Hot dip galvanizing
What is a Hot-Dip Galvanizing?
The galvanizing process consists of three basic steps;
1. Surface preparation
1.1 Caustic cleaning: A hot alkali solution often is used to remove organic contaminants such as dirt, paint marking, grease and oil from the metal surface. Epoxies, vinyl, asphalt or welding slag must be removed by grit/ sand-blasting or other mechanical means.
1.2 Pickling: Scale or rust normally is removed by pickling in a dilute solution of hot sulfuric acid or ambient temperature hydrochloric acid.
1.3 Fluxing: Fluxing removes oxides and prevents further oxides from forming on surface of metal prior to galvanizing.
– Dry galvanizing process: The steel or iron is dipped or pre-fluxed in an aqueous solution of zinc ammonium chloride. Then material will be dried before immersing in molten zinc.
– Wet galvanizing process: A blanket of liquid zinc ammonium chloride is floated on top of the molten zinc. The steel or iron being galvanized passes through the flux on its way into the molten zinc.
In this step, the material is immersed in a bath (at least 98% pure molten zinc, maintaining at 449°C) until it reach bath temperature. The zinc metal reacts with the iron on the steel surface to form a zinc/iron inter-metallic alloy. The products are withdrawn from zinc bath; the excess zinc is removed by draining, vibrating and/or centrifuging and they are cooled in either water or ambient air immediately.
The two properties with closed scrutinizing are thickness and appearance of coating. The physical and laboratory tests may be performed to determine thickness, uniformity, adherence and appearance.
Reference: American Galvanizers Association, Hot-Dip Galvanizing for Corrosion Protection of Steel Products.