New design principles for the bath towards chromate- and crack-free conversion coatings on magnesium alloys

Abstract Cracking and low thickness are major obstacles to the high corrosion performance of conversion coating on magnesium alloy. In this work, the ratio of total acidity to pH (TA/pH) was applied as an indicator, and new principles regarding the design of conversion bath were proposed. The treatment bath should be composed of species that can be categorized into two groups: the first group of species that react with Mg substrate to increase the local pH at the interface; the second group that precipitate and contributes to the growth of coating. The species belong to second group exists in a supersaturated state and its precipitation process is almost independent on the reactions of the species in first group. By this way, a thick and crack-free two-layered conversion coating is obtained. Moreover, the nature of the adjustment of TA/pH and the roles of the oxidizing agent and catalyst were discussed.