Rouging is an unwanted corrosive phenomena on stainless steel surfaces.
Stainless Steels are corrosion-resistant, but they are not completely impervious to rusting. One common mode of corrosion in corrosion-resistant steels is when small spots on the surface begin to rust because grain boundaries or embedded bits of foreign matter (such as grinding swarf) allow water molecules to oxidize some of the iron in those spots despite the alloying chromium. This is called rouging.

Unwanted corrosive phenomena on stainless steel surfaces like rouging had been observed for years occurring at certain service conditions. Rouging consists in the formation of a colored surface film containing mainly iron on metallic surfaces in contact with (high-purity) waters. In spite of numerous hypotheses and experimental examinations a clear explanation about the origin of rouging on stainless steels is currently still lacking. Because rouging occurs even at well passivated stainless steel surfaces the origin of this corrosive phenomenon should be related to the weakness of the stability of the passive film (low or no passivity).