The first step in the hot-dip galvanizing process is intended to obtain the cleanest possible steel surface by removing all of the oxides and other contaminating residues. This is achieved by first hanging the steel using chains, wires, or specially designed dipping racks, as seen in Figure 3, to move the parts through the process. There are three cleaning steps to prepare the steel for galvanizing.
First the steel is immersed in an acid degreasing bath or caustic solution in order to remove the dirt, oil, and grease from the surface of the steel. After degreasing the steel is rinsed with water.
Next the steel is immersed in an acid tank filled with either hydrochloric or sulfuric acid, as seen in Figure 4, which removes oxides and mill scale in a process called “pickling.” Once all oxidation has been removed from the steel, it is again rinsed with water and sent to the final stage of the surface preparation.
The purpose of the flux is to clean the steel of all oxidation developed since the pickling of the steel and to create a protective coating to prevent the steel from any oxidizing before entering the galvanizing kettle. One type of flux is contained in a separate tank, is slightly acidic, and contains a combination of zinc chloride and ammonium chloride. Another type of flux, top flux, floats on top of the liquid zinc in the galvanizing kettle, but serves the same purpose.
After being immersed in the degreasing, pickling, and fluxing tanks, the surface of the steel is completely free of any oxides or any other contaminants that might inhibit the reaction of the iron and liquid zinc in the galvanizing kettle.