The Myth of Rouge as a Protective Layer

In certain industries, it is believed that rouge is a protective layer that improves, instead of degrades, the durability of stainless steel. However, this is not true.

This mistaken belief is based principally on specific types of rouge assumed to be “stable” and remaining affixed to the surface — supposedly behaving in the same way as a coat of paint.

“Stable” rouge is a word used mainly for Class 3 rouge and specific categories of Class 2 rouge. With the wipe of a cloth, however, both of these are removed. The issue with the notion of “stable” rouge is two-fold, relating both to how long it will stay stable and what is happening beneath it.

In the second scenario, the rouge could be hiding worsening surface conditions underneath, or it may act as an anchor for microbial growth.

Image Credits: Shutterstock/SimoneN

A Recap on Rouge

Rouge is a type of iron oxide, like rust. In contrast to the thick, flaky kind people are familiar with on steel, rouge is usually a staining build-up on the stainless steel’s surface. In addition to a “rust red” color, rouge deposits occur in various shades ranging from orange and yellow to blue and black.

This color disparity results from the oxide's specific chemical composition, a mixture of iron (Fe), other metals, and oxygen (O) atoms. The color and type of the rouge are influenced by the water quality and temperature inside the system.

Yellow and red rouges are formed at lower temperatures. Black or gray deposits develop in steam systems. Rouge is more likely to form where temperatures repeatedly reach 60 °C or more.

Rouge can be divided into three classes:

Class 1 — Hematite (Fe2O3)

  • Red/orange, ferric oxide, loosely stuck to the surface
  • Deposition film from an upstream corrosion source in the system
  • In most cases, it  can be wiped off by hand

Class 2 — Hematite (Fe2O3)

  • Red/brown, ferric oxide, firmly stuck to the surface
  • Rouge that forms at the site where corrosion is occurring
  • The surface of the stainless steel is changed

Class 3 — Magnetite (Fe3 O4)

  • Black, ferrous oxide, firmly stuck to the surface
  • Corrosion from high-temperature applications or steam
  • Difficult to eliminate and necessitates aggressive chemistry

The Challenge that is Rouge

While rouge can be seen merely as a cosmetic issue in a number of cases, it is, at times, a more serious problem. Iron oxide corrosion creates particles that can chemically interact with products or ingredients. The FDA does not permit any visible particles, which can result in a whole production consignment being rejected.

In the absence of shedding iron oxide particle contaminants, supposedly “stable” rouge can cause issues. Its occurrence masks potential damage that is happening to the metal under the surface. It can also act as an anchor for microbial life, which, if not rectified, can contaminate the system and product. Such cases will necessitate thorough cleaning to fix the issue.

The Origin of Rouge

The source of rouge is as diverse as the system it occurs in. It occasionally results from the corrosion of a ferric iron component further “upstream” in the system. This could be due to mechanical failure, such as too much wear. Pump cavitation has also been recorded as the source of iron oxide particles in other scenarios.

Scientists have discovered elements of chromium and nickel present in rouge. As stainless steel alloy contains iron, nickel (in most grades), and chromium (at least 10%), this means that the stainless steel itself is being tainted.

Even in such a case, the reason underlying the surface’s breakdown can differ extensively. Astro Pak’s technicians have discovered situations where the clean-in-place (CIP) spray balls for vessels and tanks are spraying at excessively high pressure, resulting in the erosion of the passive layer, causing the base layer beneath to corrode.

In other examples, the water in the system destroys the thin, protective layer. Water comprising chlorides will ultimately degrade the passive layer of the stainless steel, even if it has been chemically enriched through passivation treatments. Finally, the chlorides will cause pits to develop on the surface.

In a cyclical procedure, the interaction of chloride compounds will continuously degrade the surface as iron oxide crystals occur, which settle on the surface or are shifted further into the system.

The situation becomes worse when heat is present. Temperatures beyond 60 °C (140 °F) make rouge growth more likely, and every 10 °C rise in temperature causes the corrosion rates to double.

Where That Rouge Goes

Once developed, Class 1 and most Class 2 rouges may not remain in one place. Rouge can be shifted along by the liquid's flow, causing it to enter other areas of the system. The iron oxide crystals will keep forming while a conducive situation is present.

For the Class 3 and those Class 2 rouges, which certain people may deem “stable,” the term can be deceptive. These rouges can shed debris with time as they keep growing. Each sector has its own regulations for acceptable levels of rouge. 

For example, particles below a micron in size can adversely impact a product in the semiconductor sector. In contrast, particles in the range of 5–25 μm are within regulations for most of the aerospace sector. 

At 75 μm, a particle becomes noticeable to the naked eye and is considered objectionable by the FDA for personal household care and pharmaceutical industries.

Even steam can shift rouge particles released from the surface. An Astro Pak team examined a case where black deposits developed on the inner sides of autoclaves. The client stated they had weak metallurgy in their autoclaves and were set to decommission them.

The deposits' real source was rouging in the high-temperature steam system that had remained untreated. The consequence was that the customer did not have to change the “defective” equipment at a considerable cost but was simply required to schedule a systematic derouging service for the steam system.

The Bottom Line

Rouge is not beneficial and does not act as a protective layer. It should prompt the unit operator to carry out a risk assessment.

This assessment should consider several factors, such as what is occurring beneath the rouge, the state of the protective chromium oxide layer, and if it is shedding oxide particles or anchoring microbial colonies.

These assessments must be carried out to inform ideal maintenance decisions to protect the product and durability of the system itself.

This information has been sourced, reviewed, and adapted from materials provided by Astro Pak Corporation.

For more information on this source, please visit Astro Pak Corporation.

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