Effect of annealing temperature on the corrosion resistance of MgO coatings on Mg alloy

Abstract

Abstract MgO coatings were electrodeposited on Mg alloy substrates via cathodic electrolytic technique and annealed at different temperatures. The protective performance of these annealed coatings was investigated, and a correlation drawn between annealing temperature of coatings and their corrosion resistances as well as mechanical hardness. At optimum temperature (250\u202f°C), the thermal treatment relieved internal tensile stress leading to crystallization of oxide coating layers with compact and continuous multilayered microstructures. At higher temperatures (around 500\u202f°C), phase transformations within the coating microstructure broadened inherent inter-diffusion pathways/microdefects. Surface and bulk evidences of pores and cracks are observed at 500\u202f°C as clear evidences of the consequence of over-calcination using scanning electron microscopy. Enhanced corrosion resistance for coatings are achieved at optimum curing temperature judging from electrochemical corrosion results after chloride-induced corrosion tests. The dependence of corrosion resistance on annealing temperature of coatings has also been investigated by potentiodynamic polarization and electrochemical impedance spectroscopic techniques. A modelled curing mechanism, as it relates to barrier performance at low and high thermal conditions, has been illustrated from experimental evidences.