Eddy current testing is an electromagnetic testing technique usually used to inspect non-ferromagnetic materials.

How it Works

In its most basic form — the single-element eddy current testing probe — a copper wire is excited with an alternating electrical current. This wire produces a magnetic field around itself in the direction ascertained by the right-hand rule. This magnetic field oscillates at the same frequency as the current running through the coil. When the coil approaches a conductive material, currents opposed to the ones in the coil are induced in the material — eddy currents.

A defect in the conductive material disturbs the path of eddy currents, creating a local magnetic field that changes the balance of the system. This can be detected by measuring the changes in impedance in the coil, which is a telltale sign of the presence of defects.

Benefits

Several benefits are derived from eddy current testing:

  • It is suited to volumetric flaws such as corrosion, wear, and large porosities, as well as cracking.
  • It can detect surface-breaking, near-surface, and far-surface defects.
  • ECT probes don’t need to be in contact with the part under inspection.
  • ECT can be used in applications other than flaw detection.
  • Parts under test require only minimal preparation.

Tubing Applications

Detection Capabilities According to Defect Type in Tubing

Defect/Tech ECT ECA IRIS RFT NFT NFA MFL PSEC
ID pitting

Excellent

Excellent

Excellent

Acceptable, but limited

Acceptable, but limited

Excellent

Excellent

Excellent

OD pitting

Excellent

Excellent

Excellent

Acceptable, but limited

Not suitable 

Not suitable

Acceptable, but limited 

Acceptable, but limited 

Axial cracking

Acceptable, but limited

Excellent

Not suitable

Acceptable, but limited

Acceptable, but limited

Acceptable, but limited

Not suitable

Acceptable, but limited

Circumferential cracking

Acceptable, but limited

Excellent

Not suitable

Not suitable

Not suitable

Acceptable, but limited

Acceptable, but limited

Acceptable, but limited

ID corrosion

Excellent

Excellent

Excellent

Excellent

Excellent

Excellent

Excellent

Excellent

OD corrosion

Excellent

Excellent

Excellent

Excellent

Not suitable

Not suitable

Acceptable, but limited

Acceptable, but limited

At tubesheet

Acceptable, but limited

Excellent

Excellent

Acceptable, but limited

Not suitable

Not suitable

Acceptable, but limited

Acceptable, but limited

Excellent

Acceptable, but limited

Not suitable

Sizing Capabilities According to Defect Type in Tubing

Defect/Tech ECT ECA IRIS RFT NFT NFA MFL PSEC
ID pitting

Good

Excellent

Excellent

Good

Not suitable

Excellent

Not suitable

Not suitable

OD pitting

Excellent

Excellent

Excellent

Good

Not suitable 

Not suitable

Not suitable 

Not suitable 

Axial cracking

Good

Excellent

Not suitable

Not suitable

Not suitable

Good

Not suitable

Not suitable

Circumferential cracking

Not suitable

Excellent

Not suitable

Not suitable

Not suitable

Good

Not suitable

Not suitable

ID corrosion

Good

Excellent

Excellent

Excellent

Not suitable

Excellent

Not suitable

Not suitable

OD corrosion

Excellent

Excellent

Excellent

Excellent

Not suitable

Not suitable

Not suitable

Not suitable

At tubesheet

Good

Good

Excellent

Not suitable

Not suitable

Not suitable

Not suitable

Not suitable

Excellent

Good

Not suitable

Suitability According to Tubing Material

Material/Tech ECT ECA IRIS RFT NFT NFA MFL PSEC
Non-ferromagnetic Tube

Yes

Yes

Yes

No

No

No

No

No

Integral-finned tube

Yes

Yes

Yes

No

No

No

No

No

Low ferromagnetic Tube

No

No

Yes

Yes

Yes

Yes

Yes

Yes

Integral-finned tube

No

No

Yes

Yes

Yes

Yes

Yes

Yes

Ferromagnetic Tube

No

No

Yes

Yes

Yes

Yes

Yes

Yes

Integral-finned tube

No

No

Limited

Yes

Yes

Yes

Yes

Yes

Aluminum-finned tube

No

No

Yes

No

Yes

Yes

Yes

Yes

Surface Applications

When it comes to surface applications, the performance of any given inspection technique depends greatly on the specific conditions — mostly the types of materials and defects, but also surface conditions, cleanliness, etc. However, in most situations, the following are true:

  Liquid penetrant (PT) Magnetic particles (MT) ECT ECA
Effective on coatings/paints No Yes Yes Yes
Computerized record keeping No No Partial Yes
3D/Advanced imaging No No No Yes
User dependence High High High Low
Speed Very low Very low Low Very high
Cleaning Yes Yes Application-specific Application-specific
Post-inspection analysis No No No Yes
Chemicals/Consumables Yes Yes No No