The same hot-water skid with removable insulation modules fitted over the pumps, valves and exchangers, levers and stems standing clear of the covers

Skid, right isometric, modules fitted — Inzonex Modular Insulation, hot-water skid

Hot water / CUI

Corrosion under insulation

The metal you cannot see is the metal that fails.

Insulation keeps heat in and it also keeps water in. Moisture that reaches the steel underneath cannot get away. It sits against the surface at a temperature that suits corrosion very well, and nobody sees any of it. The fault is not insulating. The fault is insulating in a way that nobody can open, drain or look at.

The mechanism

Why covered steel corrodes faster

Water gets in

Through damaged cladding, at penetrations and terminations, or from a weeping joint under the covering. Also from washdown, and by condensation on plant that cycles between hot and cold.

And then cannot leave

The insulation holds it against the surface. Wet insulation also conducts instead of resisting. The section then loses more heat than bare pipe while it corrodes. The two problems arrive together.

At a helpful temperature

Warm and wet is worse than hot and wet: above the boiling point the water is driven off. The worst cases are on plant that is warm rather than very hot, and on anything that cycles through the range repeatedly.

Temperature

The range it lives in

Carbon steel

API 583 puts the CUI-susceptible range for carbon steel at roughly −12 to 175 °C. The worst of it is in the middle, where water stays liquid against the surface. A hot-water circuit at 50 to 65 °C sits squarely inside it.

Austenitic stainless

Stainless is not immune, it fails differently. Chlorides leached from wet insulation concentrate on the surface and cause external stress corrosion cracking, broadly in the 50 to 150 °C region. It is why chloride content is specified for insulation used on stainless.

Confirm the figures

Both ranges are from the standards rather than from us, and both are revised. API 583 is the recommended practice for CUI. API 570 covers in-service piping inspection. Check the current edition before writing either range into a specification.

Where it starts

The places to look first

Supports and brackets

The insulation is compressed there, water collects there, and the pipe is in contact with another piece of metal. It is the single most productive place to look.

Low points and the underside

Water runs down and stops. The bottom of a horizontal run, the base of a vessel, the drain leg below a separator - all of them stay wet longest.

Penetrations and terminations

Wherever the covering is cut for a branch, a support, an instrument or a valve, there is an edge. Edges are where water gets in, and there are more of them than anyone remembers.

Anything opened and badly closed

A section cut for maintenance and taped back is an open invitation. That is the failure mode that connects this page to every other one on this site.

Finding it

Inspection, and the design answer

You have to look

The condition is defined by being hidden, so the inspection is either stripping the covering or a technique that sees through it. Both cost money; stripping fixed lagging costs it twice, because it is destroyed on the way in.

The cheap version of looking

A module that comes off by hand turns an inspection into a five-minute job and turns a maintenance strip into an inspection opportunity. That is an integrity argument rather than an insulation one.

Coating is the other half

Insulation is not corrosion protection. The coating protects the steel underneath, and the insulation keeps the heat in. A system that relies on the covering staying perfectly dry has one line of defence and no second.

And drain the leg

Below a separator, at a low point, under a vessel: anywhere water would stand. Treat those places differently and inspect them on purpose. Do not cover them like the rest of the run.

Reference

Where to look first

Location	Why it happens there	What to inspect
Pipe supports and brackets	Compressed insulation, trapped water, metal contact	Every support on a warm line
Underside of horizontal runs	Water runs down and stays	Low points and long straights
Penetrations and terminations	Every cut edge is an entry point	Branches, instruments, valve cut-outs
Below vessels and separators	Drain legs run cold and wet	The leg and its support
Sections opened for maintenance	Taped back rather than refitted	Anything with a joint under it

Location · Why it happens there · What to inspect. Ranges and method from API 583 and API 570 — confirm against the current edition.

Asked about corrosion under insulation

Questions.

Answers distinguish modelled heat-loss estimates from equipment selection and service guidance.

What is corrosion under insulation?

Corrosion of a pipe or vessel surface that happens beneath its insulation, where water has got in and cannot get out. It is dangerous because it is hidden. The covering looks intact and the heat loss looks controlled. The first sign is often a failure rather than a stain.

What temperature range does corrosion under insulation occur in?

API 583 gives roughly −12 to 175 °C for carbon steel. The worst of it is where water stays liquid against the surface rather than boiling off. A hot-water circuit at 50 to 65 degrees is inside that range. Confirm against the current edition of the standard before using the figures in a specification.

Does stainless steel suffer from corrosion under insulation?

Yes, differently. Chlorides leached out of wet insulation concentrate on the surface and cause external stress corrosion cracking, broadly between 50 and 150 °C. It is why insulation used on austenitic stainless has a specified chloride limit.

Where does corrosion under insulation usually start?

At pipe supports, where the insulation is compressed and water collects against metal-to-metal contact. Then the underside of horizontal runs and the drain legs below vessels. Then every penetration or termination where the covering is cut. Last, any section somebody opened for maintenance and taped back.

How do you inspect for corrosion under insulation?

Either by removing the covering or by a technique that sees through it. That is the whole difficulty - the condition is defined by being hidden. Insulation that comes off by hand makes the inspection routine instead of a project, which is an integrity argument rather than a thermal one.

On this skid