Weathering steel, or to use its technical title of “structural steel with improved atmospheric corrosion resistance”, is a high strength low alloy structural steel that, in suitable environments, may be left uncoated because it forms an adherent protective rust “patina” that minimises further corrosion. The alloys added to weathering steel compose only 2% of the steel make-up with specific alloying elements such as copper, chromium, silicon and in some cases phosphorus. The additional alloying does not diminish the structural capability of the steel, with the steel offering strength, ductility, toughness and weldability suitable for bridge construction and covered by the Australian Standard AS/NZS 3678. All structural steel corrodes, at a rate which is governed by the access of moisture and oxygen to the metallic iron. As this process continues, the oxide (rust) layer becomes a barrier restricting further ingress of moisture and oxygen to the metal, and the rate of corrosion slows down.