Exfoliation Corrosion Testing of 7075 and 7178 Aluminum Alloys—Interim Report on Atmospheric Exposure Tests

Abstract

Atmospheric exposures of 18 to 39 months have been completed at seacoast and inland industrial locations to determine the resistance to exfoliation corrosion of specially heat-treated test panels of high-strength aluminum alloys. Two seacoast locations, one at Point Judith, Rhode Island, and another at Daytona Beach, Florida, were selected, together with an industrial location near Chicago, Illinois, to represent the relatively severe atmospheric conditions to which aluminum alloy aircraft structures are subjected in service. The purpose of these tests was to demonstrate the relationship of the performances in the accelerated exfoliation corrosion tests previously evaluated by this task group with those in relatively aggressive natural environments. The exfoliation performance is being evaluated on test panels heat treated to produce three different metallurgical structures that would be expected to provide a high, low, and medium resistance to exfoliation of both a 7075 alloy extrusion and a 7178 alloy rolled plate. Results obtained thus far are gratifying in that the atmospheric exposures are ranking the test materials in the same relative order as the new ASTM Standard Method of Test for Exfoliation Corrosion Susceptibility in 7XXX Series Copper Containing Aluminum Alloys (G 34-72, EXCO test). That is, the materials that showed either a high or a low resistance to exfoliation in the EXCO test also showed the same performance in the seacoast atmosphere, and that which showed an intermediate resistance in the EXCO test showed more spotty and a slower rate of exfoliation in the atmospheric exposures. Exfoliation of the most susceptible items initiated more rapidly (within five months) at the seacoast, but the exfoliation tended to be more uniform at the industrial site. Other factors such as the climatic conditions at the seacoast, location, and exposure position of the test panels are considered.