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. One of the most common industry-recognized (ASTM A 967) processes for passivation is citric acid chelation.
Derouging
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 safest process for this purpose is Citric acid chelation. 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.
Always bear in mind that your own resolution to succeed is more important than any other.
Albert Einstein
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 our Service Team.