Q. Is rouge a contaminant that I need to deal with in my water system? It is visible sure, but there's no detectable impact
on my water quality.
Larry Gatlin
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A. Rouge is a description of corrosion deposits found in stainless-steel systems. Rouge can vary in color from a light red or
orange to a dark brick. Dark purple or black colors are normally found in steam systems with varying colors due to hotter
temperatures. Most often the light-red composite material of the deposits is primarily iron-hydrated oxides. Components of
rouge also can include chromium, nickel, and molybdenum oxides. Rouge formation is a steady process in all metallic piping
systems, but is exacerbated by high temperature and metal composition. Although, the discoloration of SS 316L piping, accessories,
and vessels due to rouge deposition is not a problem in itself, any contamination of the product from the rouge is a problem.
Very small amounts of metal ions can significantly impact the stability of some products.
Rouge can be placed into one of three categories:
- Class I Rouge is deposited corrosion product, consisting of iron oxides and hydroxides originating elsewhere in the system
and deposited downstream. The underlying stainless-steel surface beneath such deposits usually remains unaltered. This rouge
deposit can usually be easily wiped away.
- Class II Rouge is an adherent corrosion product originating in-situ on unpassivated or improperly passivated stainless-steel surfaces. By its formation, the normally passive-protective film
on the stainless-steel surface is altered.
- Class III Rouge is a blue or black, mostly iron oxide corrosion product, commonly called magnetite, which forms on surfaces
in high-temperature steam systems. On electropolished surfaces, corrosion deposit may be glossy black, stable, and adherent.
On unpassivated mechanically polished surfaces, the corrosion deposit may be powdery black and may slough off.
Changes in the rouge-formation rate may result from operational influences of flow, points of use, pressure, oscillations
in water temperatures, and supply or make-up issues. Measuring the rouge corrosion rate can help determine when a system should
be cleaned so the final product is not impacted. Modern pharmaceutical water systems are completely automated, and several
companies offer online instruments to measure rouge formation while maintaining adherence to 21 CFR part 11. It is the responsibility of the company to demonstrate that the measurement system works because an accurate measurement
method for corrosion or rouging rate will help determine the operating limits before derouging and repassivation are required.
Historically, derouging and passivation frequencies were subjectively determined (i.e., annually, biannually, every six months,
or every scheduled shutdown), but a frequency based on company risk comfort level may be too frequent thus costing the company
time, resources, and money.
Larry Gatlin, PhD, is a Biotechnology & Pharmaceutical Consultant with Patheon.
