; Matthew et al., 1996;) the T-lymphocyte infiltrate would seem to suggest the presence of an immune

Scanning electron microscopy allowed visualizing depressions and irregularities on the surface of the studied metal cover screws. Both unused cover screws and those removed from patients exhibited alterations on their surface. As mentioned previously, craters, pits, surface cracks and depressions may appear during the preparation of the sheets that will be used to manufacture miniplates (Matthew et al., 1996), and be potential sites for the initiation of corrosion. Based on this observation, it would seem advisable for professionals to handle cover screws with utmost care since the observed scratches were most likely caused during placement or removal of the cover screws and could also be potential sites for

The potential long-term biological effects of particles on soft tissues adjacent to metallic devices should be further investigated as these effects might affect the clinical outcome of

were found both inside and among epithelial cells and macrophages.

to monitor dental implant corrosion.

or cells remains unclear to date.

response mediated by cells.

the initiation of corrosion.

the implant.

been reported in oral biopsies (Koppang et al., 2007).

Corrosion is not only a local problem since the particles released during this process can migrate to distant sites. This issue is of particular interest to biocompatibility studies.

Fig. 4. A) Human oral mucosa covering an implant cover screw. Note the presence of titanium particles (→) inside cells or phagocytosed in macrophages at the epitheliumchorion interface. H-E. Orig. Mag. X1000. B) Scanning Electron Microscopy of an area of mucosa with particles. Note the fine particles (→) among connective tissue elements. Orig. Mag. X4000

### **3. Studies on the dissemination of titanium towards other biological compartments**

The local effect of corrosion and subsequent release of ions/metal-like particles into the periimplant biological milieu could compromise other biological compartments. The chemically

Systemic and Local Tissue Response to Titanium Corrosion 103

the material (Anderson et al., 2004; Lu et al., 2002; Solheim et al., 2002; Takebe et al. 2003; Xia & Triffitt, 2006)*.* Particles that are smaller than the macrophages themselves (< 10 μm) can be easily phagocytosed. However, the larger particles (10-100 μm) are ingested by giant, multinucleate cells (Brodbeck et al., 2005). The biokinetics of TiO2 and ZrO2 microparticles depends on differences in physicochemical properties of the particles, such as size, shape

Experimental studies performed at our laboratory showed the presence of titanium and zirconium particles in monocytes in the blood (Fig. 6) and blood plasma (Olmedo et al., 2003b, 2005). Several transport mechanisms have been described for titanium, e.g. systemic dissemination by the vascular system in solution or as particles (Meachim & Williams, 1973); lymphatic dissemination as free particles or as phagocytosed particles within macrophages (Urban et al., 2000), dissemination of particles to the bone marrow by circulating monocytes, or as minute particles by the vascular system (Engh et al., 1997). Several studies on the bond between metal and proteins have contributed to the understanding of the dissemination of metals. Nickel, chromium, and cobalt would seemingly migrate bound to blood cells and/or proteins in serum and tissue fluids (Brown et al., 1987; Merritt et al., 1984). Aluminum is seemingly transported by transferrin (Alfrey, 1989). Uranium is transported linked to

Fig. 6. Blood smear. Titanium particles are evident in a peripheral blood monocyte (→).

The fact that metals bind mainly to albumin would explain their widespread presence in the body. The metallic ions that result from the process of corrosion would thus disseminate to tissues, bind to albumin and enter the circulation exerting their effect at remote sites. Testing

and/or crystal structure (Olmedo et al., 2011).

proteins, to citrates and to carbonates (Leggett, 1989).

Safranin stain. Orig. Mag. X1000

**3.2 An experimental model** 

active metal ions/particles may bind to the surrounding tissues but may also bind to proteins and be disseminated in the vascular and lymphatic systems to distant organs (Jacobs et al., 1991; Woodman et al., 1984a).

Studies in the field of orthopedic implants show that titanium ions enter neighboring tissues reaching the internal milieu and are excreted through urine (Jacobs et al., 1991). A number of researchers have found metal ions in body organs and fluids. Jacobs et al. (1991) studied osseointegrated coxofemoral prostheses made of 90% titanium-6%aluminum-4%vanadium and showed that ions of all three metals entered the plasma and were excreted through urine. A study at autopsy by Urban et al (2000) demonstrated the presence of metal-like and plastic particles from coxofemoral and knee-replacement prostheses in the liver, spleen, and lymph nodes.

### **3.1 Deposition of titanium and zirconium in organs with macrophagic activity. An experimental model**

As mentioned previously, titanium and zirconium implants have a protective dioxide (TiO2 or ZrO2) layer on their surface. This layer determines biocompatibility and forms a boundary at the interface between the biological milieu and the implant, decreasing their reactivity and partially avoiding corrosion (Jacobs et al., 1998; Kasemo, 1983; Long & Rack, 1998). In order to evaluate the dissemination routes of corrosion products and estimate the intensity of the deposits in different biological compartments, our research group has developed experimental models with animals intraperitoneally injected with TiO2 or ZrO2 (Cabrini et al. 2002, 2003; Olmedo et al. 2002, 2003b, 2005, 2008a).

Though it holds true that the experimental doses employed in those studies are high in terms of a normal *in vivo* situation, they served the purpose of our studies since they allow rapid observation of the adverse effects of particles in the studied tissue (Olmedo et al., 2011). Our studies included histologic observation and quantitation of titanium and zirconium deposits in organs with macrophagic activity such as the liver, spleen, and lungs (Fig. 5 A-C), (Olmedo et al., 2002), and showed that at equal doses and experimental times titanium content in organs was consistently higher than zirconium content. Macrophages are cells that respond rapidly to *in vivo* implantation of a biomaterial, including metals, ceramics, cement, and polymers. Their response depends mainly on the size and structure of

Fig. 5. Titanium deposits in organ parenchyma of an animal injected with TiO2. A) Liver. Deposition in liver cells (hepatocytes) (→) can be seen clearly. Grenacher carmin stain. Orig. Mag. X400. B and C) Spleen and lung, respectively. Note the amount of titanium (→) deposits. Grenacher carmin stain. Orig. Mag. X400

the material (Anderson et al., 2004; Lu et al., 2002; Solheim et al., 2002; Takebe et al. 2003; Xia & Triffitt, 2006)*.* Particles that are smaller than the macrophages themselves (< 10 μm) can be easily phagocytosed. However, the larger particles (10-100 μm) are ingested by giant, multinucleate cells (Brodbeck et al., 2005). The biokinetics of TiO2 and ZrO2 microparticles depends on differences in physicochemical properties of the particles, such as size, shape and/or crystal structure (Olmedo et al., 2011).

### **3.2 An experimental model**

102 Pitting Corrosion

active metal ions/particles may bind to the surrounding tissues but may also bind to proteins and be disseminated in the vascular and lymphatic systems to distant organs

Studies in the field of orthopedic implants show that titanium ions enter neighboring tissues reaching the internal milieu and are excreted through urine (Jacobs et al., 1991). A number of researchers have found metal ions in body organs and fluids. Jacobs et al. (1991) studied osseointegrated coxofemoral prostheses made of 90% titanium-6%aluminum-4%vanadium and showed that ions of all three metals entered the plasma and were excreted through urine. A study at autopsy by Urban et al (2000) demonstrated the presence of metal-like and plastic particles from coxofemoral and knee-replacement prostheses in the liver, spleen, and

**3.1 Deposition of titanium and zirconium in organs with macrophagic activity.** 

(Cabrini et al. 2002, 2003; Olmedo et al. 2002, 2003b, 2005, 2008a).

deposits. Grenacher carmin stain. Orig. Mag. X400

As mentioned previously, titanium and zirconium implants have a protective dioxide (TiO2 or ZrO2) layer on their surface. This layer determines biocompatibility and forms a boundary at the interface between the biological milieu and the implant, decreasing their reactivity and partially avoiding corrosion (Jacobs et al., 1998; Kasemo, 1983; Long & Rack, 1998). In order to evaluate the dissemination routes of corrosion products and estimate the intensity of the deposits in different biological compartments, our research group has developed experimental models with animals intraperitoneally injected with TiO2 or ZrO2

Though it holds true that the experimental doses employed in those studies are high in terms of a normal *in vivo* situation, they served the purpose of our studies since they allow rapid observation of the adverse effects of particles in the studied tissue (Olmedo et al., 2011). Our studies included histologic observation and quantitation of titanium and zirconium deposits in organs with macrophagic activity such as the liver, spleen, and lungs (Fig. 5 A-C), (Olmedo et al., 2002), and showed that at equal doses and experimental times titanium content in organs was consistently higher than zirconium content. Macrophages are cells that respond rapidly to *in vivo* implantation of a biomaterial, including metals, ceramics, cement, and polymers. Their response depends mainly on the size and structure of

Fig. 5. Titanium deposits in organ parenchyma of an animal injected with TiO2. A) Liver. Deposition in liver cells (hepatocytes) (→) can be seen clearly. Grenacher carmin stain. Orig. Mag. X400. B and C) Spleen and lung, respectively. Note the amount of titanium (→)

(Jacobs et al., 1991; Woodman et al., 1984a).

lymph nodes.

**An experimental model** 

Experimental studies performed at our laboratory showed the presence of titanium and zirconium particles in monocytes in the blood (Fig. 6) and blood plasma (Olmedo et al., 2003b, 2005). Several transport mechanisms have been described for titanium, e.g. systemic dissemination by the vascular system in solution or as particles (Meachim & Williams, 1973); lymphatic dissemination as free particles or as phagocytosed particles within macrophages (Urban et al., 2000), dissemination of particles to the bone marrow by circulating monocytes, or as minute particles by the vascular system (Engh et al., 1997). Several studies on the bond between metal and proteins have contributed to the understanding of the dissemination of metals. Nickel, chromium, and cobalt would seemingly migrate bound to blood cells and/or proteins in serum and tissue fluids (Brown et al., 1987; Merritt et al., 1984). Aluminum is seemingly transported by transferrin (Alfrey, 1989). Uranium is transported linked to proteins, to citrates and to carbonates (Leggett, 1989).

Fig. 6. Blood smear. Titanium particles are evident in a peripheral blood monocyte (→). Safranin stain. Orig. Mag. X1000

The fact that metals bind mainly to albumin would explain their widespread presence in the body. The metallic ions that result from the process of corrosion would thus disseminate to tissues, bind to albumin and enter the circulation exerting their effect at remote sites. Testing

Systemic and Local Tissue Response to Titanium Corrosion 105

A corrosion process can decrease the fatigue resistance of the metal compromising metal resistance, which could eventually cause implant fracture (Adya et al., 2005; Guindy et al., 2004 ; Nikolopoulou, 2006 ; Tagger Green et al., 2002). It has been reported that the infiltration of saliva between the suprastructure (nickel-chromium-molybdenum alloy) and the implant (pure titanium) can trigger corrosion processes (galvanic corrosion) due to differences in electrochemical potentials. This causes the release of ions, such as nickel or chromium ions, from the alloy in the crown or bridge to the peri-implant tissues and subsequently results in bone resorption. The latter compromises implant stability,

Metal corrosion can affect the close contact between the implant and the bone tissue (osseointegration). The ions/metallic particles from coxofemoral prostheses can be phagocytosed by macrophages stimulating the release of cytokines, which contribute to bone resorption by activating osteoclasts. In addition to increasing bone resorption, the released particles may inhibit osteoblast function decreasing bone formation and

The products of metallic implant corrosion behave as haptens generating a hypersensitive reaction that involves the release of inflammatory mediators, known as cytokines, and macrophage recruitment (Hallab et al., 2001; Jiranek et al., 1993; Yang & Merrit, 1994). It remains unclear to date whether it is the hypersensitivity to metal that causes implant failure or vice versa (Hallab et al., 2001). It also remains controversial whether an inflammatory process generates corrosion or whether corrosion triggers an inflammatory reaction. Thus, hypersensitivity to titanium as an implant material in oral and maxillofacial surgery probably occurs more commonly than has been reported in the literature (Matthew & Frame, 1998). There are reports of cases where titanium allergy mainly appeared as the fundamental cause of urticaria, eczema, oedema, redness and pruritus of the skin or mucosa, either localized, at distant sites, or generalized (Sicilia et al, 2008). However, the clinical relevance of allergic reactions in patients with titanium dental implants remains

Mineral elements play a critical role in the physiology and pathology of biological systems. Titanium is a nonessential element in that (a) no enzymatic pathway has been elucidated that requires titanium as a cofactor, (b) there does not appear to be any homeostatic control of titanium, and (c) titanium is not invariably detected in the newborn (Woodman et al, 1984b). Thus, the presence of titanium in the body, titanium biokinetics, and the potential

The toxicology of titanium is a current issue of debate. According to epidemiological studies, inhalation of powder containing titanium has no deleterious effect on the lungs (Daum et al., 1977; Ferin & Oberdörster, 1985). Other studies, however, suggest an association between titanium particles and pleural pathologies (Garabrant et al., 1987), granulomatous diseases, and malignant neoplasms of the lung. Our experimental studies have shown the presence of a considerable amount of titanium particles not only in alveolar macrophages but also in hepatocytes (Olmedo et al., 2008b). The accumulation of particles in the liver could compromise liver function as described by Urban et al. (2000). The authors associated the presence of titanium particles in a patient to granulomatous reactions and hepatomegalia. Various studies have reported the presence of macrophages related to failed

eventually causing implant fracture (Tagger Green et al., 2002).

contributing to osteolysis (Allen et al., 1997; Dowd et al., 1995).

biological effects of titanium are of great interest to researchers.

debatable (Javed et al., 2011).

for titanium or zirconium in the blood (cells and/or plasma) of patients carrying an implant (coxofemoral implant, dental implant, plates and screws for fracture fixation, metallic panels for reconstructive surgery of large areas of the body) may serve as a method to detect the presence of a corrosion process of the metallic structures (Olmedo et al, 2003b).

### **3.3 Effect of titanium and zirconium deposition on the lungs: Generation of Superoxide anion (O2 - ) in alveolar macrophages. An experimental model**

It is known that trace metals can increase physiological production of reactive oxygen species (ROS) which, without a compensatory increase in antioxidative species, can lead to tissue damage (Gottschling et al., 2002; Kawanishi et al., 2002; Maziere et al., 2003). Studies conducted at our laboratory have shown the presence of titanium and zirconium particles in alveolar phagocytes immunohistochemically identified as CD68 macrophages (Olmedo et al., 2008b). Evaluation of oxidative metabolism of alveolar macrophages exposed to these oxides has shown an increase in generation of ROS. However, it must be pointed out that ROS levels in animals exposed to ZrO2 were found to be markedly lower than those of animals exposed to TiO2.

As mentioned previously, the layer of titanium dioxide is crystallographically composed of anatase or a combination of anatase and rutile. Studies on generation of superoxide anion (O2-) in alveolar macrophages performed at our laboratory showed that rutile is less bioreactive than anatase. Our results suggest that a rutile coating on metallic biomaterials would improve their biocompatibility properties (Olmedo et al., 2008a).

### **4. Clinical implications of corrosion**

The results of our studies on failed human dental implants and data obtained using the experimental models developed at our laboratory show that any titanium surface can suffer corrosion processes and release particles into the local and systemic biological milieu.

The peri-implant milieu consists of bone tissue, soft tissues, and saliva. Biochemical changes in the peri-implant environment may lead to implant corrosion (Laing, 1973). Thus, titanium implant corrosion is affected not only by the concentration of electrolytes but also by saliva pH (Duffó et al., 1999; Nikolopoulou, 2006) which can vary in areas around dental implants (Meffert et al., 1992). Lotthar et al. (1992) reported that titanium does not withstand a large number of chemical substances. These substances may be in foods, saliva, tooth pastes, and prophylactic agents. They decompose foodstuffs, change plaque metabolism, and cause corrosion (Lotthar et al., 1992; Siirilä & Könönen, 1991). The drop in pH in the electrolytic milieu as a result of local inflammatory processes would seem to stimulate the process of corrosion (Duffó et al., 1999). Abraham et al. (2006) demonstrated the presence of titanium in a wide range of concentrations in saliva and gingival fluid of patients with titanium dental implants.

Significant decreases in pH have been observed in traumatized tissues; indeed pH drops to as low as 4 during the wound healing process (Duffó et al., 1999; Laing, 1973). These low values increase tissue aggressiveness toward the metallic materials. In previous works we found that the decrease in the pH of the electrolytic milieu resulting from local inflammatory processes also stimulates the corrosion process (Duffó et al., 1999).

for titanium or zirconium in the blood (cells and/or plasma) of patients carrying an implant (coxofemoral implant, dental implant, plates and screws for fracture fixation, metallic panels for reconstructive surgery of large areas of the body) may serve as a method to detect the

It is known that trace metals can increase physiological production of reactive oxygen species (ROS) which, without a compensatory increase in antioxidative species, can lead to tissue damage (Gottschling et al., 2002; Kawanishi et al., 2002; Maziere et al., 2003). Studies conducted at our laboratory have shown the presence of titanium and zirconium particles in alveolar phagocytes immunohistochemically identified as CD68 macrophages (Olmedo et al., 2008b). Evaluation of oxidative metabolism of alveolar macrophages exposed to these oxides has shown an increase in generation of ROS. However, it must be pointed out that ROS levels in animals exposed to ZrO2 were found to be markedly lower than those of

As mentioned previously, the layer of titanium dioxide is crystallographically composed of anatase or a combination of anatase and rutile. Studies on generation of superoxide anion (O2-) in alveolar macrophages performed at our laboratory showed that rutile is less bioreactive than anatase. Our results suggest that a rutile coating on metallic biomaterials

The results of our studies on failed human dental implants and data obtained using the experimental models developed at our laboratory show that any titanium surface can suffer corrosion processes and release particles into the local and systemic biological milieu.

The peri-implant milieu consists of bone tissue, soft tissues, and saliva. Biochemical changes in the peri-implant environment may lead to implant corrosion (Laing, 1973). Thus, titanium implant corrosion is affected not only by the concentration of electrolytes but also by saliva pH (Duffó et al., 1999; Nikolopoulou, 2006) which can vary in areas around dental implants (Meffert et al., 1992). Lotthar et al. (1992) reported that titanium does not withstand a large number of chemical substances. These substances may be in foods, saliva, tooth pastes, and prophylactic agents. They decompose foodstuffs, change plaque metabolism, and cause corrosion (Lotthar et al., 1992; Siirilä & Könönen, 1991). The drop in pH in the electrolytic milieu as a result of local inflammatory processes would seem to stimulate the process of corrosion (Duffó et al., 1999). Abraham et al. (2006) demonstrated the presence of titanium in a wide range of concentrations in saliva and gingival fluid of patients with titanium

Significant decreases in pH have been observed in traumatized tissues; indeed pH drops to as low as 4 during the wound healing process (Duffó et al., 1999; Laing, 1973). These low values increase tissue aggressiveness toward the metallic materials. In previous works we found that the decrease in the pH of the electrolytic milieu resulting from local

inflammatory processes also stimulates the corrosion process (Duffó et al., 1999).

would improve their biocompatibility properties (Olmedo et al., 2008a).

**) in alveolar macrophages. An experimental model** 

presence of a corrosion process of the metallic structures (Olmedo et al, 2003b).

**3.3 Effect of titanium and zirconium deposition on the lungs: Generation of** 

**Superoxide anion (O2**

animals exposed to TiO2.

dental implants.

**-**

**4. Clinical implications of corrosion** 

A corrosion process can decrease the fatigue resistance of the metal compromising metal resistance, which could eventually cause implant fracture (Adya et al., 2005; Guindy et al., 2004 ; Nikolopoulou, 2006 ; Tagger Green et al., 2002). It has been reported that the infiltration of saliva between the suprastructure (nickel-chromium-molybdenum alloy) and the implant (pure titanium) can trigger corrosion processes (galvanic corrosion) due to differences in electrochemical potentials. This causes the release of ions, such as nickel or chromium ions, from the alloy in the crown or bridge to the peri-implant tissues and subsequently results in bone resorption. The latter compromises implant stability, eventually causing implant fracture (Tagger Green et al., 2002).

Metal corrosion can affect the close contact between the implant and the bone tissue (osseointegration). The ions/metallic particles from coxofemoral prostheses can be phagocytosed by macrophages stimulating the release of cytokines, which contribute to bone resorption by activating osteoclasts. In addition to increasing bone resorption, the released particles may inhibit osteoblast function decreasing bone formation and contributing to osteolysis (Allen et al., 1997; Dowd et al., 1995).

The products of metallic implant corrosion behave as haptens generating a hypersensitive reaction that involves the release of inflammatory mediators, known as cytokines, and macrophage recruitment (Hallab et al., 2001; Jiranek et al., 1993; Yang & Merrit, 1994). It remains unclear to date whether it is the hypersensitivity to metal that causes implant failure or vice versa (Hallab et al., 2001). It also remains controversial whether an inflammatory process generates corrosion or whether corrosion triggers an inflammatory reaction. Thus, hypersensitivity to titanium as an implant material in oral and maxillofacial surgery probably occurs more commonly than has been reported in the literature (Matthew & Frame, 1998). There are reports of cases where titanium allergy mainly appeared as the fundamental cause of urticaria, eczema, oedema, redness and pruritus of the skin or mucosa, either localized, at distant sites, or generalized (Sicilia et al, 2008). However, the clinical relevance of allergic reactions in patients with titanium dental implants remains debatable (Javed et al., 2011).

Mineral elements play a critical role in the physiology and pathology of biological systems. Titanium is a nonessential element in that (a) no enzymatic pathway has been elucidated that requires titanium as a cofactor, (b) there does not appear to be any homeostatic control of titanium, and (c) titanium is not invariably detected in the newborn (Woodman et al, 1984b). Thus, the presence of titanium in the body, titanium biokinetics, and the potential biological effects of titanium are of great interest to researchers.

The toxicology of titanium is a current issue of debate. According to epidemiological studies, inhalation of powder containing titanium has no deleterious effect on the lungs (Daum et al., 1977; Ferin & Oberdörster, 1985). Other studies, however, suggest an association between titanium particles and pleural pathologies (Garabrant et al., 1987), granulomatous diseases, and malignant neoplasms of the lung. Our experimental studies have shown the presence of a considerable amount of titanium particles not only in alveolar macrophages but also in hepatocytes (Olmedo et al., 2008b). The accumulation of particles in the liver could compromise liver function as described by Urban et al. (2000). The authors associated the presence of titanium particles in a patient to granulomatous reactions and hepatomegalia. Various studies have reported the presence of macrophages related to failed

Systemic and Local Tissue Response to Titanium Corrosion 107

make them a suitable material for biomedical implants. Because zirconium offers superior corrosion resistance over most other alloy systems, better behavior in biological environments can be presumed (Stojilovic, 2005). Nevertheless, it is not widely used as a clinical material at present (Thomsen et al., 1997), since commercial manufacture of implants from zirconium or its alloys seems to be unfeasible due to the high cost of this material (Sherepo & Red'ko, 2004). The potential uses of zirconium-based materials for prosthetics and dental applications should be strongly considered and further investigated in

Nanotoxicology is a field of applied sciences involving the control of matter on an atomic or molecular scale, i.e. between 1 and 100 nanometers. Nanotechnology allows creating materials, devices, and systems by controlling matter on a nanometric scale taking advantage of new phenomena and properties (physical, chemical, and biological) that

The aim of applying the principles of nanotechnology to biomaterials (orthopedics and dentistry) is to create materials than can be applied directly to bone tissue, mimicking the natural nanostructure of human tissues by controlling the surface of the implant at a nanometric scale. This would improve the interaction between the implant surface with ions, biomolecules, and cells, favoring the biocompatibility properties of the bioimplant (Mendonçaa et al., 2008). For example, titanium implants with nanostructured coatings, films, and surfaces that seemingly improve the integration of bone tissue with the surface of the implant (osseointegration) and decrease the risk of implant corrosion are currently being developed. Although nanotechnology and its valuable contributions seek to provide answers to the increasing demands of different areas, it is important to understand that these advances may not only bring great advantages but also problems and health risks that must be carefully analyzed and prevented. Thus, nanoparticles may involve deleterious effects to humans or the environment. The fields that study these effects are nanotoxicology (Oberdörster et al., 2005) and nanoecotoxicology (Kahru & Dubourguier,

Nanoparticles can enter the body by inhalation, ingestion, injection, and/or through the skin (Oberdörster et al., 2005). In addition, they can generate inside the body as occurs when they are released from the surface of metallic implants and biomedical devices, such as coxofemoral prostheses, grids, plates, screws, and distractors used in surgery (Revell, 2006). Little is known about the effect, biodistribution, and final destination of nanometric particles (between 1 and 100 nanometers) inside the body. Given that nanoparticles have a larger surface area per unit of mass compared to microparticles, they may be more bioreactive and potentially more detrimental to human health. Although micro and nanoparticles can be chemically similar, their particular physico-chemical properties such as size, shape, electric charge, concentration, bioactivity and stability, may cause a different biological response. Analyzing the chemistry involved in the release of nanoparticles from metallic surfaces, their size, the quantity that enter the biological milieu, the site where they are transported to, and the immediate and long-term physico-pathological consequences of these particles is

a challenge to nanotoxicology and biocompatibility studies (Fig. 7 A-C).

appear at a nanometric scale (Drexler 1986; Mendonçaa et al., 2008).

laboratory and clinical settings.

2009).

**5. Nanotechnology - nanotoxicology** 

prostheses, both orthopedic and dental (Adya et al., 2005; Langkamer et al., 1992; Lee et al., 1992; Olmedo et al., 2003; Urban et al., 2002).

As to carcinogenic potential, there are scant reports on the potential development of malignant tumors associated with prosthetic structures in humans (Jacobs et al., 1992). The carcinogenic potential of the released metal ions and the development of associated neoplasias are still controversial issues. Within this context, the need arises to record cases that will contribute to monitor the potential association between tumor development and placing of a prosthetic structure (Apley, 1989; Brien et al., 1990; Goodfellow, 1992). Features such as ionic valence, particle concentration and size and hypersensitivity have been proposed to explain the potential association between malignant transformation and a metallic implant (Jacobs et al., 1992). In the field of Orthopedics in particular, metallic biomaterials are widely used to manufacture surgical materials such as prostheses for hip replacement or internal fixation devices, and surgeons who deal with traumatic, neoplastic, and degenerative disorders of the skeletal muscle system routinely handle these materials. The potential toxicity of some of the metals most frequently employed in the manufacture of orthopedic implants (titanium, aluminum, vanadium, cobalt, chromium, nickel) has been reported (Elinder & Friberg, 1986; Gitelman, 1989; Jacobs et al., 1991; Jandhyala & Hom, 1983; Langard & Norseth; 1986; Sunderman, 1989; Urban et al., 2000; Williams, 1981). Their carcinogenic potential has been evaluated in animal experimental models (Hueper, 1952; Lewis & Sunderman, 1996; Sinibaldi et al., 1976). The development of tumors at the implant site has been described. Most of the tumors were osteosarcomas or fibrosarcomas associated with stainless steel internal fixation devices (Black, 1988a). However, few reports discuss the potential development of malignant tumors associated to prosthetic structures in humans (Jacobs et al., 1992). Several mechanisms potentially involved in implant-related sarcomatous degeneration have been proposed. However, a direct cause-effect relation between the metal and sarcomatous degeneration in patients has not been demonstrated to date (Black, 1988b; Brown et al., 1987; Case et al., 1996; Goodfellow, 1992). As regards titanium specifically, there are reports of neoplasia in association with dental implants, such as squamous cell carcinoma (Gallego et al., 2008) osteosarcoma (McGuff et al., 2008) and plasmacytoma of the mandible (Poggio, 2007). It is of note that TiO2 was classified by the International Agency for Cancer Research, as possibly carcinogenic to humans (Group 2B) (Baan et al., 2006).

In this regard, our research group reported a case of sarcomatous degeneration in the vicinity of a stainless steel metallic implant, thus adding to the pool of information that may allow determining more accurately the potential toxicity of metallic implants and the risks associated with their use (Olmedo et al., 2007b).

Regarding the use of titanium and zirconium as implantable materials, Thomsen et al. (1997) found that both titanium and zirconium have a positive effect on tissue-material interaction. A previous experimental study conducted at our laboratory on bone tissue response to an implant, showed greater peri-implant bone thickness and volume in bone surrounding zirconium implants as compared to that around titanium implants (Guglielmotti et al., 1999). Zirconium is chemically related to, and has several properties in common with titanium (Thomsen et al., 1997). According to several researchers (Johansson et al., 1994; Sherepo & Red'ko, 2004; Thomsen et al., 1997; Yuanyuan & Yong, 2007), the elasticity, corrosion resistance, and other physico-mechanical properties of zirconium and its alloys

prostheses, both orthopedic and dental (Adya et al., 2005; Langkamer et al., 1992; Lee et al.,

As to carcinogenic potential, there are scant reports on the potential development of malignant tumors associated with prosthetic structures in humans (Jacobs et al., 1992). The carcinogenic potential of the released metal ions and the development of associated neoplasias are still controversial issues. Within this context, the need arises to record cases that will contribute to monitor the potential association between tumor development and placing of a prosthetic structure (Apley, 1989; Brien et al., 1990; Goodfellow, 1992). Features such as ionic valence, particle concentration and size and hypersensitivity have been proposed to explain the potential association between malignant transformation and a metallic implant (Jacobs et al., 1992). In the field of Orthopedics in particular, metallic biomaterials are widely used to manufacture surgical materials such as prostheses for hip replacement or internal fixation devices, and surgeons who deal with traumatic, neoplastic, and degenerative disorders of the skeletal muscle system routinely handle these materials. The potential toxicity of some of the metals most frequently employed in the manufacture of orthopedic implants (titanium, aluminum, vanadium, cobalt, chromium, nickel) has been reported (Elinder & Friberg, 1986; Gitelman, 1989; Jacobs et al., 1991; Jandhyala & Hom, 1983; Langard & Norseth; 1986; Sunderman, 1989; Urban et al., 2000; Williams, 1981). Their carcinogenic potential has been evaluated in animal experimental models (Hueper, 1952; Lewis & Sunderman, 1996; Sinibaldi et al., 1976). The development of tumors at the implant site has been described. Most of the tumors were osteosarcomas or fibrosarcomas associated with stainless steel internal fixation devices (Black, 1988a). However, few reports discuss the potential development of malignant tumors associated to prosthetic structures in humans (Jacobs et al., 1992). Several mechanisms potentially involved in implant-related sarcomatous degeneration have been proposed. However, a direct cause-effect relation between the metal and sarcomatous degeneration in patients has not been demonstrated to date (Black, 1988b; Brown et al., 1987; Case et al., 1996; Goodfellow, 1992). As regards titanium specifically, there are reports of neoplasia in association with dental implants, such as squamous cell carcinoma (Gallego et al., 2008) osteosarcoma (McGuff et al., 2008) and plasmacytoma of the mandible (Poggio, 2007). It is of note that TiO2 was classified by the International Agency for Cancer Research, as possibly carcinogenic to humans (Group 2B)

In this regard, our research group reported a case of sarcomatous degeneration in the vicinity of a stainless steel metallic implant, thus adding to the pool of information that may allow determining more accurately the potential toxicity of metallic implants and the risks

Regarding the use of titanium and zirconium as implantable materials, Thomsen et al. (1997) found that both titanium and zirconium have a positive effect on tissue-material interaction. A previous experimental study conducted at our laboratory on bone tissue response to an implant, showed greater peri-implant bone thickness and volume in bone surrounding zirconium implants as compared to that around titanium implants (Guglielmotti et al., 1999). Zirconium is chemically related to, and has several properties in common with titanium (Thomsen et al., 1997). According to several researchers (Johansson et al., 1994; Sherepo & Red'ko, 2004; Thomsen et al., 1997; Yuanyuan & Yong, 2007), the elasticity, corrosion resistance, and other physico-mechanical properties of zirconium and its alloys

1992; Olmedo et al., 2003; Urban et al., 2002).

(Baan et al., 2006).

associated with their use (Olmedo et al., 2007b).

make them a suitable material for biomedical implants. Because zirconium offers superior corrosion resistance over most other alloy systems, better behavior in biological environments can be presumed (Stojilovic, 2005). Nevertheless, it is not widely used as a clinical material at present (Thomsen et al., 1997), since commercial manufacture of implants from zirconium or its alloys seems to be unfeasible due to the high cost of this material (Sherepo & Red'ko, 2004). The potential uses of zirconium-based materials for prosthetics and dental applications should be strongly considered and further investigated in laboratory and clinical settings.

### **5. Nanotechnology - nanotoxicology**

Nanotoxicology is a field of applied sciences involving the control of matter on an atomic or molecular scale, i.e. between 1 and 100 nanometers. Nanotechnology allows creating materials, devices, and systems by controlling matter on a nanometric scale taking advantage of new phenomena and properties (physical, chemical, and biological) that appear at a nanometric scale (Drexler 1986; Mendonçaa et al., 2008).

The aim of applying the principles of nanotechnology to biomaterials (orthopedics and dentistry) is to create materials than can be applied directly to bone tissue, mimicking the natural nanostructure of human tissues by controlling the surface of the implant at a nanometric scale. This would improve the interaction between the implant surface with ions, biomolecules, and cells, favoring the biocompatibility properties of the bioimplant (Mendonçaa et al., 2008). For example, titanium implants with nanostructured coatings, films, and surfaces that seemingly improve the integration of bone tissue with the surface of the implant (osseointegration) and decrease the risk of implant corrosion are currently being developed. Although nanotechnology and its valuable contributions seek to provide answers to the increasing demands of different areas, it is important to understand that these advances may not only bring great advantages but also problems and health risks that must be carefully analyzed and prevented. Thus, nanoparticles may involve deleterious effects to humans or the environment. The fields that study these effects are nanotoxicology (Oberdörster et al., 2005) and nanoecotoxicology (Kahru & Dubourguier, 2009).

Nanoparticles can enter the body by inhalation, ingestion, injection, and/or through the skin (Oberdörster et al., 2005). In addition, they can generate inside the body as occurs when they are released from the surface of metallic implants and biomedical devices, such as coxofemoral prostheses, grids, plates, screws, and distractors used in surgery (Revell, 2006). Little is known about the effect, biodistribution, and final destination of nanometric particles (between 1 and 100 nanometers) inside the body. Given that nanoparticles have a larger surface area per unit of mass compared to microparticles, they may be more bioreactive and potentially more detrimental to human health. Although micro and nanoparticles can be chemically similar, their particular physico-chemical properties such as size, shape, electric charge, concentration, bioactivity and stability, may cause a different biological response. Analyzing the chemistry involved in the release of nanoparticles from metallic surfaces, their size, the quantity that enter the biological milieu, the site where they are transported to, and the immediate and long-term physico-pathological consequences of these particles is a challenge to nanotoxicology and biocompatibility studies (Fig. 7 A-C).

Systemic and Local Tissue Response to Titanium Corrosion 109

biomedical implant is essential from a biological, sanitary, metallurgic, economic, and social

Lastly, it is important to highlight that the adverse effects of corrosion described in the present chapter will not invariably occur in all patients with implants since biological

The studies were supported by Grants: PICT 2008-1116 and 1728/OC-AR - PICT 33493 from the National Agency for the Promotion of Science and Technology; UBACyT 20020100200157, UBACyT 20020100100657, UBACyT O-009 and UBACyT O-020 from the University of Buenos Aires; CONICET PIP 6042 and CONICET PIP 11220090100117 from the

Abraham, J., Grenón, M., Sánchez, H., Pérez, C. & Valentinuzzi, M. (2006). Titanium Based

Abramson, S., Alexander, H., Best, S., Bokros, J., Brunski, J., Colas, A., Cooper, S., Curtis, J.,

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1-2, ISSN 1518-0204. Available from URL: http://www.lnls.br/ar2006/PDF/909.pdf

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York, USA

ISSN 0301-620X

Diego, California, USA

viewpoint.

response varies among individuals.

**7. Acknowledgements** 

**8. References** 

Fig. 7. A) Scanning Electron Microscopy of TiO2 nanoparticles (10nm). B) Monocyte (*m*) containing nanometric titanium particles (5nm) can be observed in the hepatic sinusoid. (*h*) hepatocyte, (*e*) erythrocyte. X4000. C) Higher magnification allows identifying particles clustered close to the nucleus (→). X16000

### **6. Conclusions**

No metal or alloy is completely inert *in vivo*. Whether noble or passivated, all metals will suffer a slow removal of ions from the surface, largely because of local and temporal variations in microstructure and environment. The potential risk of corrosion and the possible detrimental consequences of corrosion byproducts to tissues are issues of clinical importance.

The biologic effect of corrosion is a public health concern for the community of patients who have a prosthesis (orthopedic and/or dental), since these prostheses remain inside the body over long periods of time.

Evaluation of tissues around metallic devices is important since the presence of ions/particles and their potential local biological effects might affect implant outcome. Corrosion is one of the possible causes of implant failure after initial success. Metal corrosion can affect close contact between the implant and the bone tissue (osseointegration).

The issue of corrosion is not only a local problem since particles resulting from this process could migrate systemically and deposit in target organs. The long term effects of these deposits are yet to be clarified. Mineral elements play a critical role in the physiology and pathology of biological systems. Titanium is a nonessential element; thus, the presence of titanium in the body, titanium biokinetics, and the potential biological effects of titanium are of great interest to researchers.

 "In situ" degradation of a metallic implant is an unwanted event since it alters the structural integrity of the implant. Implant manufacturers must attempt to develop methods that reduce the diffusion of metal into the tissues in order to minimize the deleterious effects of corrosion.

We believe further investigation, in particular long-term research, is necessary to advance in the understanding of the factors involved in implant corrosion and establish basic guidelines for their use in clinical implantology. Handling and controlling corrosion of a biomedical implant is essential from a biological, sanitary, metallurgic, economic, and social viewpoint.

Lastly, it is important to highlight that the adverse effects of corrosion described in the present chapter will not invariably occur in all patients with implants since biological response varies among individuals.

### **7. Acknowledgements**

The studies were supported by Grants: PICT 2008-1116 and 1728/OC-AR - PICT 33493 from the National Agency for the Promotion of Science and Technology; UBACyT 20020100200157, UBACyT 20020100100657, UBACyT O-009 and UBACyT O-020 from the University of Buenos Aires; CONICET PIP 6042 and CONICET PIP 11220090100117 from the National Research Council (CONICET) and Roemmers Foundation, Argentina.

### **8. References**

108 Pitting Corrosion

clustered close to the nucleus (→). X16000

**6. Conclusions** 

importance.

over long periods of time.

of great interest to researchers.

(osseointegration).

of corrosion.

Fig. 7. A) Scanning Electron Microscopy of TiO2 nanoparticles (10nm). B) Monocyte (*m*) containing nanometric titanium particles (5nm) can be observed in the hepatic sinusoid. (*h*) hepatocyte, (*e*) erythrocyte. X4000. C) Higher magnification allows identifying particles

No metal or alloy is completely inert *in vivo*. Whether noble or passivated, all metals will suffer a slow removal of ions from the surface, largely because of local and temporal variations in microstructure and environment. The potential risk of corrosion and the possible detrimental consequences of corrosion byproducts to tissues are issues of clinical

The biologic effect of corrosion is a public health concern for the community of patients who have a prosthesis (orthopedic and/or dental), since these prostheses remain inside the body

Evaluation of tissues around metallic devices is important since the presence of ions/particles and their potential local biological effects might affect implant outcome. Corrosion is one of the possible causes of implant failure after initial success. Metal corrosion can affect close contact between the implant and the bone tissue

The issue of corrosion is not only a local problem since particles resulting from this process could migrate systemically and deposit in target organs. The long term effects of these deposits are yet to be clarified. Mineral elements play a critical role in the physiology and pathology of biological systems. Titanium is a nonessential element; thus, the presence of titanium in the body, titanium biokinetics, and the potential biological effects of titanium are

 "In situ" degradation of a metallic implant is an unwanted event since it alters the structural integrity of the implant. Implant manufacturers must attempt to develop methods that reduce the diffusion of metal into the tissues in order to minimize the deleterious effects

We believe further investigation, in particular long-term research, is necessary to advance in the understanding of the factors involved in implant corrosion and establish basic guidelines for their use in clinical implantology. Handling and controlling corrosion of a Abraham, J., Grenón, M., Sánchez, H., Pérez, C. & Valentinuzzi, M. (2006). Titanium Based Implants, Metal Release Study in the Oral Environment. LNLS, Activity report. pp. 1-2, ISSN 1518-0204. Available from URL:

http://www.lnls.br/ar2006/PDF/909.pdf


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**6** 

**Importance of Etch Film Formation** 

Maria Tzedaki1, Iris De Graeve1, Bernhard Kernig2,

Jochen Hasenclever2 and Herman Terryn1

*1Vrije Universiteit Brussel, 2HYDRO Aluminium Bonn* 

> *1Belgium 2Germany*

 **During AC Controlled Pitting of Aluminium** 

Over the last decades much research has been dedicated and reported in literature on the Alternating Current electrograining (A.C. electrograining) mechanism and the parameters that change the final pitting morphology of electrograined aluminium. The reason is the increasing industrial demand every year (currently estimated to be 800,000,000 m2/year) for high quality litho-printing and super capacitors for energy storage. Both applications relay on the production of a controlled roughened surface on aluminium foil which can be achieved through A.C. electrograining. In this case pitting is used as a surface treatment for

In the past, research focused on different conditions of the A.C. electrograining of aluminium in relation to the final pitting morphology. The charge density, the wave shape of the applied current, the electrolyte concentration, the frequency or the temperature are factors that were studied extensively. The industrial application of the A.C. electrograining process makes it very important to understand and correlate all the parameters which affect the graining. The mechanism which initiates or affects the final morphology needs to be understood. In this chapter we will present an overview on the influence of the different A.C. electrograining conditions on the final pitting morphology, with the focus mainly on the smut film formation. The latter has proven to play an important role during the A.C. electrograining but little correlation was shown so far between the smut film formation and the pitting morphology. We will present in this article the importance of the smut film formation mechanism during the A.C. electrograining of aluminium and the H2 evolution, taking place during the cathodic half cycle. Conditions, such as the applied potential, the electrolyte concentration, the frequency and the charge density are important to be presented since they play a crucial role in the creation of the final pitting morphology of aluminium.

Much effort has recently been put towards understanding the H2 evolution; therefore it is important to include in this chapter reported differences observed in gas retention within the etch smut film for different electrochemical conditions and relate these differences to the

the production of a larger surface area with uniform pits.

**1. Introduction** 

final smut morphology.

