Corrosion behaviour of laser-melted magnesium alloys

The present paper reports studies of the corrosion behaviour of magnesium alloys, AZ31, AZ61 and WE43 following laser surface melting (LSM) with a 2 kW continuous wave CO2 laser to depth about 1 mm. The laser-melted samples were immersed in 5 wt.% sodium chloride solution of pH 10.5 for 10 days, with open circuit potential measurements recorded in the initial 24 h. LSM resulted in improvement of the corrosion resistance of the alloys, with weight losses reduced by about 30, 66 and 87% for the AZ31, AZ61 and WE43 alloys, respectively. Rutherford backscattering spectroscopy (RBS) and nuclear reaction analysis (NRA) revealed that the average compositions of the alloys, following removal of surface oxide, were relatively unaffected by laser melting, indicating that any losses of magnesium or gains of oxygen only affected superficial layers. The improved corrosion resistance following LSM is associated with refinement of the alloy microstructure due to the rapid cooling of the melted layer, which results in increased concentration of alloying elements in solid solution in matrix regions and more uniform distributions of the corrosion-resistant β-phase that can accumulate as a protective layer.