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The well documented pattern of corrosion behavior for aluminum alloys is confirmed for the seven-year exposures at Corpus Christi, Tex.; Richmond, Va.; McCook, Ill.; and the Kure Beach, N. C., 800-ft site. A decrease in the corrosion rate, as determined by weight loss, was evident for all sheet alloys after about the second exposure year. Likewise, the depth of pitting decreased with time: very slight increases in depth were noted between the second and seventh years in most cases.
Results indicate that the McCook site is the most severe, with the Richmond location, the Corpus Christi exposure, and Kure Beach 800-ft site in order of decreasing severity.
The value of cladding basic alloys such as 3003 and 6061 was demonstrated again. The claddings effectively protected the core alloys from pitting and general corrosion and the clad alloys showed negligible changes in mechanical properties at all sites after seven years.
Other than the 2024 aluminum-copper alloy which showed intergranular corrosion for some hard tempers at the most severe sites, all alloys showed pitting corrosion to some extent. Maximum pit depths included 14.7 mils for the 2024-T36 alloy, 10.0 mils for the 5154-H34 aluminum-magnesium alloy, 5.2 mils for the aluminum-magnesium silicide alloy 6061-T6, and 6.3 mils for the high-purity aluminum sheet. With the exception of the 2024 metal, these deepest pits occurred at the McCook site. The deepest pit found on the Kure Beach 800-ft site panels measured 6.1 mils, and the deepest pit for most Corpus Christi panels was 5.4 mils (2024 alloy panels having some intergranular attack had measured pits as deep as 14.7 mils).
A corrosion rate of 0.175 milligrams per square decimeter per day (mdd) for 2024-T36 alloy at McCook was the highest found. If we exclude the 2024 alloys, no corrosion rate greater than the 6061-T6 rate of 0.125 mdd was found. High purity aluminum showed rates from a low of 0.004 mdd at Kure Beach to 0.044 mdd at McCook.
intergranular attack, aluminum-copper alloys, aluminum-magnesium alloys, high purity aluminum, aluminum-magnesium silicide alloys, cladding, atmospheric corrosion tests, aluminum alloys, corrosion
Senior corrosion engineerpersonal member, Reynolds Metals Co.ASTM, Richmond, Va.