Zinc corrodes in the same way as steel.

True or False?

All materials degrade over a period of time. This is a fact of life and the trick is to determine the rate of corrosion of a particular material in a specific environment. Protective coatings, whether organic or metal, are designed essentially to provide steel with barrier protection from corrosive substances in the surrounding environment. In the case of a zinc coating (e.g. hot dip galvanizing) and to a lesser degree zinc containing paints, cathodic protection is also provided, but this protective mechanism is only significant on surfaces where the steel substrate is partially exposed as a result of discontinuities in the coating.

In the case of most metals and metal coatings, resistance to corrosion is provided by the natural formation of a stable oxide film on the surface of the material while the development of this protective surface layer is largely influenced by the prevailing environment.

In the case of a hot dip galvanized coating, the outer relatively pure zinc layer of the coating immediately reacts with oxygen and moisture when a product is withdrawn from the molten zinc. For a stable protective layer to be formed on the coating surface, the presence of carbon dioxide is also essential. This results in the formation of a stable basic zinc carbonate layer on the coating surface which resists corrosion in most environments.

In circumstances where freshly galvanized components are stacked together and moisture is present, the exclusion of carbon dioxide can result in the formation of unstable zinc hydroxide, commonly referred to as white rust or perhaps more accurately, wet storage staining. Once a galvanized surface is exposed to the atmosphere, the presence of carbon dioxide prevents the further formation of soluble zinc hydroxide and a stable basic zinc carbonate film is developed.

It is important to appreciate that, when the reason for the formation of white rust is eliminated, no further attack will occur. In order to determine whether the coating has been depleted, thickness measurements should be taken to ensure that it conforms to the requirements of the relevant specification.

Structural steel is rarely affected by severe white rusting but it can cause coating damage in the case of closely packed sheet or plate, particularly in the case of pregalvanized coil and roof sheeting. Voluminous deposits of white corrosion product should be removed using a bristle brush (never use a wire brush). The dark surface discolouration which frequently remains after cleaning, will disappear with time, as a result of the natural weathering process.

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