Pitting corrosion is a localized form of
corrosion; the bulk of the surface remains unattacked. Pitting is often found in
situations where resistance against general corrosion is conferred by passive surface
films. Localized pitting attack is found where these passive films have broken down.
Pitting attack induced by microbial activity, such as sulfate reducing bacteria (SRB) also
deserves special mention.
Within the pits, an extremely corrosive micro-environment tends to be
established, which may bear little resemblance to the bulk corrosive environment. For
example, in the pitting of stainless steels in chloride containing water, a
micro-environment essentially representing hydrochloric acid may be established within the
pits. The pH within the pits tends to be lowered significantly, together with an increase
in chloride ion concentration, as a result of the electrochemical pitting mechanism
reactions in such systems.
The detection and meaningful monitoring
of pitting corrosion usually represents a major challenge. Pitting failures can occur
unexpectedly, and with minimal overall metal loss. Furthermore, the pits may be hidden
under surface deposits, and/or corrosion products.
Monitoring pitting corrosion can be further complicated by a distinction
between the initiation and propagation phases of pitting processes. The highly sensitive
electrochemical noise technique may provide early warning of imminent damage by
characteristic signals in the pit initiation phase.
A.J. Sedriks: "Corrosion of Stainless Steels, 2nd Edition", John
Wiley, New York, 1996.
Z. Szlarska-Smialowska: "Pitting Corrosion of Metals", Nace
International, Houston, 1986.
Testing for localized attack by the "Corrosion Doctors"