Effect of Testing Conditions, Gauge, and Temper on Stress Corrosion Cracking of AA7xxx Aluminum Aerospace Plate Alloys

The European Aviation Safety Agency’s recent safety information bulletin (SIB 2018-04R2) describes a brittle intergranular cracking phenomenon in 7xxx aluminum alloys and recommends testing conditions. These testing conditions are severe compared with expectations regarding, temperature, humidity, and stress in aerostructures. In this study, strong increases in time to failure of smooth specimens tested in humid air were observed when reducing either temperature or stress to levels closer to what is representative of in-service conditions. The cracking morphologies after testing in hot, humid air were identical to those described in the literature for in-service occurrences of stress corrosion cracking (SCC), including the observation of crack arrest markings consistent with a mechanism of alternate local corrosion and crack propagation by hydrogen embrittlement. The differences between such fracture surfaces and those observed in the industry standard SCC screening test, which uses a chloride-containing solution to accelerate initiation and propagation, can be attributed to the pitting of the crack surface in a chloride-containing solution in the wake of the propagating crack. The performances of 7xxx-T7 aluminum plates SCC testing in either hot, humid air or a chloride-containing aqueous solution have been compared as a function of thickness, over-aging treatment, and composition. There is a strong effect of plate thickness in both tests, with substantial decreases in initiation time and lifetime for thinner plates. The effect of plate gauge is such that “legacy” materials (such as 7010-T7651, 7050-T7651, and 7050-T7451) passing the SIB 2018-04R2 test at high thicknesses, fail at lower gauges, yet have been flying for nearly 50 y. An increase in SCC performance with increased overaging is observed in both tests. As a consequence, comparisons of SCC performance of alloy compositions should be made wherever possible at identical plate thickness and for aging treatments that result in comparable strengths. When this is done, high Zn concentration 7449-T76++ (7449-T7651 with additional aging) shows a slightly worse strength-SCC balance than 7010-T7651 and 7050-T7651. The observed trends in SCC performance in the two tests are consistent, suggesting that similar repeated initiation and propagation mechanisms are responsible for cracking in humid air and in aqueous NaCl environments.