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What Is Corrosion Under Insulation (CUI)? | Piping Engineering Glossary
What Is Corrosion Under Insulation (CUI)?
In piping engineering and process engineering, corrosion under insulation (CUI) is external corrosion that develops on piping and equipment beneath thermal insulation. Water enters through damaged jacketing, poorly sealed joints, or condensation, then stays trapped against the metal surface. Because the insulation hides the damage, CUI can thin a pipe wall for years before anyone notices.
Why CUI Is So Hard to Find
CUI is one of the leading causes of leaks in piping covered by pipe insulation. Carbon steel is most vulnerable when operating roughly between -12°C and 175°C (10°F to 350°F), especially in services that cycle through the dew point. Austenitic stainless steels face a different threat under wet insulation: chloride stress corrosion cracking. Lines with heat tracing or intermittent operation are frequent problem areas, since temperature swings repeatedly wet and dry the surface.
CUI often progresses in stages: the coating breaks down first, then general corrosion spreads beneath the wet insulation, and finally localized pits or broad wall thinning develop. Older facilities with aging coatings, coastal sites with salt-laden air, and units with frequent washdowns or deluge testing tend to see the highest rates.
Applications
Inspection Planning
CUI susceptibility is a core input to risk-based inspection programs, which rank insulated lines by temperature, coating age, environment, and consequence of failure.
Insulated Piping Design
Designers specify protective coatings, sealed jacketing, and drainage details on insulated lines to keep water away from the metal surface. Inspection ports and removable plugs are added at known trouble spots so inspectors can check the metal without stripping long runs of insulation.
Integrity Assessment
When CUI is found, engineers measure remaining wall and may run a fitness-for-service assessment to decide whether the line can stay in operation.
Benefits
Prevents Hidden Leaks
Recognizing CUI-prone lines lets plants find wall loss before it becomes a leak or rupture.
Focuses Inspection Budgets
Targeting the temperature ranges and locations most at risk avoids stripping insulation from every line.
Extends Asset Life
Good coatings, jacketing, and repairs made early can add many years of service to insulated piping.
Limitations
Damage Is Concealed
Visual inspection usually requires removing insulation, which is costly and may itself damage the system.
Screening Methods Have Limits
Techniques such as profile radiography, guided wave, and pulsed eddy current help, but most require follow-up non-destructive testing to confirm and size the damage.
Corrosion Can Be Localized
CUI often concentrates at supports, penetrations, and low points, so a few readings can miss the worst spots.
FAQ
Which parts of an insulated line are most at risk?
Low points, pipe supports, insulation terminations, nozzles, and areas around damaged or missing jacketing, where water collects and cannot drain or evaporate.
How is CUI detected?
Plants combine visual checks of jacketing with screening techniques, then confirm suspect areas with thickness measurement after removing insulation at selected locations.
Can CUI be prevented?
It can be greatly reduced with high-performance coatings, properly sealed and maintained jacketing, inspection ports, and ongoing corrosion monitoring, but no insulation system stays completely dry forever.
Does CUI affect stainless steel piping?
Yes, but differently. Wet insulation containing chlorides can cause external chloride stress corrosion cracking in austenitic stainless steel, typically at operating temperatures between about 50°C and 175°C. Using chloride-free insulation and coating stainless lines in susceptible ranges helps reduce the risk.
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