The resistance of galvanizing to atmospheric corrosion depends on a protective film which forms on the surface of the zinc mixed with the surroundings of the product when in situe, with the corrosion rate for zinc is generally linear for a given environment When the steel is withdrawn from the galvanizing bath, the zinc has a clean, bright, shiny surface. With time this changes to a dull grey patina as the surface reacts with oxygen, water and carbon dioxide in the atmosphere. Contaminants in the atmosphere affect the nature of this protective film.

The environments in most corrosion guides are necessarily general. Specific corrosivity values in the UK have been mapped by the Agricultural Development Advisory Service (ADAS). The information was based on data obtained from exposure of zinc reference samples at National Grid Reference points in a large number of 10km square reference areas of the UK.

The results indicated varying rates of corrosion for zinc in different exterior locations the uk Galvanizers Association sponsored the revision of the map, to provide specifiers with the very latest information on zinc corrosion.