Microscopically speaking, coating is a porous structure. Water in the air, corrosion media (chloride ions, etc.) will penetrate through the weak part of the coating, causing corrosion under the film, and then the coating foaming and peeling. In addition, even with the same coating thickness, the two coating is denser than the first coating. According to the foreign reports and related corrosion test results, the positive coating of more than 20 micron can effectively prevent the invasion of corrosion medium. Because of the different anticorrosion mechanism of primer and topcoat, not only the total film thickness should be specified, but also the thickness of primer (> 5 micron) and topcoat (> 15 micron) should be required respectively. Only in this way can the color coated steel be guaranteed. The corrosion resistance of different parts of the board is balanced.
Class PVDF products require thicker coating because they provide longer service life. Requirements for back coatings depend on the use of sandwich panels requiring only a layer of adhesive primer, and molded steel plates requiring two coatings, at least 10 microns thick, due to indoor corrosion conditions.
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