Influence of substrate composition on corrosion protection of sol–gel thin films on magnesium alloys in 0.6M NaCl aqueous solution

Abstract

The corrosion protection behaviour of organic–inorganic hybrid thin films on AZ31 and AZ61 magnesium alloy substrates has been studied. These films were prepared by a sol–gel dip-coating method. The organopolysiloxane precursors were γ-methacryloxypropyltrimethoxysilane (MAPTMS) and tetramethoxysilane (TMOS). An attempt was made to determine the possible relationships between the degradation of the sol–gel film and composition of the metal substrate during the exposure of the metal/coating system to 0.6 M NaCl aqueous solutions. For this purpose electrochemical impedance spectroscopy (EIS) and hydrogen evolution measurements were applied. Scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) analyses revealed that the sol–gel films formed on the surface of AZ61 alloy were far more perfect and uniform than those formed on the AZ31 alloy. This behaviour was attributed to the effect of the native oxide film initially present on the surface of the AZ61 alloy, which inhibited the attack of magnesium. Results indicated that the sol–gel coated AZ61 substrate tended to develop corrosion products slower than the sol–gel coated AZ31 substrate, trend that could change by prolonging exposure time. After 6 days of immersion, a clear inhibitive effect of the corrosion products formed during the test was observed in the case of the sol–gel coated AZ31, but not with the coated AZ61 alloy substrate, a phenomenon explained by the carbonate enrichment observed by XPS.