Effect of Na2MoO4 on bond strength of adhesive-bonded phosphate coated magnesium AZ31 sheets

A pretreat phosphate solution having the formulation of KMnO4, K2HPO4, Na2SiO3, NaF and Na2MoO4 was developed to improve the strength of adhesive-bonded magnesium AZ31 joints. The phosphate coating which has the magnesium phosphate, MgO, Mg (OH)2, MgF2 and minor Al2O3,, Al(OH)3 Al0.35–0.55Si0.10–0.48P0.13–0.35O2.1–2.2 and, Al0.35Si0.48P0.18O2.2, and Al0.52P0.48O2.2 was formed on the surface of the magnesium AZ31 after treatment. The additives Na2MoO4 in a phosphate-permanganate solution mainly act as an accelerator to speed up the coating formation. The combination of Na2MoO4 and NaF improves significantly the corrosion resistance of the magnesium AZ31 and performance of adhesive bonded joints. The joint strength and durability of the bonded Mg AZ31 joints in hot-humid environmental condition are superior to that of the phosphate treated (RPT) and CrO3 etching (CET) pretreated samples.