What Does Batch Galvanizing Mean?
Batch galvanizing is the process where an unbroken zinc coating is applied to the surface of a finished steel product or fabricated shape. The corrosion-resistant coating is achieved by immersing the object into a bath of molten zinc. This process can be done either one at a time, or with several parts in one batch.
Batch galvanizing is also known as after-fabrication galvanization.
Corrosionpedia Explains Batch Galvanizing
The three main steps for preparing objects for immersion into the zinc bath are:
- Caustic cleaning - Oils, greases and other organic contaminants are removed by a hot alkali solution
- Pickling - Removing rust and surface scale by immersing it into either heated sulphuric acid or ambient hydrochloric acid
- Fluxing - Applying a special chemical coating of zinc ammonium chloride to the surface. This coating chemically removes the last traces of any oxides just before the immersing the object into the molten zinc.
By immersing the dried object into a molten zinc bath, the steel surface becomes wetted and reacts with the zinc to form several zinc-iron alloy layers. The object is kept in the bath until its temperature equals that of the molten zinc, 833 to 870°F (445 to 465°C), to ensure a complete galvanization reaction. The object is then withdrawn at a controlled rate and air quenched or quenched in water containing potassium dichromate.
The molten zinc solidifies on the object to form a pure outer zinc coating. However, the pure zinc coating does not always form and the surface may end up a dull gray color.
The coating achieved with this process is durable, consisting of several zinc-iron alloy layers bonded to the underlying steel and then covered by an outer layer of pure zinc. The coating covers the whole object and provides excellent corrosion and abrasion resistance.
Objects coated in this process include fasteners, bolts and small components, as well as steel handrail tubing.