Relationship of Accelerated Test Methods for Exfoliation Resistance in 7XXX Series Aluminum Alloys with Exposure to a Seacoast Atmosphere

Accelerated corrosion tests are necessary and an invaluable aid to research and quality control engineers, provided that the test has adequate discriminatory ability and the test results relate to serviceability of the metal. An inherent problem associated with accelerated exfoliation tests is the definition of different degrees of susceptibility and the interpretation of borderline performances. The concern in the present instance is that truly insignificant exfoliation tendencies may be magnified out of proportion by the very rapid exfoliation tests now being used in materials specifications for exfoliation resistant 7XXX-T7 alloy products. Correlation of accelerated tests with service environments or the equivalent is necessary, and it is highly desirable that the range of performances observed in the accelerated test be related to a similar range of performances under service conditions. This paper contains the results of relatively long exposures of test panels to the salt atmosphere at a location about 100 yd from a stony beach at Point Judith, Rhode Island, where the corrosive conditions are representative of other very corrosive environments. These extended exposures are of particular interest because they furnish a basis of evaluating indications of borderline susceptibility to exfoliation revealed in the accelerated corrosion tests. Four- to eight-year exposures to this aggressive seacoast atmosphere have demonstrated the excellent resistance to exfoliation of 7075-T73 and T76 and 7178-T76 products, whereas exfoliation susceptible T6 temper materials displayed considerable exfoliation after short exposures, sometimes after only three to six months. Both the EXCO immersion test (ASTM G 34-72) and the salt spray test (modified ASTM acetic acid salt intermittent spray) developed the same basic type of corrosion that occurred in the seacoast atmosphere. It was shown, however, that the accelerated tests, especially the spray test, tend to magnify slight indications of susceptibility to exfoliation that are of doubtful practical consequence. Suggestions are made for determining more realistic visual acceptance criteria in 7XXX-T7X materials specifications.