Corrosion Protection of Friction Stir Spot Welds Produced in an AZ31B Magnesium Alloy

Magnesium alloys can be joined by friction stir spot welding (FSSW) to achieve high mechanical integrity of the weld. Previous work done on AZ31B FSSW joints has shown that the welding process reduced their corrosion resistance, and resulted in localized attack around the weldment. The present work explores the possibility of a post-welding plasma electrolytic oxidation (PEO) treatment to minimize the negative effect of welding on the corrosion resistance of AZ31B. A mass loss testing approach was adopted to assess the corrosion resistance of PEO coated base metal and welded specimens. PEO coatings were found to drastically improve the corrosion resistance of welded and monolithic AZ31B, eliminating the localized corrosion associated with welding. Further improvement to the corrosion resistance of PEO coatings was achieved by adding polyethylene glycol to the electrolyte. The scanning reference electrode technique revealed that the breakdown mechanisms of the coating was governed by cathodic activation.