Macro- and Micro-Galvanic Corrosion Mechanisms in Symmetric and Asymmetric Double-Sided Friction Stir-Welded 7A65 Aluminum Alloy Joints

The corrosion behavior of symmetric (S-joint) and asymmetric (A-joint) double-sided friction stir-welded (DS-FSW) 7A65 aluminum alloy thick plates was investigated in 3.5 wt% NaCl solution. The S-joint, produced using the same large tool (Φ30 mm) for both passes, imposes two high-heat thermal cycles that result in insufficiently fragmented intermetallic particles (IMPs), coarse grains, and severely overaged heat-affected zones (HAZs). In contrast, the A-joint, employing a smaller tool (Φ24 mm) for the second pass, reduces the total heat input and achieves a refined microstructure with fine (2–3 µm), rounded IMPs in the second-pass weld nugget (WNZ-S) and less degraded HAZs. Electrochemical measurements reveal that the HAZ-Overlap (HAZ-O) is the most anodic zone in both joints. The S-joint shows a larger potential spread (up to ~120 mV) and higher corrosion current density than the A-joint. The hierarchical galvanic coupling, where macro-galvanic corrosion between the anodic HAZ-O and cathodic WNZs drives severe localized attack, while micro-galvanic corrosion around coarse IMPs initiates trenching, is elucidated. The A-joint mitigates this damage due to its reduced galvanic driving force (smaller potential spread of ~74 mV) and improved microstructural homogeneity. The enhanced corrosion resistance of the A-joint is attributed to grain refinement, effective IMP fragmentation, and a less degraded HAZ microstructure.