The damage is ordinary. What makes CUI expensive is that the insulation hiding it costs more to remove than the inspection costs to do, so the whole discipline is built around not removing it — and every one of those methods has a blind spot.
Corrosion under insulation is not a special corrosion mechanism. It is ordinary aqueous corrosion happening in a place you cannot see, on steel that is often at the perfect temperature for it, fed by water that the insulation is very good at holding against the surface.
The reason it dominates maintenance budgets is entirely about access. Everything below is a way of finding it without stripping a live plant.
CUI needs liquid water at the steel. Dry insulation does not corrode anything, no matter how old the line or how bad the cladding looks. Two methods find water directly, and starting with them narrows a whole unit down to a handful of locations.
Wet insulation conducts heat differently from dry insulation, so on a line with any temperature difference to ambient the wet patch shows as a thermal anomaly through the cladding. Done from the ground, on line, with no shutdown and no scaffolding.
Misses: lines close to ambient, where there is no heat flow to disturb. And it tells you nothing about the steel — a wet patch is a place to inspect, not a measurement.
Neutrons scatter strongly off hydrogen, and water is full of it. A handheld probe on the cladding reads high wherever moisture is present. More direct than infrared and it works on ambient lines.
Misses: again, the steel entirely. And it carries a radioactive source, with the licensing and handling that implies.
Before any instrument comes out, walk the line. Water enters at damaged cladding, at penetrations for supports and nozzles, at the top of vertical runs, under failed caulking, and anywhere the cladding laps the wrong way. Rain runs downhill and collects at the bottom of horizontal runs and above any obstruction. The highest-value CUI inspection is often a careful visual survey of the cladding, and it costs nothing.
Reads average wall thickness through insulation, cladding, and even concrete fireproofing, with nothing removed. Ideal for vessels and large-bore lines where you can reach the surface but cannot strip it.
Misses: isolated pitting, because it averages over a footprint tens of millimetres wide. And it gives a relative value against a reference — if the reference area is already corroded, everything reads normal.
One bare band, and tens of metres of coverage in each direction. The tool when the problem is that you physically cannot reach the pipe: sleeved crossings, high racks, buried transitions.
Misses: pitting, again — it detects change in cross-sectional area, and a pit changes almost none. It cannot size anything, and its range collapses at bends, branches, supports and heavy coating.
These two are compared properly in guided wave versus pulsed eddy current.
The beam is aimed along the tangent of the pipe so the wall appears in profile at the edge of the image. Insulation stays on. It is the practical measurement for small-bore insulated lines — and small-bore is where CUI does most of its damage, because a 2-inch line has little wall to lose.
Misses: accuracy falls away as diameter grows. Above about 6 inches it becomes unreliable, and it will not measure a vessel at all. It also needs a radiation exclusion zone, which on a live unit is its real cost.
Insulation off, scanner on, thickness image out. The only method here that produces the data an API 579 assessment actually needs: the true minimum thickness and the shape of the damaged area.
Misses: nothing, within the area you scan. Which is the catch — it finds nothing outside it, and stripping insulation is what you were trying to avoid.
| Method | Insulation off? | Finds | Blind to |
|---|---|---|---|
| Infrared thermography | No | Wet insulation | The steel; ambient-temperature lines |
| Neutron backscatter | No | Wet insulation | The steel entirely |
| Pulsed eddy current | No | Average wall loss | Isolated pitting; absolute thickness |
| Guided wave | One band | Loss over long runs | Pitting; sizing; anything past a bend |
| Profile radiography | No | Wall profile, small bore | Large diameters; vessels |
| UT corrosion mapping | Yes | True minimum and shape | Everything outside the scanned area |
Steps 3 and 4 are screening. They rank locations. Neither of them can clear a line.
A clean PEC or guided wave survey means "no broad wall loss large enough for this method to see". If the mechanism on that line produces pitting — chloride pitting on stainless under wet insulation is the classic — the survey will be clean and the line will still leak. Match the screening method to the shape of the damage you expect, or the report is meaningless.
On austenitic stainless the mechanism is usually not wall loss at all. Chlorides leached from wet insulation concentrate on a hot surface and produce external chloride stress corrosion cracking — a planar flaw that removes no measurable metal.
Every thickness-based method on this page will report that line as sound. Detection needs penetrant or eddy current on a stripped, cleaned surface. If you have insulated stainless in the susceptible temperature range, thickness screening is not an inspection of it.
What corrosion under insulation actually looks like on the wall, the temperature range where it happens, and every method in the guide that detects it.
Open the mechanism libraryWho writes this. A mechanical engineer with twelve years in oil and gas — in-line inspection, fired heater and furnace inspection, and pipeline integrity. What is here comes from the published standards and from what those years in the field actually looked like. It is not written by an API-certified inspector.
This is not an assessment. Nothing on this site may be used to justify a decision about real equipment. Assessing plant requires the current editions of the applicable codes, data from a licensed source, and a competent engineer who signs for the answer. · Integrity Field Guide