Galvanising protects steel from corrosion with a zinc coating. In the strict sense this means hot-dip galvanising: after degreasing, acid pickling and fluxing, the part is dipped in a bath of molten zinc at around 450 °C. The zinc reacts with the iron and forms zinc-iron alloy layers, topped by a layer of almost pure zinc. The protection is twofold: the coating acts as a barrier, and the zinc sacrifices itself in place of the steel if a scratch exposes the bare metal.
The layer is thick, often around 45 to 100 µm depending on the steel thickness (the NF EN ISO 1461 standard sets the requirements), and it also covers the inside of hollow sections. The steel composition, in particular its silicon content, influences the thickness and the appearance, which is grey and sometimes uneven. The protection generally lasts many years outdoors, depending on the atmosphere. The process is therefore widely used for steel structures, stairs, railings, supports and guard rails exposed to the weather: plate fabrication assemblies, often put together by MIG/MAG welding before being galvanised.
The limits come from the temperature and the bath: maximum part dimensions, risk of distortion of thin sheet or large welded parts, vent and drain holes essential on hollow bodies to avoid trapping air or liquid, threads and bearing surfaces to be designed with clearance. Do not confuse it with electrolytic zinc plating, which is much thinner, nor with “cold galvanising”, which generally means a zinc-rich paint.
In a quote request
Supply the dimensions and weight of the parts, the steel grade, the sheet thickness and the conditions of use. State the appearance expected, the areas not to be galvanised, any finishing paint (for example powder coating) and the galvanising certificate required. Have the drawings reviewed by the galvanizer before manufacture: the position of vent holes is decided at design stage. See also the guide on anodising, powder coating and galvanising.