Steel gets put through several different steps, while it’s in the process of being galvanised. These important steps as outlined by the Galvanizers Association of Australia are included below.
1. Initial degreasing
The first step is ensuring that surface contaminants like oils and paint are carefully removed. This is done as a preliminary treatment, usually through a caustic solution. Some surface contaminants may need to be removed before the steel arrives at the galvanising facility.
2. Acid cleaning
After the initial degreasing, the steel then undergoes acid cleaning or ‘pickling’ which removes rust and any mill scale. Hydrochloric acid is commonly used during this step, although sulphuric acid is an alternative.
3. Hot fluxing
Once the steel has been acid-cleaned, it is then immersed in a flux solution. This solution contains zinc ammonium chloride and will be at a heat of between 50-70°C. The flux solution will remove any oxide film which forms on the steel surface after acid cleaning, preventing further oxidation. The steel is then dried so that it’s ready for hot-dip galvanising.
4. Hot-dip galvanising
In the final stages, the steel is then galvanised by being dipped into molten zinc. This involves completely immersing the steel in a galvanising bath so that the steel’s surface is completely covered by molten zinc. The molten zinc then reacts with the steel to form several zinc alloy layers.
So that the coating or ‘layers’ can form, the steel needs to remain in the galvanising bath until it reaches the same temperature of the molten zinc, 445°C to 465°C. The steel is then slowly pulled out of the bath at a controlled rate so that the outer layer of molten zinc solidifies, and forms a pure outer zinc coating.
When this process is followed, the molten zinc will cover corners and welds, and seal the edges of the steel. It will also penetrate any recesses to provide complete protection to any areas within the steel that could degrade from corrosion. The galvanised coating will be slightly thicker at corners and narrow edges to provide increased protection.
After being pulled out of the galvanising bath, the steel is then cooled in a quench solution, which will also prevent early oxidation of the zinc surface.
