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Lifetime expectancy for zinc coatings

Thickness of zinc coating

The Rotocoat galvanising plants specialise in the hot-dip galvanising of steel in accordance with NEN-EN-ISO 1461. The thickness of the zinc coating determines the duration of protection. In general: the thicker the zinc coating, the longer the expected life of the product. However, galvanising plants have only limited technical options for controlling the thickness of the zinc coating.

When galvanised steel is exposed to atmospheric conditions, there is generally a linear relationship between the thickness of the zinc coating and the lifetime expectancy. It is therefore possible to predict, for a particular environment, how the thickness of the coating will behave, and when initial maintenance will be required. To do this, it is possible to use corrosion categories or the Zinc coating life predictor.

Corrosion categories

ISO 9223 assigns atmospheric corrosion to corrosion categories C1 to C5, with each category stating a minimum and maximum corrosion rate for iron, zinc, aluminium or copper. In this way, reasonably accurate calculations can be made for particular corrosion categories in order to determine when a coating will reach the minimum value and initial maintenance will be required.

Climate category
Probability of corrosion
Average annual loss of zinc
Corrosion category outside
Corrosion category inside
very low
< 0,1 ųm - 8 ųm
Heated buildings with dry air and a clean indoor climate: hotels, offices, shops, schools.
0,1 - 0,7 ųm
Rural areas (inland), atmosphere with a low pollution level and low humidity.
Unheated buildings where mild condensation is possible: storage facilities, sports halls.
0,7 - 2 ųm
Urban and industrial area with high humidity, moderate SO² pollution. Coastal zones with low salinity.
Commercial premises with high humidity and low levels of air pollution: food industry, laundries, breweries, dairy farms.
2 - 4 ųm
Industrial and coastal areas with moderate salinity. High humidities and aggressive atmosphere. Chemical structures with constant humidity and contaminant levels.
High humidity, moderate pollution: chemical companies, swimming pools, ports, ship docks.
C5-I (Industry)
very high
4 - 8 ųm
Industrial area with high humidity and aggressive atmosphere.
Areas and buildings with almost permanent condensation and a very high level of pollution.
C5-M        (Marine)
very high
4 - 8 ųm
Coastal areas and offshore areas with an aggressive atmosphere and high salt concentrations: offshore areas, offshore wind turbines, drilling rigs.
Areas and buildings with almost permanent condensation and a very high level of pollution.

Zinc Coating Life Predictor

The corrosion of hot-dip galvanised steel by atmospheric corrosion depends, on the one hand, on climatological variables such as temperature, relative humidity and the proximity to the coastline (chlorides) and, on the other, on the presence of certain environmental pollutants in the atmosphere (mainly sulphur dioxide and nitrogen oxides).

On the basis of understanding of these parameters and taking the best knowledge currently available with respect to the corrosion of zinc, the ILZRO (International Lead & Zinc Research Organization) and Tech Cominco Metals Ltd have developed software to calculate annual zinc losses (in µm/year): the Zinc Coating Life Predictor.

All the required climatological data and environmental pollution determinants can be obtained from the Dutch National Institute for Public Health and the Environment (RIVM) and the Royal Netherlands Meteorological Institute (KNMI).

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