**3.1 Identification of the resistance to corrosion of aluminum**

Aluminum and its alloys are materials of great technical importance. Attractive physical properties of aluminum such as low density, high ductility, good thermal and electrical conductivities, relatively low production costs and its high abundance in nature make it an indispensable metal in many industries and in numerous areas of daily life, both as a pure metal and in various alloys. Aluminum, as an element of high chemical activity, shows a significant tendency to passivity, leading to high resistance of aluminum and its alloys to corrosion in many environments with low aggressiveness (Vargel, 2004).

However, the processes of alloying and heat treatments are not always sufficient to ensure the qualities of aluminum required in the modern technical applications. One way of modifying the performance of aluminum and its alloys in order to adapt them to the

Studies of Resistance to Corrosion

materials are summarized in Table 1.

of Selected Metallic Materials Using Electrochemical Methods 403

To determine the corrosion current density jcor and the corrosion potential Ecor the tangential extrapolation method was used for the polarization curves j = f(E) from the cathode and anode zones. The values of corrosion current densities and corrosion potentials for tested

> Material Ecor [mV] jcor [μAcm-2] Al -719 14.9 Al2O3 -583 0.23

It is worth mentioning that Al2O3 layer has a much higher corrosion resistance compared to aluminum. Corrosion protection of aluminum using Al2O3 layer is guaranteed by efficient isolation of the substrate material from the corrosive environment. The effectiveness of the

For further characterization of electrochemical corrosion processes at the interface between the environment of 0.5M NaCl solution and Al and Al2O3, the electrochemical impedance spectroscopy method was applied. As stated above this method allows considering the corrosion process as a combination of equivalent electric circuits. In the case of Al the study was carried out with the amplitude of the forcing sinusoidal signal of 10mV. However, in the case of Al2O3 layer the amplitude of perturbing signal in the corrosion balance was fixed at 20mV. The study was conducted in the frequency range 23kHz ÷ 16mHz. Measured impedance spectra of Al and Al2O3 layer in the corrosive environment of 0.5M NaCl

Fig. 6. Equivalent electric circuit for corrosion of bulk Al: a) corrosion system Al - 0.5M NaCl

**Rs Rct** 

**Charge transfer** 

ω=1/RctCdl

**R(ω)**

**Rs**

**X(ω)**

**Solution**

Equivalent electrical circuits obtained by minimizing the mean square error were further used for the analysis of experimentally identified frequency characteristics and a description of corrosive processes in the systems under investigation. A simple electric circuit consisting of three elements of type R and C with a single time constant was adopted as system model for Al. Figs. 6 and 7 show the circuits modeling respectively the

solution, b) an equivalent circuit scheme, c) Nyquist frequency characteristics

 **Cdl**

**Rct**

Table 1. Corrosion parameters of bulk raw Al and Al2O3 layer in 0.5M NaCl.

corrosion protection depends on the thickness and tightness of Al2O3 layer.

solution are presented in the form of Nyquist and Bode diagrams.

**b)** 

**c) 0.5M NaCl a)** 

 **Al..**

operating conditions is the production on their surfaces of a thin layer of Al2O3 by anodic oxidation process (Huang, et al., 2008).

Corrosion test has been applied to technical aluminum (99.9%) and to aluminum with Al2O3 surface layer produced by hard anodic oxidation and sealed in boiling-hot deionized water. Images of morphology and topography of the surface layer of aluminum and Al2O3 before corrosion tests using scanning electron microscope (SEM) are shown in Fig. 4.

Fig. 4. Morphology and topography of the surface of Al and Al2O3 layer before corrosion tests.

Potentiodynamic polarization distortion of the steady state technique at the interface of both Al as well as Al2O3 with 0.5M NaCl solution, was applied with the change in the potential ranging from -780mV to -450mV. The rate of the potential change during the test was 0.2mV/s. Current characteristics j=f(E) of test materials in the form of potentiodynamic polarization curves are shown in Fig. 5.

Fig. 5. Potentiodynamic polarization curves of materials of bulk raw Al and Al2O3 layer in the corrosive environment of 0.5M NaCl solution.

operating conditions is the production on their surfaces of a thin layer of Al2O3 by anodic

Corrosion test has been applied to technical aluminum (99.9%) and to aluminum with Al2O3 surface layer produced by hard anodic oxidation and sealed in boiling-hot deionized water. Images of morphology and topography of the surface layer of aluminum and Al2O3 before

 **Al2O3**

Fig. 4. Morphology and topography of the surface of Al and Al2O3 layer before corrosion

Potentiodynamic polarization distortion of the steady state technique at the interface of both Al as well as Al2O3 with 0.5M NaCl solution, was applied with the change in the potential ranging from -780mV to -450mV. The rate of the potential change during the test was 0.2mV/s. Current characteristics j=f(E) of test materials in the form of potentiodynamic


Fig. 5. Potentiodynamic polarization curves of materials of bulk raw Al and Al2O3 layer in

*E* [mV]

Al2O3

0,5M NaCl

**100 μm** 

corrosion tests using scanning electron microscope (SEM) are shown in Fig. 4.

**100 μm**

oxidation process (Huang, et al., 2008).

polarization curves are shown in Fig. 5.

0,01

the corrosive environment of 0.5M NaCl solution.

0,1

1

10

*j* [

A/cm2]

100

Al

1000

10000

100000

tests.

 **Al -**

To determine the corrosion current density jcor and the corrosion potential Ecor the tangential extrapolation method was used for the polarization curves j = f(E) from the cathode and anode zones. The values of corrosion current densities and corrosion potentials for tested materials are summarized in Table 1.


Table 1. Corrosion parameters of bulk raw Al and Al2O3 layer in 0.5M NaCl.

It is worth mentioning that Al2O3 layer has a much higher corrosion resistance compared to aluminum. Corrosion protection of aluminum using Al2O3 layer is guaranteed by efficient isolation of the substrate material from the corrosive environment. The effectiveness of the corrosion protection depends on the thickness and tightness of Al2O3 layer.

For further characterization of electrochemical corrosion processes at the interface between the environment of 0.5M NaCl solution and Al and Al2O3, the electrochemical impedance spectroscopy method was applied. As stated above this method allows considering the corrosion process as a combination of equivalent electric circuits. In the case of Al the study was carried out with the amplitude of the forcing sinusoidal signal of 10mV. However, in the case of Al2O3 layer the amplitude of perturbing signal in the corrosion balance was fixed at 20mV. The study was conducted in the frequency range 23kHz ÷ 16mHz. Measured impedance spectra of Al and Al2O3 layer in the corrosive environment of 0.5M NaCl solution are presented in the form of Nyquist and Bode diagrams.

Fig. 6. Equivalent electric circuit for corrosion of bulk Al: a) corrosion system Al - 0.5M NaCl solution, b) an equivalent circuit scheme, c) Nyquist frequency characteristics

Equivalent electrical circuits obtained by minimizing the mean square error were further used for the analysis of experimentally identified frequency characteristics and a description of corrosive processes in the systems under investigation. A simple electric circuit consisting of three elements of type R and C with a single time constant was adopted as system model for Al. Figs. 6 and 7 show the circuits modeling respectively the

Studies of Resistance to Corrosion

Rb = 100 kΩcm2.

the expression

respectively.

of Selected Metallic Materials Using Electrochemical Methods 405

value of the resistance Rb in the test case of Al2O3 protective layer is large and amounts to

The resulting impedance of the equivalent circuit (Figs. 6 and 7) adopted to describe the corrosion processes occurring in systems with bulk Al and Al2O3 layer deposited on an aluminum substrate in the corrosive environment of 0.5M NaCl solution is determined by

> s <sup>1</sup> Z R <sup>1</sup> <sup>j</sup>ω<sup>C</sup> R

C μF/cm2]

R [kΩcm2]

0,5M NaCl

The equivalent electrical circuit approach adopted maps the processes occurring in the corrosion systems and enables the determination of parameters relevant to these processes. The parameter values of individual elements of equivalent electrical circuit representing

Al 13.3 Cdl = 10.0 Rct = 0.8 Al2O3 12.5 Cb = 1.4 Rb = 100.0 Table 2. Parameters of equivalent electrical circuit for corrosion processes of bulk Al and

The frequency characteristics of corrosion systems of bulk Al and Al2O3 layer in 0.5M NaCl solution in the form of Nyquist and Bode plots obtained by the measurements and calculations based on adopted equivalent electrical circuits are shown in Figs. 8 and 9,

0 25 50 75 100

2 ]

*Z`*[kcm

Fig. 8. Nyquist diagrams of impedance spectra of corrosion systems for bulk Al and Al2O3 layer in 0.5M NaCl solution determined experimentally (point line) and as a result of

Al2O3

(2)

investigated corrosion systems are summarized in Table 2.

[Ωcm2]

Materia<sup>ł</sup> Rs

Al2O3 layer in 0.5M NaCl solution.

0

Al

25

*-Z``* [k

calculations (solid lines).

cm2

]

50

75

corrosion systems of bulk Al and Al2O3 surface layer deposited on aluminum in a 0.5 M NaCl corrosive environment.

Fig. 6 presents the system of Al corrosion in 0.5 M NaCl solution, its frequency impedance characteristic in the form of Nyquist plot and the equivalent electrical circuit. Individual parts of the electric circuit reflect the electrochemical and electrical characteristics of the corrosion systems. In this arrangement, the spectral characteristic of the impedance in the Nyquist plot has the shape of a semicircle, whose intersection with the real axis in the high-frequency range determines the electrolyte solution resistance Rs. Conversely, the intersection of the real axis in the low-frequency range corresponds to the sum of Rs + Rct, where Rct indicates the charge transfer resistance of the boundary metal/electrolyte, and characterizes the rate of corrosion. On the other hand, Cdl component of the circuit represents capacity of the double layer at the interface metal/electrolyte.

Layout of the corrosive system for aluminum with a surface layer of Al2O3 in 0.5M NaCl corrosive environment, and the designated equivalent circuit are shown in Fig.7.

Fig. 7. Layout of the corrosive system for aluminum with a protective layer of Al2O3 in 0.5M NaCl environment and its equivalent circuit.

Similarly to the previous case the element Rs of the equivalent electric circuit for this corrosion system represents the resistance of the 0.5M solution of NaCl electrolyte used as the corrosive environment. Elements in parallel in the equivalent electric circuit characterize the protective properties of Al2O3 layer deposited on the bulk Al. The element Cb specifies the Al2O3 layer capacitance, which depends on the thickness of this layer and on the dielectric properties of the material. The resistor Rb in such a system represents the resistance of the protective layer, and depends on properties of the material forming the layer, and varying with the thickness of the layer and its material composition. The

corrosion systems of bulk Al and Al2O3 surface layer deposited on aluminum in a 0.5 M

Fig. 6 presents the system of Al corrosion in 0.5 M NaCl solution, its frequency impedance characteristic in the form of Nyquist plot and the equivalent electrical circuit. Individual parts of the electric circuit reflect the electrochemical and electrical characteristics of the corrosion systems. In this arrangement, the spectral characteristic of the impedance in the Nyquist plot has the shape of a semicircle, whose intersection with the real axis in the high-frequency range determines the electrolyte solution resistance Rs. Conversely, the intersection of the real axis in the low-frequency range corresponds to the sum of Rs + Rct, where Rct indicates the charge transfer resistance of the boundary metal/electrolyte, and characterizes the rate of corrosion. On the other hand, Cdl component of the circuit

Layout of the corrosive system for aluminum with a surface layer of Al2O3 in 0.5M NaCl

**Rs**

**Cb Rb**

Fig. 7. Layout of the corrosive system for aluminum with a protective layer of Al2O3 in 0.5M

**100 μm**

Similarly to the previous case the element Rs of the equivalent electric circuit for this corrosion system represents the resistance of the 0.5M solution of NaCl electrolyte used as the corrosive environment. Elements in parallel in the equivalent electric circuit characterize the protective properties of Al2O3 layer deposited on the bulk Al. The element Cb specifies the Al2O3 layer capacitance, which depends on the thickness of this layer and on the dielectric properties of the material. The resistor Rb in such a system represents the resistance of the protective layer, and depends on properties of the material forming the layer, and varying with the thickness of the layer and its material composition. The

NaCl environment and its equivalent circuit.

**Al** 

**Al2O3**

**0.5M NaCl** 

represents capacity of the double layer at the interface metal/electrolyte.

corrosive environment, and the designated equivalent circuit are shown in Fig.7.

NaCl corrosive environment.

value of the resistance Rb in the test case of Al2O3 protective layer is large and amounts to Rb = 100 kΩcm2.

The resulting impedance of the equivalent circuit (Figs. 6 and 7) adopted to describe the corrosion processes occurring in systems with bulk Al and Al2O3 layer deposited on an aluminum substrate in the corrosive environment of 0.5M NaCl solution is determined by the expression

$$\mathbf{Z} = \mathbf{R}\_s + \frac{1}{\frac{1}{\mathbf{R}} + \text{jao C}}\tag{2}$$

The equivalent electrical circuit approach adopted maps the processes occurring in the corrosion systems and enables the determination of parameters relevant to these processes. The parameter values of individual elements of equivalent electrical circuit representing investigated corrosion systems are summarized in Table 2.


Table 2. Parameters of equivalent electrical circuit for corrosion processes of bulk Al and Al2O3 layer in 0.5M NaCl solution.

The frequency characteristics of corrosion systems of bulk Al and Al2O3 layer in 0.5M NaCl solution in the form of Nyquist and Bode plots obtained by the measurements and calculations based on adopted equivalent electrical circuits are shown in Figs. 8 and 9, respectively.

Fig. 8. Nyquist diagrams of impedance spectra of corrosion systems for bulk Al and Al2O3 layer in 0.5M NaCl solution determined experimentally (point line) and as a result of calculations (solid lines).

Studies of Resistance to Corrosion

frequency range of the forcing signal.

good corrosion protection for aluminum substrate.

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

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

potential changes during the test was 0.3mV/s.

of these systems.

practical importance.

shown in Table 3.

tests are shown in Fig. 11.

of Selected Metallic Materials Using Electrochemical Methods 407

The comparison of plots of the frequency characteristics of corrosion systems obtained experimentally and from the calculations (Figs. 8 and 9) attests that the adopted scheme for the equivalent circuits reproduces well the impedance measurements across the whole

In addition, the circuit impedance characteristics for corrosion of bulk Al and Al2O3 layer in 0.5M NaCl solution confirm the results obtained from potentiodynamic polarization studies

SEM images of material surfaces of bulk Al and surface layers of Al2O3 in the study of influences of corrosive environment of 0.5M NaCl solution are shown in Fig. 10. Images of the surfaces after corrosion tests show that in an environment of 0.5M NaCl solution Al substrate material undergoes pitting corrosion. However, Al2O3 surface layer provides a

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

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

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

Component C S P Si Mn Cr Ni Cu [%] weight 0.22 0.05 0.05 0.30 1.10 0.30 0.30 0.30

Images of topography and surface morphology of Fe iron and S235JR steel prior to corrosion

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

place, in which iron occurs primarily at two and three degrees of oxidation.

Fig. 9. Bode amplitude and phase spectra of circuit impedances for corrosion of bulk Al and Al2O3 layer in 0.5M NaCl solution determined experimentally (point line) and as a result of calculations (solid lines).

Fig. 10. SEM images of the surface of Al and Al2O3 materials after corrosion tests in an environment of 0.5M NaCl solution.

[deg]

 **20 μm 1 mm** 

0,5M NaCl

Fig. 9. Bode amplitude and phase spectra of circuit impedances for corrosion of bulk Al and Al2O3 layer in 0.5M NaCl solution determined experimentally (point line) and as a result of


log *f* [Hz]

Al

 **20 μm** 

Al2O3

Fig. 10. SEM images of the surface of Al and Al2O3 materials after corrosion tests in an

 **200 μm** 

calculations (solid lines).

1

 **Al**  f

 **Al2O3**

2

3

log I

*Z*I

4


log *f* [Hz]

Al

Al2O3

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

environment of 0.5M NaCl solution.

The comparison of plots of the frequency characteristics of corrosion systems obtained experimentally and from the calculations (Figs. 8 and 9) attests that the adopted scheme for the equivalent circuits reproduces well the impedance measurements across the whole frequency range of the forcing signal.

In addition, the circuit impedance characteristics for corrosion of bulk Al and Al2O3 layer in 0.5M NaCl solution confirm the results obtained from potentiodynamic polarization studies of these systems.

SEM images of material surfaces of bulk Al and surface layers of Al2O3 in the study of influences of corrosive environment of 0.5M NaCl solution are shown in Fig. 10. Images of the surfaces after corrosion tests show that in an environment of 0.5M NaCl solution Al substrate material undergoes pitting corrosion. However, Al2O3 surface layer provides a good corrosion protection for aluminum substrate.
