Eddy Current Testing: Cracks Through Coatings, Conductivity and Sorting

Eddy current testing detects cracks and material changes by electromagnetic induction, with no physical contact between the probe and the metal. That single characteristic is what makes it valuable: it finds surface and near-surface cracks straight through paint, galvanising and other coatings, on components where removing the finish would cost more than the inspection. itmad provides eddy current testing for crack detection, conductivity measurement, material sorting and coating thickness. itmad operates across the Middle East, Asia and Europe. Last reviewed: October 2026.

How it works

A coil carrying alternating current generates a changing magnetic field. Held against a conductive surface, that field induces circulating eddy currents in the material, and anything that disturbs their flow, such as a crack, a change in conductivity, a variation in magnetic permeability or a change in thickness, alters the impedance seen by the coil. The resulting shift in phase and amplitude is measured against reference values from calibration standards.

Crack detection through coatings

Magnetic particle and penetrant inspection both require the coating to be removed, the test to be performed and the surface to be recoated. On a painted crane boom, a structural weld or a coated pressure vessel, that sequence dominates the cost and the downtime, and it introduces a new coating whose quality then has to be verified.

Eddy current works through the coating. Lift-off is compensated in the technique, and detection of surface-breaking cracks stays reliable at the coating thicknesses normally found in service, typically up to 2 mm of non-conductive coating. For crane owners that is the difference between an inspection that takes a shift and a programme that takes the equipment out of service for a week, and it applies equally to concrete pumping equipment and other coated structures.

Other applications

Electrical conductivity, expressed in %IACS, is measured to verify alloy condition and identify material. Eddy current also verifies heat treatment, confirming that solution treatment, ageing or annealing has produced the intended condition in aluminium and titanium alloys, and assesses heat damage, detecting where aluminium has been overheated, for example after a fire or an in-service thermal event. Material sorting separates mixed batches by alloy, temper, hardness or strength where documentation has been lost, coating thickness is measured on metallic substrates for both conductive and non-conductive coatings to ASTM E376, and thin sections can be measured where access is from one side only.

Standards applied

ASME Boiler and Pressure Vessel Code Section V, Article 8, covers eddy current examination and ISO 15549 the general principles of eddy current testing. ASTM E1004 covers the determination of electrical conductivity by the electromagnetic method and ASTM E376 coating thickness measurement by magnetic-field or eddy-current methods, and client, project and manufacturer specifications and aerospace maintenance manual procedures apply where they govern.

Personnel qualification

Eddy current demands more of the technician than any other surface method. The signal is an impedance response and not a visible indication, and distinguishing a crack from an edge effect, a geometry change or a permeability variation depends on the operator's experience with that component and that material. itmad's eddy current work is carried out by technicians qualified to ISO 9712 and ASNT SNT-TC-1A Level II, with NAS 410 qualification for aerospace components and Level III oversight of procedure approval and technique validation.

Limitations

The method applies only to electrically conductive materials. Penetration depth is limited and falls as frequency, conductivity and permeability rise: standard ECT penetration is typically 1 to 6 mm, depending on material and frequency, so it addresses surface and near-surface defects and not deep subsurface ones. Ferromagnetic materials produce permeability variations that can mask genuine indications and need careful technique selection, and complex geometry, edges and fasteners generate signals that must be distinguished from defects, which is why representative calibration standards matching the component are needed.

Typical components examined

Crane booms, jibs and lattice sections; structural welds under paint; lifting equipment hooks and pins; concrete pump booms and outriggers; aircraft skin, frames and fastener holes; turbine and engine components; pressure vessel and pipework welds; non-ferrous castings and forgings; fastener holes and bolt bores; and aluminium structures and assemblies. Eddy current testing is carried out at the client's site, offshore, in fabrication yards or at itmad premises.

What the client receives

A report identifying the components examined and the areas covered, with the equipment, probe type and frequency used and the reference standards calibrated against. The coating thickness present at the time of examination is recorded where relevant, indications are recorded with location, response and interpretation, and each item has an accept or reject status against the applicable acceptance criteria. Conductivity or thickness values are included where those were the purpose of the examination, together with the technician's qualification level and certification reference.

Frequently asked questions

What is eddy current testing and what does it detect?

An electromagnetic NDT method that detects surface and near-surface cracks, material changes and coating variations in conductive materials without physical contact. A coil induces eddy currents in the material, and anything that disturbs their flow shifts the coil impedance.

Can eddy current testing find cracks through paint and coatings?

Yes. Unlike magnetic particle and dye penetrant inspection, it works through paint, galvanising and other protective coatings, because lift-off is compensated. Detection stays reliable at typical in-service coating thicknesses, up to about 2 mm of non-conductive coating.

Which standards apply?

ASME Section V Article 8, ISO 15549, ASTM E1004 for conductivity and ASTM E376 for coating thickness. Aerospace work follows applicable maintenance manual procedures and NAS 410 requirements.

What are its limitations?

It applies only to electrically conductive materials, and standard penetration is typically 1 to 6 mm, so it addresses surface and near-surface defects. Ferromagnetic materials can mask genuine indications, and edges and fasteners generate signals that must be distinguished from defects.

What else can eddy current testing do?

Measure electrical conductivity in %IACS, verify heat treatment and detect heat damage in aluminium and titanium alloys, sort mixed material batches, and measure coating and thin-section thickness. It supports material identification where documentation has been lost.

Why does eddy current demand a skilled technician?

The signal is an impedance response and not a visible indication, and telling a crack from an edge effect, geometry change or permeability variation depends on experience with that component and material. itmad's technicians hold ISO 9712 and ASNT Level II qualifications, with NAS 410 for aerospace work.