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The life expectancy of a hot dip galvanized coating is roughly proportional to the thickness of the coating. Therefore, in any given environment it is possible to predict how long a coating will last before repair is required. With a galvanized coating life expectancy is based upon the appearance of 5% red rust on the steel surface.

The graph below highlights the relationship between the protection life of a galvanized coating and its thickness.

Clearly, the resistance to atmospheric corrosion of hot dip galvanized steel depends upon climatic factors such as humidity, rainfall, proximity to the coast and the presence of pollutants. The latter can have a particularly detrimental effect upon the performance of the coating depending which pollutants are present.

Corrosion rates of hot dip galvanized steel at coastal locations (generally within 1 km of the high water mark) can be high but duplex coating in these areas can provide effective corrosion protection.

Industry generated gases such as sulphur dioxide and nitrous oxides attack the zinc coating, as do ammonia gases. In rural areas hot dip galvanized coatings can last well in excess of 80 years. This performance may be compromised where excessive application of insecticides and fertilizers occur, which in combination with moisture, attack the coating. Interestingly, galvanized coatings weather to a paler colour in marine climates and to darker colours in rural and industrial environments. In some instances the hot dip galvanized coating transmutes to a reddish hue, often misguidedly taken for rusting. This discoloration occurs as the zinc-iron alloys react with the atmosphere to form a  distinct zinc-iron alloy based protective film. In certain environments the zinc-iron alloys may even provide better corrosion control to the underlying steel than pure zinc. ISO 9223 defines six different atmospheric corrosion classifications from C1(benign) to CX (extremely aggressive). From the table provided in ISO 9223  the user is able to estimate the service life of hot dip galvanizing applicable to each corrosion category. ISO 12944 is the equivalent standard used when evaluating service life of paints or paint systems.