The hot-dip galvanizing of very thick steels (beyond 2-3 inches thick) presents process challenges to galvanizers such as a high zinc consumption rate and extra processing time. Often times, such articles are left in the galvanizing kettle for very long periods of time to allow the part to reach the galvanizing temperature and allow the coating to develop.
The requirement for an excessive immersion time (beyond an hour) is often the result of a cocoon effect which slows the heating of the steel. When the steel first enters the bath, there is a very large difference between the steel and the molten zinc temperatures. Then, the molten zinc begins to transfer heat to the steel and bring the steel to the galvanizing temperature. The molten zinc nearest the steel transfers the most heat and reaches a temporary equilibrium, leaving an unmoving cocoon of frozen zinc directly surrounding the steel article. As a result, the heat within the bath becomes unevenly distributed, and the time required to heat the steel increases significantly.
With excessive immersion times in the kettle, there is concern over the development of excessively thick and brittle coatings. The use of an excessive immersion time is especially problematic for reactive steels with high silicon/phosphorus content. For reactive steels, the alloy layers of the coating will continue to grow at a rapid rate during the immersion, while steel grades of optimal chemistry will experience a diminishing growth rate that eventually tapers off (see Figure 1). //
If the steel is of reactive steel chemistry, then coating thickness can also be controlled by blast cleaning the steel prior to galvanizing. Blast cleaning the steel causes the zeta layer of the coating to become very rough and interferes with the zinc coating growth.
