Eddy current inspection is a way to check metal parts for hidden cracks, corrosion, and other flaws without cutting into or damaging them. It works by using electromagnetic fields to detect changes inside or on the surface of conductive materials. The technique is fast, requires no direct contact with bare metal, and can even inspect through paint or coatings, which makes it one of the most versatile tools in nondestructive testing.
How Eddy Current Inspection Works
The method starts with a probe containing a small coil of wire. When alternating current runs through that coil, it generates a magnetic field that fluctuates at the same frequency as the current. Bring that probe close to a piece of metal, and the changing magnetic field penetrates the surface and induces tiny circular electrical currents inside the material. These are the “eddy currents.”
Once flowing, the eddy currents create their own secondary magnetic field that pushes back against the original one from the probe. The probe’s electronics detect this pushback as a change in the coil’s electrical impedance. In a flawless piece of metal, the eddy currents flow in smooth, predictable patterns and produce a consistent signal. A crack, void, or change in material thickness disrupts those current paths, altering the signal in a way an inspector can read on a screen. The principle behind all of this is Faraday’s law of electromagnetic induction: a changing magnetic field induces an electrical current in any nearby conductor.
What It Can Detect
Eddy current inspection excels at finding surface and near-surface defects. Cracks, pitting, corrosion thinning, and porosity all disrupt the flow of eddy currents enough to register as a signal change. Research into parts made with additive manufacturing (3D printing) has shown that the technique can pick up subsurface pores and cracks as small as 0.3 mm to 0.6 mm in size, which gives a sense of how sensitive it can be under controlled conditions.
Beyond flaw detection, the method also measures things that aren’t defects at all. Because the eddy current response depends on a material’s electrical conductivity and other properties, inspectors use it for coating thickness measurements on painted or anodized surfaces, heat treatment verification, hardness checks, and even sorting mixed-up metal alloys. A difference in conductivity between two aluminum grades, for instance, produces a noticeably different signal, making it possible to identify metals quickly in the field.
Common Probe Types
Different inspection jobs call for different probe designs, and three types cover most situations.
- Surface (pancake) probes are flat coils held against or scanned across the outside of a part. They offer excellent sensitivity and produce signals that closely mirror the shape of a crack, making them ideal for detailed inspections of flat surfaces, bolt holes, and welds. The tradeoff is speed: an inspector has to move the probe across the entire area being checked.
- Bobbin probes are cylindrical and designed to slide through the inside of tubes and pipes. They inspect the full circumference at once, making them the standard first-pass tool for heat exchanger tubing in power plants and refineries.
- Array probes combine many small coils into a single housing. Because the coils work simultaneously, an array probe can scan a wide area in one pass without the slow back-and-forth of a single pancake coil. This makes them practical for high-speed screening of large surfaces and for sizing cracks in tubing where a bobbin probe flagged something suspicious.
Where It Gets Used
Aerospace is one of the largest users of eddy current inspection. NASA’s Marshall Space Flight Center has relied on the technique for decades to inspect bolts (heads, grip length, and threads), nuts, fasteners, and bolt holes for defects. Aircraft skin panels, wing structures, and engine components are routinely checked the same way, often without removing paint or primer.
Power generation is another major application area. Thousands of thin-walled tubes in steam generators and heat exchangers need periodic screening for corrosion and cracking. Bobbin probes can be pushed through tube after tube rapidly, with array probes following up on anything suspicious. Other common uses include inspecting wire and bar stock during manufacturing, checking welds in pressure vessels, and verifying that ball bearings are free of surface cracks.
Why Inspectors Choose It Over Other Methods
Several features make eddy current inspection preferable in situations where other nondestructive methods fall short. Unlike liquid penetrant testing, which can only find cracks open to the surface and requires cleaning and drying steps, eddy current testing detects both surface-breaking and shallow subsurface flaws. It also works through nonconductive coatings like paint, lacquer, and anodizing, so there’s no need to strip a surface before testing.
Compared to ultrasonic testing, eddy current inspection needs no couplant gel or water between the probe and the part. This makes it cleaner and faster for field work. The results are also immediate: the signal appears in real time on a screen, with no film to develop and no waiting period. On the other hand, ultrasonic methods can penetrate much deeper into thick sections, which is a gap eddy current testing can’t close.
Depth of Penetration
Eddy currents are strongest right at the surface and fade exponentially as they travel deeper into the metal. The characteristic distance at which the current density drops to about 37% of its surface value is called the “skin depth.” It depends on three things: the frequency of the alternating current, the electrical conductivity of the material, and its magnetic permeability.
Lowering the test frequency increases penetration. In aluminum, which has a conductivity around 45% IACS for common 6061 alloy, a lower frequency lets the currents reach deeper. In highly conductive materials like copper (around 85% IACS), the currents crowd closer to the surface at any given frequency, reducing effective penetration. Inspectors choose their frequency based on whether they need shallow, high-resolution detection or deeper but less precise coverage. For most practical work, eddy current inspection is best suited to the outer few millimeters of a part.
Limitations Worth Knowing
The most significant limitation is that eddy current inspection only works on electrically conductive materials. Plastics, ceramics, and composites won’t support eddy currents at all, so the method simply doesn’t apply to them.
Ferromagnetic metals like carbon steel and iron present a different challenge. Their high magnetic permeability concentrates the magnetic field so heavily near the surface that the eddy currents barely penetrate, making subsurface inspection extremely difficult. The strong magnetic response also masks the more subtle signals from small flaws. Specialized techniques such as remote field testing exist for ferromagnetic tubing, but standard eddy current inspection works most cleanly on nonferromagnetic metals: aluminum alloys, titanium, stainless steels in the austenitic family, copper, and nickel-based superalloys like Inconel.
Geometry also matters. Complex shapes, edges, and changes in part thickness all affect eddy current flow in ways that can mimic or obscure real defects. Inspectors compensate with careful probe selection, calibration on reference standards with known artificial flaws, and experience reading signals. Interpretation requires training, and results depend heavily on the skill of the operator.
Standards and Certification
Eddy current inspection is governed by a network of industry standards that ensure consistency and reliability. ASTM E376 covers coating thickness measurement using eddy current and magnetic methods. ASTM E243 addresses the examination of copper and copper-alloy tubing specifically. On the international side, ISO 15548 defines requirements for both the inspection instruments and the probes themselves, covering characteristics and verification procedures. Inspectors typically hold certifications at progressive levels that qualify them first to perform tests under supervision, then to set up and interpret results independently, and eventually to develop inspection procedures for others.