Rouge is a common phenomenon in water for injection (WFI) and clean-steam systems and can often be found in equipment and piping connected to these systems. While the root cause of rouging in high-grade stainless-steel equipment is a contentious issue, it is generally agreed that rouge is a form of surface corrosion that can form on stainless steel when exposed to high-purity water at elevated temperatures. This corrosion can manifest itself as a glazed, immobile discoloration or as a powdery reddish deposit (1).

Investigators should first determine that the observed problem is indeed rouge and not product residue or another type of contamination. Reddish-brown residue from a surface swab can be analyzed using Fourier transform infrared spectroscopy (FTIR) for the presence of iron oxide, which would indicate rouge. Close visual inspection should also be used to assure that there is no pitting or flaking in the affected area, which would indicate an advanced level of corrosion beyond the scope of rouging.

If rouge is present, the most critical determination is whether the rouge is additive to the process or not. Is the rouge firmly adhered to the surface of the equipment or could it instead come off into the process stream either slowly or as a result of water hammer or another shock? If the rouge is glazed and smooth in appearance and does not wipe off or is not found on downstream filters or gaskets, then it would be safe to conclude that it is not an additive problem. If, on the other hand, the rouge wipes off easily on a swab or a glove, or is found downstream on filters or gaskets, then it can also be found in the water-for-injection or process stream. The equipment can indeed be said to be “additive, reactive, or absorptive,” which is a violation of 21 CFR 211.65(a), and further investigation and remediation is required (2).

Classifying rouge
Different companies use numerous classifications of rouge, and many do nothing to help the operator or mechanic on the floor define the problem or determine if it really is a problem. Saying that the rouge is “shiny and black” or “a glazed purple” may help to identify the visual anomaly as rouge, but it does nothing to say if the material is additive to the process. Instead, a standard operating procedure (SOP) should give steps to determine a classification using visual criteria, roughness measuring, swabbing, or other straightforward methodology to see if the rouge is mobile or fixed. An SOP should use simple categories: one category should be for rouge that can impact a process and requires an investigation and another category for rouge that does not. If the rouge is not additive, reactive, or absorptive and does not affect cleanability or operation, then it should be documented and processing should proceed.

Quality assurance personnel are not expected to be experts in the metallurgy of stainless steel, but must ensure that equipment is in good order, cleanable, and not contributing to contamination of products. Implementing clear procedures will ensure that rouging does not paralyze operations and lot-release schedules.

References
1. R.W. Revie and H.H. Uhlig, “Iron and Steel,” in Corrosion and Corrosion Control: An Introduction to Corrosion Science and Engineering, (John Wiley & Sons, Inc., Hoboken, NJ, 4th ed., 2008) pp. 115–148.
2. Code of Federal Regulations, Title 21, Food and Drugs (Government Printing Office, Washington, DC), Part 211.65(a).

John Anderson is a validation manager at Lonza in Visp, Switzerland, tel: +41 79 893 94 48, john.anderson@lonza.com.