Duplex coating measurement
Determining paint and zinc thickness in one step: this is how it's done!
Duplex coatings are often used in corrosion protection and automotive engineering because they help provide a very long service life. Duplex coatings consist of a zinc coating in combination with one or more layers of paint. The overall system of zinc and paint can protect the steel from corrosion for many years. For duplex coating measurement, the magnetic induction and phase-sensitive eddy current methods are used together in one probe. However, for zinc coatings above about 150 mm, the depth of penetration of the phase-sensitive eddy currents is no longer sufficient to reliably determine the coating thickness. Therefore, in such cases, the combination of the magnetic induction and the amplitude-sensitive eddy current method is used. The amplitude-sensitive eddy current method is also preferred for clearly pronounced intermetallic phases between zinc and steel.
This is how duplex coating measurement works.
Measuring with thin zinc layers
For measuring duplex systems with thin zinc coatings, the magnetic inductive and the phase-sensitive eddy current methods are used together.
With the magnetic inductive measuring method, all non-magnetic coatings on steel and iron can be measured. In other words, this method is used to measure the total thickness of the coating and galvanization.
The phase-sensitive eddy current method is suitable for measuring the thickness of metallic coatings on various substrates. When measuring duplex coatings, it is used to determine the thickness of the zinc layer under the coating.
The two methods are used in parallel, so that in one step the single layer thicknesses of paint and zinc are calculated and displayed from the two measured values.
Measuring thick zinc coatings or clearly pronounced intermetallic phases
For zinc coatings from about 150 µm, the depth of penetration of the phase-sensitive eddy currents is not sufficient to reliably determine the coating thickness. Even in the case of clearly pronounced intermetallic phases between zinc and steel, the amplitude-sensitive eddy current method is preferred. For this reason, the combination of the magnetic induction and amplitude-sensitive eddy current methods is used in such cases.
As in the case of thin galvanized sheets, the thickness of the entire coating system consisting of galvanizing and painting is determined by magnetic induction.
The amplitude-sensitive eddy current method is normally used to measure insulating coatings on non-magnetic metals and would not be considered for measuring on iron or steel. However, with these duplex coating measurements, the thickness of the zinc layer is so thick (typically > 70 µm) that it appears to the instrument as the substrate. Therefore, the thickness of the coating can be measured directly.
The thickness of the zinc layer can be calculated from the total thickness and the thickness of the coating.
What factors can influence the measurement?
Which standard is applied here?
Measuring duplex coating according to DIN EN ISO 2178, DIN EN ISO 2360 and DIN EN ISO 21968