4.6: PITTING CORROSION

4.6.1 DEFINITION

It is a form of localized corrosion of a metal surface where small areas corrode preferentially leading to the formation of cavities or pits, and the bulk of the surface remains unattacked. Metals which form passive films, such as aluminum and steels, are more susceptible to this form of corrosion. It is the most insidious form of corrosion. It causes failure by penetration with only a small percent weight-loss of the entire structure. It is a major type of failure in chemical processing industry. The destructive nature of pitting is illustrated by the fact that usually the entire system must be replaced.

4.6.2 ENVIRONMENT

Generally, the most conducive environment for pitting is the marine environment. Ions, such as Cl ^-, Br ^- and I ^-, in appreciable concentrations tend to cause pitting of steel. Thiosulfate ions also induce pitting of steels.

Aluminum also pits in an environment that cause the pitting of steel. If traces of Cu ²⁺ are present in water, or Fe ⁺³ ions are in water, copper or iron would be deposited on aluminum metal surface and pitting would be initiated. Oxidizing metal ions with chloride, such as cupric, ferric and mercuric, cause severe pitting. Presence of dust or dirt particles in water may also lead to pitting corrosion in copper pipes transporting seawater. With soft water, pitting in copper occurs in the hottest part of the system, whereas with hard waters, pitting occurs in the coldest...