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