Pulsed Eddy Current (PEC) Through Insulation: Capabilities and Limits

Short answer: pulsed eddy current measures the average remaining wall thickness of carbon or low-alloy steel over a footprint roughly the size of the insulation thickness, without removing insulation or jacketing. That makes it a strong screening tool for general and broad-area corrosion under insulation (CUI). It is not reliable for pitting, small localized loss or cracks, so a PEC reading is not a substitute for a UT minimum-wall reading at a CML. Treat PEC as screen-then-verify.

This guide is written for mechanical-integrity engineers, API 570 and API 510 inspectors and CUI programme owners in the USA and Canada who are deciding where PEC belongs in their insulated-equipment strategy. It explains what the technique actually measures, the standards that now describe it, where its physics gives you good data and where it quietly misleads, and how to document results so the owner's Authorized Inspector can use them.

What pulsed eddy current actually measures

PEC answers one question well: how much steel is left, on average, under this probe position? A coil in the probe is energised with a step (pulse) of current, the magnetic field penetrates the jacketing, insulation and steel wall, and when the drive is switched off the instrument records how the induced eddy currents decay in the steel. Thicker steel takes longer for the decay to reach its late-time behaviour; thinner steel decays sooner. The instrument compares that decay curve to a reference signal taken on a known area of the same component and converts the difference to a wall thickness value.

Three consequences follow directly from the physics, and every PEC decision you make should respect them:

• It is an area average, not a point minimum. The published ECNDT paper describing one of the earliest commercial systems states plainly that PEC gives a reading for the average wall thickness within the magnetic field area, in contrast to ultrasonic testing, which reports the minimum under the transducer.
• The footprint grows with distance. The same source says the footprint can be considered to be in the order of the insulation thickness. Vendors describe footprints of a few centimetres on thin insulation and considerably more on thick insulation. A 100 mm insulated line is averaged over a much bigger patch than a bare pipe.
• It is a relative measurement. Readings are referenced to a calibration point on the component, so if the reference location is itself thinned, every value on that grid is biased.

Because the measurement is non-contact and largely insensitive to lift-off, PEC can be applied through weather jacketing, insulation, fireproofing, and coatings, often with the line or vessel in service and hot. That is its value: it lets you look at the steel without the cost, scaffolding, asbestos survey and reinstatement work of stripping insulation first.

Which standards describe PEC in 2026

PEC is no longer an unwritten vendor technique. Three references matter to North American owners, and each plays a different role:

None of these documents tells you that a PEC result alone satisfies a code thickness measurement at a CML. API 570 and API 510 leave the choice of NDE technique to the inspector and owner, so the question "is PEC acceptable?" really means "has the owner's programme defined how PEC data will be used, what it can and cannot detect, and when follow-up is required?" Write that down before the first scan.

Where PEC performs well

PEC earns its place in a CUI programme in a predictable set of situations. Use it where the damage you expect is broad and the alternative is stripping long runs of insulation with no idea where the problems are.

• Long insulated piping runs in the CUI temperature window. Screening a circuit on a regular grid quickly identifies the lengths where average wall has dropped and focuses insulation removal there.
• Insulated vessels and columns. Shell courses below insulation support rings, around stiffeners and near the bottom head often hold water. PEC grids on the shell show broad wall loss without full strip-out.
• Fireproofed skirts and supports. Where fireproofing is concrete or cementitious, PEC can sometimes see the steel underneath. Embedded metal mesh affects the signal and is an active research subject, so procedure qualification on representative mock-ups matters.
• Hot equipment in service. Because there is no couplant and no contact with the steel, PEC can be applied at temperatures where conventional contact UT is difficult. Confirm the probe's own temperature rating.
• Trending. Repeat PEC on the same grid, with the same reference location and settings, can show change in average wall between surveys. Trending a consistent method is often more useful than a single absolute number.

In each case the result answers "where should we look closer?" That is screening. The answer to "how thin is the thinnest spot?" still comes from UT, profile radiography, or a corrosion map after the insulation is off.

The limits that matter: pitting, edges, geometry and jacketing

Most PEC disappointments come from expecting a screening tool to behave like a thickness gauge. The limits below are physical, not vendor-specific, though their severity varies by instrument.

Localized loss and pitting

Because the reading is a footprint average, a pit or narrow groove that takes most of the wall away over a small area may move the average only slightly. The ECNDT paper states that detection of highly localised defect types like pitting is not reliable, and ISO 20669 excludes local metal loss caused by pitting from its scope. CUI on carbon steel is frequently localized, especially at jacketing penetrations, under damaged sealant, and at low points where water collects. A clean PEC grid therefore does not prove the absence of localized CUI.

Undersizing

Even when PEC detects a wall loss area smaller than its footprint, it tends to under-report the depth because the thinned area is averaged with healthy steel around it. Practical consequence: treat the PEC value as an upper bound on remaining wall in that area, never as the minimum used in a remaining-life or retirement-thickness calculation.

Edges, nozzles, supports and fittings

The field is disturbed by nearby geometry: flanges, nozzles, saddles, support rings, elbows, tees and changes in diameter. The original system description notes the technique is unsuitable for edge or nozzle corrosion. Those are exactly where CUI likes to start, so they need a different method (often profile RT, or UT after local insulation removal).

Small diameters

Vendors report that small-diameter pipe is harder because the footprint becomes large relative to the curvature. ISO 20669's published scope stops at 50 mm diameter piping. Small-bore lines and instrument connections should not be assumed covered by a PEC campaign.

Jacketing and fireproofing

PEC is described as working through thin stainless, aluminium and galvanised sheeting, but jacketing type, overlap seams, banding and ferromagnetic content all change the signal. Metal mesh in fireproofing is known to affect results. Keep jacketing type on the scan record and qualify the procedure on a similar build-up.

Material and wall range

PEC as standardised is for ferromagnetic carbon and low-alloy steels within a wall range. Austenitic stainless steel, where external chloride stress corrosion cracking is the main CUI risk, is a different problem; PEC does not detect cracks at all.

A worked example, described qualitatively

Consider a refinery owner with an insulated carbon-steel process line, operating within the CUI-susceptible temperature window, running a few hundred feet along a pipe rack with several elbows, two drain points and a support every few metres. The jacketing is aluminium, and the inspection plan under API 570 asks for a CUI assessment of the circuit.

1. Plan. The owner's inspector and the NDE provider's Level III mark the circuit isometric with a PEC grid along straight runs, excluding a zone either side of each support, elbow and drain because geometry will distort the reading. Those excluded zones go on a separate list for targeted methods.
2. Reference. A reference position is chosen on a straight section where an inspection port allows a UT reading to confirm that the reference wall is close to nominal.
3. Screen. PEC is collected on the grid. Most of the line reads near nominal; two straight sections show a meaningful drop in average wall and one shows scattered lower readings near a jacketing seam.
4. Verify. Insulation is removed at the three flagged areas. Visual inspection and UT corrosion mapping find general wall loss at the first two and a cluster of pits at the third that is deeper than PEC suggested.
5. Cover the gaps. Profile RT or local strip-and-UT is done at the support and low-point locations PEC could not cover.
6. Record. The inspector uses the UT minimum values, not the PEC averages, for corrosion-rate and remaining-life calculations, and repairs insulation and jacketing.

PEC did its job: it found where to strip. The decisions still came from point measurements.

PEC compared with the other CUI tools

No single method answers every CUI question. The table below summarises typical roles; specific capability depends on equipment and procedure.

A defensible programme layers these: identify wet or damaged insulation, screen steel loss with PEC, guided wave or RT, then measure with UT where it counts. Our longer guide on detecting CUI without removing insulation discusses the screening toolkit in programme terms, and CUI monitoring covers repeat surveillance.

Procedure, personnel and data quality

The quality of a PEC survey is decided before the probe touches the jacketing. A written procedure, approved under the employer's written practice, should at least define:

• the component types, wall range, insulation and jacketing build-ups and temperature range the procedure covers;
• how the reference location is chosen and verified (ideally by UT through an inspection plug);
• grid spacing, the exclusion zones around geometry, and how they will be covered by other methods;
• instrument settings, probe selection, and how data quality is judged (noise, signal level, repeatability checks);
• how readings are reported: as average-wall percent of reference, with the footprint size stated, not as minimum thickness;
• the follow-up trigger the owner has chosen, for example a reduction relative to reference or relative to the previous survey.

Personnel should be qualified in eddy current under the employer's written practice, typically to ASNT SNT-TC-1A or CP-189, with technique-specific training on the PEC instrument. ASME Section V Article 21 places weight on training and on judging whether collected data is adequate, so keep training records with the survey. Data should be stored with the grid map, reference location, jacketing type and instrument serial number, so the next survey is repeatable.

Common mistakes with PEC

• Entering PEC averages as CML thickness readings. This corrupts corrosion-rate and remaining-life calculations. Keep PEC data in its own data set unless the owner's programme explicitly defines otherwise.
• Calling a circuit "clean" from PEC alone. PEC cannot rule out pitting or localized CUI.
• Ignoring geometry exclusions. Reporting readings beside supports, nozzles and elbows produces false calls and false comfort.
• Picking a thinned reference. Verify the reference location by UT first.
• Changing settings between surveys. Trend data is only valid if the grid, reference and settings are repeated.
• Forgetting the insulation itself. PEC says nothing about wet insulation or failed jacketing seals, the root cause. Pair it with visual and moisture detection.

Regulatory overlay in the US and Canada

In the US, insulated piping and vessels in covered processes fall under OSHA's Process Safety Management standard, 29 CFR 1910.119(j), which requires inspection and testing that follow recognized and generally accepted good engineering practices. API 570, API 510 and API RP 583 are commonly treated as those practices. How PEC fits is therefore an owner programme decision documented in inspection plans, with the Authorized Inspector accepting or rejecting the data's use. Some states also regulate pressure vessels through their boiler and pressure vessel offices; confirm with your jurisdiction.

In Canada, pressure equipment integrity programmes are overseen provincially, for example by ABSA in Alberta and TSSA in Ontario. Owner-user programmes there similarly describe the NDE methods they rely on. Confirm with your provincial regulator how screening data must be recorded in your integrity management system.

How Atlantis supports this

Atlantis NDT provides NDE under the owner's API 570 or API 510 programme with ASNT-certified technicians and ASNT Level III oversight. For CUI work that means the verification and companion methods a PEC campaign depends on: CUI inspection scopes, UT thickness and CML surveys, corrosion mapping, profile radiography by crews licensed where the work is, and guided wave screening. If you want PEC screening included, tell us the line list and build-up when you request a quote and we will confirm scope. Results go to your API-certified inspector, who stays the inspector of record and decides on insulation removal, intervals and repairs. Quotes are returned within 24 hours: request a CUI inspection quote.

Frequently asked questions

Can pulsed eddy current detect corrosion through insulation?

Yes, for general and broad-area wall loss on carbon and low-alloy steel. PEC measures average remaining wall through insulation, jacketing and coatings without removing them. It is weak on pitting and small localized loss, so use it to decide where to strip and measure, not as the final thickness.

What are the limitations of pulsed eddy current?

Footprint averaging, undersizing of localized loss, poor performance near edges, nozzles, supports and fittings, reduced suitability for small diameters, no crack detection, and sensitivity to jacketing type and embedded mesh. ISO 20669 excludes pitting and cracks from its scope.

How big is the PEC footprint?

It depends on probe, wall and insulation thickness. A widely cited rule of thumb from the technique's early literature is that the footprint is roughly of the order of the insulation thickness. Ask your provider to state the footprint on every report.

Is PEC accurate compared with UT?

They measure different things. UT reports a local minimum under the transducer; PEC reports an average over its footprint. PEC is repeatable for trending broad wall loss, but its average is not a replacement for a UT minimum-wall reading.

Can PEC work through galvanized or stainless jacketing?

Systems are described as working through thin stainless, aluminium and galvanised sheeting, but the jacketing affects the signal. Record jacketing type and qualify on a similar build-up.

Is pulsed eddy current in ASME Section V?

Yes. A PEC article for corrosion screening, Article 21, was added in the 2021 edition of Section V. Check the current edition for requirements.

Can PEC be used on hot pipe in service?

Often yes, because it needs no couplant or contact with the steel. ISO 20669's scope covers component temperatures from minus 100 to plus 500 degrees C; the probe and insulation surface temperature limits come from the equipment manufacturer.

Does PEC work on stainless steel?

The standardised technique is for ferromagnetic carbon and low-alloy steel. For austenitic stainless under insulation, the main concern is external chloride stress corrosion cracking, which PEC does not detect.

PEC vs guided wave for CUI: which is better?

They complement each other. Guided wave screens long lengths from one collar for cross-section change; PEC gives a grid of average-wall values at each position. Both need follow-up sizing.

Can PEC readings be used for remaining-life calculations?

Only if the owner's programme explicitly defines that use, which most do not. Standard practice uses UT or RT minimum values at CMLs for corrosion rate and remaining life, and PEC to target where those measurements are taken.

Planning a CUI campaign this turnaround? Send us your line list for a quote, or read our CUI detection methods overview, the API RP 583 guide, piping circuit CML inspection and NDE method selection by damage mechanism. For hot insulated vessels, ask us about verification NDE on your next outage.