Effect of chloride concentration on the environment-assisted cracking behavior of an Al-Mg alloy in atmospheric environments

The effect of chloride concentration on the environment-assisted cracking (EAC) behavior of AA5083-H131 in atmospheric environments was investigated using high-fidelity fracture mechanics-based testing and concurrent electrochemical potential measurements. EAC susceptibility was found to increase across all environments as chloride concentration increased, denoted by a decrease in the threshold stress intensity and faster stage II crack growth rates. However, EAC susceptibility for a given chloride concentration decreased across all chloride concentrations as cathodic limitation due to solution geometry effects increased. These results are analyzed in the context of the proposed anodic dissolution-enabled hydrogen embrittlement mechanism for EAC in Al-Mg alloys. Specifically, the increase in EAC susceptibility noted at higher chloride concentrations is postulated to arise from an increased overpotential for hydrogen production at the crack tip. Conversely, the decrease in EAC susceptibility observed as the solution geometry becomes more restrictive is attributed to cathodic limitation at the bulk surface decreasing dissolution at the crack tip, resulting in a concomitant less aggressive crack chemistry, and thus lower levels of hydrogen production and uptake at the crack tip. A close correlation between the open-circuit potential on the bulk specimen surface and the crack growth kinetics was also observed across all environments and chloride concentrations, with higher chloride concentrations and cathodic limitations resulting in larger changes in electrochemical potential. This correlates well with known electrochemical potential-dependent EAC observations for these alloys.