Eddy Current Testing (ET)

Eddy Current Testing (ET) is an electromagnetic NDT method that induces circulating currents in conductive materials using an AC coil; changes in coil impedance reveal surface and near-surface defects, conductivity variations, and coating thickness.

Definition

Eddy Current Testing (ET) uses an alternating-current coil to induce small circulating currents — eddy currents — in an electrically conductive test piece. Discontinuities, conductivity variations, and geometry changes alter the eddy current flow and therefore the coil's impedance. The resulting impedance shift is plotted on the impedance plane for interpretation.

Technical Context

Probe selection depends on application: encircling coils for tubes, bobbin coils for heat-exchanger tubes, surface coils for plates and welds, and rotating probes for bolt holes. Performance is affected by lift-off, edge effect, and fill factor.

When It Is Used

Heat-exchanger and condenser tube inspection
Aircraft skin and fastener-hole crack detection
Coating thickness and conductivity sorting

Related Standards

ASME Section V Article 8, ASTM E309, ASTM E571, ISO 15549.

Synonyms

Also called ECT. Advanced variants include multi-frequency ET, ACFM, and Eddy Current Array (ECA).

How it works

An alternating current in a coil induces circulating currents in a conductive part. Anything that disturbs those currents — a crack, a change in conductivity, a change in the gap between coil and surface — reflects back as a change in coil impedance, which is displayed as a movement in the impedance plane.

What it finds

Surface and near-surface cracking in conductive materials, including through thin non-conductive coatings, and it does so without couplant. In tubing, bobbin and array probes find wall loss, pitting and cracking at production speed. Conductivity and coating-thickness measurement use the same physics.

What it will not find

Anything deep. Penetration falls off exponentially with depth, and faster as frequency, conductivity or permeability rise, so eddy current is a surface and near-surface method whatever the probe. Ferromagnetic materials complicate it further because permeability variation swamps the flaw signal unless the material is saturated.

How it is actually done

Frequency is chosen for the depth of interest, the instrument is nulled on sound material, and the phase rotation is set on a reference standard with known artificial defects so that lift-off moves along a known direction and flaw signals separate from it. Reading the impedance plane is the skill; the equipment only presents it.

Governing codes and standards

ASME Section V Article 8 covers eddy current examination; ISO 15548 addresses equipment characterisation; ASME Section V Article 26 and the various tubing appendices cover heat-exchanger tube examination in service.

Where it goes wrong

Mistaking lift-off for a defect, or worse, phasing lift-off out so aggressively that a genuine shallow flaw goes with it. The reference standard exists so the two are separated deliberately rather than by eye.

Frequently asked questions

Can eddy current testing be used on steel?

Yes, but with care. Carbon steel is ferromagnetic, and permeability variation produces signals far larger than most flaws. Magnetic saturation probes or alternatives such as magnetic flux leakage are usually specified instead, depending on what is being looked for.

Does paint have to be removed first?

Usually not, which is one of the method's real advantages. The coating simply adds lift-off, and provided it is reasonably uniform and accounted for during setup, examination proceeds through it. Thick or irregular coatings erode sensitivity, and at some point removal becomes the honest option.

Where Eddy Current Testing fits in an inspection programme

A term is only useful when it connects to a decision. Eddy Current Testing appears in written procedures, in technique sheets, and in the records an owner or accreditation body reviews afterwards — which means the way it is defined in your documentation has to match the way it is applied on site. Where the two drift apart, audits find it. Atlantis writes and reviews procedures against the governing codes, trains inspection personnel to apply them, and builds the record-keeping that makes the evidence retrievable years later. Procedure development and code consulting · NDT training and certification · Ask us about your programme.

Related terms

Impedance Plane — The impedance plane is a 2D display in eddy current testing showing the probe coil's resistive (X-axis) and inductive reactive (Y-axis) components, on which signals from lift-off, defects, and conductivity changes trace characteristic vectors.
Lift-Off — Lift-off is the change in eddy current signal caused by a variation in the distance between the probe coil and the test surface, used both as a calibration tool and as a nuisance variable to be minimized.
Bobbin Coil — A bobbin coil is a differential or absolute eddy current probe that travels axially inside a tube, inspecting the entire tube circumference simultaneously for wall loss, pitting, and through-wall defects.
Surface Coil — A surface coil is a small handheld eddy current probe used to scan accessible surfaces of components — typically a pencil-style absolute or reflection probe — for surface-breaking crack detection.
Multi-Frequency Eddy Current — Multi-frequency eddy current testing uses two or more simultaneous frequencies whose responses are mixed in software to suppress unwanted variables (support-plate signals, tube-end effects) and isolate the defect signal of interest.
Alternating Current Field Measurement (ACFM) — ACFM is an electromagnetic NDT technique that induces a uniform AC field in a conductive component and measures the field disturbance over surface-breaking cracks to size their length and depth without surface cleaning.

More method terms

Ultrasonic Testing · Radiographic Testing · Magnetic Particle Testing · Penetrant Testing · Visual Testing · Acoustic Emission Testing · Leak Testing · Thermography / Infrared Testing · Microwave Testing · Phased Array Ultrasonic Testing

Where this comes up in practice

Terms like this one appear in three places that matter commercially: the written practice that governs how your personnel are qualified, the procedures and technique sheets that define how an examination is actually performed, and the evidence an auditor or client asks for when they want to know why an inspection was accepted. Getting the terminology right is the easy part; being able to produce the qualification record, the calibration traceability and the procedure revision that applied on the day of the inspection is the part that decides audits.

Atlantis NDT provides NDT training and certification against ASNT SNT-TC-1A and ISO 9712, ASNT Level III consulting for written practices and procedure approval, inspection management software that holds qualification, calibration and procedure-revision evidence in recoverable form, and an asset integrity platform that binds inspection results to the asset they describe. Browse the full NDT glossary or ask a Level III directly.

Eddy current probe selected by geometry — what it resolves, what it is blind to, and where it is specified
Probe or technique	Geometry it fits	Resolves	Blind to	Where specified
Bobbin coil, differential and absolute	Heat-exchanger and condenser tubing with ID access	Volumetric wall loss, pitting and general thinning at full production speed	Circumferential cracking; flaws under support plates and in the U-bend without extra channels	ASME V Art. 8; ASTM E2884
Rotating pancake or motorised probe	The same tubing, as targeted follow-up on flagged tubes	Crack orientation and circumferential defects the bobbin cannot resolve	Nothing the bobbin catches at speed — inspection rate drops by an order	ASME V Art. 8; ASTM E2884
Eddy current array (ECA)	Welds, plate, tube sheets and complex surfaces	Surface cracking with encoded C-scan coverage in a single pass	Flaws beyond the standard depth of penetration at the frequency chosen	ASTM E3052; ISO 20339; ISO 17643 for welds
Surface or pencil coil	Local weld toes, machined radii, spot checks	Fine surface-breaking cracks, weld toe cracking through paint	Anything requiring area coverage — it inspects only where it is placed	ISO 17643; ASTM E3052
Encircling coil	Bar, wire and tube moving through a production line	Continuous in-line flaw and dimensional sorting at line speed	Circumferential location of the flaw; end effects at bar and tube ends	ASTM E426; ASTM E309
Bolt-hole rotating probe	Fastener holes in aerospace structure	Cracks radiating from the hole bore, resolved layer by layer	Cracks in lower layers without disassembly or low-frequency ET	OEM NDT manual; NAS 410 personnel qualification
Magnetic saturation ET	Ferromagnetic and duplex tubing	Wall loss once permeability noise is suppressed by a saturating DC field	Flaws still masked by residual permeability variation; deep ID defects	ASTM E309

Eddy current probe selected by geometry — what it resolves, what it is blind to, and where it is specified

Probe or technique

Geometry it fits

Resolves

Blind to

Where specified

Bobbin coil, differential and absolute

Heat-exchanger and condenser tubing with ID access

Volumetric wall loss, pitting and general thinning at full production speed

Circumferential cracking; flaws under support plates and in the U-bend without extra channels

ASME V Art. 8; ASTM E2884

Rotating pancake or motorised probe

The same tubing, as targeted follow-up on flagged tubes

Crack orientation and circumferential defects the bobbin cannot resolve

Nothing the bobbin catches at speed — inspection rate drops by an order

ASME V Art. 8; ASTM E2884

Eddy current array (ECA)

Welds, plate, tube sheets and complex surfaces

Surface cracking with encoded C-scan coverage in a single pass

Flaws beyond the standard depth of penetration at the frequency chosen

ASTM E3052; ISO 20339; ISO 17643 for welds

Surface or pencil coil

Local weld toes, machined radii, spot checks

Fine surface-breaking cracks, weld toe cracking through paint

Anything requiring area coverage — it inspects only where it is placed

ISO 17643; ASTM E3052

Encircling coil

Bar, wire and tube moving through a production line

Continuous in-line flaw and dimensional sorting at line speed

Circumferential location of the flaw; end effects at bar and tube ends

ASTM E426; ASTM E309

Bolt-hole rotating probe

Fastener holes in aerospace structure

Cracks radiating from the hole bore, resolved layer by layer

Cracks in lower layers without disassembly or low-frequency ET

OEM NDT manual; NAS 410 personnel qualification

Magnetic saturation ET

Ferromagnetic and duplex tubing

Wall loss once permeability noise is suppressed by a saturating DC field

Flaws still masked by residual permeability variation; deep ID defects

ASTM E309

Which eddy current probe should a US refinery specify for heat-exchanger tubes?

Bobbin first, rotating probe or array second. A bobbin differential coil covers a full bundle at production speed and finds volumetric wall loss and pitting. It reads circumferential cracking poorly, so the accepted programme runs bobbin across the whole bundle and follows up flagged tubes with a rotating pancake or array probe. ASTM E2884 describes both stages for installed tubing.

Can eddy current inspect carbon steel piping?

Not usefully in the plain configuration. Permeability variation in ferromagnetic steel produces impedance swings larger than most flaw signals. Magnetic saturation probes suppress it on tubing, and ASTM E309 covers that case. For plant piping the working answers are magnetic flux leakage for wall loss, pulsed eddy current through insulation, and ACFM or magnetic particle for surface cracking.

What is standard depth of penetration and why does it decide frequency?

It is the depth at which eddy current density falls to about 37 percent of its surface value, given by 1/√(πfμσ). Frequency is the only term the inspector controls. Doubling frequency shrinks the skin by about 30 percent and sharpens small surface cracks; halving it reaches deeper and blurs them. Frequency selection is therefore a statement about the depth of interest.

Does coating have to be stripped before eddy current examination?

No, when the coating is non-conductive, reasonably uniform, and accounted for during setup. It adds lift-off, which phase rotation on the reference standard separates from flaw signals. Thick or irregular coatings cost sensitivity because the coil sits further from the currents it is trying to induce. Past that point stripping is the honest option, not a preference.

How is eddy current data on a tube bundle turned into a plug decision?

By the owner's plugging criterion, not by the examination standard. ASME Section V Article 8 and ASTM E2884 say how to acquire and calibrate the data. The percentage through-wall at which a tube gets plugged comes from the exchanger's design margin, the service and the owner's integrity programme. Nuclear steam generator tubing is the exception, where ASME Section XI and plant technical specifications set it.

What separates lift-off from a real flaw on the impedance plane?

The angle. On a reference standard with machined artificial defects, lift-off traces a repeatable direction and flaw signals depart from it at a phase angle that shifts with depth. Setup rotates the display so lift-off runs along a known axis. Phasing it out too aggressively takes shallow flaw signals with it, which is the most common way an examination quietly under-reports.

This entry defines the method — it is not the service page

This glossary entry exists to define eddy current testing (ET) and the vocabulary around it. If you are looking to have eddy current testing performed on your equipment rather than to understand what it is, the service page is eddy current testing — bobbin, array and rotating-probe tube inspection plus surface examination on non-ferrous and coated components. For a longer explanation of how the method works in practice, see the ET method guide.