Rough Surface on Overlapped Area of Progressively Dipped Steel
Why does the overlapped area on some progressively dipped steel have a rougher surface and thicker zinc coating than the other area of the same piece of steel?
First lets make the assumption the steel is fairly thick, as large and long structural sections requiring progressive dipping tend to be. In all likelihood, the surface of the steel dipped in first has a very typical zinc coating appearance of shiny gray to silver and has a coating thickness of eight to ten mils, very standard for high silicon, reactive steel. When the steel is flipped end-to-end and the uncoated steel is immersed in the molten zinc, it will galvanize in normal fashion, very similar to the end dipped in first. However, the overlapped surface (dipped in twice) may exhibit roughness and have a measured coating thickness of 20 mils or more.
During the second dip the overlapped surface area of the steel is once again exposed to the heat of the molten zinc and some of the free zinc Eta layer melts off. However, when the crystalline structure of the inter-metallic alloy layers of the overlapped area, in particular the Zeta layer, has more heat applied during the second dip, it grows again and this time at a different rate and in a very random pattern compared to the adjacent steel surfaces being exposed to the zinc for the first time. Essentially, the overlapped area inter-metallic layers have the opportunity (heat applied) to grow twice. So, the coating of the overlapped area is not only thicker than the adjacent area only dipped once but is rougher because of the chaotic grow of the zinc crystal structure in the Zeta inter-metallic layer.
© 2022 American Galvanizers Association. The material provided herein has been developed to provide accurate and authoritative information about after-fabrication hot-dip galvanized steel. This material provides general information only and is not intended as a substitute for competent professional examination and verification as to suitability and applicability. The information provided herein is not intended as a representation or warranty on the part of the AGA. Anyone making use of this information assumes all liability arising from such use.