**3.2 Identification of the resistance to corrosion of iron**

Iron is a metal with the greatest technical importance. Its useful physical properties include relatively high hardness, ductility and large malleability, relatively low production costs and high prevalence in nature. However, chemically pure iron has practically no direct use, while iron alloys with carbon, silicon and other metals have an enormous technical and practical importance.

Iron is a relatively reactive metal - its standard electrochemical potential is -760mV. It reacts with all diluted acids resulting in the salts of iron (II). Chemically pure iron is relatively less prone to corrosion compared to its commonly used alloys. Steels containing various alloying elements have different chemical compositions in material micro-zones. Such micro-zones in contact with the electrolyte solution lead to different electrochemical potentials and are able to create micro-cells, in which iron is most often an anode. As a result of these electrode processes the iron oxidation occurs and the formation of various corrosion products takes place, in which iron occurs primarily at two and three degrees of oxidation.

Electrochemical corrosion characteristics of iron were determined by potentiodynamic and impedance spectroscopy techniques. Tests were applied to chemically pure iron Fe made by electrocrystalization method and to carbon steel S235JR with the chemical composition shown in Table 3.


Table 3. The content of alloying elements in S235JR steel.

Images of topography and surface morphology of Fe iron and S235JR steel prior to corrosion tests are shown in Fig. 11.

In studies using potentiodynamic polarization perturbation of the steady state of the metalsolution the potential varied between -550mV to -10mV in the case of iron produced electrochemically, and in the range -723mV to -20mV in the case of S235JR steel. The rate of potential changes during the test was 0.3mV/s.

Studies of Resistance to Corrosion

intensity response.

value of the parameter n.

resulting impedance, namely

NaCl.

of Selected Metallic Materials Using Electrochemical Methods 409

The results show that pure iron produced electrochemically has much higher corrosion resistance compared to S235JR steel. Thus, alloying elements and the heterogeneity of the

For further characterization of electrochemical corrosion processes at the interface of iron and S235JR steel with the 0.5M NaCl solution environment electrochemical impedance spectroscopy was used. The study consisted of perturbing the equilibrium of the corrosion system with a sinusoidal potential signal of small amplitude (15mV) across a wide frequency range (10kHz ÷ 33mHz) and recording the changes in time of system's current

The measured frequency characteristics of electrochemically produced iron and S235JR steel in corrosive environment of 0.5M NaCl solution are presented in the form of Nyquist

Effective modeling of complex electrochemical processes of corrosion in the systems based on iron required the use of a more complex equivalent electrical circuit, i.e., circuit containing CPE - constant phase elements. Constant phase element (CPE) is characterized by a constant angle of phase shift. Impedance of the CPE is described by the following expression: ZCPE = 1/Y0(jω)n, where Y0 and n are parameters related to the phase angle. The more heterogeneous the corrosion processes occurring on the metal surface the smaller

Best matching of all designated impedance spectra for experimentally studied systems of corrosion of iron and S235JR steel in the solution of 0.5 M NaCl was obtained by using an equivalent electric circuit with two time constants, whose structure is shown in Fig. 13.

Fig. 13. The equivalent circuit for corrosion of Fe and S235JR steel in the solution of 0.5M

Rp con

CPEp

This equivalent electric circuit can be described by the following relationship defining the

<sup>s</sup> <sup>n</sup> <sup>n</sup> p dl

Each element of this circuit appropriately models the specific process or phenomenon occurring in the system investigated. In the circuit shown in Fig. 13 the resistance element Rs represents corrosive environment, i.e., 0.5 M NaCl solution. The resistance representing the charge transfer through the interface associated with the process of oxidation of iron, i.e., the corrosion element, is described by Rct, and the electrical double layer at the interface iron - 0.5M NaCl solution is characterized by a constant phase element CPEdl. The use of two constant-phase elements in an equivalent electric circuit improves the quality of model fit to

p ct 1 1 Z R 1 1 Y (jω ) Y(jω ) R R

p dl

Rct

CPEdl

(3)

diagrams (Fig. 14) and Bode plots of impedance spectra (Fig. 15).

Rs

material in the case of carbon steel activate electrochemical processes of the material.

The characteristics j = f(E) for iron Fe and S235JR steel in 0.5M NaCl solution obtained in the above potential ranges are shown in Fig. 12.

Fig. 11. Topography and surface morphology of Fe and S235JR steel prior corrosion tests.

Fig. 12. Potentiodynamic polarization curves of Fe and S235JR steel in 0.5M NaCl solution.

Each of the potentiodynamic polarization curves for iron Fe and S235JR steel in the range of potentials tested consists of two parts: the cathodic and anodic segments. Part of the reduction process corresponds to the cathodic corrosive components of H+ and O2 occurring on the metal surface. However, part of anodic potentiodynamic polarization curve is characterized by the oxidation process of metal atoms or the process of corrosion - in the case of iron the reaction is Fe - 2e Fe+2.

The corrosion test parameters of iron and S235JR steel in 0.5M NaCl solution are summarized in Table 4.


Table 4. Parameters of Fe and S235JR steel in corrosion environment of 0.5 M NaCl solution.

The characteristics j = f(E) for iron Fe and S235JR steel in 0.5M NaCl solution obtained in the

 **S235JR** 

Fig. 11. Topography and surface morphology of Fe and S235JR steel prior corrosion tests.

 **50 μm** 

<sup>100000</sup> 0,5M NaCl

Fe


Fig. 12. Potentiodynamic polarization curves of Fe and S235JR steel in 0.5M NaCl solution.

Each of the potentiodynamic polarization curves for iron Fe and S235JR steel in the range of potentials tested consists of two parts: the cathodic and anodic segments. Part of the reduction process corresponds to the cathodic corrosive components of H+ and O2 occurring on the metal surface. However, part of anodic potentiodynamic polarization curve is characterized by the oxidation process of metal atoms or the process of corrosion - in the

The corrosion test parameters of iron and S235JR steel in 0.5M NaCl solution are

Material Ecor [mV] jcor [A/cm2] Fe -445 24 S235JR -641 93 Table 4. Parameters of Fe and S235JR steel in corrosion environment of 0.5 M NaCl solution.

*E* [mV]

above potential ranges are shown in Fig. 12.

 **Fe** 

1

case of iron the reaction is Fe - 2e Fe+2.

summarized in Table 4.

10

100

S235JR

 **50 μm** 

1000

*j* [A/cm2

]

10000

The results show that pure iron produced electrochemically has much higher corrosion resistance compared to S235JR steel. Thus, alloying elements and the heterogeneity of the material in the case of carbon steel activate electrochemical processes of the material.

For further characterization of electrochemical corrosion processes at the interface of iron and S235JR steel with the 0.5M NaCl solution environment electrochemical impedance spectroscopy was used. The study consisted of perturbing the equilibrium of the corrosion system with a sinusoidal potential signal of small amplitude (15mV) across a wide frequency range (10kHz ÷ 33mHz) and recording the changes in time of system's current intensity response.

The measured frequency characteristics of electrochemically produced iron and S235JR steel in corrosive environment of 0.5M NaCl solution are presented in the form of Nyquist diagrams (Fig. 14) and Bode plots of impedance spectra (Fig. 15).

Effective modeling of complex electrochemical processes of corrosion in the systems based on iron required the use of a more complex equivalent electrical circuit, i.e., circuit containing CPE - constant phase elements. Constant phase element (CPE) is characterized by a constant angle of phase shift. Impedance of the CPE is described by the following expression: ZCPE = 1/Y0(jω)n, where Y0 and n are parameters related to the phase angle. The more heterogeneous the corrosion processes occurring on the metal surface the smaller value of the parameter n.

Best matching of all designated impedance spectra for experimentally studied systems of corrosion of iron and S235JR steel in the solution of 0.5 M NaCl was obtained by using an equivalent electric circuit with two time constants, whose structure is shown in Fig. 13.

Fig. 13. The equivalent circuit for corrosion of Fe and S235JR steel in the solution of 0.5M NaCl.

This equivalent electric circuit can be described by the following relationship defining the resulting impedance, namely

$$\mathbf{Z} = \mathbf{R}\_s + \frac{1}{\frac{1}{\mathbf{R}\_p} + \mathbf{Y}\_p \text{(joo)}^{\text{n}\_p}} + \frac{1}{\frac{1}{\mathbf{R}\_{\text{ct}}} + \mathbf{Y}\_{\text{dl}} \text{(joo)}^{\text{n}\_{\text{dl}}}} \tag{3}$$

Each element of this circuit appropriately models the specific process or phenomenon occurring in the system investigated. In the circuit shown in Fig. 13 the resistance element Rs represents corrosive environment, i.e., 0.5 M NaCl solution. The resistance representing the charge transfer through the interface associated with the process of oxidation of iron, i.e., the corrosion element, is described by Rct, and the electrical double layer at the interface iron - 0.5M NaCl solution is characterized by a constant phase element CPEdl. The use of two constant-phase elements in an equivalent electric circuit improves the quality of model fit to

Studies of Resistance to Corrosion

result of calculations (solid lines).

Fe


log *f* [Hz]

S235JR

are shown in Fig. 16.

1,2

1,6

2,0

log I

*Z*I

2,4

2,8

3,2

 **Fe** 

 **S235JR** 

polarization method.

of Selected Metallic Materials Using Electrochemical Methods 411

the test environment, which also confirms the results obtained with the potentiodynamic

[deg]

Fe

0,5M NaCl

Fig. 15. Bode amplitude and phase spectra of circuit impedance and corrosion systems of Fe and S235JR steel in 0.5M NaCl solution determined experimentally (point line) and as a

Images of destruction of corrosion test samples of Fe and S235JR steel after corrosion tests

 **200 μm 50 μm** 

 **500 μm 50 μm** 


S235JR

0,5M NaCl

Fig. 16. Images of the surfaces of Fe and S235JR steel after corrosion tests.

the observations, as shown by Figs. 14 and 15. However, this introduces two additional circuit elements whose physical meaning can be expressed as follows: CPEp - the capacity of the surface area of materials with high degree of surface development, Rp - resistance of electrolyte contained in the pores of the corroded material zone.

Analysis of impedance spectra with a fitted equivalent circuit allows the assessment of the variability of individual circuit elements with the change of potential and current intensity flowing in the corrosion system.

The values of the parameters of equivalent circuit elements that characterize the processes occurring in the corrosion of iron and S235JR steel in 0.5M NaCl solution are summarized in Table 5.


Table 5. Electrical circuit parameters of the corrosion systems of Fe and S235JR steel in 0.5M NaCl solution.

Compatibility of the actual processes in the system under study with a description of the corrosion with an equivalent circuit through which current flows with the same amplitude and same phase angle as in the corrosion system at a given excitation is illustrated in Figs. 14 and 15, respectively.

Fig. 14. Nyquist diagrams of impedance spectra of corrosive systems of Fe and S235JR steel in 0.5M NaCl solution determined experimentally (point line) and as a result of calculations (solid lines).

Nyquist diagrams (Fig. 14) in the shape of the characteristic semi-circles indicate the activation process control during corrosive material tests. Much larger diameter of the semicircle in the case of electrochemically generated iron shows high electrical resistance at the interface metal-solution, which is the result of oxidation of iron and Fe+2 ions passing into the solution. This indicates a greater corrosion resistance of iron compared to steel S235JR in

the observations, as shown by Figs. 14 and 15. However, this introduces two additional circuit elements whose physical meaning can be expressed as follows: CPEp - the capacity of the surface area of materials with high degree of surface development, Rp - resistance of

Analysis of impedance spectra with a fitted equivalent circuit allows the assessment of the variability of individual circuit elements with the change of potential and current intensity

The values of the parameters of equivalent circuit elements that characterize the processes occurring in the corrosion of iron and S235JR steel in 0.5M NaCl solution are summarized in

CPEp

Fe 16.7 5.7 212.6 0.68 1513 136.5 0.76 S235JR 15.4 114 182.4 0.68 386 48.3 0.98

Table 5. Electrical circuit parameters of the corrosion systems of Fe and S235JR steel in 0.5M

Compatibility of the actual processes in the system under study with a description of the corrosion with an equivalent circuit through which current flows with the same amplitude and same phase angle as in the corrosion system at a given excitation is illustrated in Figs.

0 200 400 600 800 1000 1200

*Z`*[cm<sup>2</sup> ]

Fe

S235JR

Fig. 14. Nyquist diagrams of impedance spectra of corrosive systems of Fe and S235JR steel in 0.5M NaCl solution determined experimentally (point line) and as a result of calculations

Nyquist diagrams (Fig. 14) in the shape of the characteristic semi-circles indicate the activation process control during corrosive material tests. Much larger diameter of the semicircle in the case of electrochemically generated iron shows high electrical resistance at the interface metal-solution, which is the result of oxidation of iron and Fe+2 ions passing into the solution. This indicates a greater corrosion resistance of iron compared to steel S235JR in

[μFsn-1/cm2] Rct

[Ω cm2]

Yp np Ydl ndl

0,5M NaCl

CPEdl [μFsn-1/cm2]

electrolyte contained in the pores of the corroded material zone.

Rp [Ω cm2]

flowing in the corrosion system.

Material Rs

[Ω cm2]

0

200

400

*-Z``* [cm2

]

600

800

Table 5.

NaCl solution.

(solid lines).

14 and 15, respectively.

the test environment, which also confirms the results obtained with the potentiodynamic polarization method.

Fig. 15. Bode amplitude and phase spectra of circuit impedance and corrosion systems of Fe and S235JR steel in 0.5M NaCl solution determined experimentally (point line) and as a result of calculations (solid lines).

Images of destruction of corrosion test samples of Fe and S235JR steel after corrosion tests are shown in Fig. 16.

Fig. 16. Images of the surfaces of Fe and S235JR steel after corrosion tests.

Studies of Resistance to Corrosion

prior corrosion tests.

**50 μm-**

**50 μm-**

 **Nin-**

 **NiP-**

 **Nim-** 

of Selected Metallic Materials Using Electrochemical Methods 413

inhibits the processes of corrosion of nickel in a certain range of the potential. However, differences in the corrosion resistance of pure nickel and its alloy result from both the

additive contained in the material alloy, as well as different material structures.

Fig. 17. Images of surface morphology (SEM), structure (TEM) and electron diffraction (SAED) of microcrystalline nickel Nim, nanocrystalline nickel Nin and NiP amorphous alloy

 **100 nm- 50 μm-**

 **100 nm-**

 **100 nm-**

Samples of iron Fe and S235JR steel subjected to corrosion test not only differed in structure and chemical composition, but also the morphology and surface topography (Fig. 11), which had also influenced the course of corrosion processes. Both the iron Fe and S235JR steel were submitted to uneven general corrosion (Fig. 16). Material pickling on the grains boundaries is clearly visible in the case of electrochemically produced iron.
