Factors Affecting Galvanizing Coating
Factors affecting the thickness and appearance of the galvanized coating. These include:
a) Chemical composition of the steel – The chemical composition of the steel is very important to the quality of the galvanized coating since a metallurgical reaction between the zinc and the iron is occurring.
As an example, silicon contents in the steel between 0.05% – 0.11% and above 0.25% will cause the zinc–iron alloy layers to grow abnormally fast. In fact, the silicon causes the galvanizing reaction to continue unabated and the outermost pure zinc layer is eventually consumed in the process. This not only causes the coating to be thicker than normal, but the coating will also most likely consist entirely of alloy layers. The increased presence of these alloy layers in the resulting coating are often brittle and appear as a dark gray, matte finish.
Steel compositions that affect the coating formation and properties in this manner are referred to as “reactive steels”.
b) The surface condition of the steel – In general, rough steel surfaces will cause the galvanizing reaction to produce thicker coatings due to the increased surface area. These thicker coatings, however, will be rough and have a generally poor appearance.
c) Bath immersion time – Galvanizing is a diffusion process. As with all diffusion processes, the reaction between the molten zinc and the steel or iron will proceed quickly at first but will slow down as the alloy layers grow and become thicker. Thus, continued immersion or dipping material more than once will not produce a significantly thicker coating except in the case of reactive steels.
d) Bath withdrawal rate – The withdrawal rate of material from the galvanizing bath has the greatest effect on the outermost layer of pure zinc. A rapid withdrawal will produce the thickest coating because the largest amount of zinc will be carried out on the material and will subsequently solidify and become the pure zinc layer. Slower withdrawal allows the zinc to effectively drain back into the kettle producing smoother, thinner, and more uniform coatings.
e) Steel cooling rate – Slower cooling rates, such as those experienced with air cooling or even with thicker sections that have been water quenched, allow the zinc/iron alloying reaction to continue. The inner alloy layers will use the outer Eta layer for their supply of zinc causing the coating to become dull, matte gray. Quickly reducing the temperature to less than 300 degrees Fahrenheit will halt the galvanizing reaction and minimize the formation of dull or matte surfaces.
f) General Considerations The generic composition of a steel to be galvanized that will produce an acceptable coating will be carbon less than 0.25%, phosphorous less than 0.05% and manganese less than 1.3%. Also, as discussed above, in order to avoid the adverse effects of silicon on the galvanized coating, silicon content in the steel should be in the range of 0-0.04% or 0.15-0.25%. Steel with these recommended levels should develop a normal galvanized coating. However, should the composition of these elements fall outside any of the previously stated ranges, the steel will most likely still able to be galvanized provided that the Galvanizer is notified in advance of any deviations.