Corrosion under insulation: what to look for

How CUI happens

CUI needs three things: bare steel, water, and a surface that stays wet long enough to corrode. Insulation supplies the third one. Water gets in through damaged or poorly sealed jacketing, then the insulation holds it against the metal like a sponge. Common ways in:

• Rain and washdown that runs behind jacketing at seams, laps and penetrations.
• Condensation on cold surfaces, which can wet insulation from the inside even when the jacketing is intact.
• Leaks from nearby steam, cooling water, a relief discharge or a fire-water system.
• Aging sealant and weathering. Caulk dries and cracks, straps loosen and jacketing gets walked on, dented or left open after earlier work.

Chlorides from the environment or from the insulation itself can make things worse, and they matter for a second reason: they connect CUI to external chloride cracking on stainless steel. Both are in RP 571, and the exam treats them as different mechanisms with different materials, so keep them apart in your notes.

Which services are most at risk

Temperature decides how often the steel is wet and how fast it corrodes. In general terms:

• Carbon and low-alloy steel in cold or cool service is exposed to condensation, and anything running near or below the dew point keeps insulation damp.
• Carbon steel in warm service, well above freezing but not hot enough to dry the insulation out, is the classic CUI case. Water stays liquid and the metal stays wet.
• Steel that cycles between hot and cold, or between operating and shutdown, wets and dries repeatedly and tends to do poorly.
• Very hot steel can keep itself dry, because water boils off, but the cool-down and the cooler spots on the same equipment bring the problem back.
• Austenitic stainless steel in warm service is susceptible to chloride stress corrosion cracking under insulation rather than general thinning.

Treat that as a map of where to look, not a table of cut-offs. The temperature ranges that RP 571 and RP 583 attach to these cases are in your own copies, so read the paragraph rather than memorize a number from a blog. On the API 510 effectivity sheet, corrosion under insulation is RP 571 paragraph 3.22, and the damage mechanisms guide shows the rest of the set. RP 583 is the dedicated document on CUI if you work with it in the field.

Where it hides on a vessel

Water goes where gravity and gaps take it, so the same spots turn up on vessel after vessel:

• Nozzles and manways, where jacketing is cut around the neck and sealant is hardest to keep tight.
• Support rings, skirts, saddles and lugs, which hold water and break up the jacketing.
• The low side of the shell and the bottom head, where water drains and sits.
• Termination points, such as the top of insulation, ladder and platform clips, and any place the insulation stops and bare shell begins.
• Small-bore connections, drains and instrument taps that pierce the insulation and are often left poorly sealed.
• Vertical vessels where water runs down the shell and collects at the first obstruction.
• Areas already damaged or repaired, including old insulation patches and earlier inspection windows that were never properly closed.

If your plant has drawings with insulation details, mark these spots on them before you walk the vessel.

What you can see from outside

You do not need to strip anything to spot the warning signs. Walk the vessel and look for:

• Torn, dented, loose or missing jacketing, and gaps at seams and laps
• Sealant that is cracked, shrunken or absent around penetrations
• Rust stains, streaks and weeping at the bottom of jacketing or below nozzles
• Bulging or sagging insulation, which can mean it is wet and heavy
• Ponding or staining at low points, and jacketing that has been walked on
• Insulation gone altogether, leaving bare, coated or rusted steel
• Wet insulation at an inspection port, or one that has been left open

None of these prove metal loss. A stained, damaged jacket is a reason to look closer, and a clean jacket is not a clean bill of health, especially on older insulation systems.

How inspectors find the damage

No single method does the whole job, so the usual approach is to screen first and then confirm where it matters. Described in general terms:

1. Visual examination of the jacketing and insulation condition, with a flashlight and the drawing in hand, to pick the suspect spots.
2. Opening the insulation at likely locations, which lets you see the steel directly. It is the most direct look you get, and it is the step that costs the most, so locations are chosen by risk rather than at random.
3. Screening methods that can cover more area without stripping everything, such as profile radiography through the insulation or other techniques that point to wet insulation or thinning. They help decide where to open, and they do not replace thickness measurement where you find damage.
4. Thickness measurement by ultrasonic readings or radiographic profiles at the suspect spots, repeated at marked points so you can calculate a rate. The thickness survey analyzer takes a list of readings with dates and shows the limiting location and remaining life, which is useful for checking CUI findings by hand.

Which methods suit your equipment depends on the insulation system, the temperature, access and the damage you are chasing. RP 572 and the guidance in API 510 are the places to read the inspection practice itself. Use your own copy at the right edition. Condition monitoring locations explained covers how to turn those readings into a rate.

How API 510 inspection planning handles CUI

API 510 puts the inspection plan in the owner/user's hands, built from the damage mechanisms the equipment is likely to see. For an insulated vessel that means CUI is something the plan has to consider, with a written answer about where you will look, how and how often. In practice that tends to come down to a few questions:

• Is the vessel insulated, and in a service where CUI is possible? The temperature, material and insulation system answer this.
• What is the condition of the jacketing and insulation? A poor system, or one with a history of leaks, raises the priority.
• What did the last inspection find, and how fast is the loss? A measured rate sets how soon you return, and the inspection interval calculator shows how remaining life drives the interval.
• Which locations get opened or screened? Pick the ones that combine the highest likelihood with the worst consequence if the wall fails.

Read Section 6 of your copy of API 510 for what it says about inspection planning and intervals, and Section 7 for evaluating the thickness you measure. Nothing in a blog post replaces those paragraphs. Our pressure vessel inspection checklist puts insulation condition in the external walk so it never gets skipped.

Prioritizing where to look first

When time or budget is limited, rank locations by two things at once: how likely is damage, and what happens if the wall fails.

1. Start with spots where insulation or jacketing is visibly damaged and the steel is in a susceptible temperature range.
2. Add the usual low points and penetrations, such as nozzles, support rings and the bottom head.
3. Weight toward equipment with a past CUI finding and toward services where a leak is serious, such as flammable or toxic fluids or high pressure.
4. Record what you open, what you found and what you closed up, so the next inspector does not start from zero.

Where this shows up on the exam

The Body of Knowledge sends you to RP 571 for damage mechanisms, and CUI is one of the paragraphs on the effectivity sheet. Questions tend to hand you service conditions and ask you to name the mechanism or to say where on the equipment it would show up. That is a recognition skill, and it improves with repetition. Run the damage mechanism quiz to practice naming the mechanism from the clues, then take the free 10-question check to see which areas to study first. When your weak spots are clear, the course drills them with fresh questions until the answers come without hesitation.

FAQ

What is corrosion under insulation?

It is metal loss on steel hidden under insulation and jacketing, caused by water that gets in and stays against the surface. It is described in RP 571 paragraph 3.22 on the current API 510 effectivity sheet.

Which temperatures are most at risk for CUI?

Carbon and low-alloy steel in cool to warm service tends to be the worst case, because water stays liquid and the insulation stays damp. Very hot steel can dry itself out. Read the exact ranges in RP 571 and RP 583 in your own copies.

Can you spot CUI without removing the insulation?

You can spot warning signs from outside, such as damaged jacketing, failed sealant and stains. Some screening methods work through insulation, but confirming metal loss usually means opening the insulation at the suspect spot and measuring the steel.

Is CUI the same as chloride stress corrosion cracking under insulation?

No. Both can happen under insulation, but CUI is general or local thinning of carbon and low-alloy steel, while chloride stress corrosion cracking affects austenitic stainless steel. RP 571 treats them as separate mechanisms.

Which RP covers CUI in detail?

RP 571 describes the mechanism and is the one the API 510 exam draws on. RP 583 is the dedicated CUI document, and RP 572 covers vessel inspection practice.