Test Environment Selection for Corrosion-Fatigue Testing of Aluminum Alloy 7075-T6

The appropriateness of a variety of test environments was determined for studies of interactions between corrosion and fatigue crack propagation (FCP) in the context of damage tolerance and durability. Because many fatigue cracks in aerospace structures involve materials in occluded geometries, measurements included occluded region potential as a function of time, boldly exposed region polarization scans, corrosion morphology comparisons, and pH. The alloy of interest was aluminum alloy 7075-T6 (UNS A97075). The test environments included four common ASTM test solutions for localized corrosion susceptibility of aluminum alloys and the lap joint simulant solution (LJSS), which was designed to reproduce in-service corrosion morphologies. The use of the ASTM standard solutions severely limited the range of potentials that can be probed during FCP testing. The LJSS was more passivating, resulting in the ability to probe a wide range of potentials without inducing localized corrosion on the boldly exposed surface. Occluded region corrosion morphology and pH measurements near the mouth of the crevice were dependent on the severity of the bulk solution. However, at interior sections, all the solutions’ pH tended toward LJSS behavior, that is, mildly alkaline.