What Is CUI — and Why Is It Dangerous?
Corrosion Under Insulation (CUI) is one of the most significant and insidious degradation
mechanisms affecting insulated piping and equipment in process plants. Water ingress beneath
insulation systems — from rain, process leaks, steam tracing, or condensation — creates a wet
environment between the insulation and the carbon steel pipe or vessel surface. In this
environment, general corrosion, pitting, and — in stainless steel systems — external stress
corrosion cracking (ESCC) can develop and advance undetected, hidden from view by the intact
insulation cladding.
CUI is responsible for a significant proportion of process piping failures in oil and gas,
petrochemical, and power generation facilities — and its danger lies precisely in its invisibility.
By the time a CUI-induced failure occurs, the corrosion has typically been active and advancing
for years beneath insulation that appeared intact from the outside.
The challenge of CUI inspection is detecting and quantifying this degradation without removing
all the insulation — which is time-consuming, expensive, and may not be practically achievable
across large insulated piping circuits during a typical inspection interval.