Electrochemistry of θ (Al₂Cu), S (Al₂CuMg) and T₁ (Al₂CuLi) and localized corrosion and environment assisted cracking in high strength Al alloys

The θ (Al₂Cu), S (Al₂CuMg), and T₁ (Al₂CuLi) intermetallic constituents and precipitates found in age-hardenable aluminum alloys play important roles in the localized corrosion behavior of Al-Cu-(Mg,Li) alloys. Most of the common understanding of the behavior of these phases is based on knowledge or assumptions of their open circuit potentials (OCP). Electrochemical measurements made on these compounds synthesized in bulk form, and detailed investigations of their dissolution from alloys reveal interesting complexities not suggested by OCP measurements. Studies show that both anodic and cathodic reaction kinetics are very fast on these intermetallics. These compounds invariably corrode by dealloying in which the more active Al, Mg, or Li components dissolve to leave behind a Cu-enriched particle remnant. The dealloyed particle remnants are themselves physically unstable and have been observed to coarsen and emit 10 to 100 nm metallic Cu clusters into corrosion product layers and surface films. Overall, a more complete and quantitative understanding of the dissolution characteristics of these intermetallic compounds has helped in understanding localized corrosion and environment assisted cracking behavior of Al alloys.