Contribution to the Mechanism of Stress Corrosion Cracking in a Welded Al-Zn-Mg Alloy

Abstract

The stress corrosion cracking (SCC) of Al-Zn-Mg welds in an aqueous solution of 3·5 wt-% NaCl (pH = 1) at 15° ± 2° C was studied with specimens under constant load as a function of the applied anodic potential and the postweld heat treatment. Various theories of SCC in high strength aluminium-based alloys (the ternary systems Al-Zn-Mg and Al-Cu-Mg or the quaternary system Al-Zn-Mg-Cu) are reviewed. They can essentially be divided into three main groups: the grain boundary (GB) dissolution model, the precipitation-free zone (PFZ) dissolution model, and the hydrogen embrittlement (HE)-assisted electrochemical model. Scanning electron microscope observations of fracture surfaces confirmed that the enhanced susceptibility to SCC due to an applied anodic overvoltage is not caused by anodic metal dissolution at the crack tip, but by hydrogen embrittlement. The fracture modes showed a tendency to be more brittle intercrystalline in character at shorter SCC failure times.