Beacon LibraryExplainerCorrosion damage

What is corrosion under insulation?

Insulation keeps the heat in. It can also hold water against the steel, and the cladding hides what the water does. Six steps through one insulated line, from the first drop to the inspector’s window.

Beacon Library · Checked by Jun Zhong as editorOct. 25, 2026Made with AI tools.

Inside an insulated lineSchematic, not to scale

BeforeThe parts. An insulated steel line in longitudinal section and in cross-section A–A: steel wall, insulation, and an outer cladding.

Corrosion under insulation, or CUI, is corrosion on the outer surface of insulated pipes, vessels and tanks, caused by water that gets through the insulation and is held against the metal. Because the cladding hides the steel, wall loss or chloride cracking can grow unseen until a leak.³

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.¹ The drawing follows that water through one line, step by step.

1Water gets in

Water can come from rain, cooling towers, sprinkler and deluge systems, or condensation.²

It finds its way through the cladding. Badly designed or fitted cladding, failed joint sealant, mechanical damage and cladding not refitted after maintenance, often around valve boxes, all open a path.¹

2It runs to the low points

Inside, the water runs down. It gathers on the underside of horizontal runs and elbows, at vessel bases, pipe supports, nozzle connections and drain legs.¹

In the field, damage is often found at the 6 o’clock position, the bottom of the pipe.²

3The insulation holds it

How much water stays depends on the material. Fibrous mineral wool, fibreglass and calcium silicate soak up water easily, while closed-cell foam glass and perlite take up less.¹

What makes CUI different from other corrosion on the outside of equipment is the insulation: it traps water against the metal and hides the result.²

4Heat keeps it active

Operating temperature has a strong effect on risk, and frequent swings between running and shutdown rewet the insulation.^1,2

The sources name different temperature ranges as the most critical. Read together, they put peak risk between about 50 and 120 °C, the band where their ranges overlap; the chart below sets them side by side.^1,2

5The damage stays hidden

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.² On austenitic and duplex grades, API RP 583 deals with it as external chloride stress corrosion cracking.³

Some cladding materials contain free chlorides, and coastal and offshore sites add more, which matters most for austenitic stainless steel.¹ None of it can be seen from outside.²

6How it is 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.¹

Other methods screen through the insulation: pulsed eddy current estimates the remaining wall, guided waves screen long runs, radiography images the wall.^1,2 API RP 583 sets out how to rank equipment by the likelihood of CUI, so the search starts where damage is most probable.³

Which temperatures carry the most risk?

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.²

The sources do not agree on one band. A 2011 circular for UK Health and Safety Executive inspectors puts the most critical range at 30 to 120 °C, and cites US data for carbon-manganese steel showing about 0.5 mm a year at 80 °C under insulation.¹ 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 a year at 60 to 120 °C.²

Two sources, one overlapMetal temperature ranges each source names as the most critical for CUI

50–120 °Cboth flag it: peak risk

HSE circular, 2011¹most critical range

30–120 °C

Cao et al. review, 2022²riskiest range, citing NACE SP0198 and API RP 583

50–175 °C

Reported rates

≈0.5 mm/y at 80 °C, US data (HSE)

≈1 mm/y, one case (Cao)

65 °C: chloride cracking to be considered on austenitic stainless (HSE)

Sources: HSE (2011); Cao et al. (2022), citing NACE SP0198 and API RP 583. Ranges from the two documents named, not the text of a paid standard; engineering decisions need the current edition of API RP 583.^1,2,3

Where water gathers

Water runs to the low points of whatever it is in. The UK circular names six places where it gathers.¹

1Underside of horizontal runs
2Elbows
3Vessel bases
4Pipe supports
5Nozzle connections
6Drain legs

Source: HSE (2011). Schematic drawings; water shown where it collects.¹

How inspectors look

Several nondestructive testing methods screen through the insulation, and others look for wet insulation rather than corrosion; the table below sets them out.^1,2 API RP 583 sets out what each method can and cannot do, and how to rank equipment by the likelihood of CUI and corrosion under fireproofing, so the search starts where damage is most probable.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.²

An offshore case shows what is at stake.

Methods and what they screen forAs described in the sources; limits as stated there
Method	Screens for	Limits and notes
Taking the insulation off
Inspection windows¹	Small cut-outs at chosen spots	Only as good as the choice of where to cut. Stripping all the insulation is very expensive; on critical lines where no leak is acceptable, full removal may be needed
Through the insulation, for the wall
Pulsed eddy current (PEC)^1,2	Remaining wall in ferritic steel	Averaged over the sensor footprint; insulation type and thickness limit it
Guided wave ultrasonics^1,2	Long or hard-to-reach runs	Picks up cross-section losses from about 10 percent
Profile and real-time radiography^1,2	An image of the wall through the insulation	—
For wet insulation, not corrosion
Infrared thermography¹	Wet or damaged insulation	Does not show the corrosion itself; drying cycles on hot lines can mislead it
Neutron backscatter¹	Wet insulation	Can screen around 300 m of insulated pipe a day

Sources: HSE (2011); Cao et al. (2022). A dash: the sources state no limit.^1,2

Is it 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.³ API RP 571 gives atmospheric corrosion its own section, 3.8, separate from CUI in 3.22.⁴ CUI is a form of external corrosion that cannot be seen from outside.²

The same recommended practice also covers corrosion under fireproofing, and applies to pressure vessels, piping, storage tanks and spheres.³ API RP 571, third edition (2020), describes CUI and chloride stress corrosion cracking as separate damage mechanisms, in sections 3.22 and 3.17.⁴

Notes and sources

UK Health and Safety Executive. Corrosion under insulation of plant and pipework, semi-permanent circular SPC/TECH/GEN/18, issued 1 March 2011 (archived copy). web.archive.org
Cao, Pojtanabuntoeng, Esmaily, Thomas, Brameld, Amer and Birbilis. A Review of Corrosion under Insulation: A Critical Issue in the Oil and Gas Industry. Metals 12(4), 561, 2022. Metals (MDPI), via MIT DSpace. dspace.mit.edu
American Petroleum Institute. API RP 583, Corrosion Under Insulation and Fireproofing, Third Edition, April 2026 (publisher’s scope as listed by a standards distributor). eshop.normservis.sk
American Petroleum Institute. API RP 571, Damage Mechanisms Affecting Fixed Equipment in the Refining Industry, Third Edition, March 2020 (edition and section numbers as listed in the API 570 Exam Publications Effectivity Sheet for February and June 2027). api.org
Bureau of Safety and Environmental Enforcement. Safety Bulletin No. 013: Severe Corrosion Found Under Insulation, 3 December 2018. bsee.gov

Adapted from Beacon Library, “What is corrosion under insulation (CUI)?”, beacon.winda.ai/library/corrosion-under-insulation. Names of standards only; no text, tables or figures from paid standards.

Made with AI tools. Checked by Jun Zhong as editor.