Corrosion under insulation (CUI)
Corrosion of a pipe or vessel beneath its thermal insulation, where water is retained against the metal and the surface is hidden from inspection. The mechanism is ordinary atmospheric corrosion; what makes it expensive is that nothing is visible until the insulation is removed.
CUI has no exotic chemistry. What it has is a reservoir: insulation that has taken up water holds it against the steel long after the surrounding air has dried, and the equipment that cycles between wet and dry is worse off than the equipment that runs continuously hot.
Because the surface is concealed, the coating under the insulation cannot be inspected on a normal round. That shifts the whole cost of the decision to specification time — the system is chosen once, for a service life nobody will verify in between.
Sources and references
- StandardNACE SP0198-2017 — Control of Corrosion Under Thermal Insulation and Fireproofing Materials: A Systems ApproachAMPP · accessed August 29, 2026
Where this is covered in depth
- Corrosion under insulation: where it startsWhy insulated equipment corrodes fastest where nobody looks, which temperature range does the damage, and what actually stops it.
- Anti corrosion coating types: choosing by exposure zoneFive coating families, and the order that picks between them: corrosivity category first, durability class second, product last.