Rouging is a thin reddish-brown (sometimes black or blue) deposit that forms on the internal surface of stainless-steel piping, especially in Purified Water (PW) and Water for Injection (WFI) loops. It occurs when the protective passive layer on stainless steel becomes damaged or weakened, leading to oxidation of the underlying iron and formation of iron oxides.
Why does it happen more with heat?
1- Hot Water Sanitization (65–80°C):
• In GMP-compliant systems, most companies perform hot water sanitization in the range of 65–80°C, ensuring that the coldest point in the loop reaches at least 60°C for a sufficient holding time to achieve microbial kill.
• This temperature range is microbiologically effective and also reduces the risk of rouging compared to prolonged exposure to higher temperatures.
2- Superheated Water / Pure Steam
• In this case, we are dealing with sterilization at 121°C under pressure, typically used as a shock treatment during emergencies or after a major contamination incident.
• The drawback is that such high temperatures accelerate the formation of tightly adherent black/blue iron oxide layers (Class III – Magnetite), which are difficult to remove.
• For this reason, it is not applied as a routine cycle but reserved for exceptional cases.
Rouging Classes and Their Impact (ASME BPE & WHO GMP):
• Class I: Light discoloration, easily removable, often introduced from other parts of the system.
• Class II: Localized deposits that begin to adhere to the surface.
• Class III: Black or blue oxide, strongly adherent, typically formed under extreme thermal conditions.
Microbial Impact:
As the surface roughens, microbial adhesion increases, making cleaning and sanitization more difficult and raising the likelihood of OOS (Out of Specification) results.
Prevention Measures:
A) Material and Surface Finish
• Use low sulfur 316L stainless steel.
• Apply ASME-BPE compliant surface finishes:
• SF1: Ra ≤ 0.5 µm
• SF4 (Electropolished): Ra ≤ 0.4 µm (smoother, easier to clean).
• Perform chemical passivation after installation or maintenance in accordance with ASTM A967/A380 (nitric or citric acid).
B) Operational Practices
• Maintain hot water sanitization cycles within 65–80°C, ensuring by validation that the coldest point exceeds 60°C.
• Avoid prolonged heating beyond validated ranges.
• Alternate sanitization methods (thermal, thermal shock, qualified chemical) based on risk assessment.
• Maintain proper flow, drainage, and eliminate dead legs.
C) Monitoring and Trending
• Monitor microbiology, conductivity, and TOC.
• Visually inspect welds and hot zones for discoloration. If persistent color appears, initiate de-rouging and re-passivation before progression.
Corrective Actions if Rouging Occurs:
• Class I & II: Chemical de-rouging followed by re-passivation.
• Class III: More difficult to remove; may require advanced chemical treatment and possibly light mechanical polishing.