Effects of second phases on the formation mechanism and corrosion resistance of phosphate conversion film on AZ80 Mg alloy

Purpose The formation and corrosion processes of a conversion film on the AZ80 Mg alloy with different second phases were compared to clarify the effect of microstructure on the quality of protective coatings. Design/methodology/approach The size and distribution of second phases in the edge and central regions of the AZ80 cast ingot exhibit a great difference. The film growth processes and their corrosion resistance on the edge and central regions of the AZ80 cast ingot were investigated by scanning electron microscope observations, immersion tests and electrochemical measurements. Findings The results indicate that second phases act as micro-cathodes and hydrogen evolution reaction occurs on their surface, which is not beneficial for the deposition of the conversion film. Originality/value The conversion film formed on the central regions of AZ80 cast ingot with a low volume fraction of second phases exhibits a more uniform surface and higher corrosion resistance than that formed on the edge regions of the sample with a higher volume fraction of second phases.