a.
Any Class 2 or 3 material which cannot be shown to be justified
under the restrictions imposed by the ASME Boiler and Pressure Vessel Code or
other Navy requirements.
b.
Magnesium and magnesium alloys.
c.
Beryllium.
d.
Steel: ASTM SA-36, SA-283, and SA-515 plate.
e.
Any material which will enter into a rapid chemical reaction with
seawater, gaseous oxygen, or any other media to be used inside the vessel.
f.
Any material whose toughness at the minimum operational temperature
is deemed insufficient.
5.
MATERIAL COMPATIBILITY. When two or more materials are to be joined or
are to be in close proximity, the designer must consider the effect of one
material on the other particularly in terms of the environment to which they
will be subjected.
a.
Welded and Brazed Joints. The past few years have seen a
pronounced tendency to join the components of critical piping systems with
welded or brazed joints. Although such joints are usually more reliable than
separable joints, the use of dissimilar materials can cause failures. The
piping designer is responsible for insuring correct specification of approved
joining materials and procedures. The ASME Code contains approved joining
materials and procedures for the materials approved by the Code. For other
materials, the piping designer must identify pertinent joining materials and
procedures approved by industry or by a military specification.
b.
Galvanic Corrosion. Care must be exercised when coupling
dissimilar metals such as steel pipe to aluminum fittings to avoid galvanic action
or galling. Coupling dissimilar metals subject to an electrolyte such as
seawater requires careful consideration of the battery action created and the
relative area affect of the materials. Good guidance is contained in The
Corrosion Handbook by Herbert H. Uhlig (Reference 4), and MIL-STD-889
Dissimilar Metals (Reference 5).
Experience has revealed accelerated general corrosion in the vapor space and
entrance igloo above wet pots due to poor material choices for hatch covers
and piping.
The practice of building a shed over bottle fields and pipe runs accelerate
corrosion in a marine atmosphere. The material under the shed is subject to
salt buildup from seawater spray and salt laden fog. The roof prevents the
rain from washing off the salt and drying from the sunshine. Most materials
and specifically weathering steel are most corrosive resistant when boldly
exposed to sun, rain, and wind. Alternate wetting and partial drying
provides the most corrosive environment.
6.
FLUID COMPATIBILITY. Two fluids found in hyperbaric chamber piping
systems, oxygen and seawater, present particularly difficult compatibility
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