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Atmospheric corrosion resistance of hot dip galvanizing

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. Industrial gases such as sulphur dioxide and nitrous oxides attack the zinc coating, as do ammonia gases. In rural areas galvanized coatings can last up to 100 years. This performance may be compromised where excessive application of insecticides and fertilizers occurs which, in combination with moisture, can attack the protective coating. Interestingly, galvanized coatings weather to a whiter colour in marine climates compared to the darker colours experienced in rural and industrial climates. In some instances the coating colours to a red, which is often mistaken for rusting. This discoloration occurs as the iron/zinc alloys react to form a protective film with the red iron corrosion product. In certain environments the iron/zinc alloys give better protection to the underlying steel than pure zinc. Studies have been carried out by the CSIR to determine the performance of hot dip galvanized coatings throughout South Africa. The map and table taken from ISO 9223 provides the potential user with a guideline as to the protection afforded by hot dip galvanizing throughout the country. Reference Michell's Plain case history (03/2005).

Case History 03/2005 - Mitchell’s Plain Train Station / Transport Interchange