Resistance to Stress Corrosion Cracking of High-Strength 7xxx Series Aluminum Alloys in 3.5% NaCl Solution

Abstract

For proper maintenance and inspection scheduling of aircraft, it is extremely important to measure the resistance to stress corrosion cracking (SCC) behavior of aluminum alloys used for aircraft structures. The SCC resistances of high-strength 7xxx-series aluminum alloys, including 7050-T7451, 7475-T7351 and 7140-T7451, were evaluated along longitudinal (L) and transverse (T) directions in 3.5% NaCl solution, using a constant testing method without polarization, and a slow strain rate testing method under a cathodic applied potential. It was found that the SCC resistance of the 7xxx series aluminum alloys in 3.5% NaCl solution varied with different testing method, chemical composition and temper condition. 7475-T7351 showed a slightly lower sensitivity to SCC than 7050-T7451 and 7140-T7451 in both anodic dissolution and hydrogen embrittlement conditions, even though the difference was not significant. The present result suggests that the Cu content and the ratio between Zn and Mg are important metallurgical factors affecting the SCC behavior of 7xxx series aluminum alloys. It was also found that pits served as initiation sties for SCC in the environment of anodic dissolution, while the small areas of hydrogen embrittlement acted as SCC initiator in the hydrogen-generating environment. The SCC behavior of each specimen was discussed based on the detailed fractographic and micrographic analyses.