Effect of Al and Ca Elements on the Atmospheric Corrosion of Field-Exposed Mg–Al–Zn–Ca–Mn Magnesium Alloys

Three different extruded Mg–Al–Zn–Ca–Mn magnesium alloys with varying alloying elements and a diecast AZ91D alloy were exposed to two coastal environments for five years. Under unsheltered exposure, Mg–Al type layered double hydroxide was primarily formed, and the mass loss corrosion rate was decreased with increasing Al content in the alloys. Under sheltered exposure, magnesium hydroxy carbonate was the main corrosion product. Additionally, Mg alloys with high Al content tended to exhibit localized corrosion. Furthermore, the negative impact of Ca addition on atmospheric corrosion occurred when the alloys were exposed under sheltered conditions.