Applying the hot dip galvanizing process to steelwork provides fabrications with a robust, durable and corrosion protective finish that under normal conditions will last for many years without maintenance of any sort. The process itself has a number of stages that are required to achieve the final finish. These are all by immersion and they may be summarized as follows (rinse stages omitted):
Degreasing – This may be carried out using either acid or alkaline based proprietory products and they may be heated or used at ambient temperatures. The target is to produce a surface, which is not contaminated with oil, or grease based products.
Pickling – This is carried out in dilute hydrochloric acid which dissolves rust and scale and produces a ‘chemically clean’ surface which will react with the molten zinc.
Fluxing – A mixture of zinc chloride and ammonium chloride in solution is the standard fluxing agent of choice. This is normally used at between 50°C and 70°C, which helps the steelwork to dry after it is withdrawn. Drying is important as it helps prevent zinc splash and a separate drying stage is sometimes employed.
Zinc Immersion – This ‘final’ stage utilizes a special bath holding molten zinc at 450°C. The clean steel is immersed in the zinc and while it is submerged it alloys with the iron in the steel to form zinc/iron alloy layers. These layers form the basis of the coating, which is then overlain with free zinc, as it is withdrawn from the galvanizing bath. The coating that is formed offers cathodic protection to the steel to which it is alloyed. As such, it will corrode preferentially to the steel extending its life by many years.