The following description of the hot dip galvanizing procedure is for our reference. These procedures will ensure higher-quality products, minimize costs and faster delivery
Production process flow overview
o Incoming inspection
o Degresing with caustic soda
o Water rinsing o Pickling with hydrochloric acid
o Water rinsing
o Prefluxing with zinc ammonium chloride
o Hot dip galvanize in molten zinc
o Quenching with sodium dichromate water
o Finishing o Final inspection BS EN ISO 1461
o Weight scale and delivery
A. Pretreatment stage
One of the most important operations in hot dip galvanizing is the surface preparation of the steel. Because the galvanizing process is a metallurgical reaction, which can occur only between a clean steel surface and the molten zinc, it is critical that the material to be galvanized has been properly prepared. Insufficient or inadequate surface preparation will result in uncoated areas.
Pretreatment usually consists of degreasing, then acid pickling, followed by prefluxing. Water rinsing is incorporated between degreasing and acid pickling and between pickling and prefluxing.
Steel products are immersed in a hot alkaline cleaner (Sodium Hydroxide – Caustic Soda) solution at a temperature of approximately 80° C to remove dirt, oil, greases, soil and soluble paints. Degreasing solutions will not, however, remove such things as epoxies, vinyls or bituminous coatings. These coatings may have to be removed by sand/grit blasting or, in some cases, by use of special solvents or by burning off.
II) Water rinsing
Following degreasing processes, products are water-rinsed before immersion into the pickling acid.
III) Acid pickling
An acid bath is used to remove surface rut and mill scale to provide a chemically clean metallic surface. Hydrochloric acid solutions are used for pickling. In general, inhibitors are added to the acid to minimize attacks on the steel. Times for products residing in the pickling acid vary, depending on the workpiece being processed. Relatively clean steel may require only a few minutes to process, but more heavily scaled steel will have to stay for up to one hour in the pickling acid.
IV) Water rinsing
The pickled product is water-rinsed to prevent carrying over of acid into prefluxing and also to remove any adherent iron salts.
The final cleaning of the steel surface is done by prefluxing, involving immersion of the workpiece in an aqueous zinc ammonium chloride solution (30 %) for a short time. The primary purpose of prefluxing is to dissolve the thin oxide film, which formed on the steel surface after rinsing and to prevent the formation of additional oxides prior to entry into the molten zinc bath for obtaining a better and more consistent finishing. Heating of the preflux solution at 90° C is highly recommended to promote a faster drying of the product.
B. Hot dip galvanizing
The dried steel workpiece is immersed into a molten zinc bath at approximately 450° – 466° C. On immersion the steel surface is wetted by the molen zinc to form a metallurgically bonded thick zinc/zinc-iron alloy layer. The period of immersion depends on size and weight of the workpiece.
After the workpiece has been withdrawn from the zinc bath at a controlled rate and the excess zinc is removed by draining, assisted in some cases by vibrating (for small items)Then the galvanized item is usually quenched in sodium dichromate water. Quenching helps removing ash particles which may have adhered to the workpice during withdrawal from the zinc bath and facilitates subsequent handling and finishing of the work.