**3.3 Identification of the resistance to corrosion of nickel**

Nickel is a metal characterized by soft, ductile, smelting and converting properties. Its standard electrochemical potential is -0.24V. The chemical compounds of nickel are mainly found in 2nd oxidation states, rather than the 3rd and 4th ones. It dissolves in mineral acids, but insensitive to bases (alkalis). In the atmospheric environment and many aqueous solutions nickel has the ability to passivity in a fairly wide pH range. Thanks to its passivity it has high resistance to corrosion in many environments (Trzaska Moszczynski, 2008).

Nickel in pure state is used for manufacturing protective coatings of products made of other metals - mainly steel - and in its fine particle form is used as a catalyst for many chemical reactions. It is also one of the major components of many alloys, which are used in a variety of current technologies. However, restrictions in uses of nickel in various products are constantly growing due to its rarity in nature.

Electrochemical corrosion characteristics of nickel were carried out by potentiodynamic polarization and impedance spectroscopy methods. Corrosion tests of nickel produced by electrocrystallization were applied to its micrometric (Nim) and nanometric (Nin) crystalline structures and for NiP amorphous alloy of nickel with phosphorus at content of 10.7% by weight (Eftekhari, 2008), (Kowalewska & Trzaska, 2006).

Images of surface topography and morphology of nickel with microcrystalline and nanocrystalline structures and of NiP alloy before corrosion tests are shown in Fig. 17.

Potentiodynamic polarization curves of all tested nickel materials were determined in the same conditions for all the above materials: during measurements the polarization potential was increased in a wide range from -750mV to +700 mV with a 0.4mV/s rate.

The potentiodynamic polarization curves j = f(E) of nickel with different crystalline structures and of amorphous NiP alloy determined from measurements are shown in Fig. 18.

Analysis of these curves indicates a noticeable influence of the structure of nickel and other ingredients contained in the material, on the process of corrosion in the test environment. The corrosion parameters of the tested materials obtained from the experiment are summarized in Table 6.

Analysis of these parameters shows that the greatest potential for corrosion and the smallest corrosion current density characterize nickel with the nanocrystalline structure. This highlights its highest resistance to corrosion in the test environment. Increased corrosion resistance of electrochemically produced nickel with the nanocrystalline structure in comparison to microcrystalline nickel may indicate a greater tendency to passivity of the nanocrystalline nickel. A passive layer that forms on the surface of nanocrystalline nickel

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

Nickel is a metal characterized by soft, ductile, smelting and converting properties. Its standard electrochemical potential is -0.24V. The chemical compounds of nickel are mainly found in 2nd oxidation states, rather than the 3rd and 4th ones. It dissolves in mineral acids, but insensitive to bases (alkalis). In the atmospheric environment and many aqueous solutions nickel has the ability to passivity in a fairly wide pH range. Thanks to its passivity it has high resistance to corrosion in many environments (Trzaska

Nickel in pure state is used for manufacturing protective coatings of products made of other metals - mainly steel - and in its fine particle form is used as a catalyst for many chemical reactions. It is also one of the major components of many alloys, which are used in a variety of current technologies. However, restrictions in uses of nickel in various products are

Electrochemical corrosion characteristics of nickel were carried out by potentiodynamic polarization and impedance spectroscopy methods. Corrosion tests of nickel produced by electrocrystallization were applied to its micrometric (Nim) and nanometric (Nin) crystalline structures and for NiP amorphous alloy of nickel with phosphorus at content of 10.7% by

Images of surface topography and morphology of nickel with microcrystalline and nanocrystalline structures and of NiP alloy before corrosion tests are shown in Fig. 17.

Potentiodynamic polarization curves of all tested nickel materials were determined in the same conditions for all the above materials: during measurements the polarization potential

The potentiodynamic polarization curves j = f(E) of nickel with different crystalline structures and of amorphous NiP alloy determined from measurements are shown in Fig.

Analysis of these curves indicates a noticeable influence of the structure of nickel and other ingredients contained in the material, on the process of corrosion in the test environment. The corrosion parameters of the tested materials obtained from the experiment are

Analysis of these parameters shows that the greatest potential for corrosion and the smallest corrosion current density characterize nickel with the nanocrystalline structure. This highlights its highest resistance to corrosion in the test environment. Increased corrosion resistance of electrochemically produced nickel with the nanocrystalline structure in comparison to microcrystalline nickel may indicate a greater tendency to passivity of the nanocrystalline nickel. A passive layer that forms on the surface of nanocrystalline nickel

was increased in a wide range from -750mV to +700 mV with a 0.4mV/s rate.

is clearly visible in the case of electrochemically produced iron.

**3.3 Identification of the resistance to corrosion of nickel** 

constantly growing due to its rarity in nature.

weight (Eftekhari, 2008), (Kowalewska & Trzaska, 2006).

Moszczynski, 2008).

18.

summarized in Table 6.

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 prior corrosion tests.

Studies of Resistance to Corrosion

0.5 M NaCl solution.

equation (5)

0.5M NaCl solution.

an environment of 0.5M NaCl solution..

of Selected Metallic Materials Using Electrochemical Methods 415

All obtained results of impedance measurements confirm the significant impact of the material structure and the additions of nickel alloy on the resistance to corrosion and are

The two methods showed that nanocrystalline nickel has the highest corrosion resistance in

Further assessment of the characteristics of the impedance at the system boundaries for nickel and its alloy in 0.5M NaCl solution was obtained by approximation of experimental data using equivalent electrical circuits. The equivalent electrical circuits most suitable to represent the measured impedance characteristics of studied systems of nickel with different structures and its alloy in corrosive environment of 0.5M NaCl solution are shown in Figs. 19, 20 and 21. The corresponding resulting impedances are described in the expressions (4) ÷ (6). For the analysis of the corrosion of microcrystalline structure nickel with the

Fig. 19. The equivalent electrical circuit for corrosion of microcrystalline structure nickel in

Rct

CPEdl

This system includes four elements: Rs - resistance of 0.5M NaCl solution, Rct - electric charge transfer resistance for phase boundary of nickel - solution, CPEdl – constant phase element characterizing the electrical properties of the double layer at the interface, and the element W - Warburg impedance, which characterizes the control of corrosion processes by

<sup>s</sup> <sup>n</sup>

ct <sup>1</sup> Z R <sup>1</sup> Y (jω ) R W

Fig. 20. Equivalent electrical circuit of corrosion of the nanocrystalline structure nickel in

Rl

CPEl

Experimentally determined impedance spectra of nanocrystalline nickel corrosion (fig. 20) are well mapped by equivalent electrical circuit with two time constants described by

dl

dl

Rct

CPEdl

(4)

consistent with the results obtained by potentiodynamic polarization method.

equivalent electrical circuit a simple layout shown in Fig. 19 was used.

Rs

diffusion of mass in the area of the electrolyte at the metal surface.

Rs

The equivalent electrical circuit is described by the resulting impedance


Table 6. Corrosion parameters of nickel and alloy NiP in 0.5M NaCl solution.

Fig. 18. Potentiodynamic polarization curves of nickel with the microcrystalline structure (Nim), and nanocrystalline structure (Nin) and of NiP alloy.

Investigations of the processes at the interface nickel-0.5M NaCl solution by impedance spectroscopy were performed with frequency changes in the range of 10kHz 2mHz. Amplitude of the sinusoidal perturbation signal was maintained at 15mV. Impedance spectra recorded for test materials are shown as Nyquist diagrams (Fig. 22) and Bode diagrams (Fig.23), in the form of two relationships: the impedance magnitude and phase angle versus frequency.

Nyquist diagrams (Fig. 22) indicate significant differences in the course of corrosion processes of the different forms of nickel and its alloy structures in the environment of 0.5M NaCl solution. In the case of nanocrystalline nickel structure and alloy NiP, the impedance spectra obtained are expressed in the form of an arc forming part of the semi-circle of very large radius. This chart indicates good corrosion resistance of nanocrystalline nickel and alloy NiP in the test environment. Impedance spectrum plot of microcrystalline nickel in the shape of semicircle of small radius ends with a fragment of straight line in the low frequency part of the forcing signal. This straight line fragment of the relationship between the imaginary component (Z'') and the real part (Z') of the impedance points to the diffusion control of corrosion processes at low frequency of the forcing signal. The smallest diameter of the semi-circle in the case of nickel with microcrystalline structure corresponds to a small value of resistance electric current flowing through the phase boundaries as a result of oxidation of nickel, which corresponds to a high rate of corrosion processes.

Material Ecor [mV] jcor [μA/cm2] Nim -568 24 Nin -340 1.5 NiP -390 4.5

Nin

NiP

Fig. 18. Potentiodynamic polarization curves of nickel with the microcrystalline structure

Investigations of the processes at the interface nickel-0.5M NaCl solution by impedance spectroscopy were performed with frequency changes in the range of 10kHz 2mHz. Amplitude of the sinusoidal perturbation signal was maintained at 15mV. Impedance spectra recorded for test materials are shown as Nyquist diagrams (Fig. 22) and Bode diagrams (Fig.23), in the form of two relationships: the impedance magnitude and phase


*E* [mV]

Nyquist diagrams (Fig. 22) indicate significant differences in the course of corrosion processes of the different forms of nickel and its alloy structures in the environment of 0.5M NaCl solution. In the case of nanocrystalline nickel structure and alloy NiP, the impedance spectra obtained are expressed in the form of an arc forming part of the semi-circle of very large radius. This chart indicates good corrosion resistance of nanocrystalline nickel and alloy NiP in the test environment. Impedance spectrum plot of microcrystalline nickel in the shape of semicircle of small radius ends with a fragment of straight line in the low frequency part of the forcing signal. This straight line fragment of the relationship between the imaginary component (Z'') and the real part (Z') of the impedance points to the diffusion control of corrosion processes at low frequency of the forcing signal. The smallest diameter of the semi-circle in the case of nickel with microcrystalline structure corresponds to a small value of resistance electric current flowing through the phase boundaries as a result of

oxidation of nickel, which corresponds to a high rate of corrosion processes.

(Nim), and nanocrystalline structure (Nin) and of NiP alloy.

0,01

0,1

1

10

*j* [

A/cm2

]

100

1000

10000

100000

angle versus frequency.

Table 6. Corrosion parameters of nickel and alloy NiP in 0.5M NaCl solution.

Nim

All obtained results of impedance measurements confirm the significant impact of the material structure and the additions of nickel alloy on the resistance to corrosion and are consistent with the results obtained by potentiodynamic polarization method.

The two methods showed that nanocrystalline nickel has the highest corrosion resistance in an environment of 0.5M NaCl solution..

Further assessment of the characteristics of the impedance at the system boundaries for nickel and its alloy in 0.5M NaCl solution was obtained by approximation of experimental data using equivalent electrical circuits. The equivalent electrical circuits most suitable to represent the measured impedance characteristics of studied systems of nickel with different structures and its alloy in corrosive environment of 0.5M NaCl solution are shown in Figs. 19, 20 and 21. The corresponding resulting impedances are described in the expressions (4) ÷ (6). For the analysis of the corrosion of microcrystalline structure nickel with the equivalent electrical circuit a simple layout shown in Fig. 19 was used.

Fig. 19. The equivalent electrical circuit for corrosion of microcrystalline structure nickel in 0.5 M NaCl solution.

This system includes four elements: Rs - resistance of 0.5M NaCl solution, Rct - electric charge transfer resistance for phase boundary of nickel - solution, CPEdl – constant phase element characterizing the electrical properties of the double layer at the interface, and the element W - Warburg impedance, which characterizes the control of corrosion processes by diffusion of mass in the area of the electrolyte at the metal surface.

The equivalent electrical circuit is described by the resulting impedance

$$Z = \mathbf{R\_s} + \frac{1}{\frac{1}{\mathbf{R\_{ct}} + \mathbf{W}} + \mathbf{Y\_{dl}} (\text{jao})^{n\_{\text{dl}}}} \tag{4}$$

Experimentally determined impedance spectra of nanocrystalline nickel corrosion (fig. 20) are well mapped by equivalent electrical circuit with two time constants described by equation (5)

Fig. 20. Equivalent electrical circuit of corrosion of the nanocrystalline structure nickel in 0.5M NaCl solution.

Studies of Resistance to Corrosion

corrosion.

of Selected Metallic Materials Using Electrochemical Methods 417

In the case of the microcrystalline structure nickel and NiP alloy in corrosive environment of 0.5M NaCl solution, a pickling of their internal structures occurred over the entire surface exposed and even its internal structures was revealed. On the other hand, corrosion of the nanocrystalline nickel in this environment takes the form of uneven local

Fig. 22. Nyquist diagrams of impedance spectra of investigated corrosion systems of nickel and its alloy in the environment of 0.5M NaCl solution determined experimentally (point

0 5000 10000 15000 20000 25000

]

*Z`*[cm2

Nin

NiP

Nin

Nim


log *f* [Hz]

NiP

0,5M NaCl

0,5M NaCl

Fig. 23. Bode diagrams of impedance spectra of investigated corrosion systems of nickel and its alloy in the environment of 0.5M NaCl solution determined experimentally (point line)

[deg]

Nin

0,5M NaCl

line) and as a result of calculations (solid lines).

0

5000

*-Z``* [

cm2

]

10000

15000

Nim

and as a result of calculations (solid lines).


log *f* [Hz]

Nim

1

2

3

log I

*Z*I

4

5

NiP

$$\mathbf{Z} = \mathbf{R}\_s + \frac{1}{\frac{1}{\mathbf{R}\_l} + \mathbf{Y}\_l \text{(jo)}^{n\_l}} + \frac{1}{\frac{1}{\mathbf{R}\_{\text{ct}}} + \mathbf{Y}\_{\text{dl}} \text{(jo)}^{n\_{\text{dl}}}} \tag{5}$$

This circuit, besides elements such as Rs, Rct, CPEdl which are needed in the equivalent electrical circuit to describe the corrosion of microcrystalline nickel contains two additional elements: CPEl - modeling capacity of the passive layer on the material surface, and Rl - describing the resistance of the passive layer.

To describe the corrosion processes occurring in the system NiP- 0.5M NaCl solution the equivalent electrical circuit shown in Fig. 21 was designed with the resulting impedance expressed by (6).

Fig. 21. Equivalent electrical circuit for corrosion of the nanocrystalline structure nickel in 0.5M NaCl solution.

$$\mathbf{Z} = \mathbf{R}\_s + \frac{1}{\frac{1}{\mathbf{R}\_{\rm ct}} + \mathbf{Y}\_{\rm dl} \text{(jo)}^{\rm dl}} \tag{6}$$

The parameters of the equivalent electrical circuits of corrosive systems of nickel materials tested in this study are summarized in Table 7.


Table 7. The parameters of equivalent electrical circuits for corrosive systems of nickel - 0.5M NaCl solution.

The agreement between characteristics predicted by the equivalent circuit methods and those obtained from measurements are illustrated in Figs. 22 and 23.

Images of the damage on the surface of nickel samples with different structure and its alloy after corrosion tests are shown in Fig. 24.

<sup>s</sup> <sup>n</sup> <sup>n</sup> l dl

This circuit, besides elements such as Rs, Rct, CPEdl which are needed in the equivalent electrical circuit to describe the corrosion of microcrystalline nickel contains two additional elements: CPEl - modeling capacity of the passive layer on the material surface, and

To describe the corrosion processes occurring in the system NiP- 0.5M NaCl solution the equivalent electrical circuit shown in Fig. 21 was designed with the resulting impedance

Rct

<sup>s</sup> dl dl

> Rct [Ωcm2]

Yl nl Ydl ndl

(6)

CPEdl [μFsn-1/cm2]

W [Ωcm2]

ct <sup>1</sup> Z R <sup>1</sup> Y (jω ) <sup>R</sup>

The parameters of the equivalent electrical circuits of corrosive systems of nickel materials

CPEl [ΩFsn-1/cm2]

Nim 14.7 – –– 1420 69 0.8 705 Nin 14.7 4708 17.6 0.9 15317 30 0.8 – NiP 12.6 – –– 20430 17 0.9 – Table 7. The parameters of equivalent electrical circuits for corrosive systems of nickel -

The agreement between characteristics predicted by the equivalent circuit methods and

Images of the damage on the surface of nickel samples with different structure and its alloy

Fig. 21. Equivalent electrical circuit for corrosion of the nanocrystalline structure nickel in

CPEdl

l ct 1 1 Z R 1 1 Y (jω ) Y(jω ) R R

Rs

Rl - describing the resistance of the passive layer.

tested in this study are summarized in Table 7.

Rl [Ωcm2]

those obtained from measurements are illustrated in Figs. 22 and 23.

Rs [Ωcm2]

after corrosion tests are shown in Fig. 24.

expressed by (6).

0.5M NaCl solution.

Materiał

0.5M NaCl solution.

l dl

(5)

In the case of the microcrystalline structure nickel and NiP alloy in corrosive environment of 0.5M NaCl solution, a pickling of their internal structures occurred over the entire surface exposed and even its internal structures was revealed. On the other hand, corrosion of the nanocrystalline nickel in this environment takes the form of uneven local corrosion.

Fig. 22. Nyquist diagrams of impedance spectra of investigated corrosion systems of nickel and its alloy in the environment of 0.5M NaCl solution determined experimentally (point line) and as a result of calculations (solid lines).

Fig. 23. Bode diagrams of impedance spectra of investigated corrosion systems of nickel and its alloy in the environment of 0.5M NaCl solution determined experimentally (point line) and as a result of calculations (solid lines).

Studies of Resistance to Corrosion

electrochemical corrosion.

avoid or mitigate corrosion.

6, Ohio, USA

ISBN 1420094629, London, GB

**5. References** 

**4. Summary** 

of Selected Metallic Materials Using Electrochemical Methods 419

The rate of corrosion processes of metallic materials in a corrosive environment depends on the chemical activity of the metal and the additional components, the structure of the

Electrochemical methods for the study of corrosion processes are based on the relationships between electrical, chemical and physical properties, which are used to identify phenomena and processes at the interface metal-corrosive environment. Electrochemical potentiodynamic polarization method allows determining the corrosion potential and corrosion current density of the metallic material in a corrosive environment. Additionally, the precise measurement method using electrochemical impedance spectroscopy (EIS) generates frequency characteristics of corrosion systems and forms the solid basis to design models based on equivalent electrical circuits, which maps the processes occurring in the corrosion system under investigation. Such an equivalent electrical circuit that meets the criteria of a mathematical (and metrological) model can also be considered as a physical model describing the phenomena and the processes occurring in a given system undergoing

The current research of corrosion phenomena appearing at the interface metal-natural environment showed that chemical re-combination of the metals to form ore-like compounds is a natural process, because the energy content of the metals and alloys is higher than that of their ores. It has to be emphasized that there are number of means of controlling corrosion. The choice of a means of corrosion control depends on economics, safety requirements, and a number of technical considerations. However, it is necessary to learn and recognize the forms of corrosion and the parameters that must be controlled to

Through the understanding of the electrochemical processes and how they can act to cause the various forms of corrosion, the natural tendency of metals to suffer corrosion can be overcome and equipment that is resistant to failure by corrosion can be designed. In this study we have shown that the measuring methods based on the electrochemical impedance

Eftekhari, A. (ed.). (2008). *Nanostructured Materials in Elektrochemistry,* ISBN 978-3-527-31876-

Huang, Y., Shih, H., Huang, H., Daugherty, J., Wu, S., Ramanathan, S., Chang, Ch. &

Kowalewska, M., Trzaska, M., Influence of Si3N4 disperse ceramic phase on the corrosion

Mansfeld, F. (2008). Evaluation of the corrosion resistance of anodized aluminum 6061 using electrochemical impedance spectroscopy (EIS). *Corrosion Science*, Vol.50,

resistance of micro- and nano-crystalline nickel layers. Physico Chemical Mechanics of Materials, Vol. 2, No. 5, (May 2006), pp. 615-619, ISSN 0430-6252 Marcus, Ph., (2011), *Corrosion Mechanism in Theory and Practice*. (2nd ed.), Taylor and Francis,

spectroscopy are able to detect the potential corrosion spots in very early stages.

No.12, (December 2008), pp.3569-3575, ISSN 0010-938X

material as well as the degree of development of their surfaces.

Fig. 24. SEM images of the surface of Nim, Nin, and NiP alloy after corrosion tests in an environment of 0.5M NaCl solution.
