The magnetic method works when the coating and the substrate material have different magnetizability. It’s primarily used to measure magnetic coatings like nickel on non-magnetic metals or plastics, but it can also be used to test non-magnetic coatings on steel or iron. Especially for thicker electroplated coatings such as chrome and zinc, the magnetic method is better suited than magnetic induction.
Magnetic measurements are based on an effect that is named for Edwin Hall. This effect occurs when a current-carrying conductor is positioned within a constant magnetic field.
As electrons move through the conductor, they also move through the static magnetic field. Thus, they are subject to the Lorentz force, which pushes the electrons to the edge of the conductor in a motion that’s perpendicular to the magnetic field. A charge separation occurs. As with a capacitor, this produces a voltage – the Hall voltage.
Magnetic materials, such as a nickel coating, strengthen the static magnetic field, which also increases the Hall voltage. This voltage is measured and converted into a layer thickness value in the measuring device using one of the probe’s characteristic curves: the functional correspondence between the measurement signal and the coating thickness.
All electro-magnetic test methods are comparative. This means that the measured signal is compared with a characteristic curve that’s stored in the device. In order for the result to be correct, the characteristic curve must be adapted to the current conditions. This is achieved through calibration.
Factors that greatly affect the results of a measurement include: the magnetic permeability of the base material, the shape of the sample and the roughness of the surface. Furthermore, the operator can also influence the result.
The magnetic permeability indicates how well a material adapts to a magnetic field. Substances such as iron or nickel have high permeability. They become magnetized themselves and strengthen the magnetic field.
Since the permeability is different for the metals and their alloys, the measuring device has to be recalibrated when the materials change.
In practice, most measurement errors occur due to the shape of the sample. With curved surfaces, the proportion of the magnetic field that passes through the air is different. For example, if a measuring device was calibrated on a flat sheet, measuring on a concave surface would lead to a lower result, whereas measuring on a convex one would lead to a higher result. The errors that occur in this way can be many times the actual value!
A similar effect can occur if the sample is small or very thin. Also in this case, the magnetic field extends beyond the sample and into the air, which systematically distorts the measurement results. To avoid these errors, you should always calibrate on an uncoated part that corresponds to the end product.
For rough surfaces, the result can be distorted depending on whether the probe pole is placed in a ‘valley’ or on a ‘peak’ of the roughness profile. With such measurements, the results vary widely and it is advisable to repeat the measurements several times in order to accumulate a stable mean. In general, coating thickness measurements on rough surfaces only make sense if the coating is at least twice as thick as the roughness peaks are high.
Last but not least, the way the measuring device is operated also plays a major role. Always make sure that the probe is set vertically on the surface and without pressure. For better accuracy, a stand can be used to automatically lower the probe onto the sample.
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