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Eddy Current Testing

ECT · PSEC · FSEC — one method, three flavours for any tube material

Technical Reference · ASME Section V / ASTM E243 / ASTM E2884 / ISO 15549

Eddy Current Testing is the workhorse for heat-exchanger tube inspection. We treat it as a single family that covers all common tube materials — only the magnet hardware in the probe changes. Conventional ECT handles non-ferromagnetic alloys (Cu-Ni, brass, titanium, austenitic stainless, Inconel, Monel — no bias magnet). When we add a DC bias magnet, the technique branches into PSEC and FSEC depending on whether full magnetic saturation of the wall is actually achieved:

  • ▸PSEC (Partial Saturation EC) — strongly ferromagnetic + heavy wall (CS, LAS, T11, T22, P91). Even with a strong magnet, the magnetic path is too "thick" to drive the wall past the saturation knee. µdiff stays > 1.
  • ▸FSEC (Full Saturation EC) — slightly ferromagnetic + thin wall (duplex, super-duplex, austenitic SS with δ-ferrite/martensite, Monel, thin ferritic SS). The wall is pushed fully over the B-H knee. µdiff → 1, AC behavior becomes paramagnetic-like.

So PSEC and FSEC describe the state we actually achieve in the wall, not the magnet on the probe shelf. Counter-intuitively this means we use PSEC on the strongly magnetic alloys and FSEC on the slightly magnetic ones — see the dedicated explainer.

The technique is governed by Faraday's law of induction. An alternating current in a sensing coil generates a primary magnetic field. When the coil is brought close to a conductive tube, this field induces circulating currents — eddy currents — in the tube wall. Any geometric or metallurgical change (pit, crack, wall loss, inclusion) disturbs the eddy-current flow, which changes the impedance of the coil. That impedance change is what the instrument records and the analyst interprets — amplitude carries defect size, phase carries defect depth and ID/OD position.

Naming convention. We name the technique after the achieved saturation state, not the probe magnet. Some vendors do the opposite — which can be confusing on heavy-wall CS where a "strong magnet" still only achieves partial saturation. Read why →

The Principle, Visualized

Longitudinal section through ONE tube — bobbin probe with 2 coils (D₁ D₂) cross-section cut Tube wall · OD side (top half of one tube) ID surface ID pit baffle plate baffle cut OD pit Tube wall · OD side (bottom half of same tube) general corrosion crack general erosion cable Bobbin probe AC ~ 1 – 500 kHz BRDG ferrite core D₁ D₂ D₁ + D₂ — bobbin coil pair switched between differential and absolute mode Primary B-field Legend: coil winding eddy current primary B-field defect disrupted path

A bobbin probe is pulled through the tube. The probe carries two coils D₁ + D₂ — and only two. The instrument switches them electronically between two modes: differential mode (the two coils form a bridge, their signals subtract — sensitive to local defects, rejects gradual changes) and absolute mode (the two coils are summed and referenced against the instrument bridge — sensitive to gradual wall changes that the differential mode cancels). The snapshot shows the probe inside the tube; defects along the tube length (ID pit, baffle cut, OD pit, general corrosion, crack, general erosion) will pass under the coils as the probe is pulled, each producing its characteristic signal. For PSEC and FSEC the same two coils sit inside a DC magnet that shifts the operating point of the ferromagnetic tube wall into a region where the AC analysis behaves predictably.

The Eddy Current Family — Three Flavours

ECT

Conventional Eddy Current

Range
Non-ferromagnetic tubing — no bias magnet needed
Materials
Cu-Ni, brass, titanium, as-drawn austenitic SS, Inconel
Magnet
None — coil current alone

The classic case. µr = 1, AC field penetrates predictably.

PSEC

Partial Saturation EC

Range
Strongly ferromagnetic — saturation is NOT fully achieved
Materials
CS, LAS, T11, T22, P91, ferritic stainless (heavy wall)
Magnet
DC bias applied, but the magnetic path (material × wall thickness) keeps the tube wall below the saturation knee

µdiff stays > 1. Phase analysis works but is more challenging — sizing is relative, calibration tube must match.

FSEC

Full Saturation EC

Range
Slightly ferromagnetic — saturation IS achieved
Materials
Duplex, super-duplex, Monel 400, austenitic SS with δ-ferrite / strain-induced martensite, thin-wall ferritic SS
Magnet
DC bias drives the wall fully over the B-H knee. µdiff → 1 — material behaves AC-wise as if paramagnetic.

The "easy" case among the bias-magnet methods. Crack signature flattens; combine with rotating/array probes when cracks are the target.

Inside the Bobbin Probe — Two Coils, Two Modes

Our bobbin probes carry only two coils — D₁ and D₂. They sit a few millimeters apart on a common ferrite core. The instrument electronically switches how they are wired into the AC bridge, so the same physical coils provide two complementary measurement modes — no separate absolute pair is needed.

Differential mode — coils opposed

The two coils are wired so their outputs subtract. When both see homogeneous tube wall, their impedances are equal — the bridge stays balanced and the differential output is near zero.

A local defect passes under D₁ first, unbalancing the bridge → spike of one polarity. Microseconds later it passes under D₂ → spike of opposite polarity. The resulting figure-of-eight on the impedance plane carries amplitude (defect size) and phase (depth, ID vs. OD). Differential mode is exquisitely sensitive to local defects (pits, cracks, small inclusions) and rejects everything common to both coils (gradual lift-off changes, slow geometry changes, temperature drift).

Absolute mode — coils summed against the instrument bridge

The same two coils are switched to an absolute configuration: their outputs are summed and referenced against a fixed impedance inside the instrument. The bridge no longer cancels for uniform tube material — the output tracks the actual wall condition under the probe.

Absolute mode detects what differential mode is blind to: gradual wall loss, baffle-plate transitions, tubesheet zones and overall corrosion. The instrument records both channels simultaneously — a typical ECT report shows the same defect on the differential and the absolute plane, each adding a different piece of information.

Skin Depth — the One Equation You Need

Eddy currents do not penetrate uniformly through the wall. They are strongest at the inside surface and decay exponentially with depth. The standard depth of penetration (skin depth, δ) is:

δ = √(2 / ω·µ·σ) ≈ 503 · √(ρ / (f · µr)) [mm, ρ in Ω·mm²/m]

Higher frequency → shallower penetration. For a typical Cu-Ni condenser tube (ρ ≈ 0.4, µr = 1) at 30 kHz, δ ≈ 1.8 mm — enough to interrogate a 1.5 mm wall fully. Frequency is chosen so δ ≈ 1.0 – 1.5 × nominal wall. For PSEC and FSEC the DC bias drives µr towards 1 (saturation flattens the permeability curve), so the same equation governs penetration — that is why the same instrument and analysis work across all three flavours.

Probe Types

Bobbin

Two coils (D₁ D₂) on a common ferrite core, switched electronically between differential and absolute mode. Pulling speed 0.3 – 1.0 m/s. PSEC/FSEC variants add a DC magnet around the coil pack.

High throughput · primary screening

Rotating

Pancake coils on a rotating head — produces a 2D scan of the tube ID. Detects longitudinal cracks and resolves circumferential location. Essential at tubesheet, U-bends and weld zones.

Verification · detailed mapping

Array

Multiple coil pairs arranged circumferentially, electronically multiplexed. Captures a full C-Scan at bobbin-like speeds — combines screening and mapping in one pass.

Modern best practice for high-value bundles

What ECT / PSEC / FSEC Detects

◉

Pitting

Localized ID/OD pits — phase angle separates ID from OD; amplitude scales with volume.

▭

General wall loss

Gradual erosion/corrosion across long sections — absolute channel amplitude tracks depth.

⊘

Cracks

Circumferential and longitudinal stress-corrosion / fatigue cracks (rotating or array probes).

◫

Baffle-cut wear

Fretting at tube-support locations — visible as amplitude spikes synchronized with the baffle pattern.

⊙

Pinholes

Through-wall perforations down to ~1 mm diameter in thin-wall tubing.

◐

Inlet/outlet erosion

Velocity-induced thinning near tube ends — accessible with bobbin probes.

Suitable Materials

Material ECT PSEC FSEC Note
Copper-Nickel (90/10, 70/30) ✓ first choice — — Sea-water condensers — ECT primary screening
Brass (Admiralty, Aluminium) ✓ first choice — — Older condensers, still very common
Titanium (Gr 2, Gr 7, Gr 12) ✓ first choice — — Aggressive chemistry — ECT ideal
Austenitic SS (304/316/321) — as-drawn ✓ first choice — ○ applicable Cold work / weld zones add δ-ferrite / martensite → FSEC if µ becomes noticeable; verify ambiguous indications with magnet probe
Inconel 600 / 625 / 825 ✓ first choice — — Heat exchangers in petrochemical service
Monel 400 — — ✓ first choice Slightly ferromagnetic — full saturation routinely reachable. FSEC is standard.
Duplex / Super-duplex SS ✕ not suitable — ✓ first choice Lower ferromagnetic content + typical wall → full saturation achievable. FSEC is first choice.
Ferritic stainless steel — thin (< 3 mm) ✕ not suitable — ✓ first choice Thin wall allows full saturation
Ferritic stainless steel — thick (≥ 3 mm) ✕ not suitable ✓ first choice ✕ not suitable Heavy wall → only partial saturation reachable
Carbon steel / LAS (heat exchanger) ✕ not suitable ✓ first choice ✕ not suitable Strongly ferromagnetic + typical 2–5 mm wall → full saturation not reachable. PSEC standard; RFT for heavy wall > 4 mm; IRIS for absolute WT.
P91 / T22 / T11 (ferritic creep-resistant) ✕ not suitable ✓ first choice ✕ not suitable High µr + thick wall → PSEC. IRIS for absolute WT datum.

Advantages (family-wide)

  • +Covers every common tube material — ECT, PSEC and FSEC share instrument and analysis
  • +Very high inspection speed — up to 100 tubes/h (ECT), 70 tubes/h (PSEC/FSEC)
  • +No couplant — dry inspection across all flavours
  • +Differential channel finds local defects, absolute channel finds gradual wall loss
  • +Phase angle separates ID from OD defects
  • +Tolerant to moderate deposits / scale
  • +Defect-type-agnostic at the screening stage

Limitations

  • −No absolute wall-thickness reading — sizing is relative to a calibration standard
  • −PSEC/FSEC: heavy magnet → straight sections only, larger probe diameter
  • −FSEC saturation flattens crack signature; combine with rotating/array probes for cracks
  • −Cold-worked SS may show metallurgical noise — magnet-probe verification recommended
  • −Reference standard must closely match production tube material and geometry
  • −Skill-dependent: experienced analyst essential for mixed indications

Typical Inspection Workflow

  1. 1

    Method selection

    Choose ECT / PSEC / FSEC based on tube material and permeability. Same instrument; only probe magnet hardware differs.

  2. 2

    Pre-job & calibration

    Reference tube matched to production (OD, wall, material). Drill calibration holes per ASTM E243 / E2884: 100 % through-hole, 4×20 % flat-bottom, OD/ID notches. Set frequency per skin-depth equation. Establish phase rotation so OD ≈ 40°, ID ≈ 140°.

  3. 3

    Bobbin screening (100 %)

    All tubes scanned with the 2-coil bobbin probe (D₁ D₂). Instrument records both channels simultaneously — differential mode for local defects, absolute mode for gradual wall loss. Constant pulling speed; encoder logs depth.

  4. 4

    Analysis & triage

    Each indication plotted on impedance plane. Sizing curves from calibration holes give % wall loss. Indications above acceptance threshold flagged for follow-up.

  5. 5

    Rotating / array follow-up

    Flagged tubes re-inspected with rotating or array probe for cracks, tubesheet zone, longitudinal defects. C-Scan recorded.

  6. 6

    Reporting & FFS

    Tube-by-tube report (% wall loss, defect type, position). If absolute WT is needed for API 579 RSF → schedule IRIS on the flagged subset.

Standards & Personnel Qualification

ASME Section V, Art. 8 Eddy Current Examination of Tubular Products
ASTM E243 Electromagnetic (Eddy-Current) Examination of Copper and Copper-Alloy Tubes
ASTM E309 Eddy-Current Examination of Steel Tubular Products Using Magnetic Saturation
ASTM E2884 Eddy Current Examination of Tubing Using Partial Saturation (PSEC / FSEC)
ASTM E2096 In Situ Examination of Ferromagnetic Heat-Exchanger Tubes (RFT)
EN 1971 Copper and copper alloys — Eddy current test for tubes
ISO 15549 Non-destructive testing — Eddy current testing — General principles
ISO 9712 Personnel qualification — Levels I / II / III for ET method

Need ECT, PSEC or FSEC for your next outage?

Our crews work refinery, petrochemical, fertiliser and power-generation tube bundles across the GCC. Bobbin, rotating and array probes with PSEC / FSEC magnet variants, full FFS support.

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DELTA TEST ME LLC · NDT Technical Services