The authors review their recent research work in an endeavour to interpret the influence of native oxide films on the corrosion behaviour of commercial AZ31 and AZ61 magnesium alloys and the oxidation kinetics of these alloys in air at 200ºC. The tendency of some of these thin films to be sufficiently protective in mild or weak corrosive environments was examined. For obtaining oxide films with different protective properties, some of the specimens were tested with the surface in the as-received condition, while others were tested immediately after mechanical polishing. The techniques applied to characterise thin oxide films of a thickness of just a few nanometres present on the surface of alloys has mainly been XPS (X-ray photoelectron spectroscopy) in combination with ion sputtering. Oxidation resistance of the alloys was quantified by thermogravimetric (TG) curves. Their corrosion rate was evaluated by Electrochemical Impedance Spectroscopy (EIS) and hydrogen evolution measurement in chloride solutions with different concentrations. Emphasis was placed on the possible effects of: (a) the different thickness of the native oxide films formed on the polished surfaces on the corrosion behaviour of the alloys; and (b) the different film homogeneity and uniformity on the oxidation results [1-5].

In parallel studies, an attempt was made to learn more about the influence of the native oxide films that covers the substrate on the subsequent growth and protective behaviour of the sol-gel coatings. In this way, the influence of heat treatment of magnesium alloy substrates on corrosion resistance of a sol-gel coating has been assessed during immersion tests in 0.6M NaCl aqueous solution. Relative differences in the chemical nature of the layers were quantified by scanning electron microscopy (SEM) and energy dispersive analysis of X-ray (EDX). Long-term immersion testing showed that the sol-gel/heat treated AZ61 substrate exhibited a superior anti-corrosion property in comparison with the sol-gel/non-heated substrate. In contrast, no significant changes have been observed between the heated and non-heated samples in the case of the sol-gel coated AZ31 substrates. A link was found between lower O/Si atomic ratios observed by EDX analysis on the sol-gel coatings after the preparation process and reduced corrosion upon the coated substrates. The thermal treatment increases the protective properties of the passive film on the surface of the AZ61 substrate and hence inhibited magnesium dissolution and hydrophilic groups formation during coating preparation [5-7].

Finally, some preliminary results are shown on our studies in progress about the modification of these sol-gel coatings with different inhibitor (L-cysteine, benzotriazole and 8-hydroxyquinoline) in an attempt to improve the corrosion performance of the AZ31 and AZ61 magnesium alloys.

Acknowledges:

This work was supported by the Ministry of Economy and Competitiveness of Spain (Project MAT2015-65445-C2-1-R).

References

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