Micro-alloyed Mg-Ca: Corrosion susceptibility to heating history and a plain problem-solving approach

The exceptionally low corrosion rate (∼0.1 mm y–1 in concentrated NaCl solution for 7 days) enables lean Mg-Ca alloys great potential for diverse applications, particularly if relevant properties (e.g. mechanical strength, electrochemical performance, etc.) can be enhanced by thermomechanical processing. However, herein it is demonstrated that the corrosion performance of lean Mg-Ca is susceptible to the heating process. The corrosion rate of Mg-0.15 wt% Ca alloy is remarkably accelerated after annealing even for a short time (4 h at 400 °C) because Fe precipitation readily takes place. Fortunately, it is found that micro-alloying with dedicated additional elements is able to solve this problem. Nevertheless, the problem-solving capability is dependent on the element category, particularly the ability of the alloying element to constrain the Fe precipitation. Among the three studied elements (i.e. Sn, Ge and In), only In shows good competence of restricting the formation of Fe-containing precipitates, thereby contributing to retention of the superior corrosion resistance after annealing even at a rigorous condition (24 h at 450 °C). The finding creates good foundation for follow-up work of developing lean Mg-Ca-based alloys combining high corrosion resistance, superior electrochemical performance with excellent mechanical properties for applications as biodegradable implants and anode materials for aqueous batteries.