1.1.1 Illustrative examples of the application of LPR in corrosion and tribocorrosion systems

A modified electrochemical noise technique, namely, electrochemical emission spectroscopy (EES) [29], offers one of the most convincing examples of the application of the LPR technique in tribocorrosion [30]. Indeed, the analysis of noise data in a potential-current plane shows the transposition of the statistical resistance due to electrochemical noise to the resistance due to linear polarization. Noise resistance is often considered equivalent to the polarization resistance, RP [31–33]. The noise resistance, RN, calculated using a method proposed by Eden et al. [33], for mild steel passive alloy in 0.05 M H2SO4 (corrosion under activation control), is of the order of 48 Ω without any sliding contact. The LPR measured on this mild steel after EES monitoring is shown in Figure 2a. The comparative value of RP obtained by the LPR technique is 50 Ω. The RN value obtained using the EES technique is therefore very close to the RP obtained by the LPR technique. Under tribocorrosion conditions (5 N normal force, 10 Hz sliding frequency, 200 μm peak-to-peak displacement amplitude), the plane plot of the potential-current data under steadystate wear-corrosion regime shows a best-fit line through the data points with a positive slope of 54 Ω (see Figure 2b), which roughly corresponds to RN (48 Ω) or RP (50 Ω in Figure 2a). Notwithstanding, no attempt has been made to relate these resistance measurements with the breakdown (i.e., depassivation) or the buildup of any kind of passive film (i.e., repassivation) on the mild steel surface subjected to either a mechanical stimuli (e.g., active-passive wear track zone area or metastable pit area) or in the absence of wear (free corrosion), characteristic phenomena of localized corrosion.

<sup>i</sup> <sup>¼</sup> <sup>i</sup><sup>0</sup> exp <sup>α</sup>nF<sup>η</sup>

Corrosion Inhibitors

zero and is related to RP as follows:

0.167 mV s�<sup>1</sup>

170

.

RT 

<sup>¼</sup> <sup>i</sup><sup>0</sup> exp <sup>α</sup>nF Eð Þ � Ecorr

RT 

� exp ð Þ <sup>1</sup> � <sup>α</sup> nF<sup>η</sup> RT

In Figure 1, the linear relationship between the polarization resistance and the corrosion rate can be easily illustrated graphically. In the small region near the corrosion potential, Ecorr, only a very small perturbation potential, usually less than �30 mV (typically �10 mV), is applied above or below the corrosion potential, yielding a linear relationship between the overpotential (η = E � Ecorr) or the polarization from the corrosion potential and the current. Due to this smooth excitation, the LPR technique is not expected to interfere with corrosion reactions. The slope of that linearized curve (i � E) is defined as the polarization resistance, RP, of a corroding electrode (in ohms cm�<sup>2</sup> if the current density is plotted or in

� exp ð Þ <sup>1</sup> � <sup>α</sup> nF Eð Þ � Ecorr RT

(4)

(5)

(6)

ohms if the current is plotted), which is mathematically interpreted as

Icorr <sup>¼</sup> <sup>1</sup> Rp

calculated from Eq. (6) if RP and Tafel constants (ba and bc) are known.

determined by the Tafel extrapolation method (cf. infra).

RP <sup>¼</sup> <sup>∂</sup><sup>η</sup> ∂i 

where i is the current density corresponding to a particular value of E.

jE�Ecorr¼0

The corrosion current, Icorr, can be calculated when the overpotential approaches

: babc 2:303ð Þ ba þ bc

ASTM G59 describes an experimental procedure required to carry out polarization resistance measurement [10]. In agreement with this standard, the potential should be scanned from �30 mV to +30 mV of the corrosion potential at a rate of

where ba and bc are the so-called anodic and cathodic Tafel slopes or Tafel parameters, respectively (cf. infra). The corrosion current density, icorr, can thus be

Many of the foregoing determined corrosion key parameters are based on empirical observations. As with any empirical method, due to the high number of factors involved in a corrosion or tribocorrosion system (e.g., environmental changes, temperature, pH, reagent as chloride ions, pressure, specimen geometry, test setup configuration, etc.), it is not uncommon to observe that the values of ba, bc, and RP are influenced by these operational parameters and are therefore subject to change. Of significance, the slope generated from the i–E curve around the corrosion potential may not be linear and may or may not be symmetrical in the anodic and cathodic regions. The symmetry of the curve (i–E) at the point of equilibrium or at open-circuit potential is obtained only when ba and bc are equal. These values are required for computing the corrosion current and are usually

It is worthy to note that the measurements of Rp can be derived potentiodynamically or by the method of stepwise potentiostatic polarization or by anodic step pulse method. In the potentiodynamic method, the potential is swept at a constant rate (typically 60 mV/h) from the active (cathodic) direction to the noble (anodic) region passing through the corrosion potential while tracking the current density continuously. More information regarding this method can be found elsewhere [3].

Figure 2.

LPR measurements on the mild steel in 0.05 M H2SO4 solution; (a) under free corrosion state, (b) under wearcorrosion steady-state phase. Reproduced from [30] with permission from Wiley Online Library.

#### 1.2 Tafel extrapolation method

In 1905, Julius Tafel [34] presented the experimental relationship between the current, I, and the overpotential, η, during an electrocatalytic test of the reduction reaction of hydrogen (i.e., protons to form molecular hydrogen) on a number of electrode metals such as Hg, Sn, Bi, Au, Cu, Ni, and so on:

$$
\eta = a + b \log I \tag{7}
$$

<sup>η</sup><sup>a</sup> <sup>¼</sup> ba log <sup>I</sup>

Electrode kinetics as expressed by the Butler-Volmer equation, plotted in a semilogarithm scale or Tafel plot

Electrochemical Techniques for Corrosion and Tribocorrosion Monitoring: Methods…

DOI: http://dx.doi.org/10.5772/intechopen.86743

showing that the corrosion current density can be obtained from the intercept.

<sup>η</sup><sup>c</sup> ¼ �bc log <sup>I</sup>

Tafel extrapolation measurements can be performed either by the potentiodynamic method or by the stepwise potentiostatic polarization method [35]. As in RP measurements, in both methods, corrosion potential is first measured, typically for 1 h (during which time corrosion potentials of most electrodes are stabilized) or until it stabilizes. After that, the potential step—at increments of �25 or � 50 or � 100 mV, every 5 min, recording the current at the end of each 5-min period—is applied (potential-step method), or the potential is scanned at a constant

rate (typically 0.6 V/h) (potentiodynamic method). In both methods, the

The polarization curve can be measured either dynamically or statically (either in the potential-controlled mode or in the current-controlled mode). The dynamic polarization techniques can be carried out relatively fast, but the drawback is that the Tafel parameters are scanning rate dependent. The static polarization techniques may produce better Tafel parameters, but they are very time-consuming.

Eq. (8) can be modified as,

Figure 3.

173

Likewise, at sufficiently larger values of η, in the cathodic direction (i.e., η = ηc),

Icorr

Icorr

(9)

(10)

where the overpotential η is defined as the difference between the potential of the working electrode, E, and the equilibrium potential.

The existence of a linear relationship between E and log I has been demonstrated when the electrode is polarized at sufficiently large potentials, and far away from the corrosion potential both in anodic and cathodic directions [34], as can be seen in the polarization curve depicted in Figure 3. The portions in which such relationships prevail are called Tafel portions or Tafel regions.

This can be mathematically expressed as

$$\begin{split} I &= I\_{corr} \left[ \exp\left(\frac{2.303\eta}{b\_a}\right) - \exp\left(-\frac{2.303\eta}{b\_c}\right) \right] \\ &= I\_{corr} \left[ \exp\left(\frac{2.303(E - E\_{corr})}{b\_a}\right) - \exp\left(-\frac{2.303(E - E\_{corr})}{b\_c}\right) \right] \end{split} \tag{8}$$

where Ecorr is the corrosion potential, E the applied potential, η the overpotential (difference between E and Ecorr), I the current, Icorr the corrosion current, and ba and bc are the Tafel constants or Tafel parameters derived from E � log I plots as the anodic and cathodic slopes in the Tafel regions, respectively.

Extrapolating from the Tafel portions of either anodic or cathodic or both, an intersection point is obtained at Ecorr, from which Icorr is readily available from the log I axis. Therefore, it is possible to obtain simultaneously the corrosion current, Icorr, and the Tafel parameters (i.e., ba and bc) from this method.

In order to obtain the Tafel portions in the anodic and cathodic regions, the electrode has to be polarized far away from its corrosion potential, e.g., �250 mV away from Ecorr. Eq. (8) can be rearranged, as appropriate, to choose one single polarization direction, either anodic or cathodic way.

At sufficiently larger values of η (100 mV ≤ η ≤ 500 mV), in the anodic direction (i.e., η = ηa), Eq. (8) can be rearranged as,

Electrochemical Techniques for Corrosion and Tribocorrosion Monitoring: Methods… DOI: http://dx.doi.org/10.5772/intechopen.86743

#### Figure 3.

1.2 Tafel extrapolation method

Figure 2.

Corrosion Inhibitors

I ¼ Icorr exp

¼ Icorr exp

In 1905, Julius Tafel [34] presented the experimental relationship between the current, I, and the overpotential, η, during an electrocatalytic test of the reduction reaction of hydrogen (i.e., protons to form molecular hydrogen) on a number of

LPR measurements on the mild steel in 0.05 M H2SO4 solution; (a) under free corrosion state, (b) under wear-

corrosion steady-state phase. Reproduced from [30] with permission from Wiley Online Library.

where the overpotential η is defined as the difference between the potential of

The existence of a linear relationship between E and log I has been demonstrated when the electrode is polarized at sufficiently large potentials, and far away from the corrosion potential both in anodic and cathodic directions [34], as can be seen in the polarization curve depicted in Figure 3. The portions in which such relation-

� exp � <sup>2</sup>:303<sup>η</sup>

2:303ð Þ E � Ecorr ba 

anodic and cathodic slopes in the Tafel regions, respectively.

Icorr, and the Tafel parameters (i.e., ba and bc) from this method.

polarization direction, either anodic or cathodic way.

(i.e., η = ηa), Eq. (8) can be rearranged as,

172

bc

where Ecorr is the corrosion potential, E the applied potential, η the overpotential (difference between E and Ecorr), I the current, Icorr the corrosion current, and ba and bc are the Tafel constants or Tafel parameters derived from E � log I plots as the

Extrapolating from the Tafel portions of either anodic or cathodic or both, an intersection point is obtained at Ecorr, from which Icorr is readily available from the log I axis. Therefore, it is possible to obtain simultaneously the corrosion current,

In order to obtain the Tafel portions in the anodic and cathodic regions, the electrode has to be polarized far away from its corrosion potential, e.g., �250 mV away from Ecorr. Eq. (8) can be rearranged, as appropriate, to choose one single

At sufficiently larger values of η (100 mV ≤ η ≤ 500 mV), in the anodic direction

(8)

� exp � <sup>2</sup>:303ð Þ <sup>E</sup> � Ecorr

bc

η ¼ a þ b log I (7)

electrode metals such as Hg, Sn, Bi, Au, Cu, Ni, and so on:

the working electrode, E, and the equilibrium potential.

ships prevail are called Tafel portions or Tafel regions. This can be mathematically expressed as

> 2:303η ba

Electrode kinetics as expressed by the Butler-Volmer equation, plotted in a semilogarithm scale or Tafel plot showing that the corrosion current density can be obtained from the intercept.

$$\eta\_a = b\_a \log \frac{I}{I\_{corr}} \tag{9}$$

Likewise, at sufficiently larger values of η, in the cathodic direction (i.e., η = ηc), Eq. (8) can be modified as,

$$
\eta\_c = -b\_c \log \frac{I}{I\_{corr}} \tag{10}
$$

The polarization curve can be measured either dynamically or statically (either in the potential-controlled mode or in the current-controlled mode). The dynamic polarization techniques can be carried out relatively fast, but the drawback is that the Tafel parameters are scanning rate dependent. The static polarization techniques may produce better Tafel parameters, but they are very time-consuming.

Tafel extrapolation measurements can be performed either by the potentiodynamic method or by the stepwise potentiostatic polarization method [35]. As in RP measurements, in both methods, corrosion potential is first measured, typically for 1 h (during which time corrosion potentials of most electrodes are stabilized) or until it stabilizes. After that, the potential step—at increments of �25 or � 50 or � 100 mV, every 5 min, recording the current at the end of each 5-min period—is applied (potential-step method), or the potential is scanned at a constant rate (typically 0.6 V/h) (potentiodynamic method). In both methods, the

experiment is started at the corrosion potential, and the cathodic polarization is first conducted by applying an overpotential of approximately 500 mV or until gas evolution (e.g., hydrogen) occurs at the electrode, at a constant rate of 0.6 V/h. Following, the corrosion potential is measured again (typically for 1 h), and then anodic polarization is conducted by applying an overpotential so that the potential at the end of the anodic polarization reaches +1.6 V versus SCE. Tafel plots are generated by plotting both anodic and cathodic data in a semilog paper as E-log I. From the plot, three values are determined: the anodic Tafel slope, the cathodic Tafel slope, and Icorr (from back-extrapolation of both anodic and cathode curves to Ecorr). The main advantage of this method is that it provides a simple, straightforward method to determine Tafel parameters, namely, ba and bc.

track area, coordinated by the coupling effects of wear and corrosion in the triboelectrochemical cell. Among the possibilities offered by the measurement of electrochemical noise sources during an electrochemical or a tribo-electrochemical system, the following can be retained: adsorption–desorption processes, e.g., formation and detachment of gas bubbles; fluctuations in the mass transport rate and in temperature; interfacial nucleation and growth processes; degradation processes due dielectric film disruption; kinetics of atom exchange at the surface sites, e.g.,

Electrochemical Techniques for Corrosion and Tribocorrosion Monitoring: Methods…

While multiple case studies on electrochemical noise have been regularly reported in recent years, even greater progress is possible, with the scope for increased breakthrough in science and technology (e.g., novel materials, precision tools on macro-to-nanoscopic scales, availability and intelligent use of these materials and tools, and so on). In particular, the main focus of these investigations is to promptly obtain in situ mechanistic information on the repassivation and breakdown of passive films and to monitor any process associated with confined corrosion and/or tribo- or bio-tribocorrosion [46, 47, 51]. It has been, indeed, suggested that the noise is caused by film breakdown and repassivation processes, and given the dynamic competition between these two processes, pitting will initiate. However, the foundation for using electrochemical noise analysis for determining the corrosion rate of an electrode is still a subject of debate within the scientific community. Indeed, the fundamental approach is not as robust as that of other techniques. On the other hand, the advantage of the noise analysis is that it is not necessary to apply any external polarization and the system is in natural corrosion conditions. This renders the technique as nondestructive and nonintrusive, capable of monitoring basic changes in an electrochemically active system. This makes it particularly suitable for online corrosion monitoring in the laboratory, especially for localized corrosion monitoring, detection of general corrosion, crevice investigation, stress corrosion cracking [12, 52, 53], fretting corrosion, or be used in the assessment of anti-corrosive organic coatings, and other surface inhomogeneity case studies [43, 46, 47]. Several approaches extend the use of electrochemical noise measurements in both pilot plant and field facilities, its use is not merely limited to the foregoing phenomena, but its development is justified especially when measurements are performed in systems with very low conductivity, where, for e.g., the impedance technique fails because of

Johnson's noise in the interfacial impedance; and so on.

DOI: http://dx.doi.org/10.5772/intechopen.86743

the loss of signal in the high resistance of the solution (cf. infra).

tribocorrosion systems

175

1.3.1 Instrumentation for electrochemical noise measurements in corrosion and

Electrochemical noise is a generic term used to describe the naturally occurring fluctuations in potential and current, which is due to spontaneous changes in electrode kinetics and mechanisms [33]. When applied to corrosion studies, electrochemical noise may be redefined as the spontaneous fluctuations observed in potential and current at the free corrosion potential. The electrochemical noise can thus be classified into potential noise and current noise. There are three major possible modes for measuring potential and current noise in a corrosion system, but the most common mode uses two nominally identical working electrodes, WE1 and WE2 (WE1 as the corroding metal and WE2 as a counter electrode), and a noise-free reference noble electrode, RE [33] (see Figure 4a). The current flowing between the two working electrodes is measured by a zero-resistance ammeter (ZRA), and their potential is monitored versus the reference electrode through a voltmeter (V) under free corrosion conditions. The two other leftover modes are two identical working electrodes WE1 and WE2 with a bias potential [54] (not shown here) and one WE coupled to a micro-counter electrode (MC, e.g., Pt wire tip) [29, 46, 47, 55]

The disadvantage of the Tafel technique is that large current densities are often required to generate the complete Tafel plots. The use of large current densities can alter the surface conditions of the specimen (e.g., permanent change or surface damage), thereby distorting the results and increasing complications due to mass transport and uncompensated electrolyte resistance. The measurement of current density over a wide potential range may also distort the results if the adsorption of some species is potential dependent. Since this method applies a large overpotential to the metal surface (e.g., anodic polarization), therefore, the technique is rather destructive and can hardly be used for online corrosion monitoring purposes and in particular in the field. An ASTM G5 standard provides a procedure for constructing an anodic polarization plot [36]. However, it does not supply a method to construct a cathodic polarization plot nor a procedure to determine the corrosion current by the Tafel extrapolation method.
