Eddy current thickness gauge detects coatings on non-ferrous metals.

Detection principle

Eddy current thickness gauges work on the principle of electromagnetic induction. When the coil in the instrument probe is connected to a high-frequency alternating current, an alternating magnetic field is generated. If the probe is close to a conductive matrix (such as aluminum, copper, etc.), the magnetic field will induce eddy currents on the surface of the matrix. This eddy current creates a reverse magnetic field that is opposite to the original magnetic field, affecting the impedance of the probe coil. When the surface of the substrate is covered with a non-conductive coating (e.g., paint, anodized film, plastic), the coating thickness changes the distance between the probe and the conductive substrate, which in turn changes the impedance of the coil. The instrument accurately measures this impedance change, calibrated and calculated by an internal algorithm to obtain the thickness value of the coating.

The measurement process can be simplified to the analysis of changes in coil impedance. The complex impedance Z of the probe coil can be expressed as:

Z = R + jωL

where R is the equivalent resistance of the coil, L is the equivalent inductance, and ω is the angular frequency of the alternating current. When there is a conductive substrate and coating, there is a specific function relationship between the impedance change ΔZ and the coating thickness d, which is determined by standard sheet calibration.

Suitable coatings

This method is mainly suitable for measuring the thickness of non-conductive coatings on non-ferromagnetic metal substrates. Common applicable combinations are shown in the table below. It should be noted that the conductivity of the matrix material, probe frequency, and coating characteristics can affect the measurement accuracy.

For coatings on ferromagnetic metal substrates, magnetic thickness measurement method should be used. If the coating itself is conductive, it is necessary to evaluate the interference of its conductivity to the measurement, and the standard eddy current method may not be applicable.

Instrument calibration

To ensure the reliability of the measurement results, the instrument must be calibrated according to the relevant standards. Calibration is usually performed on a matrix similar to the test piece using a standard sheet (foil or coated substrate) of known thickness. The main technical standards at home and abroad provide the basis for the operating procedures.

The calibration steps generally include: zero point calibration on a clean, flat, uncoated substrate; Multi-point calibration is then performed on a co-material matrix with or plated standards of known thickness to establish a thickness-to-readout curve. Before routine measurements, the zero point should be verified on the uncoated area of the workpiece being measured or on the provided matrix sample.

Operational points

In actual testing, a variety of factors may affect the accuracy of eddy current thickness gauges, and operators need to pay attention to them and take corresponding measures.

Matrix Impact:The conductivity of the matrix metal is a key factor. Different alloy compositions and heat treatment states can lead to differences in conductivity, which can lead to errors. Therefore, the matrix used for calibration should be consistent with the material of the workpiece being tested. For unknown materials, calibration should be performed on the same workpiece without coating or on substrates taken from the same batch.

Geometry:The curvature, edge effect and size of the test piece affect the magnetic field distribution. The probe should be perpendicular to the measuring surface and should be specifically calibrated or evaluated for suitability when measuring near a small radius of curvature or edge.

Surface Condition:The surface roughness of the substrate and coating affects the measurement. Rough surfaces can cause increased fluctuations in readings, usually averaged over multiple measurements. Dust, oil and other adhesions must be removed.

Key points of operation:When measuring, the probe should be in contact with the coating surface smoothly, vertically and without pressure. Each measurement point should be taken multiple readings (e.g., 3-5 times) and averaged for better representativeness. For large workpieces, sufficient measurement points should be planned to assess thickness uniformity.

Applications:

Eddy current thickness measurement technology is widely used in industrial fields where quality control and process monitoring of non-ferrous metal surface coatings are required.

Main application areas:Including but not limited to aerospace (aluminum alloy component coating), automobile manufacturing (wheel hub, radiator coating), architectural profiles (aluminum profile anodized film, spraying), electronic and electrical appliances (copper bar insulation, shell coating), and general hardware products industry.

Technical limitations:This method is not suitable for ferromagnetic material substrates. For very thin coatings (usually less than a few microns) or extremely thick coatings, it may be outside the linear measurement range of the instrument. The dielectric constant of the coating, the thin substrate thickness (matrix effect), and the strong electromagnetic interference environment can also interfere with the measurement. In practical applications, clarifying measurement needs and evaluating these limitations are prerequisites for obtaining valid data.

References

ISO 2360, Non-conductive coatings on non-magnetic electrically conductive basis metals — Measurement of coating thickness — Amplitude-sensitive eddy-current method.

GB/T 4957, Non-conductive overlay on non-magnetic matrix metals - Measurement of overlay thickness - Eddy current method.

ASTM B244, Standard Test Method for Measurement of Thickness of Anodic Coatings on Aluminum and of Other Nonconductive Coatings on Nonmagnetic Basis Metals with Eddy-Current Instruments.