Measuring Dry Film Thicknesses on Aluminum

Q: What factors influence the accuracy of dry film thickness readings on aluminum parts?

A: The most common, non-destructive test method for measuring dry film thickness (DFT) on aluminum is the eddy current technique under ISO 2178 / ASTM D7091. This test method uses a high-frequency AC field to induce eddy currents in an aluminum base. The density of the induced eddy currents corresponds to the distance from the measurement probe’s tip to the base material. Using the characteristic probe output function (the functional correlation between the measurement signal and the coating thickness), the measurement signal is converted by the instrument into the coating thickness value.

However, when using eddy current test methods, there are influencing factors that directly affect a measurement value. Two very relevant factors are the electrical conductivity of the base material, and the part curvature or shape.

Electrical Conductivity of the Base Aluminum

Because the gage uses an AC field to generate eddy currents in the aluminum base material, the electrical conductivity of the aluminum will affect the field strength. Electrical conductivity in base materials can vary greatly, depending on the alloy type and different treatment processes. The conductivity can also be affected by work hardening through bending and forming. If a given gage is adjusted (calibrated) on aluminum alloy “A,” but the measurement part is aluminum alloy “B”, the coatings inspector may see an incorrect value.

To avoid this, check with your eddy current DFT gage provider and confirm the gage you’re using offers “conductivity compensation.” Gages with conductivity compensation correct for changes in the electrical conductivity of the base material, providing the inspector with correct measurement values, independent of the alloy type.

Part Curvature

Because the probe is generating eddy currents within the sample’s base, a curve in a part “bends” or changes and affects the density of the eddy currents. If a gage is calibrated on a flat sample, the user may see large thickness changes when measuring a curved part. This shape influence becomes larger as the radius of the part gets smaller.

To avoid this problem, check with your gage supplier to confirm that your eddy current test gages offers “curvature compensation.” Probes with curvature compensation detect and correct for changes in part radius.

Thickness in mils
Part Shape	Gage A	Gage B	Gage C (curve compensated)
Flat	3.02	3.02	3.01
0.75" OD	7.8	8.01	2.92
0.40" OD	12.1	12.52	3.06
0.25" OD	16.0	16.68	3.12

(Gages were calibrated on a flat surface; tests were conducted using the same traceable, 3.02-mil standard placed on different aluminum rods made of the same alloy type.)

With standard handheld eddy current gages available on the market today, changes in the aluminum substrate and part curvature can both cause significant measurement errors. To prevent these errors and ensure accurate readings are obtained, use a gage that has automatic compensation for both substrate conductivity changes and part curvature built in.

Jay Kunick is with Fischer Technology. Visit fischer-technology.com.