Hot Dip Galvanizing
General (Batch or After Fabrication) Galvanizing
This process involves the application of zinc onto a “fabricated” shape. This means the steel is shaped into the final product; a structural beam, a large diameter pipe, or a small fastener, and then dipped into molten zinc to apply the zinc coating. These items are coated either one at a time or, in the case of small parts, as a number of parts contained in a “basket”. Hence, the terms “batch” or “after fabrication” are used to describe this process.
In many ways the general or batch process is the same as the continuous process in that the objective is to apply an unbroken coating of corrosion resistant zinc onto the surface of steel. However, these two methods have several differences.
The typical batch process involves three steps prior to the immersion of the part(s) into the molten zinc bath:
- Caustic cleaning
Caustic cleaning involves the use of a hot alkali solution to remove organic contaminants such as oils and greases. These surface contaminants need to be removed prior to pickling so that the surface can be “wetted” by the pickling solution.
Pickling involves the immersion of parts into an acid solution (typically heated sulphuric acid or ambient temperature hydrochloric acid) to remove surface scale or rust (both oxides of iron). The term “scale” is typically used to describe the oxides of iron that form at high temperatures such as during hot rolling, annealing in air, or welding. Rust is the product of corrosion of the steel surface when it gets wet. Both types of iron oxide need to be removed prior to the application of the zinc coating.
Fluxing involves the application of a special chemical coating onto the surface of the steel part. This “flux” serves the same purpose as fluxes used during soldering operations. The fluxing chemical (zinc ammonium chloride) is designed to chemically remove the last vestiges of oxides just as the steel is being immersed into the molten zinc, and allow the steel to be wetted by the molten zinc. Fluxing can be either “dry” or “wet”. Dry fluxing involves immersion of the steel part into an aqueous solution of the flux. Upon removal, the flux solution is dried prior to immersion into the zinc bath. (Note that there is a continuous galvanizing process that uses dry fluxing. It is described in GalvInfoNote 2.7). In wet fluxing, a blanket of liquid (molten) zinc ammonium chloride is floated on top of the molten zinc bath. The part to be coated is then immersed through the molten flux as it is being introduced into the coating bath. (Wet fluxing works because zinc ammonium chloride has a melting point below that of molten zinc and is less dense than molten zinc, thereby floating on the bath surface).
As with continuous galvanizing, the application of the zinc coating in batch galvanizing involves immersion of the steel into a bath of molten zinc. However,
in contrast with the continuous process wherein the steel is immersed for a very brief time, the batch process requires that the part be immersed for much longer times, typically measured in minutes, not seconds. There are two reasons for needing longer immersion times. One is to allow the part to reach the bath temperature. Immersion of a relatively cold thick-walled large pipe, for example, results in a skin of zinc freezing onto its surface when it is first immersed. For the coating to bond metallurgically to the steel, the pipe has to reach the bath temperature to “remelt” the zinc. After this, additional time is required to develop the iron/zinc alloy bond zone.
Unlike the continuous process, where the alloy layer has to be kept very thin to accommodate subsequent forming into the final shape, for batch-galvanized parts the alloy layer can be allowed to grow much thicker. In fact, a thicker alloy bond layer is often desired to provide a longer life to the final product, i.e., a longer time before the onset of rust. Like zinc itself, the alloy layer is galvanically protective to the steel part and a thicker alloy layer means longer life. Yes, the alloy layer is hard and brittle, but since the part is already fabricated, there will be no additional forming that can crack the alloy. The brittle alloy layer is not deleterious. It will not result in coating damage during shipment and subsequent handling at the jobsite. A representative photomicrograph of the alloy layer that forms while the steel is immersed in the bath is shown in Figure 1. As can be seen in this photo, the alloy layer is as much as 50% of the total coating thickness and it actually consists of two or more distinct zinc/iron layers. Each of these distinct layers combines to form the “total” alloy layer zone. Each layer actually has a specific amount of iron and zinc. The layer closest to the steel has the highest iron content while the layer immediately adjacent to the pure zinc outer layer has the lowest iron content. The composition and properties of these alloy layers are shown on Table 1.
The alloy layer grows by an intermixing diffusion reaction between the atoms of the steel and zinc. This is a time dependent process, and for most steels, a longer immersion time provides a thicker alloy layer. In fact, for batch galvanized parts, additional immersion time is often needed to achieve the final required thickness of the protective coating (the thickness is a combination of the alloy layer and the pure zinc outer coating metal).
As a result of the ability to accommodate long immersion times, the final thickness of the coating (pure zinc + alloy layer) on batch galvanized parts is often considerably thicker than the coating on continuous galvanized sheet product – at least the thickness can be much thicker if desired/required. This is one major difference between the batch galvanizing process and the continuous galvanizing process.
There are production issues that often need to be considered with respect to the maximum alloy layer thickness that can be achieved during batch galvanizing. As the alloy layer thickens, its rate of growth slows down because diffusion through the thickening alloy layer takes longer, resulting in a practical limit to the final thickness. Also, for some steel compositions, the uniformly thickening alloy bond does not form on the surface. Instead, the alloy grows to a certain thickness and then begins to spall off the steel surface. When this type of behavior is experienced, the practical maximum coating thickness is less than when the alloy continues to grow as a compact layer.
For more extensive information on the after fabrication general galvanizing process, visit your regional Galvanizing Association.