Understanding the Intergranular Stress Corrosion Cracking of an Al-Mg Alloy

Al-Mg alloys have historically been used in a variety of different engineering applications. These alloys are strengthened using work hardening and solid solution strengthening via Mg supersaturation (above 3 wt% Mg). When these alloys are exposed to sufficiently elevated temperatures, the thermodynamically stable β phase (Al3Mg2) precipitates primarily along the grain boundaries in a process known as sensitization. Sensitized Al-Mg alloys are therefore susceptible to grain boundary attack such as intergranular corrosion (IGC) or intergranular stress corrosion cracking (IGSCC) when also exposed to an electrolyte because of the known potential difference that exists between the anodic β phase and the surrounding α-Al matrix. This behavior has been extensively studied and is well understood.
IGC and IGSCC can cause pre-mature failures of engineering components, as such, significant work has been performed to understand the susceptibility of Al-Mg alloys to specific environmental conditions. Utilizing linear elastic fracture mechanics (LEFM) testing, knowledge gained from laboratory tests can easily be transferred to in-service engineering components exposed to the same mechanical driving force and environmental conditions by the principle of similitude. The LEFM testing approach has also been used to give mechanistic insights on the crack growth of these alloys. Recent studies have proposed an anodic dissolution enabled hydrogen embrittlement mechanism, where anodic dissolution of the β phase (from the electrochemical potential difference of the α and β) and α phase (from exposure of the bare metal to the corrosive electrolyte) releases Al3+ and Mg2+ ions into solution. These ions combine with surrounding water molecules to enable hydrolysis where H+ is formed as a product. Chloride ingress into the crack tip and a low pH promote an aggressive crack tip chemistry which also induces hydrogen (H) production. This hydrogen is first adsorbed onto the surface of the material and then adsorbed into the material at the crack tip. Diffusion of the hydrogen into the fracture process zone enables local embrittlement of the grain boundary and/or the β/Al-matrix interface.
This critical understanding has opened up development of mitigation strategies and informs future material development. It also helps inform the results of further testing of Al-Mg materials. The work of this thesis looks at developing a thorough understanding of an Al-Mg alloy (Alloy 1) that has not been testing using LEFM previously. Specifically, this work seeks to understand: the effect of grain directionality on the Mode I crack path, the effect of applied cathodic potential on the environment assisted cracking (EAC) kinetics, the effect of atmospheric environments (i.e. misting and wicking) compared to that of full immersion, and the effect of slightly basic, low chloride concentrated solutions on the EAC behavior compared to more acidic and higher chloride containing solutions in an atmospheric environment. All of these results are analyzed in the context of the anodic dissolution enabled hydrogen embrittlement mechanism. Collectively, these results will inform potential mitigation strategies and root cause analysis of in-service components comprised of Alloy 1. Additionally, this work will be integrated into lifetime prediction models of in-service components.
The key results show that Alloy 1 does have an applied potential dependence, as has been observed for other Al-Mg systems previously. Specifically, cathodic applied potentials decrease the IGSCC susceptibility of Alloy 1, with complete mitigation of crack growth at an applied potential of -1100 mVSCE. This allows for the development of techniques that will mitigate the susceptibility of Alloy 1 as a means of cathodic protection. The study of the grain microstructure showed that Alloy 1 had little grain directionality as compared to other Al-Mg alloys, and showed minimal effect of loading orientation on EAC kinetics. Atmospheric testing of Alloy 1 exposed to a low chloride, slightly basic solution gave enhanced crack growth kinetics over the full immersion environment because of increased oxygen transport through the thin film electrolyte. Lastly, atmospheric exposure of Alloy 1 to high chloride containing, slightly acidic solutions showed faster EAC kinetics over the more benign low chloride containing solution. However, the fastest cracking kinetics were observed during exposure to a solution of pH 2, which was explained by increased anodic current densities allowing for more active dissolution of the Al.