How to Control Zinc Layer Thickness in Continuous Hot-Dip Galvanizing of Strip Steel
Author: Robby
The core method of controlling zinc layer thickness in continuous hot-dip galvanizing of steel strip is through a combination of process setting and intelligent control. Conventional hot-dip galvanizing units use a fast heating furnace degreasing and annealing method, where the annealed strip enters the zinc bath at a specific temperature to keep the zinc liquid temperature constant. The main control of the coating thickness is the “air knife method”, i.e., a controlled nozzle is used at the exit of the zinc tank to blow compressed air (usually high-purity nitrogen) along a certain angle to the strip, in order to remove excess zinc and regulate the thickness of molten zinc on the surface of the strip.
Specifically, the steps and measures for controlling the thickness of the zinc layer include:
Setting process parameters: The control of process parameters is critical in the galvanizing process. If the parameters such as current density, temperature and time are not set properly, they may all lead to insufficient zinc layer thickness. Therefore, precise control of these parameters is required to ensure uniformity in the thickness of the coating.
Use of air knife method: The air knife method removes excess zinc liquid by blowing compressed air through a nozzle into the strip to precisely control the thickness of the plating layer. This method is effective in minimizing deviations in plating thickness.
Annealing and Passivation: Annealing improves the adhesion and corrosion resistance of the plated layer, while passivation forms a protective film on the surface of the plated layer to further improve corrosion resistance.
To add some relevant background information, the addition of bismuth to the zinc solution during the continuous hot-dip galvanizing of strip steel can reduce the surface tension and improve the wettability, which is conducive to obtaining a flat and smooth coating. In addition, bismuth can reduce the return flow of the zinc solution and lower zinc consumption