Exfoliation Corrosion Testing of Aluminum-Lithium Alloys

Abstract

A study was performed comparing the corrosion resistance of new aluminum alloys in shipboard environments with accelerated laboratory corrosion test environments. The aluminum-lithium (Al-Li) alloys tested were 2020-T651, 2090-T8E41, and 8090-T851. For comparison, 7075 aluminum was tested in the exfoliation susceptible T651 temper and the resistant T7351 temper. Shipboard exposures were performed aboard aircraft carriers deployed to the Mediterranean Sea, and to the Indian Ocean during the monsoon season. Accelerated laboratory tests performed were EXCO (ASTM G 34-86), sulfur dioxide salt fog (ASTM G 85.A4-85), and MASTMAASIS (ASTM G 85.A2-85). After shipboard exposure, the aluminum-lithium alloys in commercial tempers exhibited pitting corrosion similar to 7075-T7351. Aluminum alloy 7075-T651 exhibited severe exfoliation. During a mild shipboard exposure in which 7075-T651 developed superficial exfoliation, 8090-T651 exhibited moderate exfoliation demonstrating these alloys are not inherently exfoliation resistant. Standard accelerated laboratory tests were effective predictors of performance in 7075 aluminum, but were not as effective for aluminum-lithium alloys. For 7075 aluminum, shipboard behaviors were reproduced in sulfur dioxide and MASTMAASIS salt fog after 3 to 4 weeks, and in EXCO after 48 h. For commercial Al-Li alloys, the standard accelerated laboratory tests did not reproduce shipboard exposure results.