Effect of surface preparation on the corrosion resistance of friction stir linear lap welded AZ31B-H24

The effect of surface preparations on the atmospheric (continuous NaCl salt fog) corrosion resistance of bare (uncoated) linear friction stir lap welded magnesium (Mg) alloy AZ31B-H24 joints was investigated. The surface preparations evaluated included as-received (as-welded for the weld zones), mechanically abraded and acid-cleaned. Electrochemical potentiodynamic polarisation, X-ray photoelectron spectroscopy and Auger electron spectroscopy measurements were made to elucidate the link between the surface preparation and the relative corrosion resistance observed across the microstructural zones of the joints. Heavy metal surface contamination of the as-received material, rather than the air-formed oxide or differences in alloy grain size resulting from variations in thermo-mechanical processing history, was found to be the controlling factor affecting the corrosion mode observed and, in turn, the relative corrosion resistance across the joint. Removal of the heavy metal contamination changed the corrosion mode from pit-like to filament-like, which in turn homogenised the corrosion resistance across the joint in the continuous salt fog exposure.