This work explores the effect of humidity and galvanic coupling on the environmental assisted crack initiation and growth rate of high-strength aluminum alloys under static load. Varied relative humidity and temperature cycles, based on relevant aircraft service environments, are used to identify factors leading to peak crack growth rate for a sodium chloride salt chemistry. The influence of dynamic environmental conditions versus constant environmental conditions are being evaluated. Aerospace materials are galvanically coupled to the aluminum substrate to quantify the effect on initiation time and growth rate. Spring-actuated double cantilever beam test systems with continuous sensor monitoring are employed in atmospheric environments with smooth-notch samples to produce estimates of time to crack nucleation, crack length, crack growth rate, and crack tip stress intensity. Simultaneous zero resistance ammeter measurement of the corrosion currents between galvanically coupled aerospace materials and the aluminum samples improves understanding of the relationships between atmospheric environment and the electrochemical processes leading to crack initiation and growth. Results demonstrate clear differences between wetting and drying processes, with peaks in galvanic corrosion current observed during drying and smaller peaks observed during wetting. Peaks in crack growth rate are observed at moderate relative humidity during drying processes.
High-strength aerospace aluminum alloys, such as AA7075-T651, are susceptible to environmental assisted cracking (EAC) under the right combinations of stress, environment, and microstructure. EAC presents a serious risk to structures and equipment operated in corrosive conditions. Studies of EAC in aluminum alloys have highlighted the importance of both anodic dissolution and hydrogen embrittlement to EAC initiation and propagation.1–4 The EAC response of alloys under variable atmospheric conditions is of particular importance for assessing material performance for aerospace applications.
Characterization techniques for EAC of alloys often employ immersion testing with constant concentration, volume, and temperature conditions or testing under controlled thin electrolyte layers with static environmental conditions.5–7 There is a need for evaluation and testing of EAC performance with variable, but controlled, atmospheric environmental conditions, with in situ crack growth measurements, and electrochemical monitoring to better capture the effects of processes relevant to in-service degradation of high-strength aluminum alloy structures. Environmental cycles affect the composition and distribution of electrolyte, which can significantly impact electrochemical reactions, current distribution, and corrosion rates.8–11 Additionally, crevice and cracking conditions can result in pH and concentration gradients resulting in aggressive conditions that promote EAC.12,13