Step by step
The GL-PRO-2-FAZ and GL-PRO-6-FAZ paint thickness gauges have the function of automatic detection of the zinc coating on a steel sheet. Steel cannot be stainless. This function detects a diamagnetic or paramagnetic layer applied to a steel surface, therefore the indication "Fe + Zn" can also occur when the coating on the steel will be made of e.g. aluminum or copper. Because galvanized steel is used in the automotive industry, the indication "Fe + Zn" was adopted. The meter detects zinc when the varnish coat including the zinc layer is less than about 350um. The zinc coating must be homogeneous and continuous.
During measurements on various car bodies, it happened that on some cars, zinc is not detected despite the manufacturer's declaration that the sheet is galvanized.
This problem puzzled us because from a technical point of view continuous zinc should be detected. We decided to do our own research to determine the type of zinc that the meter can detect.
The research that was carried out was:
- macroscopic examination;
- light microscopy studies;
- scanning electron microscopy;
- microanalysis of the chemical composition of EDS.
Samples were tested in cross-section
We compared the above tests with the GL-PRO-6-FAZ meter measurements, placing an emphasis on checking when the meter detects zinc and in which case it does not detect zinc. The test results are surprising and, by the way, shed light on the types of zinc coatings on automotive metal sheets
In the following sections, we present some of the results of the tests, comparisons of sheet metal parts of various automotive brands and a piece of ordinary galvanized steel sheet "from the store" were compared.
2. Samples provided for testing
Sample "1" - galvanized steel sheet bought in a shop.
Sample "A" - a cut from the mudguard of the Asian brand "A" - year 2012-2016.
Sample "B" - a cut from the mudguard of the European brand "B" - year 2012-2016.
Sample "C" - a cut from the mudguard of the European brand "C" - year 2015-2018.
Sample "D" - a section from the mudguard of the European brand "D" - year 1991-2002.
Sample "E" - a cut from the mudguard of the Asian brand "E" - year 2017-2018.
Samples A to E were purchased on the secondary market. These are elements of car bodies purchased online and come from used cars, we do not know what particular year the sample is from, we also do not know whether the sample is from the original element or maybe already replaced in the car - for our purposes they are sufficient.

Phot.1. Samples provided for testing.

Fot.2. Determination of samples.
The measurement was carried out on body parts, from which samples A, B, C, D, E were then cut out - designation of samples by brands. The results of zinc detection by the GL-PRO-6-FAZ meter on a steel plate are as follows:
- sample "1": indication by the "FeZn" meter - detection of a zinc layer;

Fig. 3. Indication on sample "1".

Fig. 4. Display on element "A".

Fig. 5. Display on element "B".

Fig. 6. Indication on the "C" element.

Fig. 7. Display on the "D" element.

Fig. 8. Indication on "E" element.
Analyzes were carried out on cross-sections of materials. The focus was on determining the impact of zinc layer morphology on the ability to identify it when measuring with a Prodig Tech® GL-PRO-6-FAZ varnish thickness meter. przygotowane_probki_1-300x225

Fig. 9. Prepared samples, determination of samples by brands.

Fig. 10. Prepared samples.
Photo 11 shows the result of testing sample "1". You can see that the zinc layer is a continuous coating. This zinc layer is detected by the GL-PRO-6-FAZ met

Fig. 11. Result of sample "1" testing.
Photograph 12 shows the results of testing the sheet sample "A". It is clear that the zinc here is in the form of "lumps", it is not a continuity. Many gaps are visible even between zinc and steel. Some of the steel is not covered with zinc at all, which can be seen at microscopic magnification. Zinc probably obtained by spraying (?) Or similar. This type of zinc will not be detected by the GL-PRO-6-FA meter.

Fig. 12. Test result of the "A" sheet metal sample

Fig. 13. Test result of the "B" metal sheet sample

Fig. 14. Result of testing the 'C' brand sheet sample.

Fig. 15. Examination result of the sample sheet "D".

Fig. 16. Result of testing the sample of the "E" brand sheet.
5.1. The research shows that the GL-PRO-6-FAZ paint thickness gauge detects a zinc layer on a steel sheet when it is (this zinc layer) continuous. Only then are eddy currents able to "close". The discontinuous, lumpy zinc layer with numerous gaps will not be detected by the measuring device, which may be its additional advantage.
5.2. Studies have revealed the differences in zinc coatings on the tested sheets. There is a clear division into two types of galvanizing methods:
- continuous zinc coating is probably obtained by dipping method (?) Or similar, samples "B", "C";
- discontinuous, lumpy zinc coating with numerous gaps is probably obtained by spraying (?) Or similar method, samples "A", "D", "E". 5.3. In samples "A", "D", "E", the zinc coating is probably of lower quality, less resistant to chipping. Zinc lumps do not form a coherent layer, there are numerous gaps through which moisture and air can enter before applying the primer coat. Presumably, steel corrosion may develop in such places and develop without any early signs.

Fig. 17. Sample "A".

Fig. 18. Sample "D".

Fig. 19. Sample "E".

Fig. 20. Sample "1".

Fig. 21. Sample "B".

Fig. 22. Sample "C".
5.6. Research can provide information why some cars corrode very quickly despite the vehicle's young age.
5.7. It can be assumed that from a technical point of view the meter identifies a layer of zinc made continuously (it seems to be more correct).
6. Further research
In the near future we plan to carry out additional microscopic laboratory tests with more samples. Perhaps we will carry out tests for the corrosion resistance of individual samples - "splashes" will be made on the samples with equal strength for each, and then the samples will be placed in the appropriate environment for some time.