Corrosion Performance of Friction Stir Linear Lap Welded AM60B Joints

A corrosion investigation of friction stir linear lap welded AM60B joints used to fabricate an Mg alloy-intensive automotive front end sub-assembly was performed. The stir zone exhibited a slightly refined grain size and significant break-up and re-distribution of the divorced Mg17Al12 (β-phase) relative to the base material. Exposures in NaCl (aq) environments revealed that the stir zone was more susceptible to localized corrosion than the base material. Scanning vibrating electrode technique measurements revealed differential galvanic activity across the joint. Anodic activity was confined to the stir zone surface and involved initiation and lateral propagation of localized filaments. Cathodic activity was initially confined to the base material surface, but was rapidly modified to include the cathodically-activated corrosion products in the filament wake. Site-specific surface analyses revealed that the corrosion observed across the welded joint was likely linked to variations in Al distribution across the surface film/metal interface

Electrochemical behavior of magnesium alloys AZ91D, AZCe2, and AZLa1 in chloride and sulfate solutions

The influence of Cl- and SO42- on the electrochemical behavior of AZ91D, AZCe2, and AZLa1 was studied. For all alloys, there was a current plateau in the anodic polarization curves in Na2SO4 solutions. In 0.5% NaCl solution, there was a small current plateau, whereas there was none in the 3.5% and 5% NaCl solutions. This indicated that SO42- is less aggressive than Cl-. The range of the current plateau decreased with increasing SO42- concentration. For all alloys, the high frequency capacitive loop in the Nyquist plots decreased with increasing concentration consistent with the decrease in corrosion resistance with increasing Cl- and SO42- concentration