Weathering steel is a high strength low alloy steel that in suitable environments forms an adherent protective rust 'patina' to prevent further corrosion.
The corrosion rate is so low that bridges fabricated from unpainted weathering steel can achieve a 120 year design life with only nominal maintenance. Hence, a well detailed weathering steel bridge in an appropriate environment can provide an attractive, very low maintenance, economic solution in many locations. This publication highlights the benefits of weathering steel bridges, describes the limitations, and comments on both the material availability and the appearance of such bridges. It also provides advice on a range of issues including design and detailing, fabrication and installation, inspection and maintenance, and remedial measures should corrosion rates exceed those anticipated at the design stage.
Weathering steels or weather resistant (WR) steels 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 BS EN 10025-5: 2004, and have similar mechanical properties to conventional grade S355 steels to BS EN 10025-2: 2004. The most commonly used grade for bridgeworks in the UK is S355J2W+N.
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 conventional structural steels detach from the metal surface after a critical 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, which impedes further access of oxygen and moisture.