23 July 2003
The oxide film that forms on the surrounding surface will
act as a cathode and the crevice will act as an anode that
accelerates the corrosion within the crevice.
corrosion will attack one or more of the components of the
alloy, which will result in weakening of the material.
coupled in the marine environment, one will act as an
anode and the other will act as a cathode depending on the
relative positions of the metals within the galvanic
series and the relative size of the exposed metal surfaces
of each. This will accelerate corrosion. The applicant
should consider this when choosing materials to use in the
design of the system.
Stress corrosion - certain alloys are susceptible to
stress corrosion cracking, which can only occur when the
material is exposed to a corrosive environment while under
tensile stress. In the case of some high strength steels
and titanium alloys, this form of corrosion can be
propagated at highly accelerated rates, depending on
Pitting - pitting can be a serious problem in pressure
vessels and components. Pitting is normally limited to
alloys of steel and aluminum. Hatch and view port seating
surfaces have been known to leak due to pitting.
Painting, anodizing and plating are all common and cost-effective
methods of corrosion protection. These processes provide only a
thin layer protection, however. When this surface protection is
scratched, the exposed bare metal is subject to accelerated local
B.5.2 Protective Finishes. Finishes applied to pressure
containing and critical load bearing elements should not be of a
type likely to permit the development of hidden pitting. The
application of all protective finishes to surfaces in contact
with breathing gases should be performed in accordance with
written and approved procedures.
Metal applied as a surface finish, coating or cladding should be
lower on the electrochemical scale than the metal to which it is
applied. Clad welding is a process that deposits corrosion
resistant metal over the area to be protected. This process can
be used to protect critical hatch and view port seating surfaces
The applicant should justify that use of dissimilar metals in
contact, where dissimilar metals are defined by MIL-STD-889,
Dissimilar Metals, does not present an unacceptable corrosion