The Effect of Temper and Composition on the Stress Corrosion Cracking of Al-Mg Alloys

Abstract

AA5xxx-series alloys are commonly utilized as lightweight replacements for steel components in marine applications, but experience precipitation of an anodic β phase (Al3Mg2) on grain boundaries at temperatures as low as 40°C (313 K). When stressed in aggressive environments, the preferential dissolution of this intermetallic leads to intergranular stress corrosion cracking (IGSCC). Susceptibility to IGSCC is assessed in the S-L orientation for differing compositions (AA5083 and AA5456) and material tempers (-H131, -H116, and -SHTQ) sensitized at 100°C (373 K) to Nitric Acid Mass Loss Test (NAMLT) values of 24 mg/cm2 and 40 mg/cm2. The more Mg-rich AA5456 shows less susceptibility to IGSCC than AA5083 for a constant -H116 temper. In AA5083, the -H131 temper exhibits the greatest susceptibility to IGSCC, followed by the -H116, and the -SHTQ conditions. These composition and temper effects are pronounced at NAMLT 24 mg/cm2, but are less traceable at NAMLT 40 mg/cm2. Reduced IGSCC susceptibility scales with decreasing yield strength. Susceptibility to IGSCC is interpreted in the context of a Hydrogen Enhanced Decohesion model, to relate microstructural changes to the governing hydrogen-based mechanisms. The lack of similitude in IGSCC behavior at constant NAMLT values suggests that this metric is a poor proxy for IGSCC susceptibility across different compositions/tempers.