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

The phosphating of Mg alloy was performed in unagitated solutions containing 1 g·dm^-3 of Mg, Ca, Sr, and Ba as alkaline earth metals (M) and 50 g·dm^{-3 of} H₃PO₄ of pH 3 at 40°C for 3 min. The structure and formation behavior of the phosphate films were investigated. The X-ray diffraction patterns of phosphate films broadened irrespective of the type of alkaline 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 phosphating, as measured using an Sb microelectrode, was approximately 10.4, 11.4, 10.7, and 12.0 in the solutions containing Mg, Ca, Sr, and Ba, respectively. This shows that phosphoric acid dissociates to the third stage during the phosphating, and the phosphate films comprising M₃(PO₄)₂ containing an alkaline earth metal are formed. The phosphating was successively performed in two types of solutions containing different alkaline earth metals, and the cross section was analyzed. As a result, it was determined that the new phosphate films were formed at the interface between the Mg alloy substrate and phosphate films, and the previously formed phosphate films were boosted in the films.