Step by step
1. Introduction
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.
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.
3. Measurement on samples with the GL-PRO-6-FAZ meter
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;
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".
- sample "A": indication on the "Fe" meter - no zinc detected;
Fig. 4. Display on element "A".
- sample "B": indication by "FeZn" meter - zinc has been detected;
Fig. 5. Display on element "B".
- sample "C": indication by "FeZn" meter - zinc has been detected;
Fig. 6. Indication on the "C" element.
- sample "D": indication on the "Fe" meter - no zinc detected;
Fig. 7. Display on the "D" element.
- sample "E": indication on the "Fe" meter - no zinc detected;
Fig. 8. Indication on "E" element.
4. Laboratory tests of samples using a light microscope, scanning electron microscope with x-ray microanalysis of the chemical composition EDS.
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
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.
4.1. Examination of sample "1".
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
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.
Examination of the sheet sample from the "A" brand.
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.
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
4.3. Examination of a sheet sample from the "B" brand.Photo 13 shows the results of the "B" sheet sample. You can see that the zinc is a continuous layer. Zinc probably obtained by immersion method. This type of zinc has been detected by the GL-PRO-6-FAZ meter.
Fig. 13. Test result of the "B" metal sheet sample
4.4. Testing a sample of sheet metal from the "C" brand. Photo no. 14 shows the results of the "C" sheet sample. You can see that the zinc is a continuous layer. Zinc probably obtained by dip (?). This type of zinc has been detected by the GL-PRO-6-FAZ varnish meter .
Fig. 14. Result of testing the 'C' brand sheet sample.
4.5. Examination of the sheet sample from the "D" brand.Photograph No. 15 shows the results of testing the sheet sample "D". It is clear that the zinc is here in the form of "lumps", lack of continuity. There are many crevices visible. Zinc probably obtained by spraying (?) Or similar. This type of zinc will not be detected by the GL-PRO-6-FAZ meter.
Fig. 15. Examination result of the sample sheet "D".
Examination of the sheet sample from the "E" brand.Photograph No. 16 shows the results of testing the sheet sample "E". You can see that the zinc is in the form of "lumps", lack of continuity. There are many crevices visible. Zinc probably obtained by spraying (?) Or similar. ,u>his type of zinc will not be detected by the GL-PRO-6-FAZ meter.
Fig. 16. Result of testing the sample of the "E" brand sheet.
5. Conclusions from the tests performed
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.
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".
5.4. Samples "1", "B", "C" have a better quality zinc layer, the microscopic images show that there is continuity. Such zinc is obtained, among others by immersion, phases are formed between the steel and the zinc layer. There are no open gaps and pores, moisture and air do not get into the steel even before applying the primer. Such zinc is more resistant to knocks and chips.
Fig. 20. Sample "1".
Fig. 21. Sample "B".
Fig. 22. Sample "C".
5.5. When analyzing the obtained test results, it may be surprising that such different zinc coatings are used, especially taking into account the model years of the cars from which the samples were obtained (assuming that the samples come from the original elements) - from 2012 to 2018, except for the "D" sample , which comes from a car produced between 1991 and 2002. It should be noted that the car from which the "D" sample came from was always considered a non-galvanized car and even the car manufacturer did not provide information about zinc. Probably the steel from sample "D" was protected in the smelter with a spray (or similar) layer of zinc, so that it does not get corroded too quickly during eg transport or aging. Were samples "A" and "E" also protected in this way?
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.
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.