Behavior of Painted Steel and Aluminum Sheet in Laboratory Automotive Corrosion Tests

Cold-rolled steel, electrogalvanized steel (60 g/m² coating), and three aluminum alloy sheet products (Al 2036, Al 5182, and Al 6111 [UNS A92036, A95182, and A96111, respectively]) were painted with a full automotive paint system. These materials were tested in two laboratory cyclic corrosion test environments, GM9540P(B) and CCT-IV. Resistance to cosmetic corrosion was measured in terms of underfilm paint delamination on scribed, flat panels. Crevice corrosion resistance was determined in terms of pitting on lapped panels of like materials, and galvanic corrosion resistance was determined in terms of pitting on lapped panels of unlike materials. Cosmetic corrosion of the aluminum alloys was found to be much better than that of cold-rolled steel and slightly better than that of electrogalvanized steel. The CCT-IV test was found to be more severe than GM9540P(B) for cosmetic corrosion, but GM9540P(B) was more severe for galvanic corrosion. Galvanic current measurements indicated the difference was related to the salt solutions used in each test. The aluminum alloys were prone to crevice corrosion and to galvanic corrosion when coupled to steel. Results indicated that comparative evaluations of the corrosion resistance of these materials must take into account the possibility of crevice and galvanic effects.