ISPE Glossary of Pharmaceutical and Biotechnology Terminology

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In stainless steel systems utilized in the biopharmaceutical/life science industry, is a general term used to describe a variety of discolorations on the product contact surfaces, caused by variations in hydration agents and the formation of metallic (primarily iron) oxides and/or hydroxides from either external sources, or from alteration of the chromium rich “passive” layer.

Publication Source: ISPE Baseline® Guide, Vol. 4: Water and Steam Systems (First Edition)
Publication Date: 2001


A general term used to describe a variety of discolorations in high purity stainless steel biopharmaceutical systems. It is composed of metallic (primarily iron) oxides and/or hydroxides. Three types of rouge have been categorized:

Class I rouge: a rouge that is predominantly particulate in nature. It tends to migrate downstream from its origination point. It is generally orange to red-orange in color. These particles can be wiped off a surface and are evident on a wipe. Surface composition of the stainless steel under the rouge remains unchanged.

Class II rouge: a localized form of active corrosion. It occurs in a spectrum of colors (orange, red, blue, purple, grey, black). It can be the result of chloride or other halide attack on the surface of the stainless steel.

Class III rouge: a surface oxidation condition occurring in high temperature environments such as pure steam systems. The system’s color transitions to gold, to blue, to various shades of black, as the layer thickens. This surface oxidation initiates as a stable layer and is rarely particulate in nature. It is an extremely stable form of magnetite (iron sesquioxide, Fe3O4).

Publication Source: American Society of Mechanical Engineers (ASME), Bioprocessing Equipment (BPE) - 2009 Revision

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