Corrosion behavior of spray formed 7055 aluminum alloy joint welded by underwater friction stir welding

• Spray formed 7055 Al alloy had been underwater friction stir welded successfully.
• Intergranular and exfoliation corrosion behaviors of joints were studied.
• The variation of strengthening phases in joints and base metal had been analyzed.
• Corrosion mechanism of joints had been clarified compared with base metal.

Friction stir welding with water cooling method had modified the microstructure of spray formed 7055 aluminum alloy but the corrosion behavior of alloy had been affected as well. Despite the tensile property having increased 15%, the corrosion behavior of underwater joint was more complex than base metal and traditional joint. Underwater joint characterized with an atypical pattern of intergranular corrosion (IGC) that its corrosion paths were not along the grain boundary though it still suffered a rank 4 IGC. At the same time, underwater joint was slightly sensitive to exfoliation corrosion (EXCO) and crossed 3-level-leap of corrosion rank within 24 h. Tafer curves illustrated more negative corrosion potential and lower corrosion current density for underwater joint than base metal and traditional joint. High potential phase MgCu2 in traditional joint blocked corrosion process to some extend but non-uniform distribution of MgCu2 led to weak zone for corrosion. In comparison with traditional joint, underwater joint avoided the formation of MgCu2 and kept a large quantity of efficient strengthening particles (MgZn2) in the matrix. Though MgZn2 with more negative potential than that of MgCu2 would lead to a higher sensitivity to corrosion, underwater joint still showed better corrosion resistance compared with base metal. In the process of underwater friction stir welding, MgZn2 had been stirred into grains leaving grain boundary a high potential zone. Since corrosion paths were discontinuous and not along with the grain boundaries, corrosion process was blocked so it performed better resistance to corrosion and corrosion rate of underwater joint had decreased a lot compared with traditional joint and base metal.