Corrosion Detection and Measurement Using Eddy Current Methods

It is important to see the effect of thickness on the conductivity curve, as it shows the movement of the dot on the instrument display. Figure 1 shows the typical impedance plane, with the inductive reactance (XL) and resistance (R) as coordinates. With the probe in the air, the dot is at the top of the conductivity curve, and it moves down along the curve for increasing material conductivity with point B as the aluminum alloy to be inspected.

Starting at point B, a reduction in thickness will follow the thickness curve upwards.

In Figure 2 we have rotated the display (using the phase control) to set the liftoff horizontal and increase the instrument gain until only the area in the rectangular portion becomes the full display screen of the instrument. If point C along the thickness curve represents a 20% thickness reduction, the dot will move from B to C as the probe moves over the corrosion spot.

Figure 1

Figure 2

This is the easiest of the possible situations. When only one layer is present, it is possible to achieve high sensitivity detection and percentages as low as one percent loss can be displayed. Nevertheless, when using such high gain, other variations such as conductivity and even aluminum sheet rolling variations can be seen, although these are usually noticed as very slow changes. The frequency is not critical, but it is conventional to set it for one standard depth of penetration.

For this inspection, it is preferable to have a reference standard (of the same or similar material) with areas that have thickness reductions of 5, 10, and 20%, as this allows for a better estimation of shallow corrosion (see Figure 3).

The display in Figure 4 shows the indications produced by these reductions along the thickness curve. Corrosion estimates are made by comparing the readings. It may be necessary to re-null the instrument on the material under test after calibrating to the standard, due to the conductivity difference. The gain should not be readjusted.

Figure 3

Figure 4

This method relies on limiting the penetration to the first layer only, therefore avoiding any confusing airgap indications. The main limitation of this method is the low eddy current density at the far side of the first layer, making sensitivity to corrosion below 10% low. Fortunately, corrosion levels above this value is readily seen.

As the frequency is increased, the thickness indications move clockwise. Figure 5 shows the effect of probe movement from the two skin area (B) to the single skin area. Eventually a frequency is reached where the indication becomes horizontal and there is no vertical amplitude difference between the two (movement B to C).

Normally a chart is provided that relates thickness to the frequency of inspection. The operating point corresponds to approximately 1.5x the standard depth of penetration (you can use the Nortec® eddy current slide rule to calculate this).

The standard corrosion detection procedure in the airline NDT manuals relies on this method and uses calibration standards of 10, 20 and 30% thickness reduction (Figure 6). The expected display presentation is shown in Figure 7 with some horizontal gain reduction of about 6dB.

Due to variations in the nominal skin thickness, conductivity variations, and probe differences, it is also necessary to check the response on the structure to be tested. As mentioned earlier, this is done by placing the probe on a two-layer area and then a single-layer area. The dot should give the same vertical position for both, helping to ensure minimum interference from the second layer. The gain should not be readjusted.

Figure 5

Figure 6

Figure 7

To perform this inspection, it is necessary to have a dual frequency instrument and a probe that has a wide frequency range. The dual frequency method uses two separate frequencies to cancel the airgap signal. The second frequency, normally only twice the inspecting frequency still provides enough penetration to inspect both layers.

In addition to the conventional type of reference standard, a varying airgap is also needed for calibration purposes (sheets of paper are satisfactory), as seen in Figure 6.

The airgap signal detected with the second frequency F2 is adjusted to be as close as possible in amplitude and phase of the one obtained with the first frequency and then the two signals are subtracted from each other (F1-F2), thereby minimizing the airgap signal (Figure 8). Although the cancellation reduces the corrosion signal as well, there is enough phase and amplitude difference to make it detectable. Corrosion on the far side of the second layer can also be detected. Measurement of the corrosion severity is accomplished by comparison to the reference standard. The corrosion indications will look similar to Figure 9.

The main objection to this method is that the calibration has to be done with care and is time consuming. This method works better with thin layers than thick skins.

Figure 8

Figure 9

This method is used for measurement only.

The detection is done first using a frequency capable of penetrating the two layers under inspection. To do this, it is best to add their thicknesses together and use the standard depth of penetration (see the single thickness procedure described earlier in the document). As long as no rejectable signals are detected, the structure is considered acceptable.

When a suspect area is identified, the worst spot is carefully marked on the surface, and then inspected further by comparing to a known good spot used as the null point. This should be as close as possible to the point of interest to avoid other variations (see Figure 10). At each frequency step, the probe is nulled on the good spot (and the liftoff set to horizontal), and then placed on the marked suspect spot.

As the frequency is increased, the suspect corrosion spot will move clockwise (see Figure 11) until it reaches the same vertical amplitude as the null point (it will be in line and to the right of the liftoff line).

Figure 10

Figure 11