Influence of surface polishing on the degradation behavior of biodegradable Magnesium alloy

Mechanical polishing can significantly improve the surface integrity of Mg alloys to reduce the corrosion rate (CR) by minimizing the galvanic corrosion sites. In this study, an emery-polishing attachment on a lathe machine has been used for surface polishing of as-turned cylindrical ZM21 Mg alloy. Using Taguchi's L9 orthogonal array, three process variables with three levels were investigated and optimized for minimum surface roughness (Ra and Rz values) of Mg-alloy samples. The minimum surface roughness values of Ra; 195 nm and Rz; 1481 nm were obtained corresponding to emery paper (EP) grade; 2000, rotational speed; 250 rpm, and polishing time; 2 min. An in-vitro immersion study was conducted in simulated body fluid (SBF) for up to 28 days to investigate the degradation behavior of as-turned and as-polished Mg alloy samples. In-vitro study showed that the as-turned samples (surface roughness; Ra: 640 nm, Rz: 4640 nm) had a higher degradation rate (CR: 3.9 mm year-1 after 28 days) with a higher volume of H2 evolution rate, whereas the polished sample with the minimum surface roughness (Ra: 195 nm, Rz: 1481 nm) exhibited lower weight loss, H2 release rate, degradation rate (CR: 0.95 mm year-1 after 28 days) and minimum loss of mechanical strength. Based on the present study, polishing is recommended as a secondary operation after machining of Mg alloy to reduce the corrosion rate for biodegradable implant applications.