Friction stir welded joints of magnesium alloys AXM410 and AZX912, both similar and dissimilar, were exposed outdoors for three years to evaluate the effects of joint type and environment on corrosion. Unsheltered exposure caused general corrosion, whereas sheltered exposure promoted localized attack. Similar joints (AXM410/AXM410, AZX912/AZX912) showed a slight but preferential attack on the SZ/TMAZ region. In contrast, dissimilar joints (AXM410/AZX912) developed corrosion grooves up to 0.3 mm at the SZ region under unsheltered exposure, though only minor attack occurred under sheltered conditions. Electrochemical tests indicated AZX912 had a more noble corrosion potential than AXM410 in 1% NaCl, with potential difference and galvanic current increasing from untested to sheltered to unsheltered specimens. GD-OES depth profiling showed a two-layered product: an outer carbonate-rich film and inner Mg-Al LDH under unsheltered exposure, while sheltered exposure produced different carbonate films with higher NaCl content. For dissimilar joints, differences in Al concentration of base materials appear to drive macrogalvanic corrosion under unsheltered conditions, but in sheltered conditions the effect is reduced due to localized attack of the base metal.
Nakatsugawa et al. (Sun,) studied this question.