Part 1: Architect’s Guide 1 - 56 6.5.3 Protection by galvanizing The principle of galvanizing is the formation of protective layer of zinc and zinc-steel alloy on the surface of the steel parts in order to protect against corrosion. The galvanized layer which is gradually oxidized to form corrosion products that are generally adhesive. However, this form of protection is time limited and loses its effectiveness when all of the zinc coating has corroded. Speed of corrosion and life of protection Atmospheric conditions and climate have a direct impact on the rate of corrosion. Polluted industrial conditions near the sea are obviously more aggressive than the rural hinterland. Depending on atmospheric conditions, the corrosion rate varies between 0,1 microns/year and 8 microns/year. EN ISO 14713[1] provides guidance on the average annual zinc corrosion rate. This allows lifetimes to be estimated according to the coating thickness. Estimated life of protection by galvanizing: approximately 25 years Aesthetic appearance and painting All galvanized steel can be painted to alter the surface appearance. Painting greatly enhances the life of the galvanized steel. Maintenance painting of galvanized steel is very easy: a brush is used to apply new layer of paint to the damaged area. The paint can also be applied as an additional form of corrosion protection in environments where acidic solutions may attack the surface coating. Galvanizing processes Zinc can be applied either by immersion or spraying (metalizing solution) or by electrolysis. Immersion is generally used for new parts, for dimensions compatible with the size of the baths. Spraying is used instead for renovation work or for larger building elements. Electrolysis is suitable for small parts and batch processing (e.g. bolts). Some observations Welding: Before welding galvanized parts, it is recommended that the coating is removed in the area of the weld seam. After sealing, the affected area will be protected by a zinc-rich paint. Exposure to fire: A galvanized steel part that is exposed to fire behaves like uncoated steel and there is no improvement in fire resistance.
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