Barium phosphate conversion coating on die-cast AZ91D magnesium alloy
(1. State Key Laboratory for Corrosion and Protection, Institute of Metal Research,Chinese Academy of Sciences, Shenyang 110016, China;2. School of Materials and Metallurgy, Northeastern University, Shenyang 110016, China)
Abstract: Poor corrosion resistance limits the application of magnesium alloys. Conversion coating is widely used to protect magnesium alloys because of easy operation and low cost. A novel conversion coating on die-cast AZ91D magnesium alloy containing barium salts was studied. The optimum concentrations of Ba(NO3)2, Mn(NO3)2 and NH4H2PO4 are 25 g/L, 15 mL/L and 20 g/L, respectively, based on orthogonal test results. The treating time, solution temperature and pH value are settled to be 5-30 min, 50-70 ℃ and 2.35-3.0, respectively. The corrosion resistance of barium conversion coating is better than that of manganese-based phosphate conversion coating by immersion test. The coating is composed of Ba, P, O, Mg, Zn, Mn and Al by EDX analysis.
Key words: barium phosphate conversion coating; die-cast AZ91D magnesium alloy; micro-cracks; corrosion resistance