Hot dip galvanizing, has grown almost continuously since it was first used to protect corrugated iron sheets 150 years ago. Its ability to grow in the face of sophisticated competition is the result of the simplicity of the process and the unique advantages of the coating.
The galvanizing reaction will only occur on a chemically clean surface. Therefore most of the preparation work is done with this objective in mind. In common with most coating processes, the secret to achieving a good quality coating lies in the preparation of the surface. It is essential that this is free of grease, dirt and scale of the iron or steel before galvanizing. These types of contamination are removed by a variety of processes. Common practice is to degrease using an alkaline or acidic degreasing solution into which the component is dipped.
The article is then rinsed in cold water and then dipped in hydrochloric acid at ambient temperature to remove rust and mill scale. Welding slag, paint and heavy grease will not be removed by these cleaning steps and should be removed before the work is sent to the galvanizer. After further rinsing, the components will then commonly undergo a fluxing procedure. This is normally applied by dipping in a flux solution - usually about 30% zinc ammonium chloride at around 65-80°C.
Alternatively, some galvanizing plants may operate using a flux blanket on top of the galvanizing bath. The fluxing operation removes the last traces of oxide from the surface and allows the molten zinc to wet the steel.
When the clean iron or steel component is dipped into the molten zinc (which is commonly at around 450°C) a series of zinc-iron alloy layers are formed by a metallurgical reaction between the iron and zinc. The rate of reaction between the steel and the zinc is normally parabolic with time and so the initial rate of reaction is very rapid and considerable agitation can be seen in the zinc bath. The main thickness of coating is formed during this period. Subsequently the reaction slows down and the coating thickness is not increased significantly even if the article is in the bath for a longer period of time.
A typical time of immersion is about four or five minutes but it can be longer for heavy articles that have high thermal inertia or where the zinc is required to penetrate internal spaces. Upon withdrawal from the galvanizing bath, a layer of molten zinc will be taken out on top of the alloy layer. Often this cools to exhibit the bright shiny appearance associated with galvanized products.
Post galvanizing treatment can include quenching into water or air cooling. Conditions in the galvanizing plant such as temperature, humidity and air quality do not affect the quality of the galvanized coating. By contrast, these are critically important for good quality painting.