Weathering Steels

Weathering Steels All low alloy steels have a tendency to rust in the presence of moisture and air. This rust is a porous oxide layer which can hold moisture and oxygen and promote further corrosion. The rate of rust formation depends on the access of oxygen, moisture and atmospheric contaminants to the metal surface. As the rusting 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 conventional structural steels detach from the metal surface after a certain time and in the process exposes the surface once again to rusting and thus commencing the corrosion cycle again. The rate of rust formation progresses as a series of incremental curves approximating to a straight line. The slope of this straight depends on the aggressiveness of the environment. Weathering steels are weather resistant steels which work by controlling the rate at which oxygen in the atmosphere can react with the surface of the metal. These steels are high strength low alloy steels which can provide corrosion protection without additional coating. Increase in alloying elements, mainly copper, provides an arresting mechanism to atmospheric corrosion in the steel itself. The alloying elements in the steel produce a stable and durable rust layer that adheres to the base metal. This rust ‘patina’ develops under conditions of alternate wetting and drying to produce a protective barrier, which impedes further access of oxygen and moisture. This patina acts as a skin to protect the steel substrate. Section loss on the order of 100 mils (2.54 mm) may be expected before the patina sets up, but this is negligible to the structural performance. Comparison of rate of rusting in low alloy steel and...