Coating pretreatment for Mg alloy AZ91D

Coating defects and coating failure upon Mg alloy AZ91D are usually due to non-uniform coating growth, which is intrinsically caused by the microstructural heterogeneity of the Mg substrate. This is the case for both conversion coatings and electroplated (incl. electroless) coatings. In the present study, coating pretreatment for the purpose of tailoring surface chemistry on AZ91D was studied. Pretreatments to control β-phase morphology and surface homogeneity include alkaline conditioning and acid activation in four different non-toxic (F-free) acid solutions: H3PO4, HNO3, HCl and C6H8O7. The results show that careful pretreatment can do all of the following: modify surface roughness; remove β-phase; modify solid solution chemistry; and most importantly, tailor the anodic/cathodic character of the surface. This allows surface pretreatment to be customized to optimize the subsequent coating. The influence of different acids was shown to be marked.