Q. We are having a big problem with powder coating on galvanized steel sheets.
The pretreatment line was originally used for aluminum pretreatment. After the pretreatment, we could never see any signs of chromate on the GMS sheet, unlike the golden color on chromated aluminum sheets. Adhesion failure occurs when we perform cross cuts on the GMS. When we searched around, we were told that GMS has a passivation layer that prevents chromate from getting onto the substrate and we are not 100% sure how to detect this layer or remove it. Or are we even in the wrong direction?
Please help. Our project is in quite some trouble now. Thank you in advance. S.B.
A. We see a lot of questions on this subject. Galvanized steel can work well with powder coating if you treat it correctly. Chromate is typically a product made for aluminum and it may not be the right product for the galvanized. Talk with your chemical supplier about the right treatment for your metal to see what they recommend.
Adhesion loss on galvanized is often due to oxidation. The best way to avoid that problem is to lightly blast the surface prior to coating. The blast should be light enough to avoid excess loss of the zinc layer. Follow SSPC SP 7 or NACE No. 4. The Powder Coating Institute has some information in its “Guidelines to Coating Various Substrates” document.
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One very critical factor is to find out if the galvanized steel has been water or chrome quenched. Galvanizers may quench the steel at the end of the process to stop the reaction and the alloying process and ship the product without excess oxidation. This will leave a surface condition that is very poor for bonding of an organic coating. The process you refer to as passivation is probably a reference to quenching. This is not a suitable metal surface for powder coating.