Effect of Grain Size on the Corrosion Resistance of Friction Stir Welded Mg Alloy AZ31B Joints

The objective of this study was to clarify the effect of grain size on the corrosion rate across friction stir welded AZ31B joints when exposed in aqueous NaCl solutions. Grain size variations were achieved by preparing welded joints using AZ31B in different tempers. Single potentiodynamic polarization measurements made on the isolated base metal and stir zone in 0.6 M NaCl solution revealed no significant difference between the anodic kinetics, but revealed an apparent difference between the cathodic kinetics. The attempt to correlate extracted corrosion current density (icorr) values to grain size (d−0.5) revealed that the slope coefficient was insignificantly different from zero, meaning that no meaningful correlation exists (albeit during the corrosion propagation stage). Scanning vibrating electrode technique (SVET) measurements across an intact AZ31B-O welded joint revealed that grain size had a more complex effect on the anodic and cathodic kinetics of the filiform-like corrosion exhibited in 0.86 M NaCl solution. Grain size had an effect on filament initiation as it consistently occurred on the surface of the base alloy (coarser-grained structure). In contrast, grain size had no effect on filament propagation as the anodic and cathodic kinetics were unaffected when moving from the base metal (coarser-grained surface) across the stir zone (finer-grained surface).