Mg-RE alloy Electron717 (ZE10A, UNS M11600) has a nominal composition of 1.2wt % Zn, 0.25wt% Zr, <0.5%Zr, 0.007wt%Mn, 0.004wt %Fe,0.001wt%Ni and balanced Mg. Zirconium addition acts as a grain refiner, so it helps increase castability, mechanical properties and provide better corrosion resistance.¹ ZE10A alloy may have better corrosion resistance than unalloyed Mg. Even though formation of Zn₂Zr₃ type precipitates has been suggested in this type of alloy, heat treatment in the temperature range of 200 – 300 ºC for different time periods did not show significant increase in the hardness. Therefore, this alloy is considered not age hardenable. Brady et.al showed that ZE10A alloy forms a dual layered passive film with inner layer rich in MgO and outer layer of Mg(OH)₂.² The formation of two layers make it more resistant to corrosion. This presentation will focus on the corrosion behavior of the ZE10A alloy as a function of grain size, and the hydrogen evolution behavior in different electrolytes.

The alloy was investigated in three different heat-treated condition such as (1) As received (grain size of 7.90 µm), (2) Solution treated at 525^oC for 8 hours (grain size of 26.24 µm) and (3) Solution treated at 525^oC for 24 hours (grain size of 41.37 µm). Anodic and cathodic polarizations were carried out in three different electrolytes (pH unadjusted) such as:

1) 0.6M NaCl

2) 0.5M NaNO₃

3) 0.1 M Na₂SO₄

Table 1.0 Summary of anodic and cathodic polarization results of ZE10A alloy in different heat-treated conditions and in three different solutions.

Table 1 summarizes the polarization results. The polarization resistance decreased with increase in the grain size in all the electrolytes, in general. The corrosion current density increased with the grain size. Higher corrosion current densities were observed in the Na₂SO₄ solution than that in the other two electrolytes. The anodic Tafel slopes are found to be higher in the Na₂SO₄ solution than in NaCl and NaNO_3, whereas, cathodic Tafel slopes did not reveal a clear trend of variation either with the grain size or type of electrolyte. The presentation will focus on how the microstructures in terms of grain size, and secondary phase distribution influence the corrosion behavior of the ZE10A alloy based on the experimental results and literature review.

[1] N.T. Kirkland et.al JOM, 2011, 63(6)28-40

[2] M.P.Brady et. al Scripta Met. 106(2015) 38-41