Pitting and sulphide inclusions in steel

Pits are initiated at sulphide inclusions (usually MnS) in both carbon steels and stainless steels. In carbon steels, the attack starts in the matrix close to the sulphide inclusion which is more noble than the matrix. Some sulphides, termed “active”, initiate attack more easily than others, termed “inactive”. Active and inactive sulphides in carbon steels may be distinguished by a so-called micro-corrosion test (MCT). In stainless steels, the attack starts in the sulphide inclusion proper, which is less noble than the surrounding oxide film, and then spreads to the active metal below the sulphide inclusions. The propagation of a pit is, in principle, the same in both types of steel and depends upon the formation of a concentration cell in which the pit solution has a higher salt content, a higher acid content and a lower oxygen content than the surrounding bulk of the solution. The detrimental effect of S upon the corrosion properties of steels is counteracted by metals which form sulphides less soluble than FeS. These sparingly soluble sulphides may either be present in the steel (examples: TiS, CrS and VS) or form during the corrosion process (examples: the sulphides of W, Mo, Ni, Co, Zn, Cd, Sn, Pb, Cu, Ag, Bi and Hg). Of the latter metals, Cu is of particular importance.