The most important rule is that the designer, fabricator, and galvanizer should work together before the product is manufactured. This three-way communication can eliminate most galvanizing problems.The designer can better appreciate hot dip galvanizing design requirements if the basic steps of the galvanizing process are understood. Though the process may vary from galvanizer to galvanizer, the fundamental steps in the galvanizing problems are:
1. Soil and grease removal
A hot alkaline cleaner is usually used to remove oil, grease, shop oil, and soluble paints. This will not, however, remove such things as epoxies, vinyls, asphalt, or welding slag. These soils must be removed by grit blasting, or other mechanical cleaning which is normally not the responsibility of the galvanizer.
2. Pickling:An acid bath is used to remove surface rust and mill scale to provide a chemically clean metallic surface. Many galvanizers prefer the use of hydrochloric acid since it is more environmentally friendly and will not effect the surface of the steel which may be possible with the use of sulfuric acid.
3. Prefluxing:A steel article is immersed in a liquid flux predip (usually zinc ammonium chloride solution) to remove oxides and to prevent oxidation prior to dipping into molten zinc. By utilizing the dry kettle process, a galvanizer can eliminate the potential of flux inclusion or entrapment on the galvanized steel product. The wet kettle process, where the steel goes through a “flux blanket” on top of the galvanizing bath, can result in these particles adhering to the steel surface.
4. Galvanizing:The article is immersed to molten zinc at approximately 850°F (455°C). This results in a formation of a zinc and zinc-iron alloy coating which is metallurgically bonded to the steel.
5. Finishing:After the article is withdrawn from the galvanizing bath, excess zinc is removed by draining, by vibrating, or, for small items, by centrifuging. The galvanized item is then cooled in air or quenched in water. The air quenching process, which takes a bit longer than the water quenching method, will result in a better surface finish which is an important consideration in architecturally exposed steel.
6. Inspection:Thickness and surface condition inspections are the final steps in the galvanizing process. Information on inspection procedures and quality control criteria is available.
Iron and steel articles hot dip galvanized after fabrication may range in size from small pieces of hardware such as bolts and washers to large welded steel assemblies or castings weighing several tons. The ability to galvanize these articles can be improved by following the design practices recommended in this manual and by consulting with the galvanizer during the design stage of a project.