Corrosion Under Insulation (CUI): Causes and Mitigation

For carbon and low-alloy steel, the classic CUI temperature window is approximately -12 to 177 degC (10 to 350 degF), with the highest risk typically cited around 60-120 degC where water can be present as liquid long enough to drive corrosion but the equipment still operates hot enough for repeated wet-dry cycling to concentrate corrosive species. For austenitic and duplex stainless steel, the relevant window for external chloride stress corrosion cracking is roughly 60-204 degC (140-400 degF). Equipment operating continuously well above or below these windows carries meaningfully lower CUI risk, all else equal.

Equipment that cycles between hot operating temperature and ambient temperature during shutdowns or idle periods experiences repeated moisture condensation and evaporation cycles within the insulation, which concentrates chlorides and other corrosive species at the metal surface each time the water partially evaporates. Continuously hot equipment, by contrast, tends to keep any moisture that does enter in a more stable state, and equipment that stays well above the boiling point of water may simply flash off intruding moisture before it can sit against the surface for long, which is why intermittent and cyclic service is flagged as a specific CUI risk factor in inspection planning.

Yes. Austenitic and duplex stainless steel are not immune to CUI — they are susceptible to external chloride stress corrosion cracking (ESCC) under insulation, driven by chlorides that leach from certain insulation materials, from atmospheric deposition, or from the local environment, concentrating at the metal surface through the same wet-dry cycling mechanism that drives general CUI in carbon steel. Because SCC can produce fine cracking with little visible surface corrosion, CUI-related ESCC in stainless steel is often harder to detect during inspection than the more visually obvious general corrosion typical of carbon steel. See the stress corrosion cracking guide for the underlying mechanism.

The highest-risk locations are insulation terminations and penetrations (where insulation ends at a flange, valve, or nozzle and the jacketing seal is most likely to fail), pipe supports and clips where the insulation is compressed or interrupted, low points and dead legs where water can pool inside the insulation system, and any location where jacketing overlaps or seams are not properly sealed against rain or washdown ingress. Vertical runs with horizontal jacketing seams are also higher risk because those seams act as direct water collection points rather than shedding water downward.

A properly selected and applied coating system significantly reduces CUI risk but is not a complete solution by itself, since coating failures (holidays, damage during installation, degradation over time) will still allow localized attack at the failure point, sometimes at an accelerated rate due to the area ratio effect between a small coating defect and the surrounding intact coating. Effective CUI mitigation combines coating selection (such as epoxy phenolic systems or thermal spray aluminum for higher-risk cases) with insulation and jacketing design, drainage detailing, and a scheduled inspection program rather than relying on coating performance alone.

Thermal spray aluminum (TSA) is a coating applied by thermally spraying molten aluminum onto a prepared steel surface, forming both a barrier layer and a degree of galvanic (sacrificial) protection similar to hot-dip galvanizing, but capable of withstanding higher service temperatures than most organic coatings. It is increasingly specified for high-consequence, high-temperature CUI-prone equipment — particularly in offshore and high-value onshore process plant applications — because it resists the temperature limitations that restrict many conventional coating systems in the CUI-critical range.

Common screening techniques include infrared thermography, which can identify wet insulation from the surface (wet insulation shows a different thermal signature than dry insulation) without removal, guided wave / long range ultrasonic testing (LRUT) for screening long straight pipe runs, and real-time or profile radiography through the insulation at specific points. None of these fully replace direct visual inspection, so a risk-based inspection program typically combines a non-intrusive screening technique with scheduled insulation removal and direct visual/UT inspection at the highest-risk locations identified by that screening and by API 571 damage mechanism criteria.

It can, if the insulation and jacketing are not reinstalled to the same standard as the original installation — a poorly resealed jacketing seam or an improperly lapped joint after an inspection opening is a common source of new CUI initiation points. This is why CUI inspection programs specify jacketing reinstatement quality as part of the inspection procedure itself, not just as a housekeeping afterthought, and why some critical locations use removable insulation blankets or covers specifically designed to be reopened repeatedly without degrading the moisture seal each time.