Ten-Year Seawater Tests on Aluminum

Abstract

Seven aluminum alloys and panels of high-purity aluminum were exposed for ten years in seawater at Wrightsville Beach, 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-, five-, and ten-year exposures. Other than the 1199 aluminum (99.99 percent purity), seven aluminum-magnesium alloys (5000 series) were tested. Panels were degreased prior to exposure and were cleaned ultrasonically after exposure in a chromic-phosphoric acid solution. After depth of pitting was measured, tension specimens were cut from each panel, and the mechanical properties determined. Panels were heavily fouled with barnacles and other marine growth for all exposure periods. The fouling had little apparent effect on the pitting depth of the aluminum in tidal immersion, but generated heavy etching on some alloys during the 5 to 10 year exposure interval. Little change in tensile properties after ten years' exposure was noted for any of the test alloys. Tensile losses reported for several alloys after full immersion include: 5086-0 (3.7 percent), 5154-H38 (5.1 percent), and 5457-H34 (5.2 percent). The high-purity 1199 and alloys 5154-H38, 5456-0, and 5456-H321 showed losses of 16 to 27 percent in elongation while the change in elongation for the 5086-0 was a decrease of about 6 percent. Confirming other work, we found the corrosion rates to be greater for the full seawater immersion than for the tidal immersion. The lowest ten-year corrosion rate in the half-tide location was 0.014 mpy (mils per year) and the 5456-0, and the highest rate for the aluminum-magnesium 5000 series was 0.051 mpy for the 5456-H321 alloy. In full-immersion tests, the lowest rate was 0.064. For comparison, the rates for the 1199 alloy were 0.036 mpy in tidal and 0.061 mpy in full immersion. Corrosion weight losses and depth of pitting had reduced rates of growth over the interval between five years and ten 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 ten years was 72.0 mils for 5456-H321 in half-tide immersion and 131.0 mils for the same alloy (0 temper) in full immersion. Least apparent pitting depths occurred on alloys 5154-H38, 5457-H34, 5086-0 and high-purity 1199 where heavy etching tended to hide pits on the ten-year full-immersion panels.