Corrosion protection of Mg–11Li–3Al–0·5RE alloy using hybrid epoxy/silica conversion coatings

Abstract

In order to obtain effective corrosion protection for Mg–11Li–3Al–0·5 rare earth alloy, a combined epoxy/SiO₂ hybrid conversion coating was investigated in this paper. Initially ceria or phytic acid conversion coatings were applied on the substrate, then an epoxy/SiO₂ hybrid coating was applied on top of the conversion coating. Potentiodynamic polarisation curves were used to evaluate the effects of the silica content on corrosion resistance, and an optimum was confirmed at 3 wt-%. Hydrogen evolution and salt spray testing were employed to compare the corrosion resistance of the ceria and phytic acid based hybrid coatings and results indicated that the phytic acid hybrid coating was more effective than the ceria based coating. This was ascribed to reaction between the phytic acid functional groups (hydroxyl and phosphate) with the epoxy/SiO₂ hybrid coatings increasing adhesion between the coatings. The corrosion of the phytic acid based hybrid coating was investigated using electrochemical impedance spectroscopy which confirmed that a diffusion mechanism could explain the overall corrosion process and the advantages of the phytic acid hybrid coating.