Appearance Concerns with Progressively Dipping
Why do progressively dipped fabrications sometimes have one side that is bright and shiny and one side that is matte gray?
Difference Between Two Sides
When the first half of a large steel fabrication is dipped, it must heat up from ambient temperature to the galvanizing temperature of approximately 840 F. When the other half of the fabrication is dipped, it has to heat up not from ambient temperature, but a temperature near the galvanizing temperature. This means the second half reaches the galvanizing temperature in a much shorter time period than the first half. Because the second half of the fabrication is at the galvanizing temperature longer, the galvanizing reaction is able to occur for a longer time period. This usually results in a thicker galvanized coating that does not have an outer free zinc layer, like that on reactive steels.
Predicting when you will get appearance differences between the two is difficult because until you galvanize the fabrication, you do not know if the steel chemistry is reactive or not. Because of this, simply leaving the second half of the fabrication in the kettle for a shorter time than the first half is not always the best answer.
Coating Overlapped Surface
During the progressive dipping process, there will inevitably be an area that has been double dipped (dipped twice). 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 intermetallic 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 areas intermetallic layers have the opportunity (heat applied) to grow twice. So, the coating of the overlapped area is not only thicker than the area only dipped once (up to 20 mils or more), but is rougher because of the chaotic grow of the zinc crystal structure in the Zeta intermetallic layer.
The reason your customer is having the piece dipped in the first place is corrosion protection. The appearance of the surface does not significantly affect the corrosion performance of the galvanized coating. Also, the two sides will assume a more uniform appearance overall in a few months after galvanizing as the coating develops its zinc patina. Even though the two halves might initially look different, the corrosion protection will be the same and the sides will assume a more uniform appearance over time.
© 2020 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.