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Rouging in Stainless Steel Equipment

by Richard E. Avery; edited by Pietro Perrone     
February 2004

Introduction

Stainless steels are one of the most versatile and widely used materials for domestic and industrial use. The pharmaceutical industry makes extensive use of Type 316L equipment. However, when material performance questions arise and rouging of stainless steel in high-purity water systems occurs, the causes are not well understood. Industry does not have all the answers on rouging. This brief overview presents some new work that helps in identifying factors for rouging. Identifying these factors should help in minimizing the occurrences of rouging in stainless steel systems.

What is Rouging?

Rouging is a term used to describe deposits that form in stainless steel systems. The rouge can vary in color from a light-red to reddish-brown and to dark violet or black deposits. The particles that make up the deposits have been identified as predominately one of the species of iron oxide. Rouge can be in the form of a loose film that can be easily wiped off or a very tenacious film that is difficult to scrape off. Tverberg and Ledden have identified three classes of rouge.1

  • Class I Rouge - Iron oxide particles originating elsewhere and deposited downstream. The stainless steel surface and the Cr/Fe ratio of the metal surface beneath such deposits usually remain unaltered.
  • Class II Rouge - 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 Rouge - Iron oxide (or scale) which forms on surfaces in high temperature steam systems. The Cr/Fe ratio of the protective film is usually altered.

Rouge deposits can develop in a number of locations in a high purity water system and the deposit is not limited to stainless steel surfaces. Rouge deposition seems to have an affinity for Telfon® and should be one of the first places to look for signs of system rouging.2

Factors Influencing Rouge Formation

A number of factors may contribute to the formation of rouge in a highpurity water system. In recent work rouge was produced in the laboratory on 316L exposed to WFI and purged with different gases.3 Rouge developed faster when WFI was purged with nitrogen and synthetic air containing plus 1+% CO2. Little or no rouge developed in synthetic air or de-carbonated air.

Different stainless steel surfaces have an impact on rouge formation. In addition to a sheet mill finish, there are various surface treatments that can be done to stainless steel components. These include mechanical polishing, electropolishing and passivation. These treatments typically have an effect on rouge formation. In unpublished work sponsored by Nickel Development Institute, the iron release rate (which in turn could be a source of rouge) in ultra-high purity water was measured. Some of the findings were:

  • mechanically polishing to a 180 and 360 grit finish resulted in an iron release over three times that of an undisturbed 2B finish
  • iron release from the 2B finish was reduced to a nil amount by a nitric-hydrofluoric acid pickle or nitric acid passivation
  • a properly performed electropolish followed by a nitric acid passivation resulted in nil iron release

Pumps in stainless steel systems may also contribute to downstream rouge deposits. The iron release might be the result of impeller erosion due to excessively high tip speeds or from cavitation.

The presence of an alloy with a lower corrosion resistance than 316L often increases rouge formation. An example of this is a straight chromium stainless steel component in a high purity water system.

Is Rouge a Forerunner to a Corrosion Failure?

The iron that forms the iron oxide rouge particles in a total stainless steel system originates from the stainless steel. The amount of iron released is small and is generated from a large surface area. Therefore the material thickness reduction is considered insignificant. An exception to thickness reduction might be at pumps previously mentioned. Additionally, there is no proof that rouge formation promotes pitting or crevice corrosion.

The possibility of rouge adversely affecting products being processed has been raised. Opinions vary on this point, since it is influenced by factors such as the amount and form of rouge and the products being processed. However, the prudent practice would be to prevent or minimize rouge to the extent possible and when not possible, de-rouge and passivate as needed.

Concluding Remarks

This brief overview on rouging and the factors that contribute to its development in stainless steel equipment provides an introduction to this topic. When rouging is identified in a stainless steel system, there are de-rouging and passivation treatments available for fully restoring the stainless steel surface.

References

  1. Tverberg, J. C., and Ledden, J. A., "Rouging of Stainless Steel in WFI and High Purity Water Systems," Proceedings of Tube 2000, Dusseldorf, 2000.
  2. Banes, P. H., "Materials - Fundamentals of Passivation in Water Systems," Ultrapure Water, April 1998.
  3. Mathiesen, T. et al, "Using Exposure Tests to Examine Rouging of Stainless Steel," Pharmaceutical Engineering, July/August 2002.

 

Page last updated: 5 March 2009