What Is Eddy Current Testing (ET)?

Quick Answer

Eddy current testing (ET or ECT) is an electromagnetic NDT method that detects surface and near-surface defects in conductive materials without physical contact. An alternating current in a coil generates a magnetic field that induces eddy currents in the test piece. Discontinuities—cracks, pitting, wall thinning—alter the eddy current flow, producing measurable impedance changes displayed on the instrument. ET is the primary inspection method for heat exchanger tubing, condenser tubes, and mill production testing of seamless and welded pipes.

How Eddy Current Testing Works

The probe coil carries an alternating current at a selected frequency (typically 1 kHz to 6 MHz). This generates a primary magnetic field that penetrates the test material and induces circular eddy currents. Any change in material conductivity, permeability, geometry, or the presence of a defect alters the eddy current pattern. The instrument detects these changes as impedance variations, displayed on an impedance plane diagram.

ET Probe Types

Probe Type	Configuration	Application
Bobbin coil	Internal coil pulled through tube	Heat exchanger tube inspection (standard method)
Rotating probe	Motorized probe spinning inside tube	Precise flaw sizing after bobbin screening
Array probe	Multiple coils in a single housing	Fast scanning of large surfaces, welds
Pencil/spot probe	Small surface probe	Localized inspection, weld toes, bolt holes
Encircling coil	Coil surrounds the product	Mill production testing of pipe, tube, bar

Key Applications in Piping

Application	Method	Standard
Heat exchanger tube inspection	Bobbin + rotating probe	ASME V Article 8, ASTM E243
Seamless pipe mill testing	Encircling coil (online)	ASTM E309, ASTM E426
Welded pipe weld seam	Sector coil or probe on weld	ASTM E273
Surface crack detection	Pencil/array probe	ASTM E376, EN ISO 15548
Conductivity/sorting	Spot probe	ASTM E1004 (conductivity measurement)
Coating thickness	Spot probe	ASTM B244, ASTM D7091

ET vs Other Surface Methods

Parameter	ET	MT	PT
Material	Any conductive material	Ferromagnetic only	Any non-porous material
Contact required	No (coil proximity)	Yes (particles on surface)	Yes (liquid on surface)
Subsurface defects	Yes (limited depth)	Yes (near-surface)	No (surface only)
Speed	Very fast (automated scanning)	Moderate	Slow (dwell/development time)
Surface preparation	Minimal	Light cleaning	Thorough cleaning required
Permanent record	Digital data (encoded scan)	Photographs only	Photographs only
Consumables	None	Particles, contrast paint	Penetrant, cleaner, developer

Depth of Penetration

Eddy current depth of penetration depends on frequency, material conductivity, and permeability. The standard depth of penetration (where eddy current density drops to 37% of surface value) follows:

Material	Conductivity (%IACS)	Depth at 100 kHz	Depth at 1 MHz
Carbon steel	~8	~0.2 mm	~0.06 mm
Austenitic stainless steel	~2.5	~10 mm	~3 mm
Copper	~100	~0.2 mm	~0.07 mm
Aluminum	~61	~0.3 mm	~0.08 mm

Lower frequencies penetrate deeper but reduce sensitivity to small defects. Multi-frequency techniques use several frequencies simultaneously to separate defect signals from geometry or support plate signals in heat exchanger inspection.

Pro Tip

For ferromagnetic materials (carbon steel), eddy current depth is severely limited by the high magnetic permeability. Saturate the material magnetically or use remote-field eddy current (RFET) techniques to inspect carbon steel heat exchanger tubes. Standard ET is most effective on non-ferromagnetic alloys—austenitic stainless, Inconel, copper-nickel, titanium.

ET results feed into the inspection and test plan and are archived with mill test certificates for traceability.

Read the full guide to non-destructive testing