Rouging: What it is and how to avoid it

Pharmaceutical manufacturers carefully choose their process equipment to ensure a completely sanitary final product. Most commonly, manufacturers turn to stainless steel shell and tube heat exchangers and other equipment, Pharmtech.com explained. Not only is it affordable, but it’s very durable, stain resistant and has a low risk of corrosion.

Though stainless steel is highly resistant to corrosion, it’s important to realize what risk there actually is for this equipment to wear down or become unsanitary. Though titled “stainless,” the reality is that steel of any kind can actually become stained. Stainless steel just wards off this reaction for longer than others.

Stainless steel resists staining by forming a passive layer filled with oxides. The layer forms naturally when in the presence of oxygen, and it protects the stainless steel from reacting with any chemicals passing through the piece of equipment.

Rouging in stainless steel equipment

Passivation, or the formation of the passive layer, is only possible under specific circumstances. When the chromium-to-iron ratio begins to fall, passivation becomes more difficult and the all-important oxide-rich layer may not form.

Rouging in stainless steel equipment

Passivation, or the formation of the passive layer, is only possible under specific circumstances. When the chromium-to-iron ratio begins to fall, passivation becomes more difficult and the all-important oxide-rich layer may not form.

“Stainless steel is highly resistant to corrosion, but rouging can still occur.”

Eventually, the stainless steel may begin to corrode. The process usually produces a thin colorful layer with a red, orange or yellow hue. Sometimes it produces pink, purple or brown. This phenomenon is typically called “rouging” for the more commonly seen reddish colors.

Rouging is not corrosion, but rather the symptom of it. If you see rouging, you know there’s likely an underlying problem of some sort.

There’s no singular identified cause of rouging, the British Stainless Steel Association pointed out. It’s typically composed of iron oxides, though the exact chemical composition can change, leading to a rainbow of reactions that all fall under the category of rouging.

Manufacturers who have noticed rouging in their stainless steel equipment often cite causes like poor welding or construction; vulnerabilities in the passive layer; high iron content in materials that come in contact with the equipment; and surface contamination, including small steel particles or dust that lands on the equipment, according to Pharmaceutical Engineering.

The truth is, though, that no one really understands completely how to predict rouging or exactly what causes it, Michelle Gonzales explained in Pharmaceutical Engineering. Nonetheless, manufacturers can – and should – take steps to ward off this colorful phenomenon.

Choose materials of construction carefully

The most popular stainless steel to use for sanitary shell and tube heat exchangers is 316L because of its low carbon content and its ability to endure heat treatment, Gonzales pointed out. It’s an austenitic metal, which means it’s highly durable and resistant to corrosion.

Duplex stainless steel is another material that’s highly resistant to rouging. This style of stainless steel is a combination of austenitic steel, like 316L, as well as ferritic steel, which gives it greater durability under heat applications.

When having a new piece of equipment fabricated, be sure to bring up the subject of rouging with the manufacturer. Explain clearly what the conditions are in your facility and how the equipment will be used, including what products will come in contact with the metal and how you plan on cleaning. Be open to suggestions based on the engineer’s expertise.

Purchase equipment from a trusted fabricator

Pharmaceutical manufacturers have reported rouging occurring in pieces of equipment that have flawed welding or construction. The simple way to avoid this is by doing your research and choosing a fabricator that can be trusted to present you with a high-quality piece of equipment.

Find out whether the fabricator has experience with the type of equipment you need. Also ask about their experience with relevant industry regulations, such as ASME-BPE, which is commonly referred to in pharmaceutical and other bioprocessing manufacturing.

Understand common causes of rouging

Though rouging is not always predictable, there are certain conditions that are more likely to cause it than others. For example, elevated temperatures above 140 degrees Fahrenheit for long periods of time are known to cause rouging. Additionally, extreme pH levels and surface damage are all common predecessors of rouging.

“Rouging is not always predictable, and there’s no specific known cause.”

In many cases, high temperatures and solutions with a specific pH are necessary for the processes in a manufacturing plant. In these situations, it’s important to periodically check the equipment for beginning signs of rouging or surface damage.

Rough surfaces encourage rouging more than sleek ones. Electropolishing gives stainless steel a smooth surface where rouging is less likely to occur, according to BSSA. Electropolishing also provides an ideal environment for the passive layer to form and helps it maintain stability.

Once you’ve experienced rouging once or twice, you will gain an understanding of the unique conditions under which the phenomenon occurs in your specific operation. When you know this, you can be on the lookout for the first signs of rouging.

Learn how to react to rouging

When you see rouging, you know that there could be unknown chemical compositions inside your process equipment. As such, it’s important to not use any chemical cleaners or treatments right away.

Rather, manufacturers need to take the time to evaluate the rouging and determine what’s causing it, what chemicals are involved and whether it’s a danger to the final product. Gonzales pointed out that, sometimes, the rouging looks more worrisome than it really is.

Correcting rouging can be a time-consuming project, PharmTech.com pointed out. A manufacturer may want to enlist the help of an expert who can identify the ultimate cause and practical solutions to fix it.

If you’re in the market for a high-quality stainless steel shell and tube heat exchanger, reach out to the knowledgeable and helpful engineers at Enerquip. They have the knowledge and experience to help you select the appropriate material to combat rouging while meeting your codes and standards. They also can passivate your exchanger before shipping it to you.