GB/T 4957-2025 Non-conductive coatings on non-magnetic electrically conductive base metals—Measurement of coating thickness—Amplitude-sensitive eddy-current method

Scope

This document describes a method for non-destructive measurement of the thickness of non-conductive coatings on non-magnetic conductive substrate metals using amplitude-sensitive eddy current instruments.
In this document, the term "coating" applies to materials such as paint and varnish, electroplated layers, enamel coatings, plastic coatings, cladding layers, and powder coatings. This document is suitable for measuring the thickness of most anodized films, but it is not applicable to some very thin conversion film layers (see Chapter 6). It can also be used to measure the thickness of non-magnetic metal coatings on non-conductive substrates. However, the phase-sensitive eddy current method specified in GB/T 31554 is particularly suitable for this application and can provide higher-precision thickness measurement results (see Appendix A).
This document is not suitable for measuring non-magnetic metal coatings on conductive substrate metals. The phase-sensitive eddy current method specified in GB/T 31554 is particularly suitable for this application. However, in the special case of very thin coatings with very low conductivity, the amplitude-sensitive eddy current method can also be used for this application (see Appendix A).
Although this method can be used to measure the thickness of coatings on magnetic substrate metals, this application is not recommended. In this case, use the magnetic thickness measurement method specified in GB/T 4956. Only when the coating thickness exceeds about 1 mm can the amplitude-sensitive eddy current method also be used for this application (see Appendix A).

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