The immediate environment in which it is located determines the corrosion rate of any metal. Corrosion rates of hot-dip galvanized coatings in different environments have been extensively researched and documented worldwide and have been found to corrode between 1/17 and 1/80 slower than uncoated steel.
Coating life is directly proportional to the coating thickness of the zinc, and as corrosion rates in specific environments are known, the life expectancy of the hot-dip galvanized coating can be estimated.
The following information regarding general atmospheric conditions is supplied as a guide only. Specific microclimates within a general area will influence the rate of corrosion further and must also be taken into consideration. These issues can be discussed further with Hunter Galvanizing staff.
Warm, dry atmosphere
The anticipated life of galvanized coatings in an arid dry environment is long term. Protection may continue indefinitely as zinc stability is very high resulting in minimal coating loss. Zinc coatings on steels of thicknesses of greater than 6mm may remain stable for 100 years or longer.
Coating life may be affected by aerial spraying of fertilizers or insecticides. In dry form these elements will pose little threat, however in solution, fertilizer and insecticides will attack galvanized coatings. Long-term protection can be expected if not subjected to hostile chemicals with a coating loss of about 1-3 microns per year. Based on steel thickness greater than 6mm it is plausible to expect coatings to remain effective 25 to 85 years.
In light industrial areas, hot-dip galvanized coatings will generally perform well with coating loss average of 3-5 microns per year. Based on steel thickness greater than 6mm and subject to exposure to adverse contaminants, 15-30 years may be achieved. High levels of sulphuric gases and chemicals located in some heavy industrial areas will increase coating loss to about 5-8 microns per year. In this environment, a duplex coating system of hot-dip galvanizing and paint may prove beneficial.
Corrosion rates are higher in the presence of salt air. Hot-dip galvanized coatings perform well in comparison to other protective systems; however, duplex coating systems of hot-dip galvanizing and paint provide the optimal protection. Coating loss will average between 5–15 microns per year subject to the proximity to the ocean and levels of rainwater to wash marine salts from the coating surface. Life span of coatings may range between 5-10 years based on steel thickness greater than 6mm.