the joint between the metals. For example, when
aluminum pieces are attached with steel bolts and
moisture or contamination are present, galvanic
corrosion occurs around the fasteners.
The most common effect of corrosion on aluminum
alloys is pitting. It is caused primarily by variations in
the grain structure between adjacent areas on the metal
surfaces that are in contact with a corrosive
environment. Pitting is first noticeable as a white or
gray powdery deposit, similar to dust, that blotches the
surface. When the superficial deposit is cleaned away,
tiny pits or holes can be seen in the surface. These pits
may appear either as relatively shallow indentations or
as deeper cavities of small diameters. Pitting may occur
in any metal, but it is particularly characteristic of
aluminum and aluminum alloys.
Intergranular corrosion is an attack on the grain
boundaries of some alloys under specific renditions.
During heat treatment, these alloys are heated to a
temperature that dissolves the alloying elements. As the
metal cools, these elements combine to form other
compounds. If the cooling rate is slow, they form
predominantly at the grain boundaries. These
compounds differ electrochemically from the metal
adjacent to the grain boundaries. These altered
compounds can be either anodic or cathodic to the
adjoining areas, depending on their composition. The
presence of an electrolyte will result in an attack on the
anodic area. This attack will generally be quite rapid
and can exist without visible evidence.
As the corrosion advances, it reveals itself by lifting
up the surface grain of the metal by the force of
expanding corrosion products occurring at the grain
boundaries just below the surface. This advanced attack
is referred to as EXFOLIATION. Recognition and
necessary corrective action to immediately correct such
serious instances of corrosion are vital. This type of
attack can seriously weaken structural members before
the volume of corrosion products accumulate on the
surface and the damage becomes apparent.
Fretting is a limited but highly damaging type of
corrosion caused by a slight vibration, friction, or
slippage between two contacting surfaces that are under
stress and heavily loaded. Fretting is usually associated
with machined parts such as the contact area of bearing
surfaces, two mating surfaces, and bolted assemblies.
At least one of the surfaces must be metal.
In fretting, the slipping movement at the interface
of the contacting surface destroys the continuity of the
protective films that may be present on the surfaces.
This action removes fine particles of the basic metal.
The particles oxidize and form abrasive materials that
further accumulate and agitate within a confined area to
produce deep pits. Such pits are usually located where
they can increase the fatigue potential of the metal.
Fretting is evidenced at an early stage by surface
discoloration and by the presence of corrosion products
in any lubrication. Lubricating and securing the parts
so that they are rigid are the most effective measures for
the prevention of this type of corrosion.
Stress, evidenced by cracking, is caused by the
simultaneous effects of tensile stress and corrosion.
Stress may be internal or applied.
Internal stresses are produced by nonuniform
deformation during cold working conditions, by
unequal cooling from high temperatures during heat
treatment, and by internal-structural rearrangement
involving volume changes. Stresses set up when a piece
is deformed. Examples of internal stresses include
those induced by press-and-shrink fits and those in rivets
Concealed stress is a more dangerous condition than
design stress. Concealed stress corrosion is difficult to
recognize before it has overcome the design safety
factor. The magnitude of the stress varies from
point-to-point within the metal. Stresses in the
neighborhood of the yield strength are generally
necessary to promote stress corrosion cracking, but
failures may occur at lower stresses.
Fatigue is a special type of stress corrosion. It is
caused by the combined effects of corrosion and stresses
applied in cycles. An example of cyclic stress fatigue is
the alternating loads to which the connecting rod of a
double-acting piston in an air compressor is subjected.
During the extension (up) stroke a compression load is
applied, and during the retraction (down) stroke a tensile
or stretching load is applied. Fatigue damage is greater
than the combined damage of corrosion and stresses.
Fracture of a metal part due to fatigue corrosion