Study of Polarization Characteristics of Corrosion Films on Magnesium in Sulfate-Containing Electrolytes

Abstract

In this article, the results of studying the polarization characteristics of magnesium covered with corrosion film in aqueous solutions of MgSO₄ and Na₂SO₄ are presented. The absence of a corrosion-free magnesium surface was shown; in this connection, it was proposed to interpret the larger values of Tafel’s coefficients obtained in the experiment from the point of view of limiting the electrochemical process by charge transfer in the film phase. Charge transfer in corrosion films obeys the regularities of particle movement in high electric fields, and it is not only cationic. According to the impedance measurements, the resistance of the oxide and hydroxide layer of the magnesium-based corrosion film in the studied solutions was calculated. The largest contribution to the restriction of charge transfer in the initial stages of corrosion is made by a dense primary film defining the polarization resistance. Correlation of transfer parameters in high electric fields with thickness and resistance of corrosion film was demonstrated.

1. Introduction

The wide application of magnesium is limited by its low corrosion resistance. During the last decade, many researches have focused on the effect of the metal microstructure and medium on the magnesium corrosion behavior [1,2,3,4].

The standard magnesium electrode potential is −2.363 V (vs. normal hydrogen electrode (NHE)), whereas the stationary magnesium potential and its alloys is set at −1.3 V. This difference in potentials and dissolution patterns indicate a passive state of magnesium in neutral electrolyte solutions. In this case, the passivating surface layers can be sparingly soluble oxide and magnesium hydroxide. There are several researches devoted to the mechanism of corrosion of pure magnesium in aqueous solutions [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26]. Special attention should be given to the excellent review in [25]. In many works [3,4,5,12,16,21,22,24,25], the corrosion film on magnesium in different solutions is presented as a two-layered one. It consists of a thin dense layer of the MgO adjacent to metal and friable hydroxide on its surface, at the same time oxide is the main barrier layer. In [27], the relationship of the roles of these two layers using representations of their interaction with the electrolyte solution is presented in detail. Its formation is connected with metal background, i.e., with formation of the primary dense film at preparing the surface in air [4,16] and dissolution of metal is supposed only on the sites of metal which are free of MgO [5]. However, the microelectrode surface scanning data presented in the same work demonstrated a change in surface resistance in a very narrow range, whereas its value varied only one and a half times. Such a difference is hardly able to characterize the difference between a surface with a dense magnesium oxide film and without it, especially considering the declaration of corrosion blocking by that film. In some works, referred to direct study of the transverse structure of the corrosion film, its two-layer character was presented quite clearly [3,4,22,24,27].

The question of monovalent magnesium role in corrosion process is insufficiently clear as well as, except for its participation in formation of negative difference effect (NDE) [5,7,12,14,16], where, as a rule, it gets the role of intermediate short-living particle causing the emergence of NDE.

There are lots of researches dedicated to the effect of electrolyte composition on magnesium corrosion [5,6,9,10,11,14,19,22,23,26,27], where the effect of both anions, and cations on the corrosion process was described. Thus, Song et al. [17] studied the electrochemical behavior of magnesium in chloride- and sulfate-containing solutions and concluded that the presence of chloride ions accelerated the electrochemical reaction of the transition of metal magnesium to monovalent one. Sulfate ions were shown to possess less effect than chloride ions [8,17]. The ammonium salt is considered to be one of the strongest factors affecting the corrosion activity of magnesium [27]. It is assumed that the strong solvent effect of the ammonium cation appears on the dense part of the magnesium oxide film, which leads to its depassivation and corrosion.

A significant number of works are also devoted to the pH effect on the corrosion process [3,11,15,22,23,27]. The nature of the pH effect on the corrosion process through the influence at the stage of hydration of magnesium oxide and recrystallization of its hydroxide is most fully disclosed in [22].

Almost all researches of the corrosion process on magnesium as a rule use methods of investigation of electrochemical corrosion by particular polarization dependencies, sometimes supplemented with electrode impedance analysis. As a rule, the application of these methods is not accompanied by proof of the possibility of using certain equivalent schemes and approaches in the study of polarization dependencies. In some works, Ref. [12,20,27] is analyzed the mismatch of the results of corrosion rate measurement by polarization dependencies, gravimetry and volumetry. Excess of the corrosion current calculated from polarization measurements over currents from volumetric measurements is referred to anode dissolution in the form of monovalent magnesium ion. For this reason, the authors recommend investigating the possibility of applying the method of determining corrosion characteristics by polarization dependencies to magnesium very carefully. Song and Atrens in 2003 showed that for Mg-alloys, Tafel’s extrapolation does not allow a reliable estimate of corrosion rate [2]. In the article [28], this issue is discussed, and it was noted that for Mg-alloys the corrosion rates estimated by the Tafel’s extrapolation by using the polarization curves were inconsistent with corrosion rates estimated gravimetrically and by hydrogen consumption. Deviations were 50–90%, indicating the need for careful use of the Tafel’s extrapolation for Mg. If used, it was recommended to supplement these measurements with other methods. The same conclusion was made in [27] for pure magnesium.

Similarly, the corrosion currents calculated from the potentiodynamic polarization curves by the Tafel’s extrapolation methods for magnesium in KCl, NaCl, and Na₂SO₄ electrolytes decreased in this row [10]. At the same time, impedance measurements indicated that the corrosion resistance in KCl was higher than in other electrolytes, which indicated the mismatch of the characteristics obtained by different methods.

According to our opinion, using the standard method of finding the parameters of charge transfer across the interface, by means the Butler–Folmer’s equation (Tafel’s) in the cathode and anode process on the corrosion magnesium, is strongly doubtful, as the Tafel’s slopes are often very large, and the presence of a corrosion film does not allow to ignore its contribution to polarization. The presence of a corrosion film on the metal surface to determine its effect on the control of the charge and mass transfer process is obligatory. In connection with the above-mentioned, all the electrochemical measurements were interpreted herein, considering the role of surface corrosion films in charge transfer processes with an appropriate analysis of the regularities of the charge transfer through the phase of corrosion products on the surface.

2. Experimental Part

The electrochemical behavior of magnesium was studied by linear voltammetry in a three-electrode electrochemical cell using AUTOLAB PGSTAT potentiostat-galvanostat (The Netherlands). The working electrode was the end face of the magnesium rod (99.98%), with an area of 0.17 cm². The counterelectrode was a platinum plate with an area of 6.3 cm² and Ag/AgCl sat. KCl electrode was used as the reference electrode (E = 0.197V vs. the standard hydrogen electrode). Potentiodynamic polarization measurements with linear potential sweep were carried out at 10 mV/s rate of potential overlap in ±300 mV potentials’ region from the stationary potential. All the measurements were performed three times, followed by statistical analysis on blunder and averaging of the obtained result (Tafel’s coefficient).

Electrochemical impedance spectroscopy (EIS) studies were performed in a cell with a coaxial arrangement of electrodes. The area of the magnesium electrode was 5 times smaller than the area of the counterelectrode. Scanning electron microscope (SEM) studies were performed for additional characterization of the corrosion film. Prior to each measurement, the magnesium electrode was treated with SiC paper (grain size 5–7 μm), then degreased with ethyl alcohol. All the experiments were carried out in a thermostat cell at a temperature of 25 °C maintained with an accuracy of ±0.1 °C.

The following salts were used to prepare solutions of various concentrations: magnesium sulfate heptahydrate (99.5%), and sodium sulfate anhydrous (99.5%).

Initial characteristics of solutions for corrosion studies are presented in

Table 1. Electrical conductivity and pH of solutions for corrosion studies.

Electrolyte	Electrical Conductivity (γ, mSm/sm)	pH
0.5 mol/L Na2SO4	47.23	5.95
0.5 mol/L MgSO4	27.98	5.62
1 mol/L MgSO4	39.75	5.83
2 mol/L MgSO4	42.80	6.16

.

Magnesium samples were kept in solutions of magnesium sulfate and sodium sulfate for 1, 5, 10, 20, and 30 min and for longer intervals up to 1 month to form corrosion films on them for testing.

3. Results and Discussion

3.1. The Surface Condition of Magnesium during Corrosion

The micrographs of the magnesium electrode surface in the first half-hour of corrosion in an electrolyte of 0.5 mol/L magnesium sulfate are presented at Figure 1. Before immersion into the electrolyte, the magnesium surface had already been covered with a film formed by surface preparation in the atmosphere, and at washing after grinding, the film possessed a marked thickness (Figure 1a). After one minute of corrosion, the surface equalize became noticeable due to the growth of the film, and its cracking starting herewith became noticeable too (Figure 1b).

After 5 min of immersion, cracking increased and, on the surface, a new layer, also subjected to cracking, began to form; the film thickness in the cracks after 5 min may be estimated as units of micron (Figure 1c). Further, after 20 min, the cracks were gradually tightened and the second more incoherent layer was built up from above, as it had been reported in many works [3,4,5,12,16]. The observed cracking of the corrosion films demonstrated above for magnesium sulfate solutions in some investigations was associated with impact of hydrogen evolution [5].

In magnesium sulfate solutions of higher concentrations, corrosion was rapidly accelerated and in a minute the film appeared thicker and more incoherent than half an hour in a solution of 0.5 mol/L MgSO₄ (Figure 2). In 2 mol/L MgSO₄ solution, after a minute, the surface of the film became fretted as though it was subjected to dissolving.

In sodium sulfate solutions, the film growth was less expressed and its slight cracking occurred after 30 min of immersion in the electrolyte (Figure 3). However, formation of the upper crystalline layer occurred as well as in magnesium sulfate after 20 min of exposure of the sample in solution.

During corrosion, the pH of the solution changes in accordance with the main reaction as magnesium hydroxide accumulates in the solution, resulting in it equilibrating to 7.76 and 8.33 for 0.5 mol/L of sodium and magnesium sulfate, respectively, after half an hour of exposure.

This difference corresponds to an almost tenfold increase in the content of magnesium hydroxide in magnesium sulfate solutions over sodium sulfate solutions. With the same area of the electrodes and the volume of the solution, taking into account the type of surface on the micrographs, we can talk about a much higher rate of corrosion in magnesium sulfate.

3.2. Polarization Measurements

As it was demonstrated in the SEM images above, the presence of the film on the surface of the magnesium electrode was being observed from the first minutes of corrosion, also affecting its electrochemical behavior.

First of all, it was manifested in the displacement of the potential of the magnesium electrode in time to the positive side (

Table 2. Potential of the magnesium electrode (V) versus to the Ag/AgCl electrode in the first half hour of corrosion in different electrolytes.

τ, min	0.5 mol/L Na2SO4	0.5 mol/L MgSO4	1 mol/L MgSO4	2 mol/L MgSO4
1	1.68	1.52	1.72	1.89
5	1.65	1.56	1.67	1.80
10	1.63	1.66	1.63	1.74
20	1.68	1.63	1.73	1.72
30	1.62	1.48	1.65	1.70

), which characterizes the passivation of the surface with a corrosion film.

The exception was a negative shift in the first 15 min of the magnesium potential of 0.5 mol/L MgSO₄. This coincides with the trend of increasing film cracking in the time interval up to 10 min (Figure 1). Shifting the magnesium potential in magnesium sulfate solutions to the negative side also indicates less passivity due to the formation of looser films.

The anode polarization curves on magnesium possessed the form of exponent for sodium sulfate solutions with output to the limit current for magnesium sulfate solutions. Cathode curves possessed a less expressed current boost, although in the Tafel’s coordinates both anode and cathode branches were well rectified (Figure 4 and Figure 5).

Concurrently, dependence of the potential-logarithm current consists of two or even three sites for anode polarization in magnesium sulfate solution. The first site from the stationary potential to 20–40 mV overvoltage has deviation from linear dependence, connected with distortion of partial polarization curve by corrosion process in the area of compromise potential. Deviations from linear dependence for the anode process in magnesium sulfate (Figure 5a) at more positive potentials are most likely due to salt anode passivation. The absence of such passivation in sodium sulfate solutions is due to the greater solubility of the anode dissolution product (magnesium hydroxide) in these solutions [29] in comparison with magnesium sulfate solutions. Hydroxide is formed in the anode process due to monovalent magnesium [14].

The Tafel’s coefficients and polarization resistances were calculated according to polarization curves at different immersion time of the electrode in the tested solutions.

3.3. Analysis of the Laws of Charge Transfer during Polarization

In the view of classical electrochemistry, such polarization relationships should describe the charge transfer across the interface according to Butler–Volmer, when the cathode process is hydrogen evolution and the anode process is the electrochemical dissolution of the metal. However, these exponents are characterized by high value of Tafel’s coefficient (˃200 mv). The high value of Tafel’s coefficient in the cathode process is usually associated with the formation of gas bubbles at the surface resulting in an overvoltage increase [15]. However, such an explanation cannot be satisfactory, as gas bubble shielding should result in the addition of an ohmic component to the polarization while increasing the resistance of the electrolyte. At the same time, the total overvoltage will be the sum of the overvoltage of the electrochemical reaction and the voltage drop in the electrolyte:

$$E_i = E_{cor}+{\eta }_f+i{R}_{el} = E_{cor}+a+blni+i{R}_{el}$$

(1)

$$\frac{\partial E_i}{d\left(lni\right)} = b+R_{el}\frac{\partial i}{\partial \left(lni\right)} = b+i{R}_{el}$$

(2)

where E_i is the current value of potential, E_cor is the corrosion potential, η is the charge transfer overvoltage, R_el is the electrolyte resistance, and a and b are the Tafel’s equation coefficients.

Equation (2) means that the appearance of additional resistance in surface shielding does not result in an increase in the Tafel’s coefficient, but leads to a nonlinear dependence in the Tafel’s coordinates.

The same conclusion was made regarding the inclusion of a voltage drop in the corrosion film into the total polarization overvoltage, and as the corrosion film resistance was quite significant, linear dependencies in the Tafel’s coordinates would not have to be observed.

As the presence of the film on the metal surface is quite obvious, it is most likely that we register an exponential dependence characterizing the charge transfer not through a double layer according to Butler–Folmer, but through a corrosion film in the high electric fields, as it had been already reported in [18].

To analyze the charge transport through the solid electrolyte film in high electric fields, consider the equation proposed to describe this process:

$$i = zF{r}_{0}v{n}_{+}\exp \left(-\frac{W}{RT}\right)\exp \frac{r_{0}zFE}{2RT}$$

(3)

where r₀ is the distance between adjacent defects in a solid, v is the ion oscillation frequency in equilibrium position, n₊ is the concentration of defects in the solid phase, W is the activation energy of charge movement, z is the charge of the transferred particle, and E is the electric field voltage. With considerable thickness and resistance of the film, the overvoltage of the process is almost equal to the voltage drop in the film. Then, for the high electric field, the following expression may be written,

$$E = \frac{\eta }{L}$$

(4)

where η is the overvoltage process and L is the film thickness. In this case, the equation takes the form

$$i = zF{r}_{0}v{n}_{+}\exp \left(-\frac{W}{RT}\right)\exp \frac{r_{0}zF\eta }{2RTL}$$

(5)

After logarithming this equation and grouping all the constants, we get

$$lni = lna+b\eta$$

(6)

where the constant $a = zF{r}_{0}v{n}_{+}\exp \left(-\frac{W}{RT}\right)$ and the coefficient $b = \frac{r_{0}zF}{2RTL}$ are inversely proportional values to the film thickness. In this case, the value 1/b may be a parameter to estimate the thickness change of the corrosion film L in time.

If we reduce Equation (5) to the form of the Tafel’s equation, we shall see that the Tafel’s coefficient conforms to 1/b and is proportional to the film thickness.

$$\eta = -\frac{1}{b}lna +\frac{1}{b}lni$$

(7)

The dependences (1/b) on the immersion time for anode and cathode polarization curves in different solutions are presented in Figure 6a.

In most cases, the Tafel’s coefficient was higher than 200 mV, except for the initial stage of corrosion in 0.5 mol/L magnesium sulfate solution at anode polarization. In the anode process in magnesium sulfate at early terms, the Tafel’s coefficient has a value close to 60 mV, formally corresponding to one-electron transfer. However, its completely smooth change to 0.3V in 200 min indicates that this is a characteristic of a growing corrosion film. Accordingly, the Tafel’s coefficient being determined here is 1/b value for transport in high electric fields, and it reflects the thickness of the corrosion film according to Equation (5).

In sodium sulfate solutions, the Tafel’s coefficient was higher from the very outset of the corrosion process for both the anode and cathode processes. The change in the value of 1/b for almost all the curves except anode ones in sodium sulfate solutions in the first half hour of corrosion presented itself the curve of a parabolic dependence, due to this we presented these curves in 1/b to t^1/2 coordinates (Figure 6b)

Such parabolic dependences were often observed when measuring the amount of released hydrogen, as it was reported in [12]. In [22], a quantitative implementation of the parabolic law was established for the dependence of the magnesium hydroxide content on time. The parabolic law itself reflects the corrosion process, which is limited by diffusion of the reagent through the thickness L of the growing product layer according to equation $L = {\left(L_0^2+a\tau \right)}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}$ [18]. Here, L₀ is the initial film thickness and τ is the corrosion time.

In these curves, the Tafel’s coefficient change (1/b) was linear in time at the initial regions of ~30 min, except of the dependence for the cathode coefficient in magnesium sulfate having a linear site till 4 h of corrosion. Simultaneously, at all linear sites in these coordinates the periodic deviations in both sites of the straight line were observed. Comparison of emerging deviations with film behavior by microscopic studies showed that they most likely corresponded to cracking and growth of cracks in the film, as well as hydrogen evolution, when parameters of charge transfer through it could change dramatically due to such effects. The rectification of the dependence in coordinates 1/b − τ^1/2 indicated that the corrosion process was limited by diffusion of the reagent through the thickness of the growing product layer. The diffusion model underlying the parabolic law of the film growth was the diffusion of metal magnesium in ion-electron form through a layer of corrosion products to the electrolyte. This motion first happened in the primary magnesium oxide film, on the surface of which the electron reacted with water to hydrogen evolution. In the solid phase of magnesium oxide, monovalent magnesium was most likely formed, which was also a reducing agent providing hole conductivity and reacting with water to hydrogen evolution. The existence of monovalent magnesium as a reducing agent in the hydroxide layer was impossible, as well as electron conductivity.

The observed difference of the Tafel’s characteristics for the anode and cathode process was apparently determined by the nature of the particle transferred through the film, which, in accordance with Equation (6), determined the b value. In the cathode process, it was an electron being carried from the metal through the film to the solution and a magnesium cation moving in the reverse direction, whereas in the anode process, only the cation was being carried. Based on the data of the NDE study, it may be assumed that it was a single-charged magnesium cation. Higher values of the Tafel’s coefficient (1/b) for cathode sites in 0.5 mol/L magnesium sulfate solutions characterized particle transfer with a smaller value of the activated jump distance. In sodium sulfate solutions, this difference was not significant compared to the experimental spreading.

The sharp change in the shape of the curves after half an hour of corrosion after the linear site on the dependences of 1/b to time is also drawn the attention. In this area, the Tafel’s coefficient values for cathode and anode polarization began to converge dramatically and their practical matching after the one-hour corrosion became noticeable. Such behavior indicates the transformations in the film resulting in the identity of charge transfer therein in the cathode and anode processes, while there was a marked difference in the initial sites. According to SEM images for half an hour of corrosion, the surface was almost completely covered with the crystalline layer of the second product, but this did not allow to explain the change transfer parameters in cathode and anode polarization.

As the Tafel’s coefficient (1/b) determined in this experiment was a value proportional to the film thickness, it was noticed that the effective film thickness in sodium sulfate solutions decreased while for magnesium sulfate increased. The dependences of the polarization resistance of the magnesium electrode with the film in sodium sulfate solution presented in Figure 6c also confirmed the film resistance reduction. The polarization resistance, which in this case was the film resistance, was calculated from the initial site of the polarization curve (10–20 mV).

The same as on the dependence curves of the Tafel’s coefficient on time the fact of convergence and stabilization of the film resistance on magnesium in magnesium and sodium sulfate solutions after 100 min of corrosion attracts itself the attention, when the rates of their change are markedly slowed down and the values themselves are converged, demonstrating a movement towards independence from the nature of the electrolyte. This is also perfectly confirmed by the dependence of the magnesium electrode potential on time, which also passes through the acute maximum during the first half hour, and after 100 min, it does not depend on the composition of the solution, demonstrating the identity of the state in different solutions. The positive displacement of the potential of the magnesium electrode during corrosion characterizes the passivation of the surface by the corrosion film. The negative shift in the first 15 min of the magnesium potential of 0.5 mol/L MgSO₄ coincides with the trend of increasing film cracking in this time interval (Figure 1). The same interpretation of the potential shift in the negative direction, due to the introduction of an additive into the electrolyte that dissolves the oxide part of the film, is presented in [27].

A negligible period of the film growth under parabolic law ended with its transformation into another state, where parameters of cathode and anode process transfer were equalized. The most likely variant of such a conversion was the hydration of the primary film’s magnesium oxide and its conversion to hydroxide. The general perception is that the characteristics of charge transfer through the film were solely determined by its dense part, which included only the part adjacent to the metal, initially consisting of magnesium oxide and possibly a dense part of the amorphous hydroxide layer. The layers lying above had an incoherent morphology and little determine polarization. That is why there was observed a strong change of electrical characteristics during the initial period of time and the formation of the similar final product over time.

Similar regularities of behavior were demonstrated by the concentration dependence of corrosion parameters in magnesium sulfate solutions (Figure 7). When the salt concentration was increased to 1 mol/L, the coefficient of 1/b as opposed to 0.5 mol/L was no longer practically increased over time, maintaining a roughly constant value. In the electrolytes with 2 mol/L salt concentration, the value of 1/b decreased sharply from very high values at the beginning of the process to values comparable referred low concentrations in 30 min (Figure 7). Such a reduction in 1/b in the initial corrosion time period should also characterize the film thickness decrease. It also, as in the previous case, confirmed the dependence of polarization resistance on time (Figure 7b), which demonstrated the growth of film resistance in 0.5 mol/L magnesium sulfate solutions, almost constant value in 1 mol/L solutions and its decrease in 2 mol/L solution. At the same time, according to SEM, the film thickness increased distinctly with increasing the magnesium sulfate concentration from 0.5 mol/L to 2 mol/L (Figure 2). Formally, this contradicted the observed change of the Tafel’s coefficient and polarization resistance in electrolytes with a high concentration of magnesium sulfate and in sodium sulfate solutions. However, it was clear that the formed film was a rather incoherent formation, which was particularly well seen in the example of magnesium sulfate solutions of increased concentration. The state of the surface in the 2 mol/L magnesium sulfate solution was quite consistent with the dissolving film. In this case, it was logical to consider the reduction of film resistance as a consequence of the replacement of the dense primary film by the incoherent secondary film, and it was more expressed for sodium sulfate solutions and high concentrations of magnesium sulfate, in which the solubility of magnesium hydroxide [29] was higher, which allowed the primary film to dissolve.

3.4. 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 Figure 8 and Figure 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 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). This result, at first glance, contradicts the generally accepted idea of the determining role of the oxide film in the corrosion process as a barrier layer. However, the protective properties of the film are determined not by its general conductivity, but by the absence of electronic conductivity.

The polarization resistance of the film-coated electrode R_pSEI under direct current (DC) polarization conditions will be combined from the resistance of the electrolyte R_el, the resistance of the magnesium oxide film R_SEI(II), and the resistance of the hydroxide layer R_SEI(I), provided that the charge transfer is limited by the corrosion film. The resistance of the electrolyte due to the high conductivity of the solutions (Table 1) was very small and had no effect on the total value of the resistance.

The total resistance of the film parts, together with the polarization resistance R_p (LV) determined experimentally, are demonstrated at Figure 12. A fairly good match of the polarization resistance values of the magnesium electrode calculated from the EIS data and polarization curves can be also observed. The calculations for sodium sulfate solutions, where the absence of coincidence was obvious, acted as exceptions. The reason for the latter is most likely the inadequacy of the equivalent circuits for sodium sulfate solutions with a satisfactory approximation of the Nyquist plots. Based on the analysis of impedance measurements, it can be concluded that the hydroxide layer made the main contribution to polarization, whereas the oxide layer was very thin and its resistance were significant only at the initial corrosion time.

4. Conclusions

Analysis of the polarization dependences of the magnesium electrode, which is corrosive in sodium and magnesium sulfate solutions, makes it possible to conclude that the limiting stage of the electrochemical process is the transfer of charge through the corrosive film. The basis for this statement is the presence of sufficiently thick corrosion films and high values of the Tafel’s coefficients, which do not correspond to the regularities of the electrochemical kinetics charge transfer across the interface.

Analysis of the charge transport equation in high fields demonstrated that the formal Tafel’s coefficient determined from polarization curves was directly proportional to the thickness of the corrosion film. Based on the analysis of the change of the Tafel’s coefficients of the cathode and anode process in corrosion, it could be concluded that the charge transfer in the primary film formed during the preparation of the electrodes was limiting at the initial stage. Concurrently, parameters of cathode and anode process transfer differed considerably, indicating the transfer of different charged particles.

The negligible period of primary film growth under parabolic law ended with its transformation into another state, where parameters of cathode and anode process transfer were equalized. The most likely variant of this conversion was the hydration of magnesium oxide with the primary films and its conversion to hydroxide.

The characteristics of charge transfer through the film were mainly determined by its dense part from magnesium hydroxide. The layers forming above had an incoherent morphology and little determination of polarization.

In magnesium sulfate solutions of high concentration, the change in the initial state of the films was very rapid due to the higher solubility of magnesium hydroxide in them. The impedance measurements confirmed the two-phase structure of the film and demonstrated good convergence of their calculated values of polarization resistance with the voltammetry data.