Hot dip galvanizing-process description
The following description of Hot Dip Galvanizing process is given for your information and reference.
This procedure will ensure higher quality products, minimum cost and faster delivery.
A. PRETREATMENT STAGE:
One of the most important operations in hot dip galvanizing is the surface preparation of the steel because galvanizing process is a metallurgical reaction, which can only occur between a clean steel surface and the molten zinc, it is critical that the works galvanized has to he properly prepared insufficient or inadequate surface preparation will result in uncoated areas.
Pretreatment (usually) consists of degreasing, then acid pickling, followed by prefluxing. A water-rinsing step is incorporated between degreasing and acid pickling. and between the pickling acid prefluxing.
Each of this operation is described below more clearly-
I ) DEGREASING;
Articles are immersed in a hot alkaline cleaner (Sodium Hydroxide - Caustic Soda) solution at a temperature of approx. 60 deg.C to remove dirt. oil, grease, soil arid soluble paints
Degreasing solutions will not. however remove such things as epoxies, vinyl’s or bituminous coatings. These Coatings may have. to be removed by sand/grit blasting, in some cases use ~f special solvents or by burning off.
II) WATER RINSE
Following degreasing process, articles are water rinsed before immersion in the pickling acid.
III) ACID Pickling:
An acid bath is used to remove surface lust and mull scale to provide a chemically clean metallic. surface.
Hydrochloric acid solutions are used for pickling. Generally inhibitors are added to the acid to minimize attack of the steel.
Residing time in pickling acid vary
considerably, depending on the work being processed. Only few minutes may
require for relatively clean steel, but up to an hour or more for heavily scaled
steel~
IV) WATER Rinse:
Following pickling the work is water rinsed to prevent carry over of acid into preflux and also to remove any adherent iron salts.
V) PREFLUXING:
The final cleaning of steel surface is performed by involving immersion of the work for a sort time in an aqueous zinc ammonium chloride solution (20-35%). The primary purpose of the prefluxing is to dissolve the then oxide film formed on the steel surface after rinsing and to prevent formation of additional oxides prior to entry into the molten zinc bath and to obtain a better more consistent finish. Heating of the preflux solution at 60 deg .C is highly desirable to promote faster drying of the work
B. HOT DIP GALVANIZING;
The dried article is immersed into molten zinc bath at approx. 440-460 deg.C
On immersion the steel surface is wet by the molten zinc to form a metallurgical bonded thick zinc/zinc-iron alloy layers of the hot dip galvanizing. The period of immersion depends on the size and weight of the article.
C. FINISHING:
After the article is 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 generally quenched in sodium dichromate water Quenching helps with removal of ash particles which have adhered to the work during withdrawal and facilitates subsequent handling and finishing of the work