Localized Corrosion Susceptibility of Extruded Cu-Lean AA7xxx Gas Metal Arc Welding Lap Joints

A study was undertaken to evaluate the localized corrosion susceptibility of extruded Cu-lean aluminum alloy (AA) 7xxx single fillet welded lap joints prepared by gas metal arc welding exposed in a cyclic acidified salt fog environment (ASTM G85-A2). Major parameters varied including the Cu-lean AA7xxx (AA7003 vs. AA7108), heat-treated condition (as-welded T6 vs. post-weld paint-bake), and welded joint orientation (weld applied parallel vs. perpendicular relative to the AA7xxx extrusion direction). It was found that, in the absence of applied stresses, pitting corrosion (selective grain attack) was the dominant mode of corrosion that occurred across the welded joints with the heat affected zone (HAZ) being more susceptible than the unaffected base material, regardless of the joint design parameter tested. The hardness profiles measured across the welded joints agreed well with those published in the literature for which links between the hardness and precipitate structure (type, size, and density) have been established. Based on this established framework, the type and size of the precipitates (smaller Guinier–Preston zones in the HAZ versus larger η′ precipitates in the unaffected base material) likely accounted for the difference in pitting susceptibility observed.