Uniform Etch Effect. Most direct chemical attacks, as by an acid,
produce a uniform etch effect on the surface. This is first noticed
on a polished surface as a general dulling or loss of polish. If the
corrosion is not stopped, the surface becomes rough and possibly
frosted in appearance.
Pitting.
On aluminum and magnesium alloys, the usual effect of
corrosion is pitting.
A powdery white or gray deposit is the first
sign of pitting corrosion. When the deposit is removed, shallow pits
or holes can be seen in the surface. Pitting corrosion can occur on
other alloys as well.
Intergranular. Intergranular corrosion occurs at the metal's grain
boundaries. A magnified cross-sectional view of any alloy shows the
metal's granular structure.
Each grain has a clearly defined
boundary differing chemically from the metal within the grain center.
Adjacent grains of different elements reach an anode and cathode when
in contact with a conductive medium such as moisture.
Under this
condition, rapid selective corrosion at the grain boundary takes
place.
Exfoliation. Exfoliation, a form of intergranular corrosion, shows
itself by lifting a metal's surface grains. The lift is produced by
the force of expanding corrosion products at the grain boundaries
just below the surface.
Exfoliation corrosion is generally seen on
extruded sections where grain thickness is usually less than in
rolled forms.
Galvanic. Galvanic corrosion occurs when dissimilar metals are in
contact and an external circuit is completed by moisture and
The result is a corrosion buildup at the bimetal
juncture.
Concentration Cell. Concentration cell corrosion takes place when
two or more metal surface areas are in contact with different
concentrations of the same solution.
In this corrosion, three
general types of concentrations are recognized. They are the metal-
ion cells, oxygen-concentration cells, and active-passive cells.
Stress.
Stress corrosion cracking and fatigue corrosion are
related in that the latter is a special case of the former.
The
simultaneous effects of tensile stress and corrosion cause
stress corrosion cracking. Stresses can be internal or
applied.
Internal stresses are produced by nonuniform deformation
during cold working, unequal cooling from high temperatures,
and press and shrink fits, and application of rivets and bolts. The
combined effects of cyclic stress and corrosion produce
fatigue corrosion. No metal is immune to some reduction of its
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