What is corrosion under insulation (CUI)?

Why does CUI happen?

Steel corrodes when it meets water with a supply of oxygen, and insulation usually leaves a space against the metal where water can sit with air ³. Water can come from rain, cooling towers, sprinkler and deluge systems, or condensation ⁴. Five conditions explain most cases:

• Water gets in. Badly designed or fitted cladding, failed joint sealant, mechanical damage and cladding not refitted after maintenance, often around valve boxes, all open a path ³.
• Water runs to low points. It gathers on the underside of horizontal runs and elbows, at vessel bases, pipe supports, nozzle connections and drain legs ³; in the field, damage is often found at the 6 o'clock position ⁴.
• The insulation holds it. Fibrous mineral wool, fibreglass and calcium silicate soak up water easily, while closed-cell foam glass and perlite take up less ³.
• Heat and cycling keep it active. Operating temperature has a strong effect on risk, and frequent swings between running and shutdown rewet the insulation ^{3 4}.
• Chlorides are present. Some cladding materials contain free chlorides, and coastal and offshore sites add more, which matters most for austenitic stainless steel ³.

CUI grew with the use of insulation itself. Before the 1970s, insulation went mainly on lines above about 150 °C, where moisture boiled off; after the fuel shortages of that decade, insulating cooler lines to save energy became common practice, and water stayed in the insulation ⁴.

Which temperatures carry the most CUI risk?

The sources do not agree on one band. A 2011 circular for UK Health and Safety Executive (HSE) inspectors puts the most critical range for CUI at 30 to 120 °C, and cites US data for carbon-manganese steel showing about 0.5 mm per year (mm/y) at 80 °C under insulation ³. A 2022 review, citing NACE SP0198 and API RP 583, gives 50 to 175 °C as the riskiest range for both carbon steel and austenitic or duplex stainless steel, and reports one case of about 1 mm/y at 60 to 120 °C ⁴.

Read together, the two put peak CUI risk between about 50 and 120 °C, the band where their ranges overlap ^{3 4}:

Metal temperature (°C)	HSE circular (2011)	Cao et al. review (2022)	Where the sources stand
Below 30	Below the critical range; risk returns during shutdowns and cycling	Below the riskiest range	Lower risk, not zero
30 to 50	Inside the critical range	Below the riskiest range	Sources differ
50 to 65	Inside the critical range; chloride cracking possible in high-carbon stainless grades	Inside the riskiest range	Both flag it
65 to 120	Inside the critical range; chloride cracking to be considered on austenitic stainless	Inside the riskiest range	Both flag it, peak risk
120 to 175	Above the critical range; moisture should dry out at start-up	Inside the riskiest range	Sources differ
Above 175	Above the critical range	Above the riskiest range	Lower risk while hot

The ranges above come from the two documents named; neither is the text of a paid standard, and engineering decisions need the current edition of API RP 583 ^{3 4 1}.

What does CUI do to different metals?

API RP 583 deals with two kinds of damage: outside-surface corrosion of carbon and low-alloy steel, and, on austenitic and duplex stainless grades, external chloride stress corrosion cracking (ECSCC) ¹. On carbon steel the attack can be even or localized; on stainless steel it is mostly pitting and stress corrosion cracking, and duplex grades, with 20 to 28 percent chromium, resist it better ⁴.

The same RP also covers corrosion under fireproofing (CUF) and applies to pressure vessels, piping, storage tanks and spheres ¹. API RP 571, third edition (2020), describes CUI and chloride stress corrosion cracking as separate damage mechanisms, in sections 3.22 and 3.17 ².

How is CUI found?

Stripping all the insulation is very expensive, so inspectors often cut small windows, and the result is only as good as the choice of where to cut. For critical lines where no leak is acceptable, the HSE circular says full removal may be needed ³.

Several nondestructive testing (NDT) methods screen through the insulation. Pulsed eddy current (PEC) estimates remaining wall in ferritic steel, averaged over the sensor footprint, and insulation type and thickness limit it. Guided wave ultrasonics screens long or hard-to-reach runs and picks up cross-section losses from about 10 percent. Profile and real-time radiography image the wall through the insulation ^{3 4}.

Other methods look for wet insulation rather than corrosion. Infrared thermography shows wet or damaged insulation but not the corrosion itself, and drying cycles on hot lines can mislead it; neutron backscatter locates wet insulation and can screen around 300 m of insulated pipe a day ³. One field study reviewed in 2022 found severe CUI, deeper than 1 mm, once insulation moisture passed 25 percent by volume, and advised inspection above 15 percent ⁴.

API RP 583 sets out what each NDT method can and cannot do, and how to rank equipment by the likelihood of CUI and CUF so the search starts where damage is most probable ¹.

An offshore case shows what is at stake. In July 2018 the US Bureau of Safety and Environmental Enforcement (BSEE) investigated a fire inside a platform diesel generator enclosure; the cause was inconclusive, but the exhaust piping under the insulation blankets had corroded through and in places separated completely. An identical generator, shut down on BSEE's instruction, had the same damage at the same points ⁵. BSEE then recommended routine inspection of all insulated components, even where guards must be removed to reach them ⁵.

Is CUI the same as atmospheric corrosion?

No. Both act on the outside of equipment, but API RP 583 excludes atmospheric corrosion and corrosion at uninsulated pipe supports, while it does cover insulated supports ¹. API RP 571 gives atmospheric corrosion its own section, 3.8, separate from CUI in 3.22 ². What makes CUI different is the insulation: it traps water against the metal and hides the result, so CUI is a form of external corrosion that cannot be seen from outside ⁴. For the basic mechanism, see what is corrosion?.