Corrosion of cast iron which primarily destroys the ferritic and pearlitic matrix (see Metal matrix of cast iron) but retains the graphite and cementite, resulting in a spongy appearance of the affected surfaces.

It is also known as graphitization and is particularly typical for flake graphite cast iron. Spongiosis is frequently seen in underground water pipes. An affected pipe maintains its shape but loses strength. The surface assumes a spongy quality and becomes black like graphite. Fig. 1 shows a micrograph from a corroded pipe.

As can be seen, the metal matrix is decomposed, forming loose corrosion products which mainly consist of iron(ii) oxide. The graphite and the phosphide eutectic, on the other hand, are retained. Therefore, these two structural components form a kind of skeleton between the corrosion products, maintaining the casting shape.