Eddy current examination (ET) and heat exchanger tubes

Eddy current examination (ET) is an electromagnetic nondestructive examination (NDE) method. A coil induces eddy currents in an electrically conductive material, and ET reads the disturbance that surface and near-surface discontinuities — or changes in material properties — cause in those currents. ET needs no contact and no couplant, it runs fast, and it automates well. That combination makes it indispensable for high-volume examination of heat exchanger and condenser tubes.

1. How it works

Pass an alternating current through a coil and it creates a changing magnetic field around it. Bring that field close to a conductive material and it induces circular eddy currents in the material. Those eddy currents generate their own magnetic field, which opposes the coil's field and changes the coil's impedance.

A discontinuity in the material — a crack, pitting corrosion, tube wall thinning — disturbs the path of the eddy currents, and so does a change in conductivity, magnetic permeability or geometry. That disturbance appears as a measurable change in coil impedance. What ET reads, then, is how the discontinuity alters the eddy current flow.

2. Material requirement and the ferromagnetic problem

ET works only on electrically conductive materials. The method applies to both non-ferrous and ferrous conductors. However, ferromagnetic materials such as carbon steel have high and variable magnetic permeability, which generates strong magnetic noise and makes conventional ET impractical. ET therefore gives its best results on non-ferromagnetic conductors: austenitic stainless steel, copper alloys (brass, admiralty brass, copper-nickel), titanium. For ferromagnetic tubing, either apply magnetic saturation or switch to a different method — most often RFT, the remote field technique.

3. Key variables

Frequency and skin effect: Eddy currents concentrate near the surface (skin effect) and decay exponentially with depth. The standard depth of penetration (δ) decreases as frequency, conductivity and magnetic permeability increase. Select the frequency according to the depth of the discontinuity you are looking for: a high frequency gives surface sensitivity and better phase separation, a low frequency penetrates deeper.
Conductivity and magnetic permeability: These drive the signal. Alloy sorting and heat treatment verification both rely on them.
Lift-off: The distance between probe and surface. It is a strong noise source, but it appears in a known direction on the impedance plane, so you can compensate for it. The same effect is used deliberately in coating thickness measurement.
Fill factor: In tube examination, how closely the coil matches the tube inside diameter. It affects signal amplitude.
Edge effect and geometry: Edges, support plates and geometric changes produce signals you must separate out during evaluation.

4. Probe types

Surface (pancake) probes: Crack scanning on flat surfaces.
Encircling coils: Surround the outside surface of bars and tubes.
Bobbin probes: Pulled through the inside of tubes for internal examination; the standard tool for heat exchanger tubing.
Absolute and differential coils: A differential coil uses two windings, is sensitive to local and abrupt discontinuities, and suppresses gradual changes such as temperature drift. An absolute coil also sees gradual wall thinning. In practice, use both together.
Array (ECA) probes: Many elements give wide coverage and better sensitivity to crack orientation.

5. Examination of heat exchanger tubes

Heat exchanger and condenser tubes are the most common industrial application of ET. Pull a bobbin probe through the tube at a constant speed and record the impedance signals along its length. The scan detects pitting, wall thinning (erosion/corrosion) and cracking, and it also marks the support plate and tubesheet locations in the signal.

Method selection depends on the tube material:

Non-ferromagnetic tubes (copper alloys, titanium, austenitic stainless): conventional eddy current examination (ECT).
Ferromagnetic tubes (carbon steel): RFT (remote field) and MFL (magnetic flux leakage). For internal corrosion, erosion and pitting in finned air cooler (fin-fan) carbon steel tubes, NFT (near field) is widely used.
Complementary verification: Methods such as IRIS (internal rotary inspection system) for absolute tube wall thickness.

One important limitation is the end effect: a bobbin probe cannot separate the tube end signal from the signal of a discontinuity next to it. Bobbin ET may therefore fail to evaluate the region at the tubesheet reliably — the most critical initiation point for tube failures — and a complementary technique such as IRIS may be required. The bobbin probe has a second limit: it is strong on gradual wall loss and pitting, but weak on axial cracks and on crack sizing. In those cases, prefer an array probe or another technique.

6. Calibration and evaluation

ET does not size discontinuities in absolute terms; it is a comparative method that stands on its calibration. Interpret the signals against known artificial discontinuities in a calibration tube of the same material and dimensions: through holes, flat-bottom holes, circumferential grooves and the like, machined to different percentages of the tube wall thickness. Set frequency, phase and gain during calibration.

Perform the evaluation on the impedance plane (Lissajous display): the phase angle of the signal indicates the depth of the discontinuity as a percentage of tube wall thickness, and the amplitude indicates its volume. Separate support plate, tubesheet and dent signals from genuine discontinuities. Use the differential channel for local discontinuities and the absolute channel for gradual wall thinning. A ferromagnetic support plate signal can mask real wall loss underneath it, so use a multi-frequency mix channel to suppress that signal and make the discontinuity visible.

Remember the rule behind every call you make here: every defect is a discontinuity; not every discontinuity is a defect — the acceptance criteria decide which is which.

7. Advantages and limitations

Advantages: ET is fast — thousands of tubes can be scanned in a short time. It is non-contact, needs no couplant and suits automation. It is sensitive to surface cracking, pitting and wall thinning, and it can measure through thin coatings.

Limitations: It works on conductive materials only, and it struggles on ferromagnetic materials, which call for RFT or magnetic saturation. Because of the skin effect it reaches only the surface and near-surface region in thick sections — although a suitable frequency covers the full tube wall thickness in thin-walled tubing. It is sensitive to lift-off and geometry. Technique-specific constraints also apply, such as the bobbin probe's weakness on axial cracks and crack sizing, and the results demand experienced evaluation.

8. Related standards

ASME BPVC Section V, Article 8: Eddy current examination method, including the appendices covering tube examination.
EN ISO 15549: Non-destructive testing — Eddy current testing — General principles.
EN ISO 15548 (series): Equipment for eddy current examination — characteristics and verification of the instrument, the probe and the system.
ASTM E243: Standard practice for electromagnetic (eddy current) examination of copper and copper alloy tubes.
ASTM E690: Standard practice for in situ electromagnetic (eddy current) examination of nonmagnetic heat exchanger tubes.
ASTM E2096: Standard practice for in situ examination of ferromagnetic heat exchanger tubes using remote field testing (RFT).

Apply the acceptance criteria of the construction or in-service code that governs the tube bundle or the equipment. The general framework for personnel qualification is ISO 9712 (Level I/II/III).

9. Where ET stands out

The method earns its place wherever speed and automation matter on conductive materials: periodic examination of heat exchanger tube bundles, surface crack scanning on non-ferrous components, alloy sorting by conductivity measurement, and thin coating thickness measurement. For surface cracking in ferromagnetic steel, MT or PT is more practical — but on non-ferrous conductors, and especially inside tubing, ET takes the lead.

From the field

The most common mistake in ET is to mistake a clean-looking signal for a correct result. What the signal means rests entirely on the calibration and the frequency you chose. Pick the wrong frequency and you can miss a deep discontinuity. Skip the lift-off compensation and you can report noise as a discontinuity. Running conventional ET on a ferromagnetic tube is another classic error — that is where you switch to RFT, or to NFT on finned air coolers. And do not skip the site prerequisite: clean the tubes internally first, then prove the probe passes through them. A dirty or blocked tube buys you false signals and missed discontinuities. The best ET examination is the one where the technique and probe suit the material, the calibration holds, and the evaluator knows how to read the impedance plane.

Take it to the field: To keep these NDT methods, standard references and field steps in your pocket — completely offline and free — take a look at the Doawise NDT Guide app.

At DoaWise we carry out non-destructive testing services, including eddy current examination, to international standards — from heat exchanger tube bundles to surface crack scanning on non-ferrous components — producing recordable, auditable results.