Thermal insulation protects personnel, conserves energy and stabilizes process temperatures. It also hides the steel beneath it. When water finds its way under insulation, it can create an aggressive, persistently wet environment that drives corrosion for years before anyone sees it. Corrosion under insulation (CUI) is therefore one of the most significant integrity concerns for refineries, chemical plants and gas processing facilities.
CUI requires three ingredients: a susceptible metal surface, water, and oxygen or other corrosive species. Water enters through damaged jacketing, poorly sealed penetrations, deluge and rain, or condensation on cold surfaces. Insulation then holds that water against the metal, and the elevated surface temperature accelerates the reaction. Contaminants leached from the insulation or deposited from the atmosphere — notably chlorides — can make the trapped water considerably more aggressive.
The principal U.S. references are API RP 583, Corrosion Under Insulation and Fireproofing, and AMPP SP0198 (formerly NACE SP0198), Control of Corrosion Under Thermal Insulation and Fireproofing Materials — A Systems Approach. API RP 571 also describes CUI as a damage mechanism.
For carbon and low-alloy steels, API RP 583 and API RP 571 identify a susceptible operating temperature range of roughly 10°F to 350°F (approximately −12°C to 175°C). Below that range, water tends to remain frozen or corrosion is slow; well above it, the surface is usually hot enough to stay dry. Within the range, corrosion can be severe, and equipment operating at intermediate temperatures that keep water present but warm is particularly vulnerable.
Equipment that cycles through the susceptible range — or that is idle, in standby or intermittently operated — is especially vulnerable. A line that normally runs hot enough to stay dry can corrode aggressively during repeated shutdowns, when water condenses and sits against the metal.
Austenitic stainless steels do not suffer general CUI in the same way, but they are susceptible to external chloride stress corrosion cracking (ECSCC) when chlorides concentrate on surfaces that are under tensile stress at elevated temperature. Cracking can propagate rapidly and is often difficult to detect, which is why insulation materials for stainless steel are commonly specified to meet the chloride and leachable-ion limits of ASTM C795.
Because full insulation removal is expensive and sometimes hazardous (older insulation may contain asbestos), owner-users typically combine risk-based prioritization with a range of screening and inspection methods.
| Technique | What it detects | Practical notes |
|---|---|---|
| External visual of jacketing | Damage, gaps, failed sealant | Low cost; identifies water ingress points but not corrosion directly |
| Infrared thermography | Wet insulation through thermal anomalies | Requires temperature difference between process and ambient |
| Neutron backscatter | Hydrogen (moisture) in insulation | Screening for wet areas |
| Profile radiography | Wall-loss profile through insulation | Useful on smaller-diameter piping; radiation safety controls needed |
| Pulsed eddy current (PEC) | Average remaining wall thickness through insulation and jacketing | Measures averaged thickness over a footprint; may under-call small pits |
| Guided wave UT | Cross-sectional changes over long pipe runs | Screening tool; follow-up needed to size indications |
| Insulation removal with visual and UT | Direct assessment of surface and thickness | Most definitive; costly and requires reinstatement |
No single technique answers every question. A common strategy is to use screening methods to find wet insulation or suspected wall loss, then remove insulation at targeted locations for direct examination and thickness measurement.
The most effective CUI programs manage water, not just corrosion. Keep water out, give it a way to escape, and protect the steel in case it gets in anyway.
The most reliable defense is a high-performance coating on the steel under the insulation. AMPP SP0198 discusses coating systems suited to different temperature ranges, such as epoxy phenolics and other immersion-grade systems for lower temperatures, and thermal spray aluminum (TSA), which can provide long-term protection across a wide temperature range. For stainless steel, coatings or aluminum foil wrapping are used to reduce chloride stress corrosion cracking risk.
Select insulation materials with low water absorption and acceptable chloride content. Design jacketing with overlaps shed downward like roof shingles, seal penetrations carefully, and provide drainage at low points where codes and practice allow. Avoid insulating equipment that does not need it; removing unnecessary insulation, or replacing it with personnel protection cages, eliminates the problem entirely.
Many CUI programs fail not in design but in maintenance. Jacketing removed for an inspection or repair and not properly reinstated becomes the next water ingress point. Including insulation and jacketing condition in routine inspection rounds and work-order close-out is a simple, high-value control.
CUI should be treated as a defined damage mechanism within the facility's risk-based inspection program, with susceptibility based on temperature, coating age and type, insulation condition, climate and service history. API 570 and API 510 require that external inspections consider CUI, and API RP 583 provides guidance on prioritizing and selecting inspection locations. Findings should feed back into the risk model so that inspection effort tracks actual condition.
To strengthen your CUI program, consider IIME's Corrosion Under Insulation Inspection and Mitigation training course.
September 19, 2026 - BY Admin