Achieving a corrosion-resistant Mg-Ca lean alloy by solidification control to sequester parts-per-million-level Fe impurity

The influence of solidification cooling rate on the microstructure, particularly Fe-rich particles, in a Mg-0.1Ca (wt%) alloy is studied, as well as the corresponding effect on the corrosion resistance. It is revealed that a slow cooling rate (<5 K s −1 ) is required for Mg–Ca lean alloys to achieve high corrosion resistance, with corrosion rates <0.2 mm y −1 (in 3.5 wt% NaCl solution). With a low cooling rate, Fe-rich particles with a few hundred nanometers in size are found but largely wrapped by CaMgSi intermetallics at the micrometer scale introduced by Ca addition. Therefore, Fe is sequestered from the matrix, which suppresses micro-galvanic corrosion and cathodic hydrogen evolution kinetics, contributing to the improved corrosion resistance by three orders of magnitude in comparison to that of a high-purity Mg (~20 ppm Fe). However, high cooling rates (like 260 K s −1 ) result in obviously decreased size of CaMgSi, leaving more Fe-rich particles incompletely wrapped or even fully isolated, which promote cathodic hydrogen-evolution kinetics and intensify micro-galvanic corrosion. Thus, the Mg-0.1Ca alloy shows decreased corrosion resistance when the cooling rate increases. This work demonstrates the significance of Ca micro-alloying and solidification control for developing corrosion-resistant Mg by eliminating the detrimental effect associated with parts-per-million-level Fe impurity.