Rouging

Rouging is a form of corrosion found in stainless steel.[1] It can be due to iron contamination of the stainless steel surface due to welding of non-stainless steel for support columns, or other temporary means, which when welded off leaves a low chromium area.[2]

There are three classes of rouging: Class I, Class II, and Class III.[3]

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.

References
  1. Rivera, Hadziselimovic, Lopolito (2017-02-15). "Preventive and Corrective Maintenance for Rouge in Stainless-Steel Equipment". PharmTech. Retrieved 2019-04-12.CS1 maint: multiple names: authors list (link)
  2. "Rouging". corrosion-doctors.org.
  3. "Rouging of Stainless Steel in High-Purity Water", Corrosion: Environments and Industries, ASM International, 2006, pp. 15–22, doi:10.31399/asm.hb.v13c.a0004103, ISBN 9781627081849
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