Duplex systems consist of a zinc coating in combination with one or more colored coatings. Such duplex coatings are especially popular in corrosion protection and automotive engineering because of their extra-long service life. The zinc and the paint protect each other, which leads to synergetic effects. Taken together, the system can protect steel from corroding for several decades.
In automotive engineering, the steel sheets are usually weakly hot-dip galvanized (zinc thickness about 5–20 µm), while in heavy corrosion protection the zinc layers are over 70 µm thick. One needs different methods to inspect these different types of duplex coatings.
For the measurement of duplex systems with thin zinc layers, the magnetic-inductive and the phase-sensitive eddy current measuring methods are used together.
With magnetic induction, one can measure any non-magnetic coating on steel or iron. That’s why this method is used to measure the total thickness of the paint-plus-galvanization.
The phase-sensitive eddy current method is suited for measuring the thickness of metallic layers on any substrate. In duplex measurement, this is used to determine the thickness of the zinc layer under the paint.
The two methods are used in parallel, so the individual coating thicknesses of both the paint and the zinc layers can be calculated from the two measured values and displayed – all in one step.
For zinc layers 70 µm or thicker, the phase-sensitive eddy current method doesn’t penetrate deep enough to reliably determine the layer thickness. Therefore, in such cases the magnetic-inductive method is combined with the amplitude-sensitive eddy current method.
As with weakly hot-dip galvanized sheets, the thickness of the entire layer system (galvanization plus paint) is determined through magnetic induction.
The amplitude-sensitive eddy current method is normally used to measure insulating coatings on non-magnetic metals – and wouldn’t be suitable for measurements on iron or steel. But with these duplex coatings, the zinc layer is so thick (typically >70 µm) that the device interprets it as the substrate. So the thickness of the paint is measured directly.
The thickness of the zinc layer can be calculated by subtracting the paint layer from the total thickness.
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