Growth of multi-elements complex coating on AZ91D magnesium alloy through conversion treatment

The growth process of a new chromium-free multi-elements complex coating (MECC) on AZ91D magnesium alloy has been studied. The morphology and phase composition of the coatings have been determined with SEM, EDX, XRD and AFM. In addition, the thickness change of MECC layer and weight change of the specimen have been measured. On the basis of these experimental results, the coating formation is divided into initial stage, metaphase stage and late stage. At the initial stage, the compact coating is an amorphous material composed of O, Mg, Al and P. Meanwhile, coating growth is fast and mass-gain in unit area is negative owing to severe corrosion. At the metaphase stage, the coating layer consists of amorphous material of O, Mg, Al, P, Mn, and crystalline Mn5.65P3, (Mg0.66Al0.34) (Al0.83Mg0.17)2O4, and the mass-gain in unit area decreases and coating quality degenerates. At the late stage, with the quantity increase of crystalline Mn5.65P3, (Mg0.66Al0.34) (Al0.83Mg0.17)2O4, the coating surface becomes smooth and compact, and the coating grows into steady condition.