Effect of heat treatment on the mechanical and biocorrosion behaviour of two Mg-Zn-Ca alloys

The effect of heat treatment on the mechanical and biocorrosion behaviour of the Mg-1 wt.% Zn-1 wt.% Ca (ZX11) and Mg-3 wt.% Zn-0.4 wt.% Ca (ZX30) alloys was evaluated. For this purpose, three-point bending tests as well as electrochemical and immersion tests in Hank's solution were performed on both alloys in four different thermal conditions: as-cast, solution-treated, peak-aged and over-aged. Microstructural examinations revealed that the as-cast ZX11 and ZX30 alloys exhibit a microstructure composed of α-Mg grains separated by large Mg2Ca and Ca2Mg6Zn3 particles and by large Ca2Mg6Zn3 particles, respectively. During solution treatment, the Ca2Mg6Zn3 precipitates at the grain boundaries (GBs) are fully dissolved in the ZX11 alloy, but mainly redistributed to form a more connected configuration in the ZX30 alloy, showing a poor age-hardening response. Consequently, after solution-treatment, galvanic corrosion and corrosion rate decreases in the former, but increases in the latter. The peak-aged condition displays the highest corrosion rate for both alloys, maybe due to an excessive number density of fine Ca2Mg6Zn3 particles acting as cathodic sites. However, the over-aged condition shows the lowest corrosion rate for the ZX11 alloy and a very similar one to that of the as-cast sample for the ZX30 alloy. The ZX11 alloy shows generally better biocorrosion behaviour than the ZX30 alloy to its lower content in the Ca2Mg6Zn3 phase and thus reduced galvanic corrosion. The Mg2Ca phase present in the studied ZX11 alloy has been proved to exhibit an increased corrosion potential, which has been related to an observed enrichment with Zn.