The appearance of corrosion will vary with the
metal involved. The following discussion includes brief
descriptions of typical corrosion product characteristics.
These descriptions are only for the most common
materials used in gas turbine propulsion and support
Iron and Steel
Possibly the best known and most easily recognized
of all forms of metal corrosion is the familiar
reddish-colored iron rust. When iron and its alloys
corrode, dark iron oxide coatings usually form first.
These coatings, such as heat scale on steel sheet stock
and the magnetite layer that forms on the inside of boiler
tubes, protect iron surfaces rather efficiently. However,
if sufficient oxygen and moisture are present, the iron
oxide is soon converted to hydrated ferric oxide, which
is conventional red rust. Hydrated ferric oxide, red rust,
does not protect surfaces. It destroys surfaces.
Aluminum and its alloys exhibit a wide range of
corrosive attacks, varying from general etching of
surfaces to penetrating attacks along the internal grain
boundaries of the metal. The corrosion products of
aluminum are seen as white-gray powdery deposits.
Copper and Copper Alloys
Copper and its alloys are generally corrosion
resistant, although the products of corrosive attack on
copper are commonly known. Sometimes copper or
copper alloy surfaces will tarnish to a gray-green color,
while the surface will remain relatively smooth. This
discoloration is the result of the formation of a
fine-grained, airtight copper oxide crust, called a patina.
Patina offers good protection for the underlying
metal in ordinary situations. However, exposure of
copper alloys to moisture or salt spray will cause the
formation of blue or green salts called verdigris. The
presence of verdigris indicates active corrosion.
Cadmium and Zinc
Cadmium is used as a coating to protect the area to
which it is applied and to provide a compatible surface
when the part is in contact with other metals. The
cadmium plate supplies sacrificial protection to the
underlying metal because of its great activity. During
the time it is protecting the base metal, the cadmium is
intentionally being consumed. Zinc coatings are used
for the same purpose, although to a lesser extent. Attack
is evident by white-to-brown-to-black mottling of the
surfaces. These indications do NOT indicate
deterioration of the base metal. Until the characteristic
colors peculiar to corrosion of the base metal appear, the
coating is still performing its protective function.
Nickel and Chromium Alloys
Nickel and chromium alloys are also used as
They are used as electroplated
coatings and as alloying constituents with iron in
stainless steels and with other metals such as copper.
Nickel and chromium plate provide protection by the
formation of an actual physical noncorrosive barrier
over the steel. Electroplated coatings, particularly
chromium on steel, are somewhat porous. Eventually,
corrosion starts at these pores unless a supplementary
coating is applied and maintained.
TYPES OF CORROSION
As stated previously, corrosion may occur in several
forms, depending upon the metal involved, its size and
shape, its specific function, the atmospheric conditions,
and the corrosion-producing agents present. Those
corrosion types described in this section are the most
common forms found on gas turbine engines and
Direct Surface Attack
The surface effect produced by reaction of the metal
surface to oxygen in the air is a uniform etching of the
metal. The rusting of steel, tarnishing of copper alloys,
and the general dulling of aluminum surfaces are
common examples of direct surface attacks. If such
corrosion is allowed to continue unabated, the surface
becomes rough, and in the case of aluminum, frosty in
appearance. Direct surface attack is sometimes referred
to as uniform etch corrosion.
Galvanic corrosion is the term applied to the
accelerated corrosion of metal caused by dissimilar
metals being in contact in a corrosive medium.
Dissimilar metal corrosion is usually a result of
faulty design or improper maintenance practices which
result in dissimilar metals coming in contact with each
other. This is usually seen as a buildup of corrosion at