UFC 4-150-07
19 June 2001
3-5.3
and to cavitation damage. These materials are used for specialized applications
in springs, cable connectors, expansion joints, rupture disks, valves, fasteners,
heat exchangers, and piping. The high cost of these materials makes them
unsuitable for bulk construction at the waterfront.
The most common nickel alloys used are: Inconel Alloy 625 (nickel-
chrome alloy), Hastelloy Alloy "C" (nickel-chrome-molybdenum alloy), and Monel
400 (nickel-copper alloy). Inconel and Hastelloy "C" are essentially immune to
corrosion in marine environments. Monel 400 has good corrosion resistance
when it has been cathodically protected with a more active metal. If not
protected, Monel will develop pitting and crevices.
3-5.4
and alloys will corrode and develop pits and crevices under normal atmospheric
conditions. In general, these metals are not given preservative coatings, but may
be painted for color/appearance in certain uses. Corrosion rates can be greatly
accelerated when two or more dissimilar metals are in contact with each other
and exposed to a corrosive environment. Particularly when they are buried or
submerged, accelerated corrosion of one of the metals can occur due to an
electrochemical reaction called galvanic corrosion. Galvanic corrosion rates
depend on the metals' electrical properties and the medium in which the metals
are exposed. Galvanic series tables have been developed indicating lists of
metals in order of decreasing corrodibility when exposed to a certain solution or
medium. Generally, the closer one metal is to another in the galvanic series, the
lower the corrosion rate with the more active metal, and conversely, the further
apart, the greater will be the corrosion rate of this metal.
3-5.5
Preventive Maintenance for Nonferrous Metals and Alloys. PM
measures depend on whether or not dissimilar materials are involved:
Similar Nonferrous Metals and Alloys. The only formal PM measure
is regular and careful inspection to determine the condition of the
component. In the later part of a component's life expectancy, it
may prove economical to coat the metal to prevent further corrosion
rather than to replace the component.
Avoiding Galvanic Corrosion. Galvanic corrosion can effectively be
eliminated by making the structure out of as much of the same
metal as possible, placing a protective insulator between the two
dissimilar metals, and by providing cathodic protection if buried or
immersed. When galvanic corrosion cannot be effectively
eliminated, it can be reduced by using appropriate protective
coatings on the steel, ensuring not to coat the anodes. Detailed
information on treatments of galvanic corrosion is given in MIL-
STD-889B, Dissimilar Metals for metal exposed to the saltwater
environment.
3-24
Previous Page
