What Causes Rouging in Stainless Steel Equipment — and How to Prevent It
Pharmaceutical and sanitary manufacturers know that when it comes to high-purity processing, every detail matters — especially the equipment. That’s why stainless steel shell and tube heat exchangers are a go-to choice. They’re durable, cost-effective, and resistant to corrosion and staining.
But even stainless steel has its vulnerabilities.
While it’s famously tough, stainless steel isn’t invincible. Over time, under certain conditions, it can begin to show signs of wear. One such red flag — literally — is rouging.
What Is Rouging?
Rouging, pronounced ROO-jing, is a colorful surface discoloration that typically appears in shades of red, orange, or yellow — though it can also show up as pink, purple, or brown. While it may look like just a cosmetic issue, rouging is actually a sign that something’s happening beneath the surface.
Technically, rouging isn’t corrosion itself, but a symptom of it. It’s often the result of iron oxide buildup, and while the specific chemical makeup can vary, the underlying message is the same: the passive layer that protects stainless steel may be compromised.
The Role of the Passive Layer
Stainless steel owes much of its durability to a thin, naturally forming oxide layer that protects it from chemical reactions. This passive layer is formed when the steel is exposed to oxygen. But if the chromium-to-iron ratio drops — due to heat, wear, or chemical exposure — that protective layer might not form properly.
When that happens, the metal becomes more vulnerable, and rouging can follow.
What Causes Rouging?
Rouging isn’t fully understood, but some common triggers include:
- Poor welding or fabrication practices
- Contaminants like iron particles or steel dust
- High iron content in fluids or cleaning agents
- Frequent exposure to high temperatures (140°F and up)
- Extreme pH levels (either very acidic or very basic)
- Surface imperfections or damage
Even if you’re following all the right steps, rouging can still occur. The key is to minimize the risk and catch it early.
How to Prevent Rouging in Stainless Steel Equipment
Here are a few smart steps to protect your equipment — and your product:
1. Select the Right Materials
316L stainless steel is a popular choice for pharmaceutical-grade heat exchangers because of its low carbon content and strong resistance to corrosion. It’s also austenitic, meaning it can stand up to heat and stress.
For even more protection, consider duplex stainless steel, which combines the benefits of both austenitic and ferritic structures for superior durability and corrosion resistance.
2. Work with a Trusted Fabricator
Rouging often starts with improper construction. Choosing a reputable equipment manufacturer helps ensure your equipment is fabricated to meet strict sanitary standards like ASME-BPE. Be sure to discuss your application, cleaning processes, and operating environment so your fabricator can recommend the best material and surface finish.
3. Understand the Risk Factors
Rouging is more likely to happen under these conditions:
- Long exposure to temperatures over 140°F
- Cleaning or process chemicals with extreme pH levels
- Surface roughness or imperfections
Processes that require these conditions aren’t inherently flawed — they just require extra monitoring and preventative maintenance.
4. Electropolish for a Smoother Finish
A smooth surface helps reduce rouging by minimizing crevices where contaminants can settle. Electropolishing removes microscopic imperfections and supports the formation of a stable passive layer — making your stainless steel more resistant to corrosion and discoloration.
What to Do if You Spot Rouging
Don’t panic — and definitely don’t start scrubbing with harsh chemicals. First, evaluate the discoloration and look for potential sources: temperature spikes, cleaning agents, or surface wear.
In many cases, rouging looks worse than it is. Still, it’s smart to consult with an expert who can assess the situation, determine the cause, and guide you on the best path forward — whether that means cleaning, repassivation, or replacement.
Bottom Line: Rouging isn’t always preventable, but it is manageable. By understanding the conditions that cause it and taking proactive steps, pharmaceutical manufacturers can protect both their equipment and their end product.
Need stainless steel heat exchangers built to last?
Contact the experts at Enerquip. Our team is ready to help you select and customize the right solution for your process — with the durability and purity your product demands.
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