Page 37 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

LOCALIZED CORROSION 15

Localized attack

Base Noble
metal metal

Pitting Crevice corrosion

Figure 1.9 Various forms of localized corrosion. (Reproduced by permission, National Asso-
ciation of Corrosion Engineers International (3).)

these two forms of corrosion is nearly the same. The morphological differences may
be because of the different causes in the initiation step involved in pitting or crevice
corrosion (2).

1.4.1 Pitting Corrosion
Pitting corrosion may be caused by a number of factors such as mill scale, which
is a very common form of pitting corrosion of steel. Areas where a brass valve is
located in steel or galvanized pipeline form good examples. The junction of the two
areas is usually pitted, and if the pipe is threaded, the thread in close contact with
the brass valve undergoes pitting leading to a leak. This form of corrosion damage
occurs frequently in industry, as well as in homes and farms. The deep pitting of
tankers on the horizontal surfaces of cargo ballast tanks is a particularly aggravated
pitting involving deep and frequent pits. The pits result from frequent changes of
cargo and salt water that perpetrate the oxidation reduction corrosion cycle (15).
Pitting can also occur under atmospheric conditions. The corrosion can start at the
break and continue to undercut the coating, forming a rather heavy tubercle of hard
rust or scale with the pit underneath the original metal. The corrosion products help to
isolate the aggressive medium in the pit. This type of corrosion is common in marine
environments as well as in other industrial environments where strong corrosive con-
ditions exist (15). Also, pits with open mouths (uncovered) exist and are responsible
for loss of thickness and can also act as stress raisers.
Pitting corrosion is of concern in applications involving passivating metals and
alloys in aggressive environments. Pitting corrosion can also occur in nonpassivating
alloys with protective coatings or in certain heterogeneous corrosive media. Although
pitting corrosion may appear harmless and less severe in appearance, the depth of the
pit and the pit propagation rate are extremely dangerous and happen to be one of
the most serious forms of corrosion. When the pits and crevices are active over an
extended period of time, rapid dissolution of the metal occurs. The resulting pit and
crevice geometries as well as the surface states within the pits vary markedly from