Destructive Testing Methods for Coating Thickness
Coating thickness greatly affects the reliability and value of the coated product. By testing coating thickness, in addition to assessing whether the coatings with tolerance indicators or repair dimensions are reasonable, it can also directly or indirectly evaluate the coating's corrosion resistance, wear resistance, porosity and other properties. Therefore, the coating thickness tester is very important in coating quality inspection and process research.
1. Coating thickness measurement method
The inspection of coating thickness is generally divided into two methods: average coating thickness and local thickness. Since the local thickness is more economic than the average thickness in practical applications. In most cases, the local thickness and local average thickness of the coating are measured. When measuring thickness, we should at least test the thickness of three points and calculate the average value as the result.
There are two types of coating thickness inspection methods: destructive testing and non-destructive testing. The drop method, liquid flow method, chemical solvent method, coulomb method, metallographic microscope method, contour method and interference microscope method are various methods of destructive detection. The nondestructive methods include magnetic method, eddy current method and X-ray fluorescence measurement. Linshang coating thickness tester use the non-destructive method of both magnetic and eddy current method. The destructive thickness measurement method is generally applicable to non-precious coatings or products processed in large quantities, while the non-destructive thickness measurement method is applicable to some valuable or precision coating products.
2. Destructive testing of coating thickness
(1) Timed flow method
Principle: The time-flow method is a chemical method. It uses a certain test solution at a certain flow rate to form a stream. The surface of the coating under test is partially dissolved. The coating is dissolved under the action of the solution and can be visually observed. In the case of the coating, until the coating is completely dissolved, the end metal color appears as the end point. The coating thickness is calculated according to the time it takes for the part to be dissolved.
(2) Dissolution method
Principle: The coating on the test sample is dissolved by using a solution that does not cause damage to the base metal. The coating solution can be chemical or electrochemical. Coating thickness is calculated based on its weight loss and the surface area of the coating portion. In general, the weight loss of a coating can be determined by chemical analysis or weighing.
(3) Coulomb method
Principle: On a known area of the tested coating surface, the coating is used as the anode to dissolve the coating at a constant current density in the corresponding test solution. When the coating metal is dissolved and the exposed base metal or intermediate coating is exposed, A sudden change in the cell voltage indicates that the end point has been reached. The local thickness of the coating was calculated according to Coulomb's law, the area of the coating dissolved by the current consumed by the coating metal.
(4) Metallographic microscope
Principle: Metallographic microscope thickness measurement is a method that uses a special metallographic microscope with a certain magnification and a micrometer eyepiece to observe and measure the thickness of the coating cross section.
(5) Drip method
The spot method is the simplest method for coating thickness measurement. It has the advantages of simple instrument and method. However, the disadvantage is that the accuracy is low and the accuracy is generally ± 20%. Colleagues' method is not suitable for parts with small shapes and small parts, so it is only used as a reference for rough determination. The principle is a solution with a certain composition. The solution is dropped on the measured coating surface through a dropper with a tip of 1.5-2.0mm diameter. The coating is dissolved for a certain period of time until the coating is dissolved to expose the substrate and the underlying metal coating.
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