Derouging and Passivation
Pharmaceutical manufacturing facilities house some of the world’s most advanced and complex stainless steel equipment. Routine maintenance of this equipment is vital to maximizing production uptime and consistently meeting critical manufacturing and cleaning requirements. Stainless steel corrosion, or rouge, may cause issues with cleaning validation and challenge the life of critical processing equipment. Mitigating this problem can be very complicated and requires equipment and process downtime resulting in lost time and money.
A good understanding of the issues surrounding rouge, derouging and passivation will help you develop the most efficient stainless steel maintenance program.
Although stainless steel is named stainless, it is really stain resistant. This property comes from its ability to spontaneously form a chromium-oxide-rich passive layer on the surface, which helps resists corrosion. When the passive layer is damaged, the iron present in the stainless steel may begin to oxidize.
Rouge (from the French word for “red”) is the red color that sometimes appears on stainless steel surfaces after prolonged exposure to aqueous solutions. Rouge is an iron oxide species that forms due to the oxidation of the metallic iron in the stainless steel. Sometimes this visible rouge is generated elsewhere (e.g., a pump impeller) and then deposited on the stainless steel surface and can be relatively easy to remove. Under more extreme conditions (e.g., steam systems), however, the iron oxide may exist as a black species, such as iron (II/III) oxide (magnetite). This type of rouge can be extremely difficult to remove.
The presence of these contaminants is undesirable in pharmaceutical manufacturing vessels because it is a clear indication that the process vessel and/or piping are not visibly clean. Further, these deposits may affect the safety and or efficacy of the manufactured product.
Effective rouge removal, or derouging, can only be done when the stainless surface is free of organic residues. The removal of organic residues using a robust pre-cleaning using an alkaline detergent is a critical first step. Since rouge is an iron oxide, anything that will solubilize iron oxides will remove rouge; the two items most widely touted for this purpose are acids and chelants. Different acids have varying capacities for iron oxide removal. Each acid’s performance also depends on concentration and temperature. Under field conditions, the nature and amount of rouge can vary dramatically. Therefore, there is no single “recipe” for performing a successful derouging operation, and a good process should incorporate a quantification method like a colorimeter.
Passivation is the process of restoring the chromium-oxide-rich layer of stainless steel in order to enhance its corrosion resistance. While this will occur spontaneously in the presence of oxygen, it can be enhanced with the use of chemical treatments. The most common industry-recognized (ASTM A 967) chemicals for passivation use nitric or citric acid, but other chemistries (e.g. strong acids and formulated detergents) have also been shown to be effective.
Practical Considerations for Derouging and Passivation
Developing a successful procedure for derouging and passivation is a careful balance of many factors including:
1. The ability to successfully remove any visible rouge;
2. Process constraints (e.g., circulation temperatures, flow rates, etc.);
3. Maintaining the condition of the base metal (i.e. some aggressive derouging treatments can damage a highly polished stainless steel surface);
4. Operator safety when handling hazardous chemicals;
5. Environmental concerns (e.g. phosphates, volatile compounds, etc.);
6. Adherence to industry standards (e.g., ASTM A 967);
7. Concerns related to the use of chemicals that are not part of the validated process cleaning operations.
Each facility has unique challenges and needs that must be addressed. For more information on developing an effective derouging and passivation program, please contact the STERIS Technical Service Team.