SYMPOSIA PAPER Published: 01 January 1992

Modification of the EXCO Test Method for Exfoliation Corrosion Susceptibility in 7XXX, 2XXX, and Aluminum-Lithium Alloys


The EXCO test (ASTM G 34) is widely used to predict long-term exfoliation corrosion of high strength aluminum alloys in seacoast and other service environments. Recent experience has shown that the EXCO test does not always accurately predict the resistance of 7X50 and 2024 products processed to different degrees of resistance to exfoliation. The test also severely underestimates resistance to exfoliation of 2090-T8 products.

For 7X50 alloys, the standard EXCO test is too corrosive for materials in T7 types tempers. This unrealistic corrosion occurs during the first 24 h of the test, during which time the solution is highly acidic, starting at pH 0.4 and gradually stabilizing at pH 3.2. Tests in a preconditioned EXCO solution, prepared by exposing a piece of 7150 plate in fresh EXCO solution for 24 h to eliminate the period of low pH, showed better correlation with atmospheric test results. Subsequent chemical analyses of this preconditioned solution led to a modified EXCO solution capable of distinguishing the different levels of resistance to exfoliation that are inherent in the three commercial tempers (T6, T76, and T74). This modified solution contains 600 mg/L of aluminum ion plus 4M (142 g/L) chloride and 0.6M (37.2 g/L) nitrate concentrations, and has a pH of 3.2.

When conducted at a temperature of 52°C, the modified EXCO solution accurately reflected the performance of 2090 and 2024 alloys in a seacoast atmosphere.

Author Information

Lee, S
Alcoa Laboratories, Aluminum Company of America, Alcoa Center, PA
Lifka, BW
Alcoa Laboratories, Aluminum Company of America, Alcoa Center, PA
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Developed by Committee: G01
Pages: 1–19
DOI: 10.1520/STP19577S
ISBN-EB: 978-0-8031-5195-6
ISBN-13: 978-0-8031-1435-7