Weathering steel or weather resistant steel are colloquial terms used to describe structural steels with improved atmospheric corrosion resistance. These steels are high strength low alloy steels that under normal atmospheric conditions give an enhanced resistance to rusting compared with that of ordinary carbon manganese steels. Weathering steels are generally specified to EN 10025-5 , and have similar mechanical properties to ordinary structural steels to EN 10025-2 . The most commonly used grade for bridgeworks in the UK is S355J2W.
In the presence of moisture and air, all low alloy steels have a tendency to rust, the rate of which depends on the access of oxygen, moisture and atmospheric contaminants to the metal surface. As the process progresses, the rust layer forms a barrier to the ingress of oxygen, moisture and contaminants, and the rate of rusting slows down.
The rust layers formed on most ordinary structural steels are porous and detach from the metal surface after a certain time, and the corrosion cycle commences again. Hence, the rusting rate progresses as a series of incremental curves approximating to a straight line, the slope of which depends on the aggressiveness of the environment.
With weathering steel, the rusting process is initiated in the same way, but the specific alloying elements in the steel produce a stable rust layer that adheres to the base metal, and is much less porous. This rust ‘patina’ develops under conditions of alternate wetting and drying to produce a protective barrier that impedes further access of oxygen, moisture, and pollutants. The result is a much lower corrosion rate than would be found on ordinary structural steel.
The basic metallurgical difference between weathering steel and ordinary structural steel is the addition of chromium, copper and nickel alloying elements, which give the weathering steel its enhanced resistance to corrosion. Comparison of the material standards for weathering steel and ordinary structural steel shows that the specification requirements for all other elements in the steel chemistry are similar.