BUSHFIRE RESISTANCE OF GALVANIZED COATINGS 

Unlike most other applied organic coatings used to protect steel, galvanized coatings are zinc-based metallic coatings applied to steel using a variety of technologies. The performance of these zinc-based coatings when subjected to bushfire depends on a number of factors, not the least of which is the characteristics of the galvanized
coating arising from the technology with which it is applied. Pure zinc has a melting point of about 420°C, this means a coating consisting mainly of pure zinc can be expected to start melting once this temperature is reached on the metal surface.
Many galvanized products produced by a continuous galvanizing process (sheet, coil and some tube) have a coating that is largely pure zinc and is typically 15-25 microns in thickness. Hot dip galvanized coatings, applied by immersing batches of fabricated steel in a bath of molten zinc, are made up of zinc-iron alloy crystals with a thin coating of zinc on the surface. These zinc-iron alloys constitute 80-100% of the coating, depending on steel chemistry and processing techniques. These zinc-iron alloys are not only much harder than pure zinc (about 4-5X) but have a much higher melting point of around 650°C.

Typical bushfire conditions may expose steel structures to temperatures of 800°C for periods of 120 seconds. Depending on section thickness of the steel, the actual steel surface temperatures do not exceed 430°C for steel of 3mm thickness and 300oC for steel of 5mm section thickness, according to Dr. Ian Bennetts, Professonial Fellow, Centre for Environmental Safety and Risk Engineering, Victoria University of Technology. At the temperature of molten zinc (420°C), proof stress of the steel is reduced to 70% of its original value. At a temperature of 650°C, the steel will suffer a significant reduction in proof stress. Unlike most other metals, zinc-based coatings will vaporise at relatively low temperatures (about 950°C) and re-condense as zinc oxide fume below that temperature. This phenomenon is commonly seen when galvanized coatings are flame-cut or welded. In addition, zinc coatings are generally reflective and being metallic, have high emissivity characteristics. For this reason, a galvanized surface will not absorb heat at the same rate as an organically coated or non-metallic surface. Another metallurgical phenomenon takes place with galvanized coatings, in that if there is any free zinc in the galvanized coating and the steel temperature approaches but does not exceed the melting point of zinc, a `solid state’ reaction can take place and the free zinc will be converted into zinc-iron alloy.

In summary, if the bushfire flame duration and intensity is not high enough to compromise the structural strength of the steel, then the hot dip galvanized coating should remain largely unaffected through a bushfire event.

When you build your fence with Waratah steel post & Longlife wire products, you can be safe in the knowledge that you have chosen products that have been made in Australia for Australian conditions.

* An extract from 'Coatings & Bushfires' article 06 - Industrial Galvanizers Corporation Pty Ltd