Properties of oxide-silane composite coating on AZ31 magnesium alloy

Various oxide films have been grown on AZ31 magnesium alloy by microarc oxidation in alkaline phosphate solution with addition of different compounds (Na2SiO3, NaAlO2). Because of the anodic oxide porosity, arising from the sparking treatment, organo-functional silane coatings have been applied to seal pores and cracks and achieve a more effective protective system. The surface and cross-section morphologies of the anodic films were analysed by scanning electron microscopy. The phase composition was determined by X-ray diffraction. Bare and anodised AZ31 specimens were post-treated with silanes, namely N-phenil-3-aminopropyltrimethoxysilane and Bis (trimethoxysilylethyl) benzene. The corrosion resistance of anodic oxides and oxide-silane composite coatings was evaluated in dilute Harrison solution by means of single-cycle anodic polarisation. The results showed that the addition of different salts to the alkaline phosphate solution affects the surface morphology and the phase composition of the coatings and, as a consequence, the corrosion resistance behaviour. Both silane post-treatments provide further corrosion protection to samples anodised in aluminate containing solution.