Understanding The Corrosion Response of Biodegradable Magnesium Alloys Through Microstructure Design

Abstract

Widespread use of Mg-based alloys as temporary biomedical implants remains a scientific and technological challenge because of their rapid corrosion kinetics that leads to premature loss of mechanical integrity. Several methods have been explored to control the degradation rate of Mg alloys mainly through tailoring their microstructure or adding alloying elements. However, literature still shows contradictory results on the effect of microstructural features such as grain size, texture and secondary phases on the mechanical and corrosion response of Mg alloys. Therefore, this work aims to provide a deeper understanding of the effect of microstructural features on the in vitro corrosion behavior and mechanical integrity of Mg alloys. Three commercially available Mg-Zn-Zr alloys, namely ZK40, ZE41 and EZ33 were investigated, in addition to high purity (HP) Mg. HP Mg was subjected to three different routes of equal channel angular processing to develop unique textures with similar grain sizes. Results revealed that grain refinement enhanced corrosion resistance due to the development of a stable surface film. Furthermore, a dominant non-basal texture coupled with a wide range of angle misorientations exhibited the most improved corrosion resistance. Post-corrosion surface morphologies indicated the presence of filiform corrosion which was linked to micro-galvanic coupling occurring between basal and non-basal grains. Finally, subsequent annealing resulted in a decrease in residual strain and uniform distribution of grain boundary misorientations which improved the film’s ability to regenerate. The study of the corrosion behavior of ZE41 and EZ33 under physiological conditions revealed that although both alloys showed similar microstructure, size and distribution of precipitates played a significant role on its corrosion response. An increase in RE concentration in EZ33, relative to ZE41, had a positive effect on corrosion rate that subsequently controlled alloy mechanical integrity and biocompatibility. Finally, two alloys were processed (ZK40 and EZ33) to assess the combined influence of altering the grain size in the presence of secondary phases on the corrosion and mechanical properties of the base alloys. It was concluded that heterogeneity in grain size and precipitate distribution deteriorates the corrosion response of Mg alloys, counteracting the corrosion enhancement caused by grain refinement.