Whether it is the FDA, NASA or some other external agency – many organizations have mandates to ensure that corrosion (rust or rouge) is not present in their product and distribution systems, tanks, vessels or equipment. Therefore, stainless steel systems for water, steam or products must be free of corrosion. Removing corrosion also referred to as ‘derouging’, is critical for product safety and also extends the life of systems and equipment. Rouge not only can cause surface degradation but also expose contaminants into the final product. In addition, rouge areas may exacerbate bioburden issues within the system itself causing greater contamination.
What is Rouge?
A general term used to describe a variety of discolorations in high purity stainless steel systems. It is composed of metallic (primarily iron) oxides and/or Hydroxides. Three types of Rouge are as follows:
Type 1 Rouge originates from an external source. It usually appears as a lightly-adhered red or orange “dusting” of the surface. Most Type 1 Rouge is metal oxide particles that are broken loose by the erosion of rouge or cavitation at pump impellers or internals. This type of rouge is relatively easy to remove chemically or by electropolishing. Generally, a wipe may or may not remove Rouge Type 1. However, it is best to passivate the surface after Type 1 rouge removal to ensure Rouge removal.
Type 2 Rouge forms at the site where it is seen and where chloride exposure is induced. It typically has a red, reddish-orange, or brownish color. A chemical process removes the rouge or it may need more aggressive derouging formulations or by electropolishing. The surface must be passivated after Type 1 rouge removal.
Type 3 Rouge normally contains a form of iron oxide called magnetite, which is black in color and forms in high-temperature steam systems. It often has relatively high silica content in many cases which can make it resistant to chemical removal, necessitating the use of more aggressive chemicals. On shiny surfaces, it may appear tightly adhered to and shiny, but on rough surfaces, it is less tightly adhered and may have a powdery appearance. Removal of Type 3 Rouge, includes aggressive chemical solution, or mechanical polishing and/or electropolishing.
Each type of rouge has specific chemistry and process for remediation. Post derouging, it is imperative to Passivate in order to provide enhanced corrosion resistance.