01 March 2005
should be limited to metal surfaces from which the
cadmium has been depleted. Care should be taken not
to remove undamaged cadmium plate adjacent to the
3-10.8. CHROMIUM. Chromium is used as a protective
plating. Chromium plating is also used to provide a
smooth, wear-resistant surface and to reclaim worn
parts. When corrosion resistance in a marine
environment is required, a nickel under-coat is used.
The degree of protection is dependent upon plating
A blue oxide coating will form at 700 to 800F (370 to
thickness. Chromium forms a continuous oxide coating
425C), a purple oxide at 800 to 950F (425 to
that can be polished to a high luster and still protect not
510C), and a gray or black oxide at 1000F (540C) or
only itself but any underlying metal. Chromium coatings
higher. These are protective discolorations and should
contain cracks, and corrosion may originate at the base
not be removed. Since titanium is the less active member
(cathodic) of most dissimilar metal couples, it can greatly
results of a failed chromium plate.
accelerate corrosion of a dissimilar metal coupled to it.
Electrical insulation between titanium and other metals
3-10.9. NICKEL. Nickel is important as a plating metal,
is necessary to prevent galvanic corrosion of the other
an additive to stainless steel, and a base for nickel
metal. Titanium in contact with a corroding metal can
alloys. Pure nickel is used as an electroless coating and
absorb hydrogen and become brittle. Frequent
is subject to pitting corrosion. Flaking of the nickel
inspection of such areas is required to insure that
coating can also occur when an underlying metal
insulation failure has not allowed corrosion to begin.
corrodes. When added to stainless steel alloys, the
3-10.7. CADMIUM. Cadmium is used as a coating to
protect steel hardware, such as bolts, washers, and
contents above 10%. Nickel based alloys are used in
screws, and as plating on electrical connectors. It is
high temperature areas (engines, afterburners), but
used as a plating on high strength steel parts (e.g.
they are subject to hot corrosion attack and embrittlement
landing gear) to improve resistance to corrosion fatigue.
when sulfur containing gases are present.
Cadmium may also used to provide a compatible surface
when a part is in contact with other materials. Cadmium,
3-10.10. COPPER AND COPPER ALLOYS. Copper
when coupled with steel, is anodic and protects the steel
and copper-based alloys (brass and bronze) are
by galvanic action. Corrosion on cadmium is evident
considered corrosion resistant, with corrosion usually
by white to brown to black mottling of the surface.
limited to staining and tarnish. Copper and copper-
When cadmium plate shows mottling and cracks in the
based alloys are generally used in avionic systems as
coating, the plating is still performing its protective
contacts, springs, leads, connectors, printed circuit
board (PCB) conductors, and wires. Generally, changes
cadmium plate on iron or steel is still protecting the
in surface conditions are not dangerous and should
base metal until signs of rust begin to appear. Even
ordinarily have no effect on the part. Copper corrosion
then, any mechanical removal of corrosion products
is evidenced by the accumulation of blue or blue-green