Aluminum Alloys After Five Years in Seawater

Abstract

Eight aluminum alloys and panels of high-purity aluminum were exposed for five years in seawater at Harbor Island, N. C. Replicate 4 by 12-in. panels of each material were exposed in half-tide immersion and full immersion. Removals in triplicate were made at the end of one-, two-, and five-year exposures. Other than the 1199 aluminum (99.99 percent purity), seven aluminum-magnesium alloys (5000 series) and one aluminum-zinc-magnesium-copper alloy 7079 were tested. Panels were degreased prior to exposure and were cleaned ultrasonically in a chromic-phosphoric acid solution after exposure. After depth of pitting was measured, tension specimens were cut from each panel, and the ultimate tensile and yield strengths and elongations were determined. All panels were heavily fouled with barnacles and other marine growth for all exposure periods. The fouling apparently had some effect on the pitting depth of the aluminum in tidal immersion. Little change in tensile properties after five-years' exposure was noted for the 1199, 5083-O, 5086-O, or 5454-H32. Tensile losses were recorded for several alloys as follows: 5086-H112 (4.8%), 5154-H38 (6.2%), and 5457-H34 (4.3%). The high-purity 1199 and alloys 5154-H38, 5086-H112, and 5456-H321 showed losses of 6 to 10 percent in elongation while the change in elongation for the 5457-H34 was a decrease of about 16 percent. The 7079-T6, which can only be used in seawater when protected, had severe exfoliation corrosion. Confirming other work, we found the corrosion rates to be greater for the full-seawater immersion than for the tidal immersion. The lowest five-year corrosion rate in the half-tide location was 0.07 mdd (milligrams per square decimeter per day) for the 5083-O, and the highest rate for the aluminum-magnesium 5000 series was 0.11 mdd for the 5454-H32 alloy. In full-immersion tests the lowest rate was 0.12. For comparison, the rates for the 7079-T6 alloy were 0.79 mdd in tidal and 3.4 mdd in full immersion. Corrosion weight losses and depth of pitting had reduced rates of growth over the interval between two years and five years. Maximum depth of pitting for an alloy was generally at least four times the magnitude of the average of the twenty deepest pits. The deepest pit found for any alloy after five years was 33.0 mils for 5456-H321 in half-tide immersion and 38.0 mils for the same alloy in full immersion. Least pitting depth occurred in 5457-H34 (less than 4.5 mils for both exposures).