TO 35-1-3 - Robins Air Force Base

TO 35-1-3
Figure 1-7. Example of Pitting Corrosion
Figure 1-6. Example of Galvanic Corrosion - Steel Hinge
Bolted to an Aluminum Door
1.7.3 Pitting Corrosion. The most common corrosion on
aluminum and magnesium alloys is called pitting corrosion
(see Figure 1-7). It is first noticeable as a white or gray powdery
deposit, similar to dust, that blotches the surface. When
the deposits are cleaned away, tiny pits or holes can be seen in
the surface. Pitting corrosion may also occur in other types of
alloys. The combination of small active anodes to large passive
cathodes causes severe pitting. Pitting starts as an
advanced form of uniform etch. It can worsen into very severe
corrosion that results in localized and relatively deep holes in
the metal. Pits are isolated holes that can vary in shape and
size and usually take several months to a year before they
become visible. Pitting is an extremely destructive and insidious
form of corrosion. It causes equipment to fail because of
perforation with very little weight or material loss on the
entire structure. It is also difficult to detect because the pits are
often covered with corrosion products. Pitting usually initiates
at anodic points on a metal surface such as breaks in protective
coatings or films, scratches, and irregularities that are exposed
to an aggressive electrolyte. Even though a pit is slow to initiate,
once started, it will progress at an ever-increasing rate
and will tend to undercut the surface as it grows.
1.7.4 Intergranular Corrosion. Intergranular corrosion (see
Figure 1-8) is an attack on the grain boundaries of the metal. A
highly magnified cross-section of any commercial alloy shows
the granular structure of the metal. This consists of quantities
of individual grains, each having a clearly defined boundary
that chemically differ from the metal within the grain. The
grain boundaries are frequently anodic (i.e., tend to corrode
more easily) to the metal within the grain. When in contact
with an electrolyte, rapid corrosion occurs at the grain boundaries.
1.7.5 Exfoliation Corrosion. Exfoliation corrosion (see Figure
1-9) is an advanced form of intergranular corrosion and
occurs when the surface grains of a metal are lifted up by the
force of expanding corrosion products occurring at the metal
grain boundaries. The lifting up or swelling is visible evidence
of exfoliation corrosion. Exfoliation occurs on extruded,
rolled, wrought, and forged high strength aluminum and magnesium
parts.
1.7.6 Concentration/Crevice Corrosion. Crevice corrosion
(see Figure 1-10 and Figure 1-11) occurs when the electrolyte
in a crevice has a different concentration than the area
adjacent to the crevice. This type of corrosion is also known as
concentration cell corrosion. Electrolyte inside the crevice
contains less oxygen and more metal ions than electrolyte just
outside the crevice. As a result, the metal surfaces have different
activities, even though they may be part of the same metal,
and corrosion occurs inside the crevice. This form of corrosion
often occurs between faying surfaces or when a surface is
covered by a foreign material (such as dirt) or under gaskets,
rubber, or plastic tape. The mechanisms involved in crevice
corrosion are very similar to pitting corrosion. Crevice corrosion
most often occurs where there are stagnant solutions
(electrolytes) in holes, gasket surfaces, lap joints, under surface
deposits (sand, dirt, corrosion products, etc.), and in crevices
under bolt and rivet heads. To become a site for corrosion,
a crevice must be wide enough to allow liquid entry but narrow
enough to keep the liquid stagnant. As with pitting, crevice
corrosion is slow to start, but once started, progresses at
ever increasing rates (due to various electrochemical factors).
Crevice corrosion is also most intense in solutions containing
chlorides (saltwater). Metals susceptible to pitting (stainless
steel, aluminum, magnesium) are also susceptible to crevice
corrosion. However, not all metals that are susceptible to crevice
corrosion will be susceptible to pitting. Three general types
of crevice corrosion are: Metal ion concentration cells, Oxygen
concentration cells, Active-passive cells.
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