22 Feb The Galvanizing Process
Hot dip galvanizing is the process of coating iron or steel with a layer of zinc by immersing the metal in a bath of molten zinc at a temperature of around 842°F (450 °C). During the process, a metallurgically bonded coating is formed which protects the steel from harsh environments, whether they be external or internal.
Galvanized steel is widely used in applications where corrosion resistance is needed without the cost of stainless steel and can be identified by the crystallised pattern on the surface (often called a ‘spangle’). Galvanizing is probably the most environmentally friendly process available to prevent corrosion.
The galvanizing reaction will only occur on a chemically clean surface. In common with most zinc 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 before galvanizing. These types of contamination are removed by a variety of processes and common practice is to degrease first using an alkaline or acidic solution into which the component is dipped. The article is then rinsed in cold water to avoid contaminating the rest of the process. The article is then dipped in hydrochloric acid at ambient temperature to remove rust or mill scale. Welding slag, paint and heavy grease will not be removed by these cleaning steps and should be removed by the fabricator before the work is sent to the galvanizers. 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.
Post galvanizing treatment can include quenching into water or air cooling. Conditions in the galvanizing plat such as temperature, humidity and air quality do not affect the quality of the galvanizing coating.
By contrast, these are critically important for good quality painting. No post treatment of galvanized articles is necessary and a paint or a powder coating may be applied for enhanced aesthetics or for additional protection where the environment is extremely aggressive. Chemical conversion coatings and other barrier systems may be applied to minimise the occurrence of wet storage stain.