Influence of Al Content on Corrosion Resistance of Mg-Al-Zn Alloys in Chloride Environments

Influence of Al content on corrosion resistance of Mg-Al-Zn alloys containing 1 to 9 mass%Al was investigated in chloride environments. The corrosion rate decreased with increasing the Al content in the alloys regardless of the testing methods such as salt immersion test (SIT) and salt spray test (SST). The immersion tests in buffered solutions at constant pH also clarified that the effect of the Al content on the corrosion resistance is largest around pH 10, and diminished when pH is raised up to 12, where Mg-Al-Zn alloys are passivated by the formation of Mg-hydroxide on the surface. XPS analysis revealed that Al was concentrated in the surface film, especially around pH 10, and its concentration was increased with the Al content in the alloys. The high corrosion resistance of Mg-Al-Zn alloys with high Al can be attributed to the formation of the Al-rich surface film in chloride environments.