Comparison of corrosion performance of AA7075 and AA2070 in various test environments

Abstract

Accelerated cabinet style corrosion tests are developed to determine the susceptibility of metals to specific types of corrosion such as pitting, intergranular corrosion (IGC), and exfoliation. However, this work shows that not all tests result in an accurate representation of the corrosion attack found in long-term outdoor exposures at both industrial and sea coast locations. This is particularly true for the third-generation Al-Li alloy 2070. To better understand the propensity of Al-Li alloys to particular forms of corrosion attack in various exposure environments, the corrosion morphology and extent of attack for AA7075-T6 and AA2070 in several tempers are compared between laboratory-accelerated cabinet and outdoor corrosion tests. The variation in the corrosion type of attack and depth as determined using cross-sectional analysis and white light interferometry is discussed. Results show severe exfoliation on AA7075-T6 after an accelerated ASTM G34 (EXCO) test while ASTM B117 testing reveals pitting. For AA2070, it was observed that the corrosion morphology varied by temper after the ASTM G85-A2 accelerated test, where the underaged AA2070-T3 exhibited both IGC and grain lifting, and peak aged and overaged AA2070 exhibited pitting. Results suggest that the ASTM G85-A2 dry bottom modified ASTM acetic acid salt intermittent spray test correlates better with outdoor seacoast and industrial exposures than either ASTM B117 or EXCO.