Stress Corrosion Cracking of Al-Mg and Mg-Al Alloys Available to Purchase

Al- and Mg-based alloys are being used for reducing the weight of automobiles. For structural applications, they must have adequate stress corrosion resistance, and yet, under some circumstances, stress corrosion cracking can occur in both alloy systems. Precipitation of the Mg-rich β-phase (Al₃Mg₂) at grain boundaries of Al-Mg alloys and the γ-phase (Mg₁₇Al₁₂) at grain boundaries of the Mg-Al alloys is a critical factor in their stress corrosion performance. In Mg-Al, the β-phase is cathodic (noble) to the matrix, while in the Al-Mg case, the β-phase is anodic (active) to the matrix. These phases produce localized galvanic-induced corrosion that leads to intergranular stress corrosion cracking and crack growth rates of 350 and 1,800 times faster than the solution-treated condition, for Al-Mg and Mg-Al, respectively.