ssc-452 aluminum structure design and fabrication guide ship

Aluminum Marine Structure Guide
Vreeland and Ferrara (1969) made an inspection of 72 aluminum-hulled vessels and the
maintenance records of 88 Vietnam-based PCF patrol craft. On the PCF’s, significant corrosion
of the 5456-H321 plate in the interior bilge areas was observed after 3 to 30 months of service,
with the average being 14 months. The corrosion consisted of exfoliation of the plate, with no
exfoliation or other corrosion observed on either extrusions or weld zones in the same areas. Ten
hulls of vessels constructed of 5086 alloy were also examined, with some pitting but no
exfoliation observed. However, these vessels all had service times of two years or less.
As mentioned above, the H116 temper of exfoliation-prone alloys was developed to
eliminate the exfoliation problem. As was discussed in section 2.2.1, it is important that an alloy
with a magnesium content greater than 3 percent be tested through the ASTM G 66 test to ensure
that it has no exfoliation corrosion susceptibility. However, the ASTM B 928 covers the H116
and H321 tempers, and the IACS requirements discussed above cover the 0, H111, H112, H116
and H321 tempers. All of these requirements are for rolled plate only, not extrusions. Also,
because the 6xxx-series are not prone to exfoliation, there are no testing requirements for these
alloys.
2.2.3 Intergranular Corrosion
As mentioned above, precipitation of magnesium to grain boundaries will increase the
susceptibility to intergranular corrosion. It is important that an alloy with a magnesium content
greater than 3 percent be tested through the ASTM G 67 (NAMLT) test to determine the
material’s susceptibility to intergranular corrosion.
Susceptibility to intergranular corrosion can result from sensitization of the alloy over
time in service or through improper treatment during production. In testing programs, the
method generally used to evaluate the propensity of a 5xxx-series alloy to sensitize is to heat it to
100 0 C (212 0 F) and hold at that temperature for one week. This thermal exposure is used to
simulate precipitation that would occur in long-term service at ambient temperatures.
2.4.4 Stress-Corrosion Cracking
Czyryca and Vassilaros (1972) provide a description of stress-corrosion cracking. When
alloys of certain susceptible metallurgical structures are subjected to a sustained tensile surface
stress (external or residual) in a corrosive environment, failure by stress-corrosion cracking can
occur with time. The mechanism of stress corrosion involves both electrochemical and
mechanical processes. Local cell action creates a sharp pit, which may induce mechanical
tearing at the root, thus exposing a fresh metal surface to accelerated corrosion. Further
corrosion deepens the crack and continues the process. Stress corrosion in aluminum alloys is
characterized by intergranular cracks normal to the metal surface and stress direction. The
electrochemical processes involved in stress-corrosion cracking of the alloys are the same as
those in intergranular corrosion; i.e., selective attack of grain boundaries sensitized by
precipitates or of areas adjacent to grain boundaries which are highly anodic due to depletion of
certain elements. However, not all alloys subject to intergranular corrosion will undergo stress
corrosion. Stress corrosion causes a brittle-type failure in an otherwise ductile material.
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