Rouging refers to a form of corrosion found in stainless steel. It can be due to iron contamination of the stainless steel surface due to welding of ferrous steel for support columns, or other temporary means, which when welded off, leaving a low chromium area.[1]
There are three classes of rouging: Class I, Class II, and Class III.
Class I - stainless steel surface and the Cr/Fe ratio of the metal surface beneath such deposits usually remain unaltered.
Class II - Iron particles originating in-situ on unpassivated or improperly passivated stainless steel surfaces. By their formation the Cr/Fe ratio of the metal surface is altered.
Class III - Iron oxide (or scale) which forms on surfaces in high temperature steam systems. The Cr/Fe ratio of the protective film is usually altered.