ssc-452 aluminum structure design and fabrication guide ship

Aluminum Marine Structure Guide
Figure 2-3 Example of exfoliation on the edge of a waterway bar
(Dye and Dawson, 1974).
No further corrosion problems were noted for many years with aluminum ships and craft
when constructed and maintained to avoid corrosion problems. (See Chapter 12 for problems
associated with maintenance and repair.) There have been recent problems, however. In 2001,
one producer of aluminum plate made a slight change to the processing of 5083-H321 plate. The
resulting plate was susceptible to intergranular corrosion and intergranular stress corrosion
cracking. Consequently, numerous vessels required either extensive replacement of plate, or in
many cases, complete scrapping of the hull structure. The corrosion was especially noted at
welds, and intermittent welds showed more of this type of corrosion and associated stress
corrosion cracking than did continuous welds below the waterline.
Intergranular corrosion of 5xxx-series aluminum alloys is caused by -phase precipitating
into the grain boundaries of the metal. Intergranular corrosion is defined as “Preferential
corrosion at or adjacent to the grain boundaries of a metal or alloy.” (ASTM G15-93). Figure 2-
4 is an example of intergranular stress corrosion cracking, showing a typical semi-circular crack
at an intermittent weld, what investigators called “smiley face cracks.” The cracks initiated at
localized stress concentrations such as the ends of intermittent welds and at other details that had
higher stress concentrations.
Resistance to intergranular, exfoliation, and stress corrosion also is influenced by the
grain structure of the product, which is determined by the fabrication process. Some fabrication
paths result in a fibrous elongated grain structure that is unrecrystallized, while others result in a
recrystallized equiaxed grain structure. If continuous grain boundary precipitate networks are
avoided, either microstructure will be resistant to intergranular forms of corrosion. However, if a
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