Rouge is a by product of corrosion and occurs on stainless steel surfaces when exposed to a corrosive environment either from an external source or from the destruction of the passive layer.

The primary constitutes of rouge are iron oxides (ferric oxide FE203, or ferrous oxide FE304), but may include iron, chromium, nickel and other elements.

Initially, rouge may slough off, but eventually it will adhere to the surface as a glossy coating that is not removed by wiping.

The surface corrosion may eventually contribute to increased microbial contamination and further deterioration of the stainless surface.