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Graphitic corrosion is a form of de-alloying or selective leaching that occurs in cast iron material. This corrosion mechanism is unique to gray cast irons and is caused by the graphitic matrix present. The graphite is cathodic to the iron matrix. Exposure to an electrolyte results in leaching of the iron matrix, leaving behind a porous mass of graphite flakes.
Ductile, malleable, and white cast irons do not experience graphitic corrosion because of the absence of a graphitic network. A network is necessary to keep the corrosion products in place.
Graphitic corrosion often occurs in buried gray cast iron pipes, although exposure to an aqueous environment is all that is necessary for the de-alloying to occur. The process of de-alloying in cast iron typically is long-term, taking several years to occur. In fact, many cases of graphitic corrosioncaused failure in cast iron piping occur in piping that has been in service for 50 years or more.
Underground water mains and sewer treatment piping are examples of systems often subjected to graphitic corrosion. It is particularly evident in cast iron exposed to clay-based soils and soils containing sulfates.
Several cases...