Effect of K2HPO4 Concentration on the Formation, Structure, Composition and Protectiveness of Conversion Coating Deposited on AZ31 Magnesium Alloy

Abstract

In deaerated 0.05 M, 0.1 M, 0.2 M and 0.5 M K₂HPO₄ solutions with pH 9.5, AZ31 magnesium (Mg) alloy was subjected to potentiostatic polarization at − 0.8 V_SCE to deposit a phosphate conversion coating. The morphology, structure, chemical composition, and protective performance of the conversion coating during the formation process were characterized. The results showed that amorphous Mg(OH)₂ and MgHPO₄ and crystallized KMgPO₄·6H₂O (struvite–K) were successively deposited on the surface of AZ31 Mg alloy in the four solutions. MgHPO₄ was converted from Mg(OH)₂, while struvite–K was transformed from MgHPO₄. The distribution of Mg(OH)₂, MgHPO₄ and struvite–K along the thickness of the coating differs with K₂HPO₄ concentration. As the concentration of K₂HPO₄ increased, the coating time was gradually shortened, and the coating was gradually thinned. Meanwhile, the ratio of the nucleation rate to the growth rate of struvite–K crystal nuclei increased, resulting in a decrease in the size of struvite–K crystal. When the concentration of K₂HPO₄ increased from 0.05 to 0.1 M, the content of struvite–K increased, and the protective performance of the coating was enhanced. However, as the concentration of K₂HPO₄ continued to increase to 0.5 M, the content of struvite–K and the protective performance of the coating decreased.