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The stress-corrosion-cracking (SCC) susceptibility of Al 7075-T6 alloys has been studied in a controlled-atmosphere chamber utilizing the slow-strain-rate technique. The effects of high-humidity air with small additions of sulfur dioxide, nitrogen dioxide, and surface salts in different combinations have been investigated to predict the behavior of industrial environments. The slow-strain-rate technique has been used to optimize the simulated environments which result in accelerated crack growth and failure by SCC in high-strength Al 7075-T6 alloy. These controlled environmental tests provide initial data on the environmental susceptibility needed for the development of realistic accelerated-corrosion testing methods. The design of the environmental chamber for providing controlled atmospheres is discussed and initial experimental results are presented.
Most tests were conducted at 80 to 90% relative humidity. The SO2 concentration was varied from 10 to 1000 ppm, while NO2 was maintained to 100 ppm. Small amounts of surface salts were obtained by rapid immersion in aqueous salt solutions. The susceptibility of the alloy shows significant differences that are dependent upon the specific environmental contaminants. Synergistic effects in several environments were found to maximize the environmental effects upon crack growth in stress corrosion. In a related study, these effects were demonstrated in corrosion-fatigue experiments under similar conditions. The qualitative estimates of embrittlement in the fracture surfaces were consistent with slow-strainrate results. The data provide the basis for the development of accelerated corrosion tests in realistic atmospheres.
slow-strain-rate testing, controlled atmospheres, stress-corrosion cracking, quasi-cleavage, intergranular separation, embrittlement
University of Dayton Research Institute, Dayton, Ohio
Group Leader, Office of Naval Research, Arlington, Va.