A recurring myth in some industries is that “rouge is a protective layer” that enhances, rather than degrades, stainless steel’s durability. This mistaken belief is primarily based on certain types of rouge being considered “stable” and remaining affixed to the surface – being perceived as acting similar to a coat of paint.
“Stable” rouge is a term used primarily for Class 3 rouge as well as some categories of Class 2 rouge. Neither of these are easily removed with a simple wipe of a cloth. However, the problem with the concept of “stable” rouge is twofold: How long will it remain stable, and what is going on underneath it? In the latter case, the rouge may be hiding deteriorating conditions of the surface beneath it, or it may be serving as an anchor for microbial growth.
A quick recap on rouge
Rouge is a form of iron oxide, similar to rust. Unlike the thick, flaky form most people are familiar with on steel, rouge is generally a staining accumulation on the surface of stainless steel. In addition to appearing as a “rust red” color, rouge deposits can range in hue from orange and yellow to blue and black. This color variation is due to the particular chemical makeup of the oxide which are all various combinations of iron (Fe), other metals, and oxygen (O) atoms. The type and color of the rouge that forms is heavily influenced by the temperature and water quality within the system. Red and yellow rouges appear at lower temperatures with the gray or black deposits forming in steam systems. Furthermore, rouge is more likely to occur where temperatures regularly reach 60°C or more.
Rouge falls into three Classes:
Class 1 – Hematite (Fe2O3)
- Deposition film from an upstream corrosion source in the system
- Orange/red, ferric oxide, loosely adhered to the surface
- Often times can be manually wiped off
Class 2 – Hematite (Fe2O3)
- Rouge that occurs at the site of where corrosion is taking place
- Red/brown, ferric oxide, tightly adhered to the surface
- Surface of the stainless steel is altered
Class 3 – Magnetite (Fe3O4)
- Corrosion from steam or high temperature applications
- Black, ferrous oxide, tightly adhered to the surface
- Difficult to remove and requires aggressive chemistry
The problem with rouge
While in a number of cases, rouge can be seen merely as a cosmetic issue, it is at times a more serious problem. Iron oxide corrosion produces particles that can chemically interact with ingredients or products. Beyond that, the FDA does not allow any visible particles which can result in the rejection of an entire production batch.
Even without shedding iron oxide particle contaminants, so-called “stable” rouge can cause problems. As noted earlier, its presence masks any ongoing damage to the metal beneath the surface. Or, it can serve as a breeding ground for colonies of microbial life, which if left untreated can contaminate the entire system as well as the product. Such instances will require even more extensive cleanings to remediate the issue.
Where is the rouge coming from?
The source of rouge is as varied as the system it is found in. Sometimes, it comes from the corrosion of a ferric iron component further “upstream” in the system. This could be due to mechanical failure, such as excessive wear. Pump cavitation has also been identified as the source of iron oxide particles in other situations.
However, researchers have found elements of nickel and chromium appearing in rouge. As stainless steel alloy consists of iron, chromium (at least 10%) and nickel (in most grades), this indicates that the stainless steel itself is being degraded.
Even in such a situation, the reason behind the breakdown of the surface can vary widely. Astro Pak’s technicians have found cases where the clean-in-place (CIP) spray balls for tanks and vessels are spraying at too high of a pressure and have caused erosion of the passive layer, allowing the underlying base layer to corrode.
In other situations, it is the water in the system itself that attacks the thin, protective layer. Water containing chlorides will eventually destroy stainless steel’s passive layer, even one that has been chemically enhanced through passivation treatments. Eventually, the chlorides will cause pits to form in the surface. In a cyclical process, the interaction of chloride compounds will continue to attack the surface as iron oxide crystals form, which are then deposited on the surface or carried further into the system. The situation gets worse when heat is involved. As noted above, temperatures above 60°C (140°F) makes the formation of rouge more likely. But in general, for every 10oC increase in temperature, the corrosion rate doubles.
And where does that rouge go?
Once formed, Class 1 and most Class 2 rouges may not stay in place. It can be carried along by the flow of the liquid spreading it to other parts of the system. The iron oxide crystals will continue to grow as long as a favorable situation exists.
As for the Class 3 and those Class 2 rouges which some may consider “stable”, the term can be misleading. Even if it’s not readily visible, these rouges do shed debris over time as they continue to grow. Each industry has its own tolerances regarding what is acceptable. For example, in the semiconductor industry, particles less than a micron in size can negatively affect a product, while particles measuring between five and 25 microns are within tolerances for much of the aerospace industry. At 75 microns, a particle becomes visible to the unaided eye and are considered unacceptable by the FDA for things such as pharmaceuticals and household personal care industries.
Even steam can transport rouge particles that have been loosened from the surface. In one case, an Astro Pak team diagnosed a situation where black deposits were forming on the interiors of autoclaves. The client believed that they had poor metallurgy in their autoclaves and were ready to decommission them. The actual source of the deposits was found to be rouging in the high temperature steam system that had gone untreated. The result was that the customer did not have to replace the “defective” equipment at great expense, but rather only needed to schedule regular derouging service for the steam system
The bottom line
It is important to remember that the presence of rouge is not a good sign and certainly does not serve as a protective layer. Instead, it ought to prompt the plant operator to make a risk assessment. This assessment should consider a number of factors such as what is going on underneath the rouge, what is the condition of the protective chromium oxide layer, and is it harboring microbial colonies or shedding oxide particles? These evaluations need to be made in order to make the best maintenance decisions to mitigate the risk to the product as well as the longevity of the system itself.