pitting corrosion

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.

References/Literature:

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.

Links:

Testing for localized attack by the "Corrosion Doctors"
www.corrosion-doctors.org/Localized/Localized.htm