Effect of Zn coating on microstructure and corrosion behavior of dissimilar joints between aluminum alloy and steel by refilled friction stir spot welding

Refilled friction stir spot welding of aluminum alloy to galvanized and uncoated steel was conducted, and the microstructure and the corrosion behavior of joints were investigated. Al5FeSi phases distributed in TMAZ and broke during tool stirring in NZ, which induced the pitting corrosion in 3.5 wt% NaCl solution. The maximum corrosion depth of ST-Al/ST06 Z joint was up to 21.3 μm at the mixed layer, which was 2.8 μm deeper than that of ST-Al/ST16 joint at the interface between aluminum alloy and steel. Zn and Mg segregated at the grain boundaries of the mixed layer, resulting in serious intergranular corrosion. The corrosion current density of ST-Al/ST06 Z joint ((8.1 ± 0.2) × 10–6 A/cm−2) is higher than that of ST-Al/ST16 joint ((7.9 ± 0.4) × 10–6 A/cm−2) after immersion for 1 h in 3.5 wt% NaCl solution. The corrosion current densities of samples were minimum for 6 h immersion time, which was related to the passivation films and corrosion products formed on the joint surface.