Every insulated pipe and vessel in a plant has a layer nobody can see through. Corrosion under insulation, usually shortened to CUI, is what happens in that layer when water finds its way in and stays. This page explains the mechanism in plain terms, the conditions that make it likely, and why deciding where to look is most of the problem.
A working definition
Corrosion under insulation is external corrosion of carbon steel, low-alloy steel and, in a different form, stainless steel, occurring on surfaces that are covered by thermal insulation or fireproofing. The metal corrodes because the insulation system holds moisture against it and keeps the surface wet, warm and, often, salty for far longer than an exposed surface would ever be.
Two features make CUI different from ordinary atmospheric corrosion. It is hidden, because the jacketing that protects the insulation also hides the steel, so the first visible sign is often a stain, a bulge in the jacketing or a leak. And it is localized. Water collects at specific places, so the wall loss is concentrated in pits and patches rather than spread evenly, and a line that reads fine at nine thickness points can be close to failure at the tenth.
How the mechanism works
Three things have to line up: a path for water to get in, something to hold it against the steel, and a metal temperature that keeps the reaction going.
Water gets in
Rain, deluge tests, wash-down, cooling-tower drift, steam-trace leaks and condensation all supply water. It enters through jacketing seams and laps, at penetrations for supports and instruments, where jacketing is dented or missing, at insulation terminations around flanges and valves, and anywhere sealant has aged out. Once inside, gravity carries it to low points, dead legs and the bottom of vertical runs.
The insulation holds it
Absorbent insulation such as calcium silicate and mineral wool acts like a wet sponge against the pipe. Closed-cell materials such as cellular glass hold far less water, which is why insulation choice is one of the main preventive levers. Some insulation materials can also leach chlorides, which matters for stainless steel.
Temperature keeps it going
Temperature changes how long water remains at the metal and how quickly corrosion mechanisms proceed, but it is not a stand-alone clearance test. Within commonly cited susceptibility bands, wet insulated steel can corrode quickly. Shutdowns, steam-outs, cycling and damaged weather barriers can also wet equipment whose normal operating temperature sits outside those bands. The insulation makes matters worse by trapping water and deposits against the surface.
Temperature ranges that matter
API's public standards plan lists the third edition of API Recommended Practice 583, Corrosion Under Insulation and Fireproofing, with an April 2026 publication date. The public AMPP overview below gives a commonly cited carbon-steel range and explicitly warns that CUI can occur outside it. Use the licensed current standard, the owner's procedure and site experience for an actual assessment; these are screening cues, not safety thresholds.
| Material | Public screening guidance | Important limitation |
|---|---|---|
| Carbon and low-alloy steel | A commonly cited band is about −12 °C to 175 °C (10 °F to 350 °F). | CUI can occur outside this band. Wetting history, cycling, shutdown state, coating condition and geometry still matter. |
| Austenitic and duplex stainless steels | This public summary does not assign a universal numeric band. | Potential external chloride stress corrosion cracking depends on alloy, stress, chloride exposure, wet-dry history and temperature. Apply the current owner procedure and licensed standard. |
Normal operation below or above the commonly cited band may reduce time spent wet at temperature, but it does not clear an item. Water ingress, cooling during shutdown, steam-out, intermittent operation and cycling can move the metal through susceptible conditions while wet. Temperature therefore belongs in a multi-factor screening review, never as the sole reason to omit inspection.
Where it starts
CUI is a geometry problem as much as a chemistry one. The places that fail first are the places water reaches and cannot leave:
- Pipe supports, shoes and saddles, where the insulation is compressed and the jacketing is interrupted.
- Low points and dead legs, drains and the bottom of vertical sections where water pools.
- Penetrations for nozzles, instruments, tracing and structural steel.
- Insulation terminations at flanges, valves and vessel skirts, and around manways.
- Damaged, dented or missing jacketing, and horizontal seams that face upward.
- Vessel heads, stiffening rings and the areas below leaking flanges or steam traps.
- Steam-traced lines, which are both warm and a source of water when a trace fails.
This is why a good CUI program is not a matter of stripping whole lines. It is a matter of knowing which circuits carry the conditions above, and which points on those circuits deserve the first look.
Why it stays hidden
An uninsulated line shows rust; an insulated one shows jacketing. Wet insulation does not always stain the outside, and a bulge can look like a dent. Routine thickness monitoring on insulated piping is usually done at a small number of established points, and CUI rarely sits under one of them. Meanwhile the insulation keeps the surface wet in exactly the temperature band where the loss rate is highest, so by the time a leak appears the pit has had years to grow.
That combination, invisible and fast, is the reason industry practice treats CUI as its own damage mechanism with its own recommended practice, rather than as a special case of external corrosion.
Prevention: the four levers
AMPP SP0198 (formerly NACE SP0198), Control of Corrosion Under Thermal Insulation and Fireproofing Materials, and API RP 583 both come down to the same set of controls.
- Coating
- An immersion-grade coating applied to the steel before insulation is the last barrier once water is in. Its condition and age drive most susceptibility estimates.
- Insulation choice
- Non-absorbent, low-chloride materials hold less water against the metal. Where absorbent insulation is used, drainage and thickness matter.
- Weather barrier
- Jacketing designed to shed water: overlaps facing down, sealed penetrations, drip rings, and inspection ports that can be opened without cutting.
- Design and housekeeping
- Remove insulation that is not needed for process or personnel protection, keep supports and terminations sealed, fix steam-trace and flange leaks quickly, and repair jacketing damage when it is found rather than at the next turnaround.
Finding it: the order is the decision
No plant can strip and inspect every insulated circuit. The practical question is which circuits to open, screen or scan first. The inputs are things a plant already tracks: material, operating temperature and whether it cycles, insulation and jacketing type, coating age, environment (marine, cooling-tower plume, deluge zones), the number of supports and penetrations, and the inspection and repair history.
API RP 581 turns these into a CUI damage factor inside a risk-based inspection framework, and our note on risk-based inspection for CUI walks through how that ranking is built. The inspection techniques themselves, from stripping windows to pulsed eddy current and profile radiography, are covered in CUI inspection methods.
We built a small workbench that takes a circuit list with those attributes and returns a ranked, explainable look-here-first list, with a blind backtest against the findings from a previous campaign. It runs on a synthetic sample unit and is meant to be pressure-tested, not believed.
Open the CUI Priority WorkbenchWhere to read more
- API standards plan, which lists API Recommended Practice 583, Corrosion Under Insulation and Fireproofing, third edition, April 2026. Obtain the licensed current edition for mechanism, design, inspection and program requirements. Our summary.
- API Recommended Practice 581, Risk-Based Inspection Methodology. Contains the CUI damage factor used to rank insulated equipment.
- AMPP SP0198, Control of Corrosion Under Thermal Insulation and Fireproofing Materials — A Systems Approach. Coatings, insulation and design requirements. AMPP also publishes an official overview of CUI causes, detection and prevention.
- European Federation of Corrosion publication 55, Corrosion Under Insulation (CUI) Guidelines. A European perspective with a strong section on inspection strategy.
- API 570, Piping Inspection Code, and API 510, Pressure Vessel Inspection Code, for how CUI inspection fits into external inspection requirements and intervals.