CUI is one of the most expensive and least visible failure modes in industrial piping. The right insulation material — and the right chemistry — is your first line of defense.

The Problem Nobody Sees Coming

In industrial facilities, some of the most expensive maintenance failures start somewhere nobody is looking: underneath the insulation.

Corrosion under insulation — CUI — is exactly what it sounds like: external corrosion forming on metal pipe surfaces while the insulation sits on top, hiding the damage until it’s severe. By the time CUI becomes visible, a pipe may have lost significant wall thickness, or already be leaking.

The financial scale of the problem in the U.S. alone is staggering.

That $276 billion figure — roughly 3% of U.S. GDP — covers all industrial corrosion, but CUI represents a major and largely preventable share of it. Indirect costs, including production downtime, emergency repairs, and regulatory consequences, are estimated to potentially double that number.

At the facility level, the impact is just as stark:

One in ten dollars of an industrial facility’s entire maintenance budget goes to repairing CUI damage. For contractors and engineers who specify insulation on hot-service piping, that number reframes what “good” insulation selection is actually worth.

A 2002 case study illustrates the real-world cost: a large chemical company spent over $5 million replacing 304 stainless steel equipment due to chloride stress corrosion cracking under insulation — before accounting for the production downtime. Cases like this are commonplace across refining, petrochemical, power generation, and general industrial process piping

How Moisture Gets Trapped: The CUI Mechanism

The insulation itself doesn’t cause corrosion. The problem is what happens when moisture finds its way in — and can’t get out.

On insulated piping, several pathways bring moisture into contact with the metal surface:

Once moisture penetrates the insulation, the system acts like a sponge — holding water against the metal surface and preventing it from evaporating. The pipe is then in continuous contact with water and oxygen, driving the oxidation reactions that produce rust on carbon steel. On stainless steel, chloride ions — concentrated by coastal atmospheres or cooling tower drift — cause a different but equally destructive failure mode: stress corrosion cracking.

Temperature range matters too. Carbon steel is most vulnerable to CUI between roughly 25°F and 350°F (-4°C to 175°C) — a range that covers an enormous amount of industrial piping. Stainless steel faces chloride-induced stress corrosion cracking at temperatures above 140°F (60°C). Both ranges are common in refineries, petrochemical plants, power generation, and general industrial process piping.

What makes CUI particularly insidious is that the insulation and jacketing hide the damage entirely. Inspections require removing insulation — a costly, labor-intensive process — and many facilities only discover the problem when a pipe fails.

Why Insulation Material Selection Is the First Line of Defense

Protective coatings applied to the pipe surface are a valuable tool against CUI, but they have limits — most have maximum service temperatures between 300°F and 400°F, and they degrade over time. The insulation material itself, chosen correctly, can actively work against corrosion even when moisture intrusion occurs.

There are two broad strategies for CUI mitigation through insulation material selection: keeping moisture out, and neutralizing it when it gets in.

Strategy 1: Hydrophobic Insulation — Keep Moisture Out

Hydrophobic insulation materials are engineered to repel water rather than absorb it. The logic is straightforward: if moisture can’t be held against the pipe surface, the corrosion mechanism can’t sustain itself.

Hydrophobic expanded perlite is one of the most proven materials in this category. With a long track record in Gulf Coast refineries — some of the most demanding CUI environments in the world — hydrophobic perlite resists water absorption and has demonstrated excellent corrosion performance on hot-service piping. Its high compressive strength also provides structural support to the metal jacketing system, reducing the risk of “fish-mouthing” at overlaps — a common entry point for rainwater.

Cellular glass is the other hydrophobic standout, with a completely impermeable closed-cell structure. It doesn’t absorb water at all — making it highly effective for CUI mitigation, particularly on below-ambient or cycling temperature systems. Its limitation is service temperature: cellular glass is generally appropriate up to 450°F to 500°F, above which other materials are preferred.

It’s worth noting that hydrophobicity has limits. Hydrophobic treatments on mineral wool and similar products are organic in nature and can degrade at sustained elevated temperatures. No hydrophobic material is fully immune to moisture intrusion over the life of an installation — which is why active corrosion inhibition is increasingly specified alongside hydrophobic properties.

Strategy 2: Active Corrosion Inhibitors — Fight Moisture When It Gets In

The more sophisticated approach to CUI prevention involves insulation materials formulated with active corrosion inhibitors — chemicals that activate in the presence of water and work directly against the corrosion mechanism at the pipe surface.

The leading technology in this category is the XOX Corrosion Inhibitor, developed by Johns Manville’s Industrial Insulation Group and incorporated into Thermo-1200 calcium silicate.

The XOX system works on two levels:

This is a meaningful departure from passive hydrophobic approaches. Rather than simply trying to keep moisture out, XOX-treated insulation treats moisture intrusion as an anticipated condition and responds to it chemically. Thermo-1200 has been independently tested using the NACE TG 516 accelerated corrosion protocol, which cycles between wet/dry conditions and chloride-rich environments, and has demonstrated significantly reduced corrosion rates compared to untreated materials. The XOX Corrosion Inhibitor remains an integral part of the insulation formulation and continues providing corrosion protection throughout the life of the installation.

Calcium Silicate: The High-Temperature Workhorse

Calcium silicate has been the go-to high-temperature industrial insulation for decades — specified for operating temperatures up to 1,200°F on process piping, steam lines, power generation equipment, and petrochemical systems.

Historically, traditional calcium silicate was associated with CUI risk because its cementitious nature makes it water-absorbent. When wet, older formulations could actually leach compounds that accelerated corrosion on the pipe surface.

Modern calcium silicate with XOX Corrosion Inhibitor changes that equation entirely. The inhibitor package converts the water-absorption characteristic from a liability into an active defense mechanism — when the material absorbs water, the inhibitors activate and protect the pipe. Thermo-1200 also incorporates water-resistant features that reduce the rate of absorption compared to traditional calcium silicate.

For applications requiring:

…calcium silicate with XOX is the specification most engineers reach for on high-temperature industrial piping.

Hydrophobic Expanded Perlite: The High-Temperature Hydrophobic Option

For applications above the cellular glass service range — particularly hot, humid environments where CUI risk is highest — hydrophobic expanded perlite with XOX Corrosion Inhibitor offers a unique combination: a material that is both hydrophobic (resisting moisture absorption) and actively inhibiting (fighting corrosion when moisture penetrates).

Expanded perlite has a long history of use at Gulf Coast refineries precisely because it performs well in high-humidity, high-temperature environments. With the XOX package, it provides both passive and active CUI protection — making it one of the strongest tools available for the most demanding industrial environments.

Where Other Materials Fit

Not every industrial piping application requires the most aggressive CUI mitigation. A brief material comparison:

The Full CUI Prevention System

No single material or product eliminates CUI risk in all circumstances. A complete CUI prevention approach combines several elements:

Material selection is where the contractor and specifier have the most influence at the design stage — and where the choice of a standard calcium silicate versus a CUI-inhibiting calcium silicate can make the difference between a system that performs for 20 years and one that fails in five.