The effect of thermal-mechanical processing of the exfoliation corrosion resistance of an aluminum-zinc-magnesium-copper alloy

"The objective of this investigation was to evaluate the effect of thermal-mechanical processing on the exfoliation corrosion resistance of an aluminum-zinc-magnesium-copper alloy. the aluminum alloy used was commercially available 7178 alloy in two thicknesses, 0.390 inch thick and 0.250 inch thick. this material was reduced 10, 25 and 40% in thickness in both the annealed and solution heat treated conditions. The material was then aged to a high strength condition, T6, or T8, in the case of the material reduced in the solution heat treated condition. The effect of the thermal-mechanical processing on the exfoliation corrosion resistance was then evaluated by exposing 3 by 6 inch specimens to both an intermittent acidified salt spray test and a salt spray plus sulfur dioxide gas test. Increasing amounts of mechanical deformation of material in the solution heat treated condition resulted in progressively lower resistance to exfoliation corrosion. As the amount of deformation of the annealed material increased to a point where recrystallization resulted during the subsequent solution heat treatment, the exfoliation corrosion resistance improved. The aging schedules developed for the T8 condition resulted in strengths equivalent to the high strength T6 condition without significant losses in ductility. It was also demonstrated that the salt spray plus sulfur dioxide test was a considerably more severe exfoliation test environment than the intermittent acidified salt spray test"--Abstract, page ii.