It is critical to ensure that any item to be hot dipped galvanised is completely clean in order for the molten zinc to form a sound and complete metallurgical bond with the item. To ensure the material surface is completely clean, the material is passed through a pickling process prior to entering the galvanising kettle. In this process any 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. Rolled steel surfaces covered by heavy mill scale may require abrasive blast cleaning prior to acid cleaning.
The acid-cleaned steel article is immersed in a flux solution, usually 30% zinc ammonium chloride with wetting agents, maintained at about 65 degrees celsius. The flux solution removes the oxide film which forms on the highly reactive steel surface after acid cleaning, and prevents further oxidation before galvanising. The work is then dried and ready for galvanising.
On immersion in the galvanising bath, the steel surface is wetted by the molten zinc, and reacts to form a series of zinc-iron alloy layers. To allow formation of the metallurgic bond, the work remains in the bath until its temperature reaches that of the molten zinc, in the range of 445-465 degrees celsius. The work is then withdrawn at a controlled rate and with it carries an outer layer of zinc which solidifies to form the relatively pure outer zinc coating.
The period of immersion in the galvanising bath varies from several minutes for relatively light articles, up to half an hour or longer for major structural members.
The resulting galvanised coating is tough and durable, comprising relatively pure zinc and zinc-iron alloy layers bonded metallurgically to the underlying steel, completely covering the article externally and internally, providing an unmatched resistance to abrasion. The fact that hot dipped galvanising provides internal as well as external protection is a major advantage when compared to normal paint only systems.
An important advantage of the galvanising process is that visual inspection shows that work is completely protected and gives an excellent guide to coating quality.