Galvanised and Coated Steels
Galvanized and coated steels are widely used corrosion resistant materials. Covering mild steel with a protective coating allows a relatively cheap material to be used in environments where it would normally rust quite quickly. The most well known type of coated steel is galvanized steel, that is, steel with a layer of zinc on the surface. The main processes for applying the zinc are hot dipping, electroplating, and metal spraying. This type of steel is used for all sorts of applications in mildly corrosive conditions, for example in vehicles, heating and ventilation, architecture, and electricity pylons. Welding galvanized is carried out regularly and most processes can be used. Porosity is a major problem as is exposure to the zinc oxide fume. Other types of coating used for steel protection include chromium plating, tin plating, cadmium plating, paint, enameling, or vitrification (glass). Chromium plating involves applying an initial layer of nickel (this is what provides the corrosion resistance) followed by a layer of chromium to give it the bright, shiny appearance. Both layers are applied by electroplating. Welding over chromium plating destroys the protective layer and it is not easy to effect a repair. Tin plate is applied by electroplating or hot dipping and is used mainly in production of tin cans for food storage. Welding of tin plate is uncommon. Cadmium plating is also used to protect some engineering steels. Welding can cause health problems, as the fume from cadmium welding is particularly toxic. Whether welding over paint is possible but depends on the type of paint being used. Some weld through primers are fairly tolerant to welding whilst the two-pack epoxy paints and marine anti-fouling paints are not. Fume create by welding or cutting onto these materials can cause health problems and ideally should be avoided. Welding enameled or glass covered steel is not viable.
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