Eddy current testing for tubes, cracks and coatings

A coil carrying alternating current creates a changing magnetic field. Bring it close to a conductive material and it induces circulating currents, the eddy currents, in the surface layer. A crack, a change in wall thickness, a different alloy or a change in coating thickness disturbs those currents, which changes the coil's impedance. The instrument shows that change, usually on an impedance plane display, and a trained technician separates real indications from lift off, edges and geometry. Frequency controls depth: high frequencies stay in the first fraction of a millimeter and suit fine surface cracks, low frequencies reach further but with less resolution.

• Pencil and surface probes for cracks at edges, radii and around fasteners
• Rotating bolt hole probes for cracks inside fastener holes
• Bobbin probes pulled through tubes for wall loss, pitting and cracking
• Array probes (ECA) with many coils that scan a wide strip in one pass and produce a C scan image

Tube inspection is the largest industrial use. Bobbin probe ECT of non ferromagnetic tubes such as copper alloys, stainless steels and titanium in condensers, chillers and feedwater heaters finds pitting, wall loss, fretting at support plates and cracking at speed, often hundreds of tubes per shift. Carbon steel tubes are ferromagnetic and usually call for remote field testing, near field testing, magnetic flux leakage or ultrasonic IRIS instead, so tell the lab the tube material first. Outside tubing, aerospace relies on eddy current for fastener holes, wheels and skins around rivets; power generation uses it on turbine components and generator parts; and manufacturing uses it inline to sort bars, tubes and fasteners.

• Heat exchangers, condensers, chillers, boilers with non ferromagnetic tubes
• Aircraft structure, wheels, landing gear and engine parts in maintenance
• Welds in carbon steel inspected through paint with array probes
• Production lines for bar, wire, tube and machined parts

Eddy current gauges measure non conductive coatings such as paint, powder coat and anodize on non ferrous metals, and are used next to magnetic induction gauges for coatings on steel. Conductivity meters report percent IACS and are a fast, non destructive check that an aluminum part received the right temper, a standard step after heat treatment in aerospace. These measurements are quick and cheap per point, and they need calibrated reference blocks of the same alloy family.

• ASME Section V Article 8 for eddy current examination of tubular products in pressure equipment
• ASTM practices for tube inspection, conductivity measurement (ASTM E1004) and coating thickness, chosen by the job
• Aircraft manufacturer NDT manuals for aerospace inspections
• Technician qualification to SNT-TC-1A, CP-189 or NAS 410, with Level II analysts for tube data and Level III review on critical programs
• NADCAP NDT accreditation when an aerospace customer requires it

Ask which reference standard the lab will use. ECT is a comparative method: calibration on a standard with machined notches or holes of the same material and geometry is what makes the readings mean something.

1. You share tube material, outside diameter, wall thickness, bundle drawing and tube count.
2. The provider builds reference standards and chooses probes and frequencies.
3. Tubes are cleaned so the probe passes freely.
4. Probes are pushed or pulled through each tube while data is recorded.
5. Analysts review the data, often with a second independent analysis on critical plants.
6. You receive a tube map with indications by depth category and recommendations for plugging.

Surface crack work on parts is simpler: calibrate, scan the area, evaluate indications and report.

Cost depends on tube count and access, material, required analysis level, reference standards, outage timing, travel and night or weekend shifts.

Reference standards for unusual tube sizes can add several hundred dollars. More context in NDT inspection cost.