Screening of Salt Fog Environments for Mg Alloy SCC Susceptibility

U-bend samples of Mg alloy AZ31B were subjected to continuous near-neutral salt fog exposure to evaluate the effect of a variable salt content on stress corrosion cracking (SCC) susceptibility. Using the ASTM B117 standard environment as a baseline (5 wt% NaCl (aq) at 35°C), a two order of magnitude reduction in NaCl content coincided with a change in the dominant corrosion mode from localized filament corrosion only to transgranular SCC only. An 11 d exposure period in the 0.05 wt% NaCl (aq) fog was sufficient to cause fracture by transgranular SCC. The resistance of the intact surface film and subsequent cathodically-activated enhanced hydrogen gas evolution likely were the critical factors controlling the transition of the dominant corrosion mode (from filament corrosion to SCC). Filament corrosion initiation likely required a Cl− ion-induced (i.e., chemical) break down of the intact film, whereas SCC likely required a stress-induced rupture (i.e., mechanical) break down of the intact film.