Formation Behavior and Structure of Chemical Conversion Coating of Alkaline Earth Metals on Magnesium Alloy

Formation behavior, structure, and corrosion resistance of chemical conversion coatings containing alkaline earth metals of various kinds formed on the AZ91D Mg alloy were investigated. The pH near the AZ91D surface was measured using a Sb microelectrode probe, which revealed that the pH increased from 3.0 to 10.5 during the chemical conversion reaction. The XRD results indicated that the precipitates obtained from the chemical conversion baths adjusted to pH 10.5 containing Ca, Sr, Ba, and Mg ions respectively corresponded to apatite, apatite-Sr, Ba3(PO4)2, and amorphous. Surface and cross-sectional SEM observations indicated that the chemical conversion coating containing Mg was the thickest among all the coatings, and showed many penetrative cracks reaching the substrate. In contrast, the coatings respectively containing Ca, Sr, and Ba were less defective and showed only small cracks. The salt spray test showed that the coatings respectively containing Ca, Sr, and Ba showed excellent corrosion resistance.