Thomas R. Hay (PhD, PE) and Suhaib Zafar
Eddy Current Testing (ECT) utilizes the phenomenon of electromagnetic induction — a magnet inducing electric currents in a conducting material (typically metals) that is moving relative to the magnet, or an alternating current (AC) causing a time-varying magnetic field . The induced current is known as eddy current, and this in turn affects the current in the magnet. Typically, the magnet used is a coil, with alternating current (AC) flowing through it.
The eddy current will then develop its own (secondary) magnetic field, which will oppose the (primary) magnetic field of the coil. This affects the current and the voltage in the coil, by varying its electrical impedance. This variation can be used to detect changes in metal thickness or defects since these aberrations impact the amplitude and pattern of the eddy current. Figure 1 shows the working principle of ECT .
Figure 1 Schematic showing working principle of ECT 
The strength of eddy currents is dependent on the electrical conductivity of the test piece, the AC frequency, dimensions of the coil and the test piece, and the distance between the coil and the test piece. Eddy current density is highest near the surface of the test specimen, which is the region of the highest resolution as a result. This translates into a huge advantage of using ECT for detecting surface flaws on a painted or coated surface, with minimum preparation required and quick analysis. On the downside, however, ECT can only be used on conductive materials, and is very susceptible to variation in magnetic permeability.
Figure 1: Example multi-frequency eddy-current tube inspection data showing likely wear scars.
Eddy Current Testing (ECT) of Tubes
Eddy Current Testing (ECT) is one of a few methods that may be used to inspect the individual tubes in a heat exchanger. ECT is applied exclusively to conductive non-ferrous heat exchanger tubes for detection of cracks, generalized corrosion and pitting corrosion. The four different non-destructive testing techniques used for heat exchanger tube inspections are internal rotary inspection system (IRIS), eddy current testing (ECT), magnetic flux leakage (MFL), and remote field eddy-current testing (RFET). IRIS may be used on ferrous and non-ferrous tubes and detects corrosion, erosion, pitting and wear scars. Due to their orientation relative to the ultrasonic sound wave, tube cracks may be undetected by this method. The travel speed is in the 2 in/s range.
In contrast, eddy current testing (ECT) of tubes is a very fast technique with travel speeds up to 36 in/s. ECT is applicable to stainless steel, titanium, Inconel, and copper nickel alloys. Like IRIS, ECT can detect corrosion, erosion, and pitting. An added advantage is that ECT will detect cracks in the axial and circumferential directions provided the correct ET probe is used.
For thicker steel tube and smaller piping found in boilers, RFET may be applied with reliable results. Comparable sensitivity is achiever of ID and OD flaws. Tubes up to 0.25” make be tested with RFET. Travel speed is up to 12 in/s.
MFL inspection is limited to ferromagnetic and ferrous stainless steel tubes but it inspects at a high travel speed of 30 in/s. This technique can detect pitting, circumferential cracks, and generalized wall loss. Defect sizing is limited since the signal quality is dependent on a sensor constant travel speed which is difficult to maintain under certain inspection circumstances.
Figure 2: Example eddy current tube inspection field inspection process.
Alternating Current Field Measurement (ACFM)
The Alternating Current Field Measurement (ACFM) is an electromagnetic NDT method that locally induces a uniform current into the test component and measures the magnetic flux density over the component surface. Surface defects perturb the induced current, and ACFM measures variation in the flux density: one component is for position of the defect, and the other is for its sizing.
ACFM has significant benefits relative to conventional eddy current testing (ECT), including better scan pattern, superior response to variations in thickness due to coatings and most importantly the ability to distinguish and characterize surface breaking defect up to a depth of 25 mm. However, ACFM is generally less sensitive to shallow defects when compared with conventional eddy current testing (ECT). ACFM has applications in the oil and gas, mining and transportation industries.
Figure 3: Example Alternating Current Field Measurement (ACFM) from a fatigue crack in a welded tube.
Pulsed Eddy Current Testing (PECT)
The Pulsed Eddy Current Testing (PECT) method measures the differences between the conductivity and permeability of different metals, and the quantity of those metals in comparative readings. The usual means of conducting a test is to inspect the insulated component, identify a consistent area of thicker metal, and place the reference point (RP) in the middle of that area. The customer then makes this area accessible for by removing the insulation so that a corresponding ultrasonic thickness testing (UTT) measurement and used as a reference point for normalization. The software then normalizes all data points, and compares them to the RP thickness, and converts them all from a percentage to an average wall thickness in inches. It is critical that the UTT be done at exactly the RP.
Figure 4: Example pulsed eddy current testing A-scan data. PCET A-scan data is amplitude versus time and show the material response to the pulsed eddy current inserted.
An eddy current sensor is placed on top of the thermal insulation. By means of a low frequency (pulsed) magnetic field, eddy currents are generated in the material. By measuring the duration of the eddy currents, a thickness calculation is made. This thickness calculation gives the average thickness of the material, within the enclosed magnetic field (footprint). The diameter of the footprint varies between 45 mm (1.75 in.) and 200 mm (4 in.) depending on wall thickness and thickness of the insulation.
high temperature objects
heavily coated objects
rough (corroded) surfaces
insulated and coated objects
object protected by concrete/fireproofing
object covered with marine growth or fouling