Improved corrosion resistance of Mg alloy by a green phosphating: insights into pre-activation, temperature, and growth mechanism

Abstract

Poor corrosion resistance of magnesium alloys remains a major obstacle to their extensive application. Phosphate conversion coating (PCC) is one of the most direct and effective strategies to enhance the corrosion resistance of magnesium alloys. To overcome the environmental damage of traditional phosphating technology, a PCC free of fluorine, chromium and nitrite was prepared in the present work. The effects of surface pre-activation process and preparation temperature on the surface morphology and corrosion resistance of PCC on ZK60 magnesium alloy were investigated. Surface pre-activation could significantly reduce the ultimate grain size of phosphate. At 90 °C, the prepared PCC showed perfect morphology and corrosion resistance. To better understand the phosphating nucleation and growth process, the surface and cross-sectional morphologies of PCC prepared for different times were observed. The phase composition of the prepared PCC was detected to be Hureaulite (Mn5(PO4)2(PO3OH)2·4H2O) and the coating growth mechanism was suggested in the end.