## 3 Results and Discussion
### Determination of Charge Transfer Parameters Using EIS  
The electrochemical impedance technique was also used to addition analysis of charge transfer through corrosion films. The Nyquist plots for the magnesium electrode in the investigated solutions are shown in Figures 8 and 9. The presence of two semicircles in the hodographs of electrochemical impedance most often refers to RC circuits describing the double electric layer and dense part of the film [5; 30; 31]. Figure 8 shows the EIS data of electrochemical impedance of the surface film on the magnesium electrode in 0.5 mol/L magnesium and sodium sulfate solutions obtained at open-circuit potentials at different immersion times. Nyquist plots have one form type for a single solution. It can be seen that with the increase of the electrode holding time in the solution, the radius of the semi-circle increases, which indicates an increase in the resistance of the RC circuit. The difference between the Nyquist plots in sodium sulfate solutions was proved by the presence of an inductive loop appearing after the indication of transition to the second half-circle. The EIS Nyquist plots of magnesium electrode in higher concentration magnesium sulfate solutions are shown in Figure 9.  
The proposed versions of electrical equivalents of electrochemical processes for magnesium electrodes contain a serial connection of RC-circuits, where the capacity is represented by an element with distributed parameters of Constant Phase Element (CPE) for describing a complex nature of charge transfer through a film and double layer [30]. When considering the case of partial coating of  
Figure 8: The Nyquist plots at 25 \({}^{\circ}\)C and at different immersion times on Mg-electrode: (**a**) in 0.5 mol/L MgSO4 solution and (**b**) in 0.5 mol/L Na2 SO4 solution. Figure 9: The electrochemical impedance spectroscopy (EIS) data measured at 25 \({}^{\circ}\)C and different immersion times on Mg-electrode: (**a**) in 1 mol/L solution MgSO4; (**b**) in 2 mol/L solution MgSO4. the surface with a dense corrosion film, their parallel connection is used [5; 31]. Application of addition elements responsible for the inductance loop in the circuits is more as an adjustable element to obtain a better approximation of the experimental data and does not have sufficient justification [5; 27]. In our process of designing the equivalent scheme, we assume that the above-mentioned RC-circuits characterize two parts of the film: dense oxide part and more incoherent hydroxide part. This understanding is based on the defining role of the film in the charge transfer process shown in this work and the undetectable effect of the double interface layer on it. In addition, the Warburg element is introduced in the equivalent scheme describing the loose part of the film to reflect the diffusion transfer in it. An equivalent electrical circuit of charge transfer in corrosion films formed in magnesium and sodium sulfate solutions is shown in Figure 10. In view of the fact that the inductive loop was expressed in sodium sulfate solutions, an inductive element for the entire volume of the film was introduced into the circuit. The equivalent circuits presented in Figure 11 satisfactorily approximate the experimental EIS data for solutions of magnesium sulfate and slightly worse in the low-frequency region for sodium sulfate. The resistance of oxide and hydroxide parts of the film at corrosion in magnesium and sodium sulfate solutions calculated from the Nyquist plots is shown in Figure 12. The resistance of the magnesium oxide film [32] was almost always less than the resistance of the hydroxide film and possessed a small maximum on its dependence on the corrosion time. The resistance of the hydroxide oxide layer was growing relatively stable except for the 2 mol/L magnesium sulfate solution due to its dissolution as can be seen from the images (Figure 2).