Corrosion Under Insulation (CUI): Detection and Prevention is the practice of finding and controlling external corrosion that develops on piping and equipment beneath thermal insulation, weatherproof jacketing or fireproofing, where the metal is hidden from routine visual inspection. CUI is a leading cause of unplanned piping failures in refineries, chemical plants and power stations, not because the mechanism is unusual, but because it happens where nobody is looking. A line can look intact under its jacketing while much of its wall thickness is already gone.
No jacketing system stays perfectly sealed for the life of a plant. Rain, wash-down water, steam leaks and condensation find their way in through damaged cladding, poorly sealed penetrations, or breaks around supports and nozzles. Once past the jacketing, the insulation holds water against the pipe wall like a wet sponge, and unlike bare pipe, an insulated line can stay wet for weeks since the cladding blocks evaporation. Trapped water, oxygen and metal are all corrosion needs.
CUI on carbon and low-alloy steel is a problem of intermediate operating temperatures. NACE SP0198 frames the overall susceptible range as roughly -12 to 175 degrees C (10 to 350 degrees F): below that, water tends to freeze rather than sustain corrosion, and above it, surfaces generally run hot enough to stay dry. The most severe attack, sometimes exceeding 1 millimeter per year of wall loss, concentrates between about 60 and 120 degrees C. Equipment that cycles between ambient and this range, such as intermittent-service and standby lines, is especially vulnerable because wetting and drying repeat continuously.
Austenitic stainless steel has a separate, more insidious mode: chloride stress-corrosion cracking (Cl-SCC) under insulation. Chlorides leach out of some insulation materials or arrive with wash and rainwater, then concentrate under the insulation as moisture evaporates and re-wets, attacking stainless under tensile stress. API RP 583 identifies this risk as active above roughly 60 degrees C (140 degrees F) with moisture and chlorides present, overlapping the carbon steel CUI window. Cl-SCC produces fine, branching cracks far harder to detect than general wall loss, and can cause sudden failure with little warning.
CUI is invisible by design: the insulation that makes a pipe efficient also makes it impossible to inspect visually without removal. A line can look pristine for years while wall thickness thins underneath, or cracks propagate through a stainless nozzle. It rarely reaches an operator's radar the way a leak or vibration problem would, so it tends to surface during a turnaround, by chance, or through failure. On piping and vessels carrying flammable, toxic or high-energy fluids, such failures can mean fires, releases or injury, which is why regulators and insurers treat CUI as a top-tier integrity risk.
CUI does not distribute evenly. Inspectors focus first on locations where water ingress is most likely:
Because removing insulation from every line is not economical, CUI programs rely on risk ranking. API RP 583, Corrosion Under Insulation and Fireproofing, is the industry reference: it covers identifying susceptible systems, ranking them by likelihood and consequence, and selecting detection methods accordingly.
| Method | Insulation removal required | What it identifies |
|---|---|---|
| Profile (real-time or film) radiography | No | Wall-thinning profile through jacketing |
| Pulsed eddy current scanning | No | Averaged wall loss on ferromagnetic piping |
| Infrared thermography / neutron backscatter | No | Wet insulation zones (indirect indicator) |
| Visual inspection and thickness measurement | Yes, at selected points | Direct confirmation of wall loss |
| Liquid penetrant testing on stainless welds | Yes | Surface-breaking Cl-SCC cracking |
Prevention is more cost-effective than detection because it addresses the root cause, water ingress, rather than chasing damage after the fact. Effective programs combine:
CUI only stays under control if findings, risk rankings and corrective actions are tracked systematically rather than living in individual inspectors' notes. A CMMS or OEE platform such as Fabrico can hold the asset-level CUI risk register, schedule insulation-removal inspections on a recurring interval, log thickness readings against specific pipe segments, and flag overdue high-risk locations before a turnaround is planned. Book a Fabrico demo to see how a structured workflow handles CUI alongside other hidden-damage inspection programs.
Roughly 60 to 120 degrees C, within the broader -12 to 175 degrees C range. Equipment that cycles in and out of this band sees the worst attack.
No. It avoids the general wall loss carbon steel suffers, but is susceptible to chloride stress-corrosion cracking above roughly 60 degrees C with moisture and chlorides present, and that cracking is often harder to detect than wall thinning.
Partially. Radiography and pulsed eddy current scanning screen through insulation to flag likely wall loss, and thermography or neutron backscatter can spot wet zones. Confirming actual damage, especially stainless cracking, still requires targeted removal at flagged locations.
A framework for identifying which insulated systems are CUI-susceptible, ranking them by likelihood and consequence, and selecting inspection intervals and methods, used alongside a plant's broader inspection program.
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