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Watch this video for a general explanation of the galvanizing process and its uses.
Scale, rust, oil, paint and other surface contaminants are carefully removed from the steel by suitable preliminary treatment and subsequent acid cleaning or pickling in sulphuric or hydrochloric acids, followed by rinsing.
The acid-cleaned steel article is immersed in a flux solution, usually 30% zinc ammonium chloride with wetting agents, maintained above 65°C. The flux solution removes the oxide film which forms on the highly reactive steel surface after acid cleaning, and prevents further oxidation before galvanizing. The work is then dried ready for galvanizing.
Prepared items are galvanized by immersion in molten zinc. On immersion in the galvanizing bath the steel surface is completely covered by the molten zinc, which reacts with the steel to form a series of zinc-iron alloy layers, producing a uniform coating. The thickness of these layers is determined principally by the mass of the steel being galvanized. This is an important advantage of the galvanizing process – a standard minimum coating thickness is applied automatically regardless of the operator.
To allow formation of the coating the work remains in the bath until its temperature reaches that of the molten zinc, in the range of 445°C to 465°C. The work is then withdrawn at a controlled rate and carries with it an outer layer of molten zinc which solidifies to form the relatively pure outer zinc coating.
The molten zinc in the galvanizing bath covers corners, seals edges, seams and rivets, and penetrates recesses to give complete protection to areas which are potential corrosion spots with other coating systems. The galvanized coating is slightly thicker at corners and narrow edges, giving greatly increased protection compared to organic coatings which thin out in these critical areas. Complex shapes and open vessels may be galvanized inside and out in one operation.
The period of immersion in the galvanizing bath varies from a few minutes for relatively light articles, or longer for massive structural members.
Upon extraction from the galvanizing bath the item is then quenched to cool.
Galvanizing fasteners and small components
Fasteners and small components are loaded into perforated cylindrical steel baskets. After acid pickling and prefluxing, baskets are lowered into the galvanizing bath. On withdrawal from the molten zinc, baskets are raised without delay into a centrifuge or 'spinner' and rotated at high speeds for 15 to 20 seconds. Excess zinc is thrown off, providing a smooth, uniform coating and maintaining the integrity of threaded items.
Metallurgy of Galvanizing
When the cleaned and fluxed steel surface contacts the molten zinc of the galvanizing bath the protective flux layer is removed leaving a clean steel surface which is immediately wetted by the zinc. This results in a reaction between zinc and steel with the formation of zinc-iron alloy layers.
The photomicrograph below shows a section of typical galvanized coating which consists of a progression of zinc-iron alloy layers bonded metallurgically to the base steel, with the relatively pure outer zinc layer.
Abrasion resistance of galvanized coatings
The photomicrograph below shows that the delta and zeta zinc-iron alloy layers are actually harder than the base steel, resulting in galvanizings outstanding resistance to abrasion and mechanical damage. Abrasive or heavy loading conditions in service may remove the relatively soft eta layer of zinc from a galvanized surface, but the very hard zeta alloy layer is then exposed to resist further abrasion and heavy loading.
For more information on Abrasion Resistance of Galvanized Coatings go to the FAQ page
About Hot-Dip Galvanizing
For a copy of the brochure "Hot dip galvanizing - The best protection inside and out" please click here to download a copy.