Salt deposits are a major cause of zinc corrosion. In industrial areas, the presence of sulphurous gases and chemicals can be corrosive, because when they mix with moisture they tend to attack zinc. These risks can be mitigated by painting. It is important to understand that different geographical locations display widely differing corrosive factors.
For example, the False Bay coastline is more corrosive than the Atlantic coastline due to the prevailing onshore South East wind that blows in the summer months. There is no wash-down of salt deposits (chlorides) in rainfall from the onshore South East wind and structures near the sea that are directly exposed receive a sand, sea salt and wind spray battering. In winter, the prevailing wind is from the North West and the side of the galvanised item facing the sea will corrode much quicker than the side that is facing inland.
On the Atlantic Table Bay coastline, the prevailing summer wind (South East) blows offshore on galvanised structures, so any salt spray is continually washed down and diluted by the North West wind.
For extremely corrosive environments (CATEGORY A in the Corrosion Profile) , a galvanised coating must be accompanied by a minimum three coats superior duplex paint system, with a dry film thickness (dft) of 200 µm, as this is a very aggressive environment. Please refer to the accompanying Corrosion Profile PDF below.
For highly corrosive environments (CATEGORY B and C in the Corrosion Profile), the galvanised coating should be followed by a two or three coat duplex system for optimum performance.
The ability of zinc to protect the steel is enhanced by the paint. As long as the paint coating is intact, the zinc is protected from the aggressive environment. To repaint structures on the sea can be extremely difficult due to foreign contaminants, so it is necessary to paint them correctly first time.
In the presence of atmospheric moisture, such as that found in suburban areas, the zinc oxide film is quickly converted to zinc hydroxide and carbon dioxide normally present in the air reacts to form basic zinc carbonates. These stable inert compounds resist further action and ensure long life for the protective zinc coating.
In warm, dry rural atmospheres, the stability of zinc is remarkable. The zinc-oxide film formed during initial exposure remains intact and prevents further reaction between the zinc coating and the air, and protection continues indefinitely.
The figures in the Corrosion Profile document (link below) are an attempt to give engineers and clients a guideline to estimate the potential lifetime of the zinc coating in various proximities to the sea in the Western Cape region of South Africa. Precise comparisons of the corrosion behaviour of the galvanised coating in various atmospheric conditions is influenced by many factors such as prevailing wind direction, density of corrosive fumes and pollutants, amount of salt sea spray, number of wetting and drying cycles, duration of exposure to moisture and the amount of shelter the article receives from these corrosive conditions. Please note that galvanising is only effective in an atmosphere in which the pH ranges from 5.5 to 12.0.