Types of Eddy Current Testing Techniques: ECT, ECA, PEC, RFT & More

Table of Content

1. Introduction
2. The Eddy Current Technique Family at a Glance
3. Conventional Eddy Current Testing (ECT)
4. Eddy Current Array (ECA)
5. Pulsed Eddy Current (PEC)
6. Remote Field Testing (RFT)
7. Near Field Testing (NFT)
8. Alternating Current Field Measurement (ACFM)
9. Related Electromagnetic Techniques That Are Not Eddy Current
10. How to Choose the Right Technique
11. Conclusion
12. FAQs

Introduction

Eddy current testing (ECT) is an electromagnetic non-destructive testing (NDT) method used to detect a range of discontinuities in conductive materials, including cracks, corrosion and metal loss. Over time, conventional ECT has developed into a family of specialized techniques, each created to solve an inspection problem the original method could not handle well.

Inspectors today can choose from Eddy Current Array (ECA), Pulsed Eddy Current (PEC), Remote Field Testing (RFT), Near Field Testing (NFT) and Alternating Current Field Measurement (ACFM). Each has distinct strengths depending on the material, the location of the expected damage, the inspection environment and the type of asset. This guide looks at these techniques, explains how they differ, and shows where each one belongs.

The Eddy Current Technique Family at a Glance

The eddy current family is usually presented as six main techniques: Conventional Eddy Current Testing (ECT), Eddy Current Array (ECA), Pulsed Eddy Current (PEC), Remote Field Testing (RFT), Near Field Testing (NFT) and Alternating Current Field Measurement (ACFM). Each was developed to address a particular shortcoming in conventional ECT, and each serves different industrial applications.

How the Techniques Relate to Each Other

All six belong to the broader family of Electromagnetic Testing (ET). Conventional Eddy Current Testing is the foundation, working by inducing eddy currents in a conductive material and detecting flaws through the disturbance they cause.

Understanding why the other techniques exist means understanding one constraint: standard depth of penetration, also called skin depth. Eddy currents are strongest at the surface and weaken rapidly with depth. How rapidly depends on test frequency, electrical conductivity and, above all, magnetic permeability. Carbon steel and other ferromagnetic materials have permeability high enough to crush eddy currents into a very thin surface layer, which is why conventional ECT struggles on them. Most of the specialized techniques below are workarounds for that single problem.

Conventional ECT had its limits, and as inspection requirements grew more complex, specialized techniques appeared to address them. ECA uses multiple coils to increase inspection speed. PEC detects corrosion through insulation. RFT inspects ferromagnetic tubes. NFT detects internal corrosion and erosion on the inside surface of carbon steel tubes. ACFM detects and sizes surface-breaking cracks through coatings and underwater.

ACFM is the odd one out here, since it measures field perturbation rather than coil impedance, but it is conventionally grouped with the family and it is easier to compare alongside the others.

These are not competing techniques. They complement each other, and each addresses a different inspection problem.

Conventional Eddy Current Testing (ECT)

Conventional Eddy Current Testing is the most common eddy current inspection technique and the starting point for most of the advanced electromagnetic methods that follow. The principle is straightforward: an alternating current applied to a probe coil produces a changing magnetic field, which generates eddy currents in the conductive test material. Cracks, corrosion or material changes disturb those currents and register as a change in electrical impedance.

ECT detects surface and near-surface defects well, which makes it suitable for aircraft structures, heat exchanger tubes, fastener holes, welds and machined parts. It requires no couplant and minimal surface preparation, so inspections are quick and results are immediate.

Advantages

1. Sensitive to very small surface cracks
2. Immediate inspection results
3. Portable equipment
4. No couplant required
5. Handles complex component geometries

Limitations

1. Limited penetration because of the skin effect
2. Small inspection area with single-coil probes
3. Ineffective on ferromagnetic materials without magnetic saturation
4. Requires skilled operators for signal interpretation

For localized inspections aimed at small surface defects, conventional ECT is still the preferred method.

Eddy Current Array (ECA)

Eddy Current Array is an advanced form of conventional ECT that houses multiple coils in a single probe. Instead of scanning one narrow path at a time, the probe covers a wide swath in a single pass, which was the major advance: far greater productivity with no loss of detection capability.

Data from each coil is acquired independently, and the software combines the signals into detailed inspection images. Because ECA scans are encoded, they produce permanent digital records that make reporting, traceability and comparison against previous inspections much easier.

ECA has been widely adopted for aircraft structural inspection, welded joints, pipelines, pressure vessels, storage tanks and other large metallic surfaces. In many surface crack detection applications it is now an effective alternative to Liquid Penetrant Testing (PT) and Magnetic Particle Testing (MT).

Advantages

1. Scans a much larger area in one pass
2. Faster than conventional ECT on large surfaces
3. Produces permanent, encoded digital records
4. Improves inspection consistency
5. Reduces overall inspection time

Limitations

1. Higher equipment cost and greater training requirement
2. More complex data analysis
3. Penetration depth is limited, as with conventional ECT
4. Requires an encoder and often geometry-specific probes

ECA suits inspections that demand high productivity, wide coverage and reliable digital documentation.

Pulsed Eddy Current (PEC)

Pulsed Eddy Current is designed to detect wall loss and corrosion under insulation, coatings and fireproofing without removing the protective layer. PEC differs from conventional ECT in the signal it uses: instead of a continuous sine wave, it sends a broadband electromagnetic pulse into the material and analyzes the decay of the returning signal. This allows inspectors to identify corroded areas even when the steel surface is completely inaccessible.

PEC is commonly applied to insulated pipelines, storage tanks, pressure vessels and process equipment in the oil and gas, petrochemical and power generation sectors. It is especially valuable for Corrosion Under Insulation (CUI) programs, where it sharply reduces the need to strip and replace insulation.

One point deserves emphasis. PEC is a screening process, not a thickness gauge. It reports an average wall thickness across a relatively large footprint and is used to pinpoint areas where wall loss may be present, so that follow-up inspection by a method such as Ultrasonic Testing (UT) can confirm it.

Advantages

1. Identifies corrosion through insulation and coatings
2. Minimizes insulation removal
3. Covers large areas quickly
4. Saves inspection time and cost

Limitations

1. Screening tool, not a thickness measurement
2. Primarily applicable to carbon and low-alloy steel
3. Lower resolution than localized NDT methods such as UT
4. Averages across the probe footprint, so small isolated pits may be missed

Remote Field Testing (RFT)

Remote Field Testing is an eddy current variant developed to inspect ferromagnetic tubes such as those used in boilers, heat exchangers and condensers. Conventional ECT is not effective on these materials because their high magnetic permeability sharply reduces eddy current penetration. RFT overcomes this by spacing the exciter and receiver coils roughly two tube diameters apart, so the receiver measures a field that has crossed the tube wall twice rather than the direct field.

That distinctive operating principle gives RFT near-equal sensitivity to internal (ID) and external (OD) defects, which makes it very effective for detecting corrosion, erosion, pitting and wall thinning in carbon steel tubing.

RFT is most often applied in power plants, refineries and industrial heat exchangers, where tube integrity is critical to safe operation.

Advantages

1. Near-equal sensitivity to ID and OD defects
2. Suited to ferromagnetic tubing where conventional ECT fails
3. Accurately identifies corrosion and general wall loss
4. Reliable for heat exchanger tube inspection

Limitations

1. Limited to ferromagnetic tube materials
2. Lower spatial resolution than conventional ECT
3. Less sensitive to fine cracks
4. Not suitable for non-ferromagnetic materials

RFT is one of several specialized techniques for ferromagnetic tube integrity — see the full comparison of tube inspection methods, including IRIS and advanced electromagnetic options.

→ Read: What is Remote Field Testing

Near Field Testing (NFT)

Near Field Testing is another specialized eddy current technique, designed for carbon steel fin-fan and air cooler tubes. The transmitter and receiver coils sit very close together, which confines the sensed electromagnetic field to the tube's inner surface and makes the method extremely sensitive to internal corrosion and erosion.

NFT is not a full-wall inspection like RFT. It is deliberately an inside diameter (ID) technique. That makes it well suited to applications where internal corrosion is the main concern, such as air-cooled heat exchangers in refineries and petrochemical plants.

Note that NFT is not sensitive to external (OD) damage and is therefore the wrong choice if external corrosion is the primary inspection target.

Advantages

1. Excellent detection of internal corrosion
2. Rapid screening of carbon steel tube inner surfaces
3. Ideal for fin-fan and air cooler tubes
4. Minimal tube preparation needed

Limitations

1. Cannot reliably detect external corrosion
2. Limited to specific tube applications
3. Not suitable for general surface inspections

For deeper insight into defect morphology in ferrous tubes, Remote-Field Array (RFA) technology takes electromagnetic tube inspection a step further — see how it works.

→ Read: Near Field Testing (NFT)

Alternating Current Field Measurement (ACFM)

Alternating Current Field Measurement is a non-destructive crack detection method that detects perturbations in a uniform induced electromagnetic field rather than changes in coil impedance. Because of that, protective coatings and marine growth do not need to be removed for inspectors to detect and size surface-breaking cracks.

ACFM is commonly used for welded structures, offshore platforms, pressure vessels, storage tanks and marine assets. It works through coatings and underwater, which cuts preparation time and returns reliable crack sizing.

Advantages

1. Detects and sizes surface-breaking cracks, returning both length and depth
2. Works through coatings and paint without removal
3. Can be used underwater
4. No surface cleaning required, unlike magnetic particle or penetrant testing

Limitations

1. Detects surface-breaking defects only
2. Requires correct probe positioning
3. Does not detect volumetric flaws

Want to get certified in ACFM inspection? This Level I course covers electromagnetic theory, probe operation, and signal interpretation for real-world crack detection.

→ Read: Alternating Current Field Measurement (ACFM) Level I Course

Related Electromagnetic Techniques That Are Not Eddy Current 

Not every electromagnetic inspection technique belongs to the eddy current family. Two are commonly grouped alongside it but work on different physical principles.

Magnetic Flux Leakage (MFL) does not induce eddy currents at all. It saturates a ferromagnetic component with a magnetic field, and wherever metal is missing, some of that flux escapes the surface where sensors pick it up. Anyone who has watched a tank floor scan will recognize the arrangement: a trolley pushed across the plate with magnets fore and aft and a sensor bridge between them. The same principle scales up to pipeline in-line inspection tools and down to wire rope testers.

Electromagnetic Acoustic Transducer (EMAT) is a stranger case. The transducer is electromagnetic, but what it produces is sound. A magnetic field interacts with currents induced in the surface, and through Lorentz force or magnetostriction the material itself becomes the source of an ultrasonic wave. No couplant is needed, which is why EMAT turns up on hot, rough or coated surfaces where conventional UT would struggle.

Both are electromagnetic NDT techniques, but neither is considered eddy current testing, because they rely on different physical principles.

How to Choose the Right Technique 

The eddy current technique you use depends on the inspection goal, the material and the condition of the asset. Rather than starting from the technique, start from what you need to inspect.

If You Need to Inspect Surface Cracks

For small, localized surface or near-surface defects in conductive materials, Conventional Eddy Current Testing is usually the best choice. It is highly sensitive and gives immediate results, which suits aircraft parts, fastener holes and machined components. There is little reason to bring an array system to a twenty-hole spot check.

If You Need Faster Coverage Over Large Areas .

For large surfaces, welded joints or any inspection that needs a digital record, Eddy Current Array is the preferred choice. The multi-coil probe scans a much larger area in one pass, increasing productivity without sacrificing detection capability. That traceability is often the real reason for buying it, not just the speed.

If Corrosion Is Hidden Beneath Insulation 

Pulsed Eddy Current is the most practical solution when the inspection target is carbon steel under insulation, coatings or fireproofing. It scans rapidly for wall loss without stripping the insulation and flags the areas that need detailed follow-up. Keep the expectation right: it tells you where to look more closely, not how thick the wall is.

If You're Inspecting Carbon Steel Tubes

For ferromagnetic tubing, Remote Field Testing gives balanced sensitivity to internal and external defects, which matters because you rarely know in advance which surface is corroding. Near Field Testing is the specialist option for internal damage in fin-fan or air cooler tubes, being far more sensitive to the inside diameter than the outside. If there is any real chance of external damage, use RFT instead, since NFT will not see it.

If You Need to Detect and Size Surface Cracks

Alternating Current Field Measurement is a strong option for welded structures, offshore assets or painted parts where coating removal is not feasible. It detects and sizes surface-breaking cracks through coatings and underwater without surface preparation, which cuts inspection time considerably.

No single eddy current method works for every application. The right choice depends on material, defect type, accessibility and inspection requirements.

Conclusion

Eddy current testing has advanced considerably, growing from conventional ECT into a family of specialized techniques tailored to specific inspection problems. ECT remains the preferred method for localized surface inspections. ECA increases the speed and traceability of surface inspections. PEC detects corrosion under insulation. RFT inspects ferromagnetic tubes. NFT finds internal corrosion in carbon steel tubes. ACFM detects and sizes surface cracks through coatings.

Understanding the strengths and weaknesses of each method allows inspectors to select the most effective and efficient approach for a given asset. These techniques are not mutually exclusive. Used together, they are significant tools in modern asset integrity and non-destructive testing programs.

FAQs

1. What is the difference between ECT and ECA?

Coil count, mostly, and what follows from it. Conventional ECT inspects with one coil or a few, scanned by hand. ECA packs multiple coils into one probe, which gives faster inspection of larger areas and generates digital inspection records. ECA costs more and takes longer to set up, so it is not the right answer for a quick spot check.

2. What is the most suitable eddy current method to be used for heat exchanger tubes?

It depends on the tube material. For ferromagnetic heat exchanger tubes, RFT is the better choice. For non-ferromagnetic tubing such as austenitic stainless, titanium or copper alloy, conventional ECT is more common. Note that ferritic and duplex stainless are ferromagnetic despite the name, so check the actual grade. For detecting internal corrosion in carbon steel fin-fan tubes, NFT is preferred.

3. Which eddy current technique works through insulation? 

Pulsed Eddy Current. PEC inspects carbon steel through insulation, coatings and fireproofing, which makes it ideal for corrosion under insulation screening. The reason is economic as much as technical: it avoids stripping and replacing insulation across long runs of pipework.

4. Which eddy current technique is suitable for carbon steel? 

It depends on the application. Conventional ECT performs poorly on carbon steel because high magnetic permeability limits penetration, so it is restricted to surface-breaking defects. PEC identifies corrosion under insulation. RFT inspects the full wall of ferromagnetic tubing. NFT covers internal corrosion in tubes. ACFM handles surface crack detection in welds.

5. Is ACFM an eddy current testing technique? 

It is usually treated as a close relative rather than a member of the family. Eddy current methods read changes in coil impedance, whereas ACFM induces a uniform current field and measures how a crack distorts it, which is what lets it size crack depth and tolerate thick coatings. Most people file it under eddy current for convenience, but the mechanism is genuinely different.

6. Can one eddy current technique replace all the others? 

No. Each technique addresses a different inspection task, and the choice depends on the material, the type of defect and the requirements of the inspection. Most mature programs run several in combination.