The Role of Hydrogen in the Stress Corrosion Failure of High Strength Al-Zn-Mg Alloys and Sensitized Austenitic Stainless Steels.

An investigation into the effect of exposure of Al-Zn-Mg alloys to water vapour has shown that hydrogen, produced by the reaction is absorbed by both the alloy grain boundaries and matrix. The amount of absorbed hydrogen depends on the local chemistry and the metalOxide interface and is influenced by alloying additions. Attainment of a critical concentration of dissolved hydrogen in the boundaries leads to brittle intergranular fracture on the subsequent application of stress. Attainment of a critical hydrogen concentration in the matrix leads to transgranular cleavage fracture. Depending upon the alloy composition and heat treatment bubbles of gaseous hydrogen can be formed at suitable sites within the alloys such as grain boundary particles. The formation of grain boundary bubbles reduces embrittlement by reducing the local concentration of dissolved hydrogen. It is proposed that the interaction of stress and environmental variables with the above criteria governs the environmental failure of Al-Zn-Mg alloys in aqueous and water vapour containing environments. It is suggested that such failures are hydrogen embrittlement dominated.