Sensitization and stress corrosion cracking characteristics of 5059 alloy vis-a-vis 5083 alloy after reversion treatments

Moderately higher than room temperature exposures can sensitize marine grade 5xxx series aluminum alloys, making them susceptible to intergranular corrosion and stress corrosion cracking in marine environments. Sensitization occurs due to precipitation of active, magnesium-rich beta phase along the grain boundaries. Higher strength, high-magnesium 5059 and 5083 grade alloys were artificially sensitized by thermal exposures at 60 degree celsius, 80 degree celsius and 150 degree celsius for various durations of 1, 4, 15, 41, and 100 days. Sensitization and stress corrosion cracking responses are compared. 5059 alloy exhibited a higher degree of sensitization of 86 +/- 1 mg/cm(2) as compared to 64 +/- 2 mg/cm(2 )of 5083 alloy, for the similar sensitization condition of 150 degree celsius-15 days. This is attributed to the higher content and therefore super-saturation of magnesium in the former. In order to restore the corrosion resistance, reversion treatments involving re-exposing the sensitized alloys to elevated temperatures for short durations of 10 to 30 min were performed. Degree of sensitization after reversion treatment is evaluated leading to identification of conditions for obtaining safe values (<15 mg/cm(2)) without compromising the mechanical properties much. Optimum reversion treatment employed 30 min of exposures at 230 degree celsius.