Effect of temperature
Hot dip galvanized coatings will withstand continuous exposure to temperatures of approximately 200°C and occasional excursions up to 275°C without any effect on the galvanized coating.
Above these temperatures there is a tendency for the outer zinc layer to separate, but the alloy-layer, which usually comprises much of the coating, remains. Adequate protection may often be provided up to the melting point of the alloy layer (around 650°C).
Bushfires are a special case. In most cases, the temperature of the fire is not sufficiently high enough to damage the steel, or the heat is only applied briefly to the steel. In these cases, the galvanized coating may anneal and the galvanizing diffusion reaction restart, creating a dark zinc-iron alloy layer at the surface. If this occurs, the steel is undamaged, and the galvanized coating will provide ongoing corrosion protection consistent with the remaining galvanized coating thickness. It will not be possible to clean the surface back to the original colour.
A question that is commonly asked is whether the structure, such as a transmission tower, crash barrier or steel bridge element, has been damaged. The answer is straightforward: if a galvanized coating can be measured; the fire was not sufficiently hot to damage the steel and the structure can remain in place. This is because the galvanized coating will melt or vaporise before the steel is damaged. Determining whether the galvanized coating remains can be done via the same straightforward and non-destructive testing method used in the factory using a magnetic thickness gauge.