Mechanism of Stress Corrosion Cracking and Corrosion Fatigue of Precipitation Hardening Aluminium Alloys

Abstract

Aluminium and aluminium alloys, including precipitation hardening alloys, basically are corrosion resistant due to the presence of a passivating oxide film. At the same time, all are subject to pitting, and some alloys moreover are subject to intergranular corrosion after heat treatment methods common for actual service aims. Moreover, some alloys especially suitable as aircraft structural materials, such as alloys with zinc, magnesium and copper, and recently also with lithium, commercially heat treated for precipitation hardening, are susceptible to intergranular cracking under both static and dynamic loading. The danger is particularly pronounced for zinc-containing alloys, as very little crack-tip corrosion is sufficient to trigger discontinuous events of crack extension by hydrogen embrittlement. Other alloys only crack at more positive values of the electrode potential by the onset of stress-enhanced anodic metal dissolution in precipitation-free zones at grain boundaries.The electrode kinetics of the combination of the various types of crack growth mechanisms, together with the kinetics of cathodic partial reactions at oxide films of widely differing properties, suffice to at least qualitatively model the behaviour, with respect to cracking, of commercial alloys of types AlZnMg, AlCu, AlCuMg and AlLi as a function of environmental conditions. Quantitative predictability is difficult due to the extreme dependence of the rate of cracking on composition and heat treatment of the alloys.