Weathering steels offer improved resistance to corrosion thanks to the addition of copper during manufacture. Additional alloying elements can be added to increase the steel’s tensile strength or make forming processes easier. Weathering steels are classed as high strength low alloy (HSLA) carbon steels. Their characteristics are specified in the European standard EN 10025-5 and in the American standard ASTM G101:04 (American Society for Testing and Materials). The metallurgical composition of weathering steels includes less than 0.2% carbon. Alloying elements (mainly copper, chromium, nickel, phosphorus, silicon, and manganese) typically comprise less than 5% of the steel. Weathering steels are known as corrosion resistant steels. Like standard carbon steels, weathering steels oxidise when exposed to the atmosphere. Due to their specific chemistry, the corrosion rate of weathering steels is generally much lower than that of standard carbon steel.
Weathering steels can be classified into two categories: those with limited phosphorous content (typically less than 0.035%); and those with a higher phosphorous content. Weathering steels with a phosphorous content of between 0.06 and 0.15% are identified by the letter P at the end of the product name. High levels of phosphorous improves the corrosion resistance of weathering steels. Phosphorous is not used in heavy plate for structural uses as it can form iron phosphide (FeP3) during welding. This can hamper weldability and cause the weld zone to become brittle. For this reason, phosphorous weathering steels are usually only available in thicknesses lower than 12 mm*.