**2. Electrochemical methods for testing the susceptibility to corrosion of metallic materials**

Both processes of oxidation and reduction simultaneously occur at the metal-corrosive environment interface during an electrochemical corrosion. The basic processes occurring during the electrochemical corrosion of metallic materials are:


In the reduction processes in natural environments hydrogen ions H+ and oxygen O2 are most often involved, and the related reduction processes can be written as following:

	- in alkaline and neutral environment: O2 +2H2O + 4e → 4OH-
	- in an acidic environment: O2 + 4H+ + 4e → 2H2O.

Oxidation and reduction processes are accompanied by the flow of electric charge through the interface metal-corrosive environment. In metals the charge carriers are electrons while in the corrosive environment charge flow is due to ions. Thus an active assessment of electrochemical corrosion processes can be achieved by assessing the electrical charge transfer process. In the reactions of corrosion that are controlled by the rate of charge transfer, the current - potential relationship can be described by the Butler-Volmer equation:

$$\mathbf{j} = \mathbf{j}\_0 \left[ \exp\left(\frac{\mathbf{a}\_\mathrm{A} \mathbf{nF}}{\mathrm{RT}} \boldsymbol{\eta}\right) - \exp\left(-\frac{\mathbf{a}\_\mathrm{K} \mathbf{nF}}{\mathrm{RT}} \boldsymbol{\eta}\right) \right] \tag{1}$$

where: j - current density, j0 - exchange current density, η = E-E0 - overpotential (voltage), n number of electrons, αA and αK - transfer coefficients, respectively, at the anode and cathode, F - Faraday constant, R - gas constant and T – absolute temperature (Marcus, 2011).

The disruption of the steady state corrosion by the electrical signal and the measurement of its response to the stimulation allow determining the set of electrical quantities providing valuable information about electrochemical processes occurring in the system under study. The most common approaches for the electrochemical characterization of corrosion processes are non-stationary methods, which are easy to automate and computer-control (Trzaska & Trzaska, 2007).

The present study of corrosion processes of metallic materials uses variable current technology, namely the electrochemical polarization potentiodynamic and electrochemical impedance spectroscopy (EIS) techniques. The basis of polarization potentiodynamic electrochemical technique is the stimulation of the corrosion system by a potential, whose value varies linearly in time and the recording of the instantaneous value of current flowing in the system. The electrochemical impedance spectroscopy consists of a perturbation of the

Two electrochemical methods were used to characterize the corrosion properties of the materials under investigation: potentiodynamic polarization and impedance spectroscopy.

Both processes of oxidation and reduction simultaneously occur at the metal-corrosive environment interface during an electrochemical corrosion. The basic processes occurring

In the reduction processes in natural environments hydrogen ions H+ and oxygen O2 are

Oxidation and reduction processes are accompanied by the flow of electric charge through the interface metal-corrosive environment. In metals the charge carriers are electrons while in the corrosive environment charge flow is due to ions. Thus an active assessment of electrochemical corrosion processes can be achieved by assessing the electrical charge transfer process. In the reactions of corrosion that are controlled by the rate of charge transfer, the current - potential relationship can be described by the Butler-Volmer

A K

(1)

<sup>α</sup> nF <sup>α</sup> nF j j exp <sup>η</sup> exp <sup>η</sup> RT RT

where: j - current density, j0 - exchange current density, η = E-E0 - overpotential (voltage), n number of electrons, αA and αK - transfer coefficients, respectively, at the anode and cathode,

The disruption of the steady state corrosion by the electrical signal and the measurement of its response to the stimulation allow determining the set of electrical quantities providing valuable information about electrochemical processes occurring in the system under study. The most common approaches for the electrochemical characterization of corrosion processes are non-stationary methods, which are easy to automate and

The present study of corrosion processes of metallic materials uses variable current technology, namely the electrochemical polarization potentiodynamic and electrochemical impedance spectroscopy (EIS) techniques. The basis of polarization potentiodynamic electrochemical technique is the stimulation of the corrosion system by a potential, whose value varies linearly in time and the recording of the instantaneous value of current flowing in the system. The electrochemical impedance spectroscopy consists of a perturbation of the

F - Faraday constant, R - gas constant and T – absolute temperature (Marcus, 2011).

most often involved, and the related reduction processes can be written as following:


**2. Electrochemical methods for testing the susceptibility to corrosion of** 

during the electrochemical corrosion of metallic materials are:

in an acidic environment: O2 + 4H+ + 4e → 2H2O.

0

computer-control (Trzaska & Trzaska, 2007).




**metallic materials** 

equation:

steady state corrosion by applying the sinusoidal alternating potential signal of small amplitude, but in a wide range of frequencies and the automatic recording of current intensity responses of the system. Investigations of corrosion by those methods, based on a change in the relationship between potential and current were implemented in the threeelectrode system (Fig. 1).

Fig. 1. Three-electrode system for corrosion studies: a) measuring system, b) circuit diagram.

Research of metallic materials corrosion was carried out by means of computerized measuring systems, which generated in a digital form an electrical signal of a certain shape to stimulate the system and simultaneously analyze the response of the corrosion test (Figs. 2 and 3).

Fig. 2. Block diagram of the electrochemical corrosion tests.

In the three-electrode system, used in the current study, the examined metal takes the role of the active electrode. A calomel electrode, Hg/Hg2Cl2/KCl characterized by the potential +244 mV, was used as the reference electrode. Auxiliary electrode was made of platinum (Pt). The tests were carried out in the corrosive environment of 0.5M NaCl solution at pH = 7 and a temperature of 293K (Trzaska Trzaska, 2010).

Studies of Resistance to Corrosion

signal frequency are shown by

systems investigated.

investigation.

**materials** 

of Selected Metallic Materials Using Electrochemical Methods 401

The experimental results expressing the dependence of impedance spectra on the applied



Impedance is an essential characterization of the current intensity response of the corrosion system to the sinusoidal perturbation of the potential applied to the metal. The results of impedance measurements made in a suitably wide range of frequencies provide valuable information about the system and electrochemical corrosion occurring therein. The majority of electrochemical as well as physical processes can be interpreted within the impedance spectroscopy method as elements of electrical circuits with appropriate time constants. Thus, to interpret the results of electrochemical impedance measurements surrogate models

Experimentally determined frequency characteristics were used to map the corrosion processes using models based on suitable equivalent circuits. Each element of such a circuit models the specific process or phenomenon occurring in the corrosion system under

Metals are very commonly used materials in various technologies and applications. Properties of metallic materials are shaped by their composition and structure. Moreover, most natural metals are found in chemical combination with other elements. In the current study, resistance to electrochemical corrosion tests were applied to metallic materials with different properties and structures: aluminum (Al), aluminum with a surface layer of oxide aluminum (Al2O3), iron (Fe), S235JR steel, nickel (Ni), microcrystalline nickel (Nim), nanocrystalline nickel (Nin), and amorphous alloy of phosphorus-nickel (NiP). The choice of

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

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

**3. Electrochemical characteristics of corrosion resistance of metallic** 

these materials was due to the universality of their applications in technology.

corrosion in many environments with low aggressiveness (Vargel, 2004).

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

φ = f2(log(ω)), where Z(jω) = |Z|ejφ, |Z| - impedance magnitude,

of electrical circuits, known as Randles models, can be used.

Fig. 3. The set of appliances for the electrochemical corrosion testing by EIS technique.
