Corrosion under insulation: the damage you insulated out of sight
Corrosion under insulation (CUI) is external corrosion of piping, vessels and tanks that happens beneath thermal insulation or fireproofing when water gets past the cladding and is held against the metal. Because the insulation hides it, CUI usually progresses unseen until a leak, a bulge in the jacketing or an inspection finds significant wall loss.
The mechanism is ordinary — wet steel corrodes — but the conditions favour it and the damage is hidden by design. AMPP (formerly NACE) standard practice SP0198 approaches corrosion under insulation as a systems problem covering design, coatings, insulation, jacketing and inspection, and API RP 583 is dedicated to corrosion under insulation and fireproofing for refinery and petrochemical equipment.
How corrosion under insulation works
Insulation is not waterproof, and jacketing does not stay watertight for long. Rain and snowmelt, deluge testing, wash-down hoses, leaking steam tracing and condensation get in through damaged cladding, open seams, unsealed penetrations and terminations — and once inside, water does not leave easily.
Three factors then work together:
- Trapped water. Wet insulation acts as a sponge. It keeps an electrolyte against the steel far longer than an exposed surface would stay wet, and it keeps oxygen supplied through the pores.
- Heat and thermal cycling. A warm surface corrodes faster while it is wet. Equipment that heats and cools, or runs intermittently, goes through repeated wetting and drying: each cycle evaporates water and concentrates whatever salts it carried.
- Contaminants. Chlorides and other species come from the environment (de-icing salts, coastal or cooling-tower drift) and from the insulation material itself, leached out by the water that soaks it.
On carbon and low-alloy steel, the result is general and localized wall loss, often with deep pits under a layer of corrosion product. The mechanism behind localized attack is the one described in our article on pitting corrosion: once chlorides concentrate in a small anodic area, the attack drives inward rather than spreading.
On austenitic stainless steels (304, 316 and similar grades), the threat is different. Chlorides concentrated on a warm surface under tensile stress can cause external chloride stress corrosion cracking. The cracks grow from the outside surface with little visible metal loss, so a stainless line can look clean at a strip-out and still be cracked. This is why insulation placed on austenitic stainless is normally specified for low leachable chloride content — ASTM C795 is the usual reference for that requirement.
Where CUI concentrates: the at-risk locations
CUI is not spread evenly along an insulated line. It concentrates where water enters, where it collects, and where the operating temperature lets it stay liquid on the metal.
| Location | Why it is at risk |
|---|---|
| Pipe supports, hangers and saddles | Insulation is cut or compressed, jacketing is hard to seal, water runs down to the contact point |
| Penetrations: nozzles, vents, drains, instrument connections, small-bore branches | Every break in the jacketing is a water entry point |
| Insulation terminations and flange boxes | Open ends and poorly sealed end caps let water in and hold it |
| Low points, bottom heads, lower side of horizontal runs | Water that entered anywhere upstream drains and collects here |
| Vertical-to-horizontal transitions, stiffening rings, vessel skirts | Water running down a vertical surface is stopped and pooled |
| Insulated dead legs and equipment in intermittent service | Temperature swings through wet and dry conditions, concentrating salts |
| Cold service cycling around the dew point | Condensation forms under the insulation whenever the vapour barrier is breached |
| Areas downwind of cooling towers or steam vents | Constant external wetting and airborne contaminants |
Temperature governs susceptibility. Equipment that runs well below freezing, or hot enough to keep the surface dry, is less exposed. The at-risk band covers equipment operating between those extremes, where water stays liquid on the steel — plus any equipment, whatever its design temperature, that cycles through that band during start-ups, shutdowns or intermittent service. AMPP SP0198 and API RP 583 give the temperature ranges for carbon and stainless steels; screening your circuits against those documents, rather than against a rule of thumb, is the starting point of any CUI program.
Finding CUI: inspection methods
Every method trades direct confirmation against screening long lengths without removing insulation.
| Method | What it tells you | Limits |
|---|---|---|
| Visual inspection of jacketing | Where water can enter: damaged cladding, open seams, missing sealant, bulging or stained jacketing | Says nothing about the metal itself |
| Insulation removal and direct examination | The actual surface condition, with ultrasonic thickness and pit depth measurement | Costly, requires reinstatement, and only covers the stripped area |
| Profile radiography | Wall thickness and corrosion at the pipe edges in the image, through the insulation | Small area per shot; radiation safety constraints; mainly for piping |
| Real-time radiography | Rapid screening along a line for wall loss and wet insulation | Screening only; findings need confirmation |
| Pulsed eddy current | Average wall thickness through insulation and jacketing | Averages over a footprint, so small isolated pits can be missed |
| Guided wave ultrasonics | Screening of long lengths of pipe from one location for changes in cross-section | Screening only; sensitivity depends on coating, supports and geometry |
| Infrared thermography | Wet insulation, detected by its thermal signature on hot or cold equipment | Finds the water, not the corrosion; depends on temperature difference |
The logic is screen, then confirm. Visual surveys and screening tools narrow the search to the suspect locations; insulation removal and direct thickness measurement confirm the condition and feed the condition monitoring locations used for corrosion rate and remaining-life calculations. The inspection codes expect exactly that: API 570 for piping and API 510 for pressure vessels both require susceptible insulated equipment to be identified and inspected for CUI, with the extent set by the code edition and your inspection plan.
CUI prevention: coating, insulation design and jacketing
Water will get in over the life of a plant. Durable CUI prevention makes sure it finds a barrier when it does.
Coating under insulation
The coating on the metal is the last line of defence and the one that lasts. It has to be selected for immersion-like wet service at the operating temperature, and for the thermal cycling the equipment actually sees — not for the atmospheric exposure a painted, uninsulated line would get. The families referenced in SP0198 include high-temperature epoxy phenolic and epoxy novolac systems, coatings formulated for high-temperature or cyclic service, and thermally sprayed aluminium (TSA). Final selection comes from the operating envelope and the manufacturer's qualification data for that service.
Surface preparation decides whether the coating performs. Abrasive blast cleaning to the degree the system specifies — often near-white metal, as described in our NACE No. 2 surface preparation guide — plus a check for soluble salts on the steel. On equipment that has already suffered CUI, chlorides remain in pits after blasting; left in place, they drive osmotic blistering and early failure of the new coating.
Insulation and jacketing design
- Remove what is not needed. Insulation installed only for personnel protection can often be replaced by guards or cages. Insulation that no longer serves a process purpose is pure risk.
- Choose insulation for the service. Low-chloride materials on austenitic stainless, and materials with low water absorption where wetting is likely.
- Shed water. Jacketing overlaps lapped so water runs off, seams on the lower side of horizontal runs, sealed penetrations, properly designed end caps and weather hoods at nozzles and supports.
- Keep the vapour barrier intact on cold service. On equipment below ambient temperature, a breached vapour barrier lets humid air reach the cold surface and condense continuously.
- Plan for inspection. Removable covers or inspection plugs at known high-risk points make future inspection cheaper and less destructive.
Maintenance inspection
Jacketing damaged during maintenance and never resealed is a common entry point. A routine walk-down of insulated systems, with cladding, sealant and end-cap repairs logged and tracked, is the cheapest CUI control there is.
Repairing CUI damage
Once wall loss is confirmed, the first question is structural: can the equipment stay in service as it is? A fitness-for-service assessment under API 579-1/ASME FFS-1 evaluates the remaining wall against the operating conditions, using the parts of the standard that address general metal loss, local metal loss and pitting. The outcome is one of three decisions: continue in service with a monitoring plan, repair, or replace.
Repair options depend on the extent of damage and on the code governing the equipment: replacement of spools or components, welded repairs, or engineered composite repairs designed to ASME PCC-2 where local wall loss must be reinforced without hot work. Whatever the structural fix, the surface is then prepared, coated with a system suited to CUI service, and reinsulated with the entry points that caused the damage corrected. Reinstating the original detail restarts the clock.
A risk-based CUI management program
A plant cannot strip every metre of insulation. A CUI program focuses effort where the probability and the consequence of failure are both high, within the inspection framework the site already uses (API 570, API 510, and risk-based inspection following API 580 and API 581 where adopted).
- Inventory insulated equipment: material, operating and cycling temperature, insulation type, jacketing and coating condition.
- Screen susceptibility against the criteria of SP0198 and API RP 583.
- Rank circuits by likelihood and consequence of failure.
- Inspect high-ranked circuits: visual survey, screening, then targeted insulation removal and thickness measurement at the at-risk locations.
- Assess findings through fitness-for-service and set intervals from measured corrosion rates.
- Mitigate at the source — recoat, redesign jacketing details, remove unnecessary insulation — and record what was done.
FAQ
What temperature range is most at risk for CUI? The range in which water stays liquid on the metal, plus any equipment that cycles through it. The specific ranges for carbon steel and austenitic stainless steel are given in AMPP SP0198 and API RP 583, and your inspection plan should cite them rather than a remembered figure.
Can CUI be detected without removing the insulation? It can be screened for. Pulsed eddy current, guided wave ultrasonics, real-time and profile radiography, and thermography for wet insulation all work through or around the insulation. Confirming the extent and depth of damage still generally requires removing insulation at the suspect points and measuring directly.
Does stainless steel suffer from corrosion under insulation? Yes, but differently: the risk is external chloride stress corrosion cracking rather than wall loss. Low-chloride insulation and a suitable coating are the usual defences.
Is a new jacketing enough to stop CUI? No. Jacketing reduces how much water gets in; it never keeps all of it out, and it degrades. The durable protection is a coating on the metal designed for wet, hot and cycling service.
When should a CUI-damaged line be recoated rather than replaced? When a fitness-for-service assessment shows the remaining wall, or the wall after a local repair, is adequate for the operating conditions and the planned service life. In that case, proper surface preparation, salt removal and a CUI-rated coating stop the attack from continuing.
In practice
CUI is controlled by what is on the steel under the insulation, and that is where Induscoat works: surface preparation, soluble salt removal and application of coating systems for insulated piping and vessels, carried out during shutdowns for plants in Quebec and Ontario as part of our protective coatings service and our corrosion protection work. When an inspection campaign turns up CUI, send us the findings, the operating temperatures and the insulation type to scope the recoating: request a quote. Technical data sheets for the Induscoat range are published on the brand site, induscoat.com.
Hicham M, P Eng, PMP
Engineer and project manager (PMP) at Induscoat. Over 16 years of experience in industrial coatings, composite repairs and wear protection on mining, energy and petrochemical sites in Canada and internationally.