Designing for the chemical conversion coating with high corrosion resistance and low electrical contact resistance on AZ91D magnesium alloy

The designing idea for the chemical conversion coating with high corrosion resistance (CR) and low electrical contact resistance (ECR) on AZ91D Mg alloy was presented. The main characteristics of the conversion bath include acidic solution, strong oxidizing agent, proper pH value and cation concentration (determined by thermodynamic calculation). The desired performances are attributed to the unique microstructure of conversion coating, i.e., β phase protruded from the surface and covered by a thinner passive film about 30 nm, contributing to low ECR, meanwhile, a thicker coating of 200-300 nm without cracks was deposited on α-phase, providing a good corrosion resistance.