Corrosion tests in laboratory accelerated environments and in seacoast and inland industrial atmospheres have been conducted on specimens from 1 3/8 in thick hot rolled plate of several, high strength, heat treatable, aluminum alloys and tempers.
Alloy-tempers 2020-T651, 2024-T851, 2219-T851 and 7075-T7351 were shown to be highly resistant to stress-corrosion cracking. No failures occurred with specimens stressed as high as 75 percent of the respective yield strengths, even in the short-transverse direction, in any environment studied. Alloy-tempers 7001-T7551 and 7178-T7651 also were resistant in longitudinal and long-transverse directions; but showed some susceptibility to high sustained stresses acting in the short-transverse direction. The “threshold” stress for these two alloy-tempers, below which no failure is expected in 3.5 percent NaCl alternate immersion or in the atmosphere, is estimated at 27 ksi or 40 percent of the average short-transverse yield strength measured on these plates. In contrast, alloys 7075 and 7079 in the T651 temper showed some susceptibility to stress-corrosion cracking in the longitudinal and long-transverse directions at high sustained stresses, 75 percent of the yield strength and were susceptible in the short-transverse directions at a stress of 15 percent of the yield strength. Previous investigations have placed the threshold stress for 7075-T651 and 7079-T651 plate at 7 ksi or about 10 percent of the short-transverse yield strength.
All alloy-tempers were highly resistant to exfoliation with the exception of 7079-T651, which showed a very mild degree of susceptibility and 7075-T651 which showed medium susceptibility.
