ssc-452 aluminum structure design and fabrication guide ship

Aluminum Marine Structure Guide
exposure. Because of the magnitudes of the corrosion rates reported by Ailor above, it is
possible that those samples were in a sensitized condition.
The general corrosion rate of 6xxx-series is not as well established, despite anecdotal
evidence that it is not as good in seawater as the 5xxx-series. One source of information comes
from testing 6061-T6 in flowing seawater for one year (Rausch, 1958) who reported a corrosion
penetration rate of 0.037 mm (1.2 mils) per year, which is comparable to 5xxx-series alloys. In
another study (Wacker and Chu, 1970) the measured depth of corrosion was 0.025 mm (1 mil)
per year for 6061-T652 forgings after 6 months in flowing seawater.
The difference in corrosion resistance of 6xxx-series alloys compared to 5xxx-series is
exhibited more in the extent and depth of pitting that occurs after exposure. Goddard et al.
(1967) report that the maximum pit depth in three 5xxx-series alloys (5052, 5056, and 5083) was
0.18 and 0.86 mm (7 mils and 34 mils) after five and ten years of immersion in seawater,
respectively. In the same tests, 6061-T6 had 1.65 and 1.30 mm (65 and 51 mils) of pit depth
when samples were removed after 5 and 10 years of immersion.
The maximum pit depth of the 5xxx-series alloys studied by Ailor (1967) were in the
5456-H321 alloy, which had maximum pit depth of 0.74, 1.14, 0.96 mm (29, 45, and 38 mils)
after 1, 2, and 5 years of exposure. Leveau (1965) reported a maximum pit depth in several
5xxx-series alloys of 0.86 mm (34 mils) after 8 years of exposure to seawater, and 0.36 mm (14
mils) maximum pit depth in 6061-T6 after 8 years. Taylor et al. (1984) had samples of 6061-T6
immersed in quiet seawater for 120 days, after which the measured depth of the deepest pit was
0.034 mm (1.3 mils). Basil (1957) reported pits in 6061-T6 that was exposed for one year in
quiet seawater that were mostly 0.076 mm (3 mils) deep, but one pit was 0.64 mm (25 mils)
deep.
In comparative testing of 6005A-T61 extrusions and 6061-T6 extrusions in accordance
with ASTM G85 Annex 3, (Tower, 2006) the 6005A showed maximum pit depth of 0.8 mm
(0.03 inches) and 6,800 pits per square meter (632/ft 2 ) and the 6061 showed maximum pit depth
of 1.5 mm (0.06 inches) and 8,000 pits per square meter (743/ft 2 ). The ASTM G85, Annex 3 test
is a cyclic spray test in acidified seawater. A 5 percent solution of synthetic seawater is acidified
with acetic acid to a pH between 2.8 and 3.0 and atomized as a fog into a heated cabinet
maintained at 120° F (49° C) in a cycle consisting of a 30-min. spray followed by a 90-min. soak
period at above 98% relative humidity. This test is used to determine time to perforation
primarily on aluminum alloy brazing sheet used to manufacture heat exchangers and is
presumably harsher than the ASTM B117 test, which is 120-day test in a continuous saltwater
mist. This comparative test of two 6xxx-series alloys points up the need for a standardized test.
An aluminum extruder, anxious to promote his product, used what seemed to be a good
standardized test. Unfortunately, is has no basis for comparison in a marine context, so the
results, while of some value on a comparative basis should not be relied upon for material
selection.
alloys.
The data on general corrosion or pitting do not support limiting the use of 6xxx-series
Lacking a standard for acceptable corrosion rates or extent of pitting, it is difficult to
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