Influence of Ca-addition amount on corrosion behavior of Ca-added Mg alloys

In recent years, Ca-added Mg alloys has been developed to improve the flame-resistance. Thus Ca-added Mg alloys is expected to be used as the component materials for transport industry and electronic devices because of their safety and lightweight. However, it has been revealed that the corrosion resistance was lowered if Ca was added to Mg alloys. In this study, we aimed to investigate the corrosion behavior of Ca-added Mg alloys in NaCl aqueous solution. FE-SEM and XRD measurements demonstrated that Al-Mn compound and MnAl₂O₄ were precipitated in AZ61 (0 wt% Ca) and Al₂Ca phase was present in AZCa611 (1 wt% Ca) and AZCa612 (2 wt% Ca). And the existence amount of Al₂Ca phase in the Ca-added Mg alloys increased with an increase in Ca-addition amount in the AZ61 Mg alloy substrate. It is known that Mg₁₇Al₁₂ (β phase) is present in AZ61, AZCa611, AZCa612. KFM measurements revealed that the surface potential difference between Al₂Ca and α-Mg was larger than that between Al-Mg compound, MnAl₂O₄, or Mg₁₇Al₁₂ (β phase) and α-Mg. The topographic images of the samples after the immersion tests in NaCl aqueous solution demonstrated that Mg(OH)₂ was deposited on each precipitate. Therefore, each precipitate acts as a cathode site of local cells. Thus Al₂Ca could be one of the causes lowering the corrosion resistance of Ca-added magnesium alloys.