Eddy-current testing (also commonly seen as Eddy Current Testing and ECT) is one of many electromagnetic testing methods used in nondestructive testing (NDT) making use of electromagnetic induction to detect and characterize surface and sub-surface flaws in conductive materials.
In its most basic form, the single element ECT probe, a coil of conductive wire is excited with an alternating electrical current. This wire coil produces an alternating magnetic field around itself in the direction ascertained by the right-hand rule. The 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, which follow circular paths. These circular currents are called eddy currents.
Variations in the electrical conductivity and magnetic permeability of the test object, and the presence of defects causes a change in the flow pattern, intensity and phase of eddy currents.In turn acorresponding change in phase and amplitude of the coupled magnetic field that can be detected by measuring the impedance changes in the coil. This is a telltale sign of the presence of defects and the basis of standard ECT (using pancake coil).
The two major applications of eddy current testing maybe broadly classified as surface inspection and tubing inspections.
Tubing inspection is generally limited to non-ferromagnetic tubing and is known as conventional eddy current testing. Conventional ECT is used for inspecting steam generator tubes in nuclear plants and heat exchanger tubes in power and petrochemical industries. The technique is very sensitive to detect and size pits. Wall loss or corrosion can be detected but sizing is not accurate.
Tube inspection can be performed by techniques which employelectromagnetic phenomena (as in Eddy Current Testing) as detailed below:
Surface inspection is used extensively in the aerospace industry wherein very sensitive ECT techniques are implemented to detect tight cracks. Surface inspection can be performed both on ferromagnetic and non-ferromagnetic materials.
Eddy current testing is a noncontact method used to inspect nonferromagnetic tubing. This technique is suitable for detecting and sizing metal discontinuities such as corrosion, erosion, wear, pitting, baffle cuts, wall loss, and cracks in nonferrous materials.
Remote field testing (RFT) is being used to successfully inspect ferromagnetic tubing such as carbon steel or ferritic stainless steel. This technology offers good sensitivity when detecting and measuring volumetric defects resulting from erosion, corrosion, wear, and baffle cuts. Remote field probes are used all around the world to successfully inspect heat exchangers, feed-water heaters, and boiler tubes.
This feature provides more flexibility for mixing and defect validation. The detection and sizing of flaws at the support plate is made easier with multifrequency inspections and dual-driver operations.
The high frequency available in the market extends RFT inspection to thin materials with low permeability, such as 400-series stainless steel, and other ferromagnetic alloys.
Near field testing (NFT) technology is a rapid and inexpensive solution intended specifically for fin-fan carbon-steel tubing inspection. This new technology relies on a simple driver-pickup eddy current probe design providing very simple signal analysis.
NFT probes can be successfully used to inspect carbon steel heat exchangers, and air cooler tubes.
NFT is specifically suited to the detection of internal corrosion, erosion, or pitting on the inside of carbon steel tubing. The NFT probes measure lift-off or "fill factor," and convert it to amplitude-based signals (no phase analysis). Because the eddy current penetration is limited to the inner surface of the tube, NFT probes are not affected by the fin geometry on the outside of the tubes.
Magnetic flux leakage (MFL) is a fast inspection technique, suitable for measuring wall loss and detecting sharp defects such as pitting, grooving, and circumferential cracks. MFL is effective for aluminum-finned carbon steel tubes, because the magnetic field is almost completely unaffected by the presence of such fins.
NFT probes can be successfully used to inspect feed water heater tubes, air cooler tubes and carbon steel heat exchanger tubes