Review of electrochemical properties of hybrid coating systems on Mg with plasma electrolytic oxidation process as pretreatment

Fabricating adhesive coatings such as organic coatings with adequate corrosion protection has been a concern due to the presence of oxide-hydroxide-carbonate deposits on the Mg alloys surface. Although plasma electrolytic oxidation (PEO) can solve the problems of insufficient adhesion of organic coatings by creating a hard relatively thick coating with a high adhesion strength, but the pores and microcracks in the PEO coating structure reduce its corrosion resistance. Today, PEO-based coating systems are utilized to provide a proper corrosion protection on the surface of Mg alloys. PEO coatings have been used is several works for the surface preparation of top layer such as organic coatings, sol-gel and silane layers, conversion coatings, electrophoretic and electroplating coatings. Moreover, mentioned topcoats are used to seal PEO coatings in order to improve the corrosion resistance. In this review paper, we review different types of PEO-based complex coating systems for improving protective properties of Mg alloys and the mechanism of increasing corrosion resistance.