CUI is external corrosion of pipe and vessel walls under wet insulation — consistently ranked among the costliest integrity problems in process industry. Mechanism: water enters through damaged jacketing or unsealed penetrations, the insulation holds it against hot steel (the 50–175 °C wall range is the classic worst zone — hot enough to concentrate, not hot enough to stay dry), and corrosion proceeds invisibly until inspection or failure. Defences, in order of effectiveness: keep water out (jacket integrity), use non-absorbent insulation where wetting is inevitable (cellular glass; hydrophobic aerogel), use inspectable systems — removable covers turn a destructive inspection into a two-minute open-and-look — and maintain coating systems under the insulation. Standards context: NACE SP0198 / EN 16961 cover CUI control practice.
Compute your own case: free heat-loss calculator · materials library: 12 datasheets · related: What is economic thickness of insulation? · R-value vs k-value (λ) — what's the difference? · What insulation works at 600 °C? · Can insulation touch the hot pipe directly?
