Corrosion Under Insulation (CUI): Causes, Risk Range & Prevention

Updated 10 June 2026 · ASTM C680 / ISO 12241 basis · by the Inzonex engineering team

Direct answer: Corrosion under insulation (CUI) is corrosion of pipework and equipment caused by water trapped under the insulation. It is most aggressive on carbon steel roughly between −4 °C and 175 °C (where water stays liquid against the metal) and on austenitic stainless via chloride stress-corrosion cracking. The biggest enabler is insulation that cannot be removed to inspect — which is exactly what removable insulation solves.

Why CUI happens

Water ingress — rain, wash-down, condensation or steam leaks soak the insulation; damaged cladding lets it in and traps it.
Temperature window — worst where the metal sits between about −4 °C and 175 °C (water stays liquid). Cyclic and intermittent service is especially bad.
Hidden — the corrosion is under the insulation, out of sight, so it is found late — often as a leak or failure.
Chlorides — on stainless steel, wet chloride-bearing insulation can cause stress-corrosion cracking.

How removable insulation reduces CUI risk

Fixed, cut-and-weld lagging is rarely removed once fitted, so the surface underneath is never inspected — CUI grows unseen. Removable (modular) insulation unzips in minutes, so operators can inspect the metal at every turnaround and refit the same cover with no damage. It also dries out and re-seals rather than staying permanently wet. Insulating the awkward parts (valves, flanges, fittings) that are normally left bare or badly lagged removes the cold/wet spots where CUI starts.

CUI prevention checklist

Use removable covers on anything that needs periodic inspection.
Keep cladding and seams weather-tight; repair damage promptly.
Inspect high-risk ranges (−4 to 175 °C, cyclic service, dead legs) on a schedule.
Don't leave valves and flanges bare — cold bare spots condense moisture; insulate them (removable).
Specify proper coatings under the insulation on carbon steel.

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FAQ

What is corrosion under insulation (CUI)?

CUI is corrosion of insulated pipes and equipment caused by water trapped under the insulation against the metal. Because it is hidden, it is often discovered only when a leak or failure occurs.

At what temperature is CUI worst?

On carbon steel, roughly −4 °C to 175 °C, where trapped water stays liquid; cyclic and intermittent service is especially aggressive. Stainless steel is at risk of chloride stress-corrosion cracking when wet.

How do you prevent corrosion under insulation?

Keep water out (intact cladding/seals), inspect high-risk ranges on schedule, coat carbon steel, and use removable insulation so surfaces can actually be inspected and dried rather than sealed up permanently.

Does removable insulation help with CUI?

Yes — it is one of the few practical ways to inspect for CUI. Removable covers unzip for inspection and refit without damage, so the metal can be checked at every turnaround instead of being hidden under fixed lagging for years.