Corrosion Inhibition of AZ31 Mg Alloy by Aqueous Selenite (SeO₃²⁻)

Figure 1 shows 24-hour OCP measurements of AZ31 in selenite-bearing chloride solutions with concentrations ranging from 0.5 to 50 mM Na₂SO₃. The black curve shows the variation of AZ31 OCP during exposure to pH 9, 0.1 M NaCl with no selenite addition. In this case, a sharp break is evident in the curve at 0.5 hours. This phenomenon has been associated with passive film breakdown.³⁷ Thereafter, the OCP remained relatively steady until about 16 hours when an increase in potential and in signal noise indicated onset of new or renewed electrochemical reactivity. The noise episode was accompanied by the appearance of a dark corrosion product on the surface, which is attributed to localized corrosion.¹

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In the presence of Na₂SeO₃, initial OCP values are shifted to more negative potentials at the outset of the exposure and were observed to rise quickly during the first two hours. The OCP variation in 0.5 mM and 1.0 mM Na₂SeO₃ solutions were similar increasing in the first 2 hours and then stabilizing at about −1.52 V_SCE after eight hours. The OCP behavior in the 10.0 mM Na₂SeO₃ solution is substantially similar in its evolution over time, but the steady-state potential is about 60 mV more negative than the OCP values recorded for the more dilute selenite concentrations. The OCP variation for the 50.0 mM Na₂SeO₃ solution was distinct rising through the entire 24-hour exposure period with some variation in rate of increase. There were no sudden drops in OCP suggesting surface film breakdown in any of the measurements conducted in Na₂SeO₃-bearing solutions. The pH of each solution was measured before and after the 24-hour exposure period. In all cases, the final pH had increased no more than 0.3 pH units.

Images of sample surfaces exposed for 24 and 72 hours are shown in Figure 2. The control sample exposed in aerated 0.1M NaCl for 24 hours was heavily corroded and covered with dark gray and black corrosion product (Fig. 2a). Figures 2b, 2d, 2f, and 2h show surfaces from samples exposed to selenite-bearing solutions for 24 hours. Film formation is prominent on these samples, and there is little to no evidence of localized corrosion or significant general corrosion. The film formed in 0.5 and 1.0 mM selenite solutions was uniform across the surface and gray with a slight pink coloration. There was no localized corrosion evident under the coating when it was removed. The film formed during exposure to 10 mM selenite was also uniform across the surface, but had a brown-black coloration. No localized corrosion was found under this coating either. The film formed during exposure to 50 mM selenite was uniform, reddish-brown in coloration and marked with several transparent crystals that were firmly embedded in it. These crystals were not soluble in water.

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Samples exposed for 72 hours in selenite-bearing solutions (Figs. 2c, 2e, 2g, 2i) exhibited film formation that mostly mirrored observations made for the 24-hour exposure. Notable differences included the emergence of black and reddish film formation on part of the surface exposed to the 0.5 mM Na₂SeO₃ solution (Fig. 2c), and some slight indications of localized corrosion on the samples exposed to the 1.0 and 50 mM Na₂SeO₃ solutions (Figs. 2e and 2i).

Figure 3 shows EIS spectra collected every 4 hours over a 24-hour period from AZ31 samples immersed in the Na₂SeO₃ – NaCl solutions. The spectra were fit to the equivalent circuit model for a defective coating shown in Figure 3f.³⁸ In this model, R_s represents the bulk solution resistance. R_p is the pore resistance representing the ohmic resistance through the coating or corrosion product layer. Q_c is the reactance associated with the coating or corrosion product layer. R_ct is the faradaic resistance associated with electrochemical reaction below the coating or corrosion product layer, and Q_dl is the interfacial reactance that develops due to charge separation at the interface between the metal and the coating, corrosion product layer and penetrating solution,^39,40 respectively. The total resistance (R_tot) represents the sum of R_p and R_ct.

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Figure 3a shows the Nyquist plots of the spectra collected from AZ31 exposed to the 0.5 mM Na₂SeO₃ solution. The plot shows a two-time constant response at all times and the total impedance increases significantly after 8 hours of exposure increasing from 12 to 24 kΩ. The impedance does not change significantly with further exposure beyond 8 hours.

In 1.0 mM Na₂SeO₃ (Figure 3b), the total measured impedance increases steadily over the course of about 20 hours with little change between 20 and 24 hours, suggesting an approach to steady state conditions. Also, of note in the spectra collected in the 1.0 mM selenite solution is the low frequency capacitive loop that is present in the 4-hour data, whose articulation diminishes as exposure time increases.

In the 10.0 mM Na₂SeO₃ solution (Figure 3c), the spectra show an increase of impedance with time, although the total impedances are clearly lower than those measured at lower selenite concentrations. Additionally, a low frequency capacitive loop is distinctly articulated even at 24 hours.

Data collected from the samples in the 50.0 mM Na₂SeO₃ (Figure 3d) show the lowest total impedances among all four selenite concentrations examined. The spectra are all fairly similar in the high frequency range and do not show a clear increase with time, with the exception of the spectrum collected after 4 hours of exposure, which is somewhat lower. These data do show the presence of a low frequency capacitive loop that decreases in size with increasing exposure time.

To facilitate comparison, Figure 3e shows the dependence of the total resistance measured by EIS on selenite concentrations. The data show that the highest R_tot values were obtained in the lowest concentrations of Na₂SeO₃ (0.5 and 1.0 mM). Additionally, Fig. 3e illustrates that an increase in R_tot with time was observed for the two lowest concentrations (0.5, 1.0 mM), a slight increase for the 10 mM selenite concentration and a slight decrease for the 50 mM selenite concentration.

Overall, these results show that while there is film formation at all selenite concentrations examined, the most protective films are forming in the single millimolar selenite concentration range.

Both anodic and cathodic polarization curves were collected on samples immersed in a control solution (0.1 M NaCl, adjusted to pH 9.0) and in 1.0, 10.0, and 50.0 mM Na₂SeO₃ in 0.1 M NaCl. Based on the immersion testing and EIS data, experiments in 0.5 mM Na₂SeO₃ were not performed. Representative results of those scans are shown in Figure 4, including the scans resulting from curves collected in the chloride-only solution.

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With the exception of the measurement in the chloride-only solution, anodic polarization curves were collected after an 8-hour hold at the OCP. The scan in the chloride-only solution was collected after a 1-hour OCP hold. All of the anodic polarization curves obtained in Na₂SeO₃-bearing solutions exhibited a shift to lower current densities, and an increase in the breakdown potential indicating anodic inhibition. Suppression of anodic current increased with decreasing concentration—in accord with the trend observed in EIS measurements, with the 1.0 mM Na₂SeO₃ solution providing the greatest degree of inhibition (Figure 4a). The anodic current arising from the sample in 1.0 mM Na₂SeO₃ was approximately one order of magnitude less than that measured in the chloride-only solution. The breakdown potential shifted slightly to increasingly higher potentials as the concentration of selenite increased.

Representative cathodic polarization curves, performed after a 30-min hold at OCP, are shown in Figure 4b. A 30-min hold was employed, versus the 8-hr hold for anodic polarization experiments, to avoid the formation of a passive film before initiation of cathodic measurements. The presence of Na₂SeO₃ suppressed cathodic kinetics and the inhibiting effect appeared to be independent of concentration. The inhibiting effect was significant. Selenite additions inhibited the hydrogen evolution reaction and decreased the cathodic current density by an order of magnitude compared to the chloride-only case. The effect of anodic and cathodic inhibition conferred by selenite is a substantial decrease in the corrosion rate and the corrosion potential of AZ31.

Figure 5 illustrates the effect of low-level selenite additions to chloride solutions on the polarization characteristics of AZ31. Values for the control experiment (not plotted) were approximately 7 × 10⁻⁶ A/cm² for corrosion current density and −1.51 V_SCE for corrosion potential. These plots show some dependence on the concentration of Na₂SeO₃ on all three parameters. The breakdown potential, observed at approximately −1.32 V_SCE at all three concentrations signifies a breakdown of a protective film.⁴¹ Although little difference was observed in the breakdown potential values for samples tested in 10.0 mM versus 50.0 mM solution, the lowest concentration of selenite, 1.0 mM, yields the lowest average breakdown potential. However, within the margins of error for these experiments neither breakdown potential nor corrosion potential show a strong dependence on selenite concentration. The slight downward trend in corrosion potential with increasing concentration, observable on this plot, is in agreement with the OCP measurements in Figure 1. In the presence of Na₂SeO₃, corrosion current density, however, clearly decreases as its concentration decreases. The lowest concentration of Na₂SeO₃ (0.1 mM) produced a corrosion current density of approximately 9 × 10⁻⁷ A/cm², which was about one-half of the current observed in the 50.0 mM solution and almost an order of magnitude lower than the current observed in the control experiment.

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In order to determine if surface films formed during exposure to selenite-bearing solutions possess latent corrosion protection, an AZ31 sample was immersed in 1.0 mM Na₂SeO₃/0.1 M NaCl solution for 30 minutes, removed, and rinsed in DI water. This treated sample was then subjected to a cathodic scan in 0.1 M NaCl (without added Na₂SeO₃). As can be seen in Figure 6, the resulting cathodic kinetics were lower than kinetics measured on a control sample scanned in 0.1 M NaCl, but they were not as low as the sample scanned in the 1.0 mM Na₂SeO₃/0.1 M NaCl solution.

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Figure 7 shows Raman spectra of AZ31surfaces that were immersed in the selenite-chloride solutions for 3 days. After exposure, the surfaces were rinsed with DI water and air dried. Three spectra, representing exposure to 1.0, 10 and 50 mM selenite concentrations, each display a scattering band at 251 cm⁻¹, which indicates the presence of amorphous Se⁰ (a-Se).^42,43 The spectra from surfaces exposed to 10 and 50 mM selenite also contain a distinct band at 234 cm⁻¹, which indicates the presence of crystallized trigonal Se⁰ (t-Se).^42,43 Based on the approximate relative intensity of those two bands (reproducible in a minimum of three sets of spectra) the ratio of t-Se to a-Se increased in films formed in the 50 mM Na₂SeO₃ solution. No evidence of the presence of t-Se was present in the spectrum of a sample exposed in the lowest concentration (1.0 mM), however, due to the weakness of the spectrum arising from the film formed in that concentration, the presence of t-Se is not completely ruled out.

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Micro-Raman analysis of one of the crystals trapped in the film formed in the 50.0 mM solution produced a distinctive spectrum, which is also shown in Figure 7. Here, sharp bands were observed at 795 and 400 cm⁻¹, as well as a number of smaller sharp bands below 225cm⁻¹. Diffuse bands were observed at 650 and 400 cm⁻¹. These features were not observed in the spectra collected from the uniform film that covered the surface. These bands match well with scattering bands arising from lab-synthesized bulk MgSeO₃ hydrate (Fig. 7e), which was formed by titration of a MgCl₂ solution with Na₂SeO₃ to induce precipitation of the compound. The spectrum of the trapped crystal also contains weak scattering associated with a-Se, and t-Se, which were picked up due to scattering arising from the film entrapping and surrounding the crystal.

Surface film cross sections were also examined by SEM imaging of sections prepared by focused ion beam (FIB) milling. Figures 8a through 8c show plan views of surfaces after FIB removal of the cross sections and Figures 8d through 8f show the cross section of films on AZ31 formed during 24-hour immersion in 0.1M NaCl solution with 1.0, 10, and 50 mM Na₂SeO₃ additions respectively. Estimated coating thicknesses are 150 nm for the film formed in 1.0 Na₂SeO₃, 250 nm for the 10 mM solution, and 1000 nm for the 50 mM solution. There are variations in contrast in the cross section of the film formed in the 1.0 mM solution that are consistent with coating porosity. These variations increase in number for the film formed in the 10 mM solution, but are not visually apparent in the film formed in the 50 mM solution. The film formed in the 50 mM solution is cracked significantly due to dehydration in the microscope column. Whether the observed porosity evolves into shrinkage cracks or is obscured by different imaging physics associated with cracked film is not clear.

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These samples exposed for 24 hours were subjected to STEM-EDX mapping, providing elemental analysis of the cross sections (Figure 9). These data show that immersion in each of the three selenite concentrations resulted in films that contained Mg, O, and Se. Immersion in the 1.0 mM solution appears to have resulted in a film with less, but more evenly dispersed Se. The film formed on the sample immersed in 10.0 mM solution had more but less evenly dispersed Se than the sample immersed in the 50.0 mM solution. The thicker film formed in the 50.0 mM solution contained both voids or cracks and areas of highly concentrated Se (Figure 9c). These results indicate an intermixing of the Mg and Se through the thickness of the coating.

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The shrinkage cracking phenomenon, discussed above, is revealed more clearly in plan view imaging in Figure 10, which shows coatings formed at each concentration during a 72-hour immersion period. Shrinkage cracking is an artifact of surface film dehydration in the microscope column, porosity is likely not. The EIS spectra shown in Figure 3 indicate that the surface films that form possess defects that lead to the two-time constant response. It should also be noted that shrinkage cracking increases as coating thickness increases. The observation of shrinkage cracking in the microscope column is likely associated with dehydration of the surface film. This in turn suggests that the surface films that are forming are predominantly hydrated gels.

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Plan view EDS mapping of the samples from the 72-hour exposure indicate some lateral chemical heterogeneity in the surface films (Figure 11). This is mainly associated with the presence of Al-rich intermetallic particles. There is an enhanced Al EDS signal after exposure to the 1.0 mM selenite solution, while this may in part be due to X-ray signals from the intermetallic particle under the coating, Figure 10b shows the distinctive morphology of the double layered Mg-Al hydroxcarbonate compound indicating that some aluminum has dissolved from the intermetallic to participate in film formation. Otherwise, the dispersions of Mg, Se and O are uniform across the surface within the detection limits of the EDS technique.

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Figure 12 shows the XPS spectra of samples immersed at OCP for 3 days in a 1.0 mM selenite and 0.1 M chloride solution. Surface films formed under these conditions were compact with very little shrinkage cracking under an applied vacuum.

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As indicated by the Mg_2p spectrum, the corrosion of Mg results in the formation of MgO and Mg(OH)₂ on surface. Mg⁰ arises from the underlying metallic substrate. Selenium is present as reduced Se⁰ and unreduced Se⁴⁺. Substantial fractions of SeO₂ and SeO(OH)₂ (or possibly SeO₂^.H₂O) are detected in the surface film. The ratio of Se to Mg is 1:2.64 on an area fraction basis, indicating that Mg and Mg corrosion products make up a much greater fraction of the surface film than Se and its associated reaction products, consistent with the EDS maps shown in Figure 9.

The results of this study show that selenite is both an anodic and a cathodic inhibitor of the magnesium corrosion process, and that the extent of inhibition in dilute chloride solutions is strongly dependent and inversely related to concentration between 1 and 50 mM.

Both EIS and anodic polarization measurements indicated the use of lower versus higher concentrations of selenite to promote corrosion resistance. Specifically, in EIS experiments, exposure to 0.5 mM and 1.0 mM Na₂SeO₃ yielded similar values of R_tot, which were significantly greater than those obtained at higher concentrations (Figure 3). Likewise, in anodic polarization experiments a 1.0 mM selenite solution produced the lowest corrosion current density, albeit with breakdown potentials that were similar to those observed in both lower and higher concentrations (Figure 5).

Anodic inhibition was concentration-dependent and appeared to depend also on the characteristics of the film, which were themselves concentration-dependent. Exposure of AZ31 at higher concentrations (or for longer periods) resulted in a thicker, but less protective film. Plan view observations of surfaces exposed to 50 mM selenite solution showed defects in the form of MgSeO₃ crystals and small pits that might work antagonistically against the protectiveness of a thicker film. STEM cross sections of coated surfaces showed that films formed in the highest concentration (50.0 mM) were not as compact and may have contained subsurface voids and shrinkage cracks that might have been detrimental to corrosion protection; an idea elaborated on below.

Strong cathodic inhibition was observed in these experiments, with the remarkable feature that the extent of inhibition was independent of selenite concentration in solution (Figure 4); at least at short exposure times. The fact that the pre-exposure period prior to measurement was only 30 minutes is an important condition attached to this result.

The formation of a protective film, presumably the Mg-Al double layered hydroxide on noble Al-Mn particles, (Figure 11) is notable in longer-term exposures. Its presence would suppress hydrogen reduction at these particles, but its existence in the 30-minute exposures was not ascertained.

The cathodic inhibition noted in this work bears similarities to the work of Eaves et al., who studied corrosion inhibition of Mg by arsenate.²² Arsenate and selenite have some similarities in that they both form precipitates in the presence of aqueous Mg²⁺ (Mg₃(AsO₄)₂ and MgSeO₃, respectively) and they both can be reduced in the presence of Mg to form As⁰ and Se⁰. The work of Eaves et al. demonstrated that at low pH, the reduction of arsenate to solid arsenic, rather than precipitated Mg₃(AsO₄)₂, acts to poison hydrogen reduction on iron particles contained in a Mg matrix. Although, in the case of arsenate, the reduction reaction was limited to low pH conditions. In this study, Raman analysis indicated that in the case of selenite, low pH was not required for the reduction of Se⁴⁺ to Se⁰ to proceed.

The XPS results pointed to a mixed Mg-Se oxyhydroxide that is probably a gel-like substance upon initial formation. Raman analyses point to additional complexity and the possibility of structural transformation that may affect corrosion protection over time. The Raman work confirms the presence of Se⁰ in all the films regardless of SeO₃²⁻ concentration in solution. The formation of Se⁰ is expected when SeO₃²⁻ is exposed to a strongly reducing Mg alloy surface. Se has several allotropes and, similar to sulfur, these allotropes are all polyatomic units, making them Raman active. The Raman analysis also confirms that differences in the color of the films, observable in Figure 2, can be attributed to the ratio of a-Se (amorphous) to t-Se (crystallized).

With time or with increased temperatures, thermodynamically unstable brick-red a-Se will naturally convert to the more thermodynamically stable dark gray or black t-Se.⁴⁴ These properties agree with the visual results in Figure 2 and the Raman spectral results shown in Figure 7. Furthermore, the visual data agree with the Raman results which indicate that samples immersed in higher concentrations formed films that contained a higher percentage of t-Se (231 cm⁻¹ peak) versus a-Se (251 cm⁻¹ peak). Both visual inspection and Raman analysis indicate that the films formed at higher concentrations contained more of both allotropes of Se. However, they were darker due to a higher percentage of the dark gray t-Se.

Lindberg et al. studied the transformation process of t-Se (deposited on an Si substrate) to t-Se using Raman mapping and atomic force microscopy.⁴⁵ It was found that the transformation to t-Se will distort the surrounding a-Se, leaving gaps and cracks in the film. This effect, if it occurs in the mixed Mg-Se film that are indicated by XPS, may explain the decrease in corrosion protection observed for films formed in the 50 mM selenite solutions.

Although pH might have an effect on the ability of selenite to inhibit corrosion, in this work the bulk pH remained relatively stable between and 8 and 9 throughout all the experiments. This stability might be attributed to either the small surface to volume ratio in these experiments or suppression of water reduction, though the results of control experiments of AZ31 immersed in 0.1 M NaCl (without selenite inhibitor) seem to favor suppression of water reduction. In the control experiments, the pH of the solution climbed from 5.8 to 6.5 after one hour and it rose to 7.9 after 24 hours. Therefore, the apparent lack of pH increase observed in the SeO₃²⁻ plus 0.1 M NaCl solutions seems to point to suppression of hydrogen reduction.

When visually comparing the films formed under the two highest concentrations, the somewhat lighter appearance of the film formed in 50.0 mM solution versus 10.0 mM solution (Figure 2) can be attributed to the presence of precipitated colorless crystalline MgSeO₃ that became trapped in the film. Raman indicated the presence of MgSeO₃ in the film formed in the 50.0 mM solution only, which did not protect as well as the films formed at lower concentrations, however, the presence of MgSeO₃ should not be ruled out as a corrosion inhibitor. It merits further study as it may have properties similar to the protective Mg₃(AsO₄)₂ films reported by Eaves et al.²²