The effect of zirconium grain refinement on the corrosion behaviour of magnesium-rare earth alloy MEZ

Corrosion performance of sand cast magnesium alloy MEZ was investigated for unrefined (MEZ_U) and Zr-grain-refined (MEZ_R) microstructures in 5% NaCl solution using salt spray, immersion, in situ examination of the corrosion morphology, ESEM, electron probe microanalysis, hydrogen evolution and polarisation curves. MEZ_U demonstrated higher rates of anodic dissolution and cathodic hydrogen evolution than MEZ_R. The central zirconium-rich areas within the grains of the MEZ_R microstructure was more corrosion resistant than the outer zirconium-depleted areas of the grains, whereas MEZ_U showed little difference in corrosion between the centre and the edge of the grains. Based on the analyses of the corrosion process, polarisation behaviour and the microstructure of these two alloys, it is postulated that zirconium acts in a number of ways to improve corrosion resistance. Zirconium stabilises the solid solution and makes it inactive in anodic dissolution, significantly passivates the precipitated particles reducing cathodic hydrogen evolution and increases the barrier effect of the grain boundary phase through a finer grain size and thus a more continuous layer of the grain boundary phase.