Crack Chemistry Control of Intergranular Stress Corrosion Cracking in Sensitized Al-Mg

Fracture mechanics experiments and occluded crack chemistry modeling validate the mechanism for intergranular stress corrosion cracking (IGSCC) of sensitized Al-Mg alloys: dissolution of discontinuous grain boundary β (Al₃Mg₂) precipitates activates Al-Mg (α) solid solution dissolution to acidify the crack solution for crack tip hydrogen embrittlement. Slow-rising displacement experiments with precracked specimens establish the strong effect of applied potential on IGSCC kinetics for sensitized AA5083-H131 (S-L orientation, 22 mg/cm²) in neutral NaCl solution. Anodic polarization increases growth rates through enhanced α dissolution for crack acidification and H uptake, whereas cathodic polarization below the pH-sensitive α breakdown potential reduces growth rates by limiting Al dissolution, and thus crack acidification and the overpotential for H production (η_H). Polarization below the β breakdown potential eliminates IGSCC by precluding β dissolution and crack acidification. Cathodic polarization could provide a practical means to mitigate IGSCC in sensitized Al-Mg alloys. As a second validation, AA5083-H131 without β is susceptible to IGSCC in either acidic AlCl₃/MgCl₂ simulated crack solution or neutral NaCl with anodic polarization, but resists cracking in NaCl at near the open circuit potential and in NaOH with strong cathodic polarization. Growth rates and calculated crack tip H solubility systematically increase with increasing η_H, estimated from crack pH and ohmic drop, to confirm the proposed mechanism. As a corollary, severe IGSCC at near open circuit potential occurs above a critical degree of sensitization, as required to produce a sufficient volume of grain boundary β for dissolution and acidification to trigger intergranular H cracking.