In Situ X-Ray Radiographic Study of Intergranular Corrosion in Aluminum Alloys

In situ x-ray radiography was used to characterize inter-granular and exfoliation corrosion in high-strength Al alloys. The samples were either exposed to sodium chloride (NaCl) solution at a controlled potential or to high humidity after an electrochemical pretreatment in NaCl solution. Intergranular corrosion (IGC) growth rates in solution for samples encased in epoxy and exposed to solution were distributed over a range with an upper limit equal to the rates determined by the foil penetration technique. Unencased AA2024-T3 (UNS A92024) samples that were held at a controlled potential in solution exhibited IGC and then exfoliation corrosion. AA2024 and AA7178 (UNS A97178) samples encased in epoxy developed sharp intergranular fissures during exposure to 96% RH following an electrochemical pretreatment. Unencased samples given the same treatment exhibited exfoliation in the high-humidity environment.