Rouge Types: What Are The Three Main Classes Of Rouge?

Stainless steel tubes and applications can sometimes develop rust-coloured surface corrosion, or rouge, under certain conditions. Removing this corrosion – or derouging stainless steel– is crucial for preventing damage to your systems and equipment and maintaining product safety, as untreated rouge can degrade surfaces and contaminate products. In this article, we’ll look at the three types of rouge and how they can be treated.

Class One – Yellow / Orange Rouge:

Class one, or ‘migratory rouge’ is a type of powdery deposit that frequently originates from pump erosion or corrosion within stainless steel pipework. If untreated, the rouge can migrate throughout the system, causing further contamination and potentially damaging your products. Pharmaceutical water systems are particularly vulnerable to this type of rouge contamination. Fortunately, class one rouge is also one of the easiest to treat. Because of the weak attachment of the rouge particles to surfaces, the reddish or orange dust can be easily removed through chemical cleaning, followed by passivation to prevent recontamination. The colour of class one rouge is caused by a high proportion of iron oxides, which can also cause discolouration in water systems, making them appear a rusty yellow or orange hue.

Class Two – Gold / Red / Brown Rouge:

Class two rouge consists mainly of hematite (Fe2O2) and exists both in particulate and surface-bound form. The colour spectrum of the rouge layer ranges from orange-red through blue and lilac to grey. Can be removed only partly by wiping. As a rule, chemicals must be used for removal. Class 2 rouge is commonly found in systems that contain liquids that have a low oxygen concentration, at elevated temperatures (WFI for example). The dynamic formation and decomposition of the passive layer is disturbed and the chromium-oxide matrix is converted to an iron-oxide matrix .

Class II rouge can be more challenging to remove than class one because it involves chemical reactions that compromise the integrity of the stainless steel’s protective layer. Traditionally, it would require aggressive, crossive chemistry to remove. However, to ensure equipment longevity, a non-aggressive, pH-neutral product that takes the rouge off without damaging the base material is ideal. Post-removal passivation is needed to restore the chrome-oxide layer.

Class Three – Grey / Black / Purple Rouge:

Class three rouge is recognised by its black or deep purple colour, and is typically associated with high-temperature environments, such as steam pipework and heating systems. The black colouration arises from the high concentration of magnetite/iron oxide and silica components that form at these higher temperatures, and its silica content makes class three rouge resistant to standard chemical removal. Class three rouge sticks tightly to shiny surfaces, whereas on rough surfaces it can appear powdery. Removal can require aggressive chemical cleaning or mechanical polishing, followed by re-passivation.

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At Inox, we offer a range of services for passivation and derouging stainless steel tubes, applications, and equipment. To find out more or to request a quote, please get in touch today.