**2.1 Potentiodynamic polarization method**

In any process of electrochemical corrosion oxidation processes, i.e. the anodic processes, as well as the reduction i.e. cathodic processes occur on the surface of the metal. In the measurement system these processes occur simultaneously during the polarization for each of the applied potential values but at different speeds. The resultant speed of the processes constituting a sub-inflicted response to the potential changing at prescribed rate is recorded by the measurement system in the form of instantaneous current values. The characteristics j = f(E) of the current intensity as function of the potential obtained in this way are called potentiodynamic polarization curves.

Potentiodynamic polarization method was applied to a wide range of potential changes to characterize the current-potential relationship j = f(E) in corrosion systems under investigation. The corrosion current density jcor and potential Ecor of tested metallic materials were further determined based on extrapolation of tangents to the curves of the cathodic and anodic polarization zones.

#### **2.2 Impedance spectroscopy method**

The perturbation of the equilibrium of the corrosive systems composed by metal - 0.5M NaCl solution, was obtained through time-varying sinusoidal signal described by the following relationship E(t) = E0cos(ωt), where E(t) is the instantaneous potential value [V], E0 – potential magnitude [V], t - time [s]. The response of the corrosion system to such an interfering signal was the current intensity signal, which is the effect of transferring electrical charge between the corrosive metal - an electron conductor, and the electrolyte ionic conductor. This response is described by the time-varying current signal I(t) = I0cos (ωt + φ), where I(t) – instantaneous current value [A], ω = 2πf - pulsation [rad/s], f - frequency [Hz], φ - phase shift [rad.]. The measuring system digitally generates the excitation having the above sinusoidal form and measures the current system response as a function of frequency. Then the plots of |Z(ω)| and φ(ω) were generated , i.e., amplitude and phase spectra of impedance, called Bode plots, and curves X(ω) = F(R(ω)), called amplitude-phase characteristics or Nyquist plots (Orazem & Tribollet, 2008), (Sword et al., 2007).

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

In any process of electrochemical corrosion oxidation processes, i.e. the anodic processes, as well as the reduction i.e. cathodic processes occur on the surface of the metal. In the measurement system these processes occur simultaneously during the polarization for each of the applied potential values but at different speeds. The resultant speed of the processes constituting a sub-inflicted response to the potential changing at prescribed rate is recorded by the measurement system in the form of instantaneous current values. The characteristics j = f(E) of the current intensity as function of the potential obtained in this way are called

Potentiodynamic polarization method was applied to a wide range of potential changes to characterize the current-potential relationship j = f(E) in corrosion systems under investigation. The corrosion current density jcor and potential Ecor of tested metallic materials were further determined based on extrapolation of tangents to the curves of the cathodic

The perturbation of the equilibrium of the corrosive systems composed by metal - 0.5M NaCl solution, was obtained through time-varying sinusoidal signal described by the following relationship E(t) = E0cos(ωt), where E(t) is the instantaneous potential value [V], E0 – potential magnitude [V], t - time [s]. The response of the corrosion system to such an interfering signal was the current intensity signal, which is the effect of transferring electrical charge between the corrosive metal - an electron conductor, and the electrolyte ionic conductor. This response is described by the time-varying current signal I(t) = I0cos (ωt + φ), where I(t) – instantaneous current value [A], ω = 2πf - pulsation [rad/s], f - frequency [Hz], φ - phase shift [rad.]. The measuring system digitally generates the excitation having the above sinusoidal form and measures the current system response as a function of frequency. Then the plots of |Z(ω)| and φ(ω) were generated , i.e., amplitude and phase spectra of impedance, called Bode plots, and curves X(ω) = F(R(ω)), called amplitude-phase

characteristics or Nyquist plots (Orazem & Tribollet, 2008), (Sword et al., 2007).

**2.1 Potentiodynamic polarization method** 

potentiodynamic polarization curves.

**2.2 Impedance spectroscopy method** 

and anodic polarization zones.

The experimental results expressing the dependence of impedance spectra on the applied signal frequency are shown by


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

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 investigation.
