Hot-dip galvanisation

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The industrial-scale application of hot-dip galvanising will soon be 200 years old. Nevertheless, it remains a very efficient way of protecting steel from rust. The process involves immersing steel in a bath of molten zinc at 450°C.

To rid the steel of fats and oils, it is first dipped into a degreasing bath. After rinsing, all surface oxides, such as rust and mill scale, are removed in a pickling bath consisting of diluted hydrochloric acid. Rinse again and the perfectly cleaned steel is ready to be “fluffed”. The flux bath consists of ammoniacal zinc chloride of which a thin layer protects the steel surface against oxidation while waiting for contact with the hot zinc. The flux layer melts in this, which ensures excellent metallic contact and good irrigation of the steel by the zinc. When dipping into the zinc bath, the steel gradually takes on the temperature of the zinc. Diffusion occurs and gives rise to the formation of zinc-iron alloys. During this process, a layer of pure zinc is deposited on top of the alloy layers. Not all gates and fences can be hot-dip galvanised. The processing of perforated or solid steel sheets means that, if they are hot-dip galvanised, on the one hand the perforations are full of zinc and on the other hand (if the surface is larger) the sheets are warped by the high temperature. This type of gates and fences should be metallised.

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