Stainless steel’s corrosion resistance is a key reason for its use in everything from household goods to aerospace and manufacturing systems where high purity is critical. “Corrosion resistance” is the key term. Stainless steel can and will corrode if not properly maintained. The cause of corrosion varies based on the chemical make up of the steel and the operating environment it is in. In the biotech and pharmaceutical industries, one such type of corrosion is referred to as is collectively a trio of iron-based compounds that form on the surface of stainless steel structures. Their formation may not immediately damage the underlying metal, but their presence indicates that some part of the larger system is experiencing corrosion. If left untreated, rouge can lead to structural damage or system failure, but even before then it can serve as a source of contamination for the product.
All Rouge is Not the Same
While all rouge contains iron, their chemical make up is different. Rouge can be iron oxide (FeO2), iron hydroxide (Fe(OH)2) or iron carbonate (FeCO3). In addition to chemical differences, they differ visually and form under different conditions. As a result, they require varying removal techniques. Additionally, the source of the rouge needs to be identified and remediated to prevent a quick return.
The variation in the chemical compounds is due to the numerous valence states iron can take on as well as the hydration of the chemical compound itself. To briefly take you back to your high school chemistry class, “valence” is a measure of how easily an element (iron or “Fe” in this case) can combine with other elements to form a chemical compound. “Hydration” is simply a chemical reaction involving water. You are most familiar with common rust, which is what happens when an iron molecule binds with two oxygen molecules in the presence of water to form FeO2. High temperatures and the presence of chlorides in the water can accelerate the formation of rouge as well.
The term “rouge” comes from the French word for “red”. And the most familiar version of rouge, runs from orange to red-brown in color. However, the variation in chemistry and source can lead to deposits appearing as shades of blue, yellow, gray or even black. Given that stainless steel used in high purity industries is typically highly polished, this discoloration can be easy to spot regardless of what color it takes on. In areas which cannot be readily accessed, it is often the appearance of iron in water samples will first alert an operator to the presence of rouge. There are three classes or types of rouge and the color provides an initial indication as to the nature of the rouge:
Blue – This rouge is high in chromium, the material that gives stainless steel its resistance to chemical interaction. This form of rouging is an early indicator that corrosion is taking place and that the passive layer (CrO2) is starting to degrade, although it has not yet sloughed off from the surface of the stainless steel. Because it is still technically part of the stainless steel surface, it cannot be merely wiped off.
– Red, Orange, Brown – This is the most common. It is rich in iron oxide as crystals are forming on the surface of the metal. This can be easily wiped off, but it can also just as easily be dislodged while product is being processed resulting in particulate contamination. These crystals start out at sizes smaller than a micron, but can easily grow to measure 100 microns (0.1mm).
– Gold, Red, Brown
This type of rouge forms at the site it is discovered, and if dislodged, it may reveal the bright surface of the stainless steel underneath. It is generally caused by the presence of chlorides in the water and it often appears on surfaces where the passive layer has become weak or where passivation was never performed. It can be harder to remove than the similar-looking Type 1 rouge. It cannot simply be wiped off, but be treated with a derouging process. More severe cases may require chemical electropolishing to completely eliminate it from the surface. Regardless of the cleaning process used, the surface must be passivated afterwards to restore the passive layer which will have been damaged by the rouge as well as the removal process.
– Gray or Black
Gray or Black rouge is unique to high temperature steam systems and is a form of magnetite. It typically gets darker as it grows. It is formed by the iron interacting with the hot water vapor, first binding with the oxygen to form ferrous oxide (FeO) which gives it its black color along with ferric oxide (Fe2O3). Further chemical interaction takes place between the two to form the more stable Fe3O4. Its appearance depends on the surface it is on. With a rough surface such as un-passivated, mechanically polished steels, it may appear loose and powdery, while on a smooth surface it can be shiny and can be In fact, the presence of Type 3 rouge is often detected when it appears on filters or autoclaved items. As far as removal, it may have a relatively high silica content which makes it even more resistant to the usual chemical treatments. Often, more aggressive chemical treatments are called for as well as mechanical polishing or electropolishing. Passivation is also an essential step to be taken after derouging.
Addressing Multiple Problems
Removing the rouge is only half the job. Identifying the cause, and if necessary, locating the source of the rouging and remediating the situation so that it doesn’t immediately return is just as essential. Some metallurgists have suggested that rouge itself is an anti-corrosive compound, particularly when referring to the Type 3 form, which can only grow to a certain size before reaching the point where further saturation cannot occur. However as mentioned above, this ignores the very real problem of the larger crystals breaking off and being carried deeper into the system where they can cause further mechanical damage, if not contamination.
The chromium-rich blue rouge is a warning sign that something has damaged the passive layer, and if that layer is lost, then Type 2 rouge can form as the un-passivated surface is chemically attacked and damaged. As the rouge grows, it too can have the iron crystals break off and be carried downstream where they will either contaminate the product, or come to rest on a surface and create a patch of Type 1 rouge.
A Source of Additional Contamination
Regardless of the type of rouge, it creates a rough surface where biological contaminates can find a safe harbor. Given time, these organisms grow into colonies and generate a protective biofilm. Like the rouge particles, these organic objects can become detached and spread throughout the system offering yet another contamination risk. Ultimately, this can lead to loss of product and productivity as the system is shut down for a thorough cleaning and removal of rouge and/or bioburden throughout the system rather than at one or two localized sources.
The compounding problem of recurring rouging and spreading contamination can be reduced or removed entirely with the removal of the rouge followed by a restoration of the metal back to its original smooth, passivated state. However, it is important to remember that even after removing the rouge, repairing the damage and addressing the cause, rouge will return at some time in the future without a plan in place to proactively prevent it from forming. A science-based preventative maintenance schedule of cleaning and passivation can be set up to protect both the system as well as the product, resulting in a longer service life, reduced unscheduled downtime and a resultant reduction in costs associated with emergency repairs.