Formation behavior of phosphoric acid-based chemical conversion coating containing alkaline earth metals on magnesium alloy

The phosphate coating for Mg alloy was performed in the unagitated solutions containing 1 g·dm^-3 of Mg, Ca, Sr and Ba as alkali-earth metal and 50 g·dm^-3 of H₃PO₄ of pH 3 and 40°C for 3 min, and the structure and formation behavior of the phosphate films were investigated. The X-ray diffraction patterns of phosphate films broadened regardless of the kinds of alkali-earth metal, which was a characteristic of an amorphous structure. XPS analysis revealed that the valence of P in the phosphate films was pentavalent. The pH in the vicinity of the Mg alloy during the phosphate coating, as measured using an Sb microelectrode, was approximately 10.4, 11.4, 10.7, 12.0 in the solutions containing Mg, Ca, Sr and Ba, respectively, which shows that the phosphoric acid dissociates to the third stage during the phosphate coating and the phosphate films composed of M₃(PO₄)₂ containing alkali-earth metal (M) are formed. The phosphate coating were successively performed in the two kinds of solutions containing different alkali-earth metals and the cross section was analyzed, as a result, it was suggested that the new phosphate films were formed at the interface between Mg alloy substrate and phosphate films and the previously formed phosphate films were boosted up in films.