Influence of critical surface defects and localized competition between anodic dissolution and hydrogen effects during stress corrosion cracking of a 7050 aluminium alloy

Aluminium alloys used in the aeronautical industry, and in particular the high strength Al-Zn-Mg(-Cu) alloys of 7XXX series, can be susceptible to stress corrosion cracking (SCC). In the numerous studies reported in the scientific literature on this subject, two basic mechanisms have been proposed to model SCC: anodic dissolution and hydrogen embrittlement. However, there is currently no consensus on the precise mechanism. By coupling discriminating slow strain rate tests and scanning electron microscopy (SEM) examinations, it is shown that: (1) both anodic dissolution and hydrogen embrittlement operate during the SCC process of a 7050 aluminium alloy stressed in a chloride solution (NaCl 3%); (2) the main role of anodic dissolution is to produce critical defects which promote subsequently localized hydrogen discharge, entry and embrittlement; and (3) the relative influence of these two mechanisms depends on the main parameters that govern cracking, i.e. the microstructure, the electrochemical potential and the strain rate.