Eddy Current Testing (ECT): Complete Guide to Principles, Types, Standards & Applications

Eddy current testing and eddy current inspection: the practical answer

Eddy current testing (ET) finds surface and near-surface cracks, pitting and wall loss in conductive metals. A coil induces currents in the part, and flaws disturb them. In the US it does three jobs most often: fastener-hole and structural checks on aircraft, tube inspection in heat exchangers and condensers, and crack checks on coated welds. Choose the technique by material and geometry first, because each eddy current technique fails somewhere specific.

Which eddy current technique fits the job

"Eddy current inspection" on a purchase order can mean at least five different techniques. The table below is the selection logic a Level III uses before writing a procedure. IRIS is included because buyers often weigh it against eddy current for tubing, but it is an ultrasonic technique, not ET.

Two practical rules come out of the table. First, never let one technique stand in for another just because the crew already has the kit on site. A bobbin probe pushed through carbon steel tubing produces data, but not data you can trust. Second, a screening technique gives you locations, not numbers. PEC and RFT tell you where to look; IRIS or UT thickness gives the measurement that an engineer can use in a remaining-life calculation.

For carbon steel welds where coating removal is the real cost, two electromagnetic options exist. Alternating current field measurement (ACFM, ASTM E2261) and eddy current array built to ASTM E3052 both detect and size surface-breaking cracks without stripping the coating. ASTM E3052's scope covers nonconductive coatings up to 5 mm thick. Magnetic particle testing can also work through thin nonmagnetic coatings, but only with a demonstrated AC-yoke technique such as the one in ASME Section V Article 7.

Eddy current inspection in US industry: three workloads

Aerospace and aviation

Aviation eddy current work is driven by the airframe or engine maker's NDT manual, service bulletins and airworthiness directives. The inspection is written for you: probe, frequency, reference standard, scan path and reject level. Typical tasks are bolt-hole inspection with rotating scanners, surface ET around fastener rows and wheel bead seats, ECA on lap joints, and conductivity checks to ASTM E1004 to screen aluminum for heat damage.

The personnel side is different from industrial work. Most US aerospace employers certify technicians under their own written practices, aligned to customer flow-down and Nadcap AC7114. Atlantis builds aerospace written practices on SNT-TC-1A and trains to them; certification comes from your employer's program. Method background for this sector is on our eddy current testing in aviation page.

Heat-exchanger, condenser and boiler tubing

Gulf Coast refineries, chemical plants and power stations do most tube inspection inside turnaround windows. Tube material decides the technique:

• Copper alloys, admiralty brass, copper-nickel, titanium and austenitic stainless: bobbin ET, with array probes added where cracking is the concern. ASTM E690 covers in-situ eddy current examination of nonmagnetic exchanger tubes. ASTM E243 covers copper and copper-alloy tube at the mill.
• Carbon steel: RFT to ASTM E2096 for full-bundle screening, then IRIS on a sample of tubes to size what RFT found.
• Duplex and other partly magnetic alloys: magnetically biased (saturation) probes, or a different technique. Check this before mobilization, not on the tube sheet.
• Finned air-cooler tubes in carbon steel: usually an ultrasonic or near-field technique, chosen per bundle.

The output is a tube sheet map and an indication list with estimated percent through-wall. The owner, or the owner's engineer, decides which tubes to plug. The inspection company reports against the criteria the contract names. More on tube methods: eddy current tube inspection and our ECT, IRIS, RFT and MFL comparison.

Weld eddy current (ECA and ACFM)

Weld ET is growing on coated carbon steel: offshore structures, cranes, bridges, storage spheres and in-service pressure equipment. The gain is that no blasting or recoating is needed, and ECA produces an encoded C-scan record instead of a pass/fail note. The limits are real, though. It finds surface-breaking cracks only. Toe geometry, spatter and weld caps that are not ground can all produce signals. The procedure must be demonstrated on weld samples with notches of known depth in the same steel and coating thickness. Treat weld ET as a crack-detection screen at the toes. It does not replace volumetric UT.

Eddy current standards and codes US contracts cite

A common error is to cite ASTM E309 as a general eddy current practice. E309 is specific to steel tubular products examined with magnetic saturation.

One change matters for anyone who cites article numbers. The 2025 edition of Section V was published in July 2025 and became mandatory on January 1, 2026. It added a new Subsection C for in-service techniques, and PEC is among the techniques reported as relocated there. If your ET, ACFM or PEC procedure cites an article number, check it against the edition your contract names. Our ASME Section V guide maps the articles.

Eddy current qualification under SNT-TC-1A

In SNT-TC-1A, electromagnetic testing (ET) is the method, and it is divided into three techniques: eddy current, alternating current field measurement and remote field testing. Magnetic flux leakage is listed as a separate method. A technician certified for eddy current is not automatically qualified for RFT. The employer's written practice must say which techniques each person holds.

• Training and experience. SNT-TC-1A gives recommended minimum hours by technique and level. The written practice adopts them or justifies changes. Experience has to be in the technique, logged and verifiable.
• Examinations. Level II takes general, specific and practical exams. The practical should use specimens with known flaws that look like the work: tubing standards for tube analysts, notched plate or weld samples for surface work.
• Vision. Near-vision acuity checked at least annually, plus a color-contrast check as the written practice requires.
• Acquisition vs analysis. In tubing work, the person pushing probes and the person calling indications are often different. Many owners require analysts to be Level II or III with tube-specific practicals. Nuclear steam generator work adds EPRI's Qualified Data Analyst performance demonstration.
• Procedures. When ASME applies, a Level III approves the ET procedure. The 2025 edition of Section V adopts the 2024 edition of SNT-TC-1A, so written practices for Code work should be updated to match.

Atlantis has trained 1,000+ technicians under SNT-TC-1A, at Level I, II and III, self-paced online or onsite at the employer. There is no physical training centre to travel to. The ET Level 2 course covers the method and technique content. Teams in the Gulf states usually start from NDT training in Texas. Our published US training fees list the course costs.

Request a training plan for your ET Level II candidates, with hours mapped to your written practice.

How to buy eddy current testing services

Most poor eddy current results trace back to the request for quotation, not the field crew. A quote is only as good as the information behind it. Run the purchase in this order:

1. State the question. Crack detection, wall-loss trending, screening ahead of replacement, or a code-required examination. Each needs a different technique and a different report.
2. Send the asset data. Tube material and specification, outside diameter and wall gauge, tube count and length, U-bends, baffle spacing, prior inspection reports and the damage mechanism you suspect. For surface or weld work, send the material, coating type and thickness, and access.
3. Agree the technique and procedure up front. Ask for the procedure number, revision and Level III approval before mobilization, not after the report.
4. Settle the reference standards. Reference tubes must match the bundle's material, diameter and wall. Decide who supplies them and how long they take to make. Lead time on a custom tube can drive the schedule.
5. Define reporting thresholds. Agree what percent through-wall gets reported, how indications are classified and who makes plugging or repair decisions.
6. Confirm cleaning and access. Hydroblasting quality decides data quality. Agree an acceptance check for cleanliness before probes go in.
7. Specify deliverables. Ask for raw data files as well as the PDF, so the next inspection can be compared signal for signal.

Questions worth asking any provider: Who analyses the data, and at what level? Is there a secondary analysis? How do you handle tubes the probe cannot pass? What will you do if the tube material turns out to be magnetic? Atlantis performs ET with ASNT-certified technicians under ASNT Level III oversight. If a bundle needs a technique outside our scope, we say so in the quote rather than force a technique that does not fit. Field teams work from Houston across Texas, Louisiana and California. Pricing is quote on request, and we return a quote once we have a complete scope.

Request an inspection quote for tube, surface or weld eddy current work.

Common eddy current mistakes we see in the field

• Reference tube from the wrong alloy or gauge. Calibration on a tube that does not match makes every depth call wrong by an unknown amount.
• Screening reported as measurement. PEC and RFT percentages written into a remaining-life calculation without UT or IRIS confirmation.
• No baseline. A new bundle that is never inspected leaves the first in-service result with nothing to compare against.
• PDF-only reports. Without the data files, the next crew cannot re-analyse old signals against new ones.
• Surface-qualified technicians on tubing. Surface ET certification does not make someone a tube data analyst.
• Outdated code references. Procedures that still cite article numbers or SNT-TC-1A editions the current contract no longer uses.

Every one of these is cheaper to fix in the scope than in the report. If you are not sure which technique your asset needs, start with the eddy current testing service overview, then request an inspection quote with your asset data attached.

Frequently asked questions

How much does eddy current testing cost?

Cost depends on the technique, the tube count or surface area, cleaning and access, the analysis level (single or independent secondary), reference standard fabrication, mobilization and standby time. Atlantis prices ET as quote on request and replies within one business day.

How do I choose between absolute and differential eddy current probes?

Choose by the flaw you expect. Absolute coils respond to gradual changes such as general wall thinning, wear and conductivity shifts. Differential coils compare two adjacent zones, so they ignore slow changes and respond sharply to small, abrupt flaws such as pits and cracks. Tube inspection usually records both channels at once and reads each for what it does best.

What makes up an eddy current testing system?

An eddy current system has four parts: the instrument, the probes, the reference standards and the analysis software. For tubing, the instrument is usually multi-frequency, so a mix channel can suppress support-plate and tube-sheet signals that would otherwise hide flaws. Portable single-channel instruments suit surface and bolt-hole work, and array instruments add encoded C-scan imaging.

What is high frequency eddy current testing used for?

High frequencies, roughly 100 kHz and up, keep the currents close to the surface. That makes them the right choice for fine surface cracks, thin-wall parts and fastener holes. They are the wrong choice for anything below the surface, because penetration drops as frequency rises.

Is electromagnetic testing the same as eddy current testing?

Not quite. Electromagnetic testing (ET) is the method name in SNT-TC-1A, and it includes three techniques: eddy current, alternating current field measurement and remote field testing. Magnetic flux leakage is a separate method. Certifications and procedures should name the technique, not just "ET."

Do I need ASME Section V Article 8 for heat exchanger tube inspection?

Only when a construction code or your own specification invokes it. Much in-service bundle work runs to an owner specification that borrows from Article 8 and the ASTM practices. If your contract calls Section V, the procedure, calibration and personnel requirements of the cited article apply in full.

Who certifies eddy current technicians in the US?

Under SNT-TC-1A and CP-189 the employer certifies its own technicians against its written practice. ASNT certifies Level III individuals and runs the ASNT 9712 central certification program. Aerospace employers do the same, aligned to customer flow-down and Nadcap AC7114. A certificate from a training provider documents training. It is not a certification by itself.

Is eddy current testing better than ultrasonic IRIS for tubes?

Neither is better in general. ET and RFT screen a whole bundle quickly. IRIS measures wall thickness directly but is slower and needs very clean tubes. The usual approach on carbon steel is to screen every tube with RFT, then size a sample with IRIS. On nonferrous tubes, bobbin ET alone usually does the job.