Surface & Near-Surface NDT · ECT
Eddy Current Testing
Induced currents that flinch at every flaw.
Eddy current testing (ECT) detects surface and near-surface defects in electrically conductive materials. A coil carrying alternating current induces circulating “eddy” currents in the component; a crack or thinning interrupts their flow, changing the coil’s impedance — a shift the instrument detects instantly, without couplant or surface contact.
The explainer
Watch how it works.
Schematic · loops automatically
1Energise the coil
Alternating current in the probe coil creates an oscillating magnetic field beside the component surface.
2Induce eddy currents
The field induces circulating currents in the conductive material — flowing in closed loops just beneath the surface.
3A defect diverts the flow
A crack forces the currents to detour around it; the disturbed field alters the coil’s impedance.
4The impedance plane tells the story
The instrument plots the change; cracks, lift-off and thinning each trace distinct signatures, separating defects from noise.
5Scan and record
The probe is scanned over the inspection area — or drawn through tubing — with signals recorded against position for reporting.
The physics
Pass alternating current through a coil and it generates an alternating magnetic field; bring that field near a conductor and it induces circulating currents in the material — eddy currents — which generate their own opposing field. The net effect loads the coil: its electrical impedance settles at a value set by the material’s conductivity, permeability, geometry and the probe lift-off.
A defect rewrites that balance. Eddy currents must flow around a crack rather than through it; the disturbed current paths change the reflected field, and the instrument plots the coil impedance on a phase plane. Crack, lift-off, and wall thinning each move the point along characteristically different trajectories — which is how a trained technician tells a real defect from a probe wobble.
Frequency is the depth control: high frequencies concentrate eddy currents at the surface for maximum crack sensitivity; lower frequencies drive them deeper (the skin effect) for sub-surface response and tube-wall inspection. The method needs no couplant and tolerates thin coatings, making it fast on painted structures and ideal for heat-exchanger tubing, where internal probes inspect metres of tube per minute.
Where it earns its keep
Typical applications
- Heat exchanger and condenser tubing surveys
- Crack detection through paint on aluminium and stainless structures
- Weld inspection on non-ferrous and duplex materials
- Rapid screening where couplant-free testing is needed
Standards & schemes
| Designation | Covers |
|---|---|
| ISO 15549 | Eddy current testing — general principles |
| EN ISO 17643 | Eddy current testing of welds by complex-plane analysis |
ECT is performed by ISO 9712-certified eddy current technicians; tubing inspection campaigns are reported with per-tube condition grading for maintenance planning.
Why choose eddy current over penetrant or MPI?
Speed and surface tolerance: no couplant, no chemicals, works through thin coatings, and gives an instant electronic signal that can be logged. On non-magnetic conductive materials — where MPI is impossible — ECT is often faster and cleaner than penetrant, especially for repetitive scanning.
How deep can eddy currents see?
The skin effect concentrates current near the surface; penetration falls off exponentially with depth and increases with lower frequency. In practice ECT is a surface and near-surface method — typically the first few millimetres in non-ferrous metals, less in steel — not a volumetric one.
Can it inspect our exchanger tubes without pulling the bundle?
Yes — internal bobbin probes are drawn through each tube from the header, screening full lengths rapidly and grading defects by phase and amplitude. It is the standard method for condenser and exchanger tube health across power and process plants.
Related methods
Delivered through
Testing · Inspection · Certification
Need ECT on your asset?
Describe the component, the material and the question — a specialist confirms the technique, the standard and the turnaround.