Weathering steel should not be used in atmospheres where high concentrations of corrosive chemicals or industrial fumes, specifically SO
Weathering steel should not be used in atmospheres where high concentrations of corrosive chemicals or industrial fumes, specifically SO2, are present. Such environments with a pollution classification above P2 to BS EN ISO 9223 (i.e. SO2 > 80mg/m2/day) would rule out the use of weathering steels. However, this is an extreme level, which is rarely encountered today, under present limits on industrial pollution of the atmosphere.
Concern has been expressed in the past about the effect of diesel fumes on the long-term performance of a weathering steel bridge over a railway. Whilst diesel fumes contain airborne sulphur compounds, within limits they actually have a beneficial effect in forming insoluble corrosion products by reacting with the alloying elements in the steel.
Data from Tata Steel research programmes have shown that the corrosion rates of weathering steel in highly polluted industrial (sulphur bearing) atmospheres averaged at 2 μm/year compared to 50 μm/year for conventional structural steel. In these natural exposure tests, the industrial environment was comparatively extreme, i.e. in close proximity to an industrial coking plant.
By comparison, a microclimate beneath a bridge as created from passing trains is unlikely to produce similar environmental conditions. Increased pollution levels may be anticipated when locomotives are stationary directly beneath the bridge but it is unlikely that the duration of exposure and the concentration of sulphur compounds on the steelwork would exceed the 200mg/m2/day limit. In addition, the slightly oily nature of the deposits from diesel exhaust fumes may also act as a barrier to water and reduce corrosion of the steel.
Rolled sections in weathering steel are less readily available in the UK than ordinary structural steel sections. This is particularly true for small quantities. ArcelorMittal do offer such sections, but there is a key sentence in their Arcorox brochure - “Minimum order quantity is subject to agreement”. As such, designers are advised to confirm the availability of the required weathering steel rolled sections with the manufacturer at an early stage.The potential lack of availability of rolled sections in weathering steel does not pose any problem for the supply of main girders as they can be fabricated from plate (even with ordinary structural steel, rolled sections are rarely used). However, angles, channels and hollow sections are often used for bracing elements on ordinary structural steel bridges, so for weathering steel bridges, alternatives must be considered.
The possible options are:
Use of unpainted ordinary steel for temporary bracing, which is removed afterwards
Use of painted ordinary steel bracing that is left in place
Use of plan bracing within the depth of the slab
Fabrication of angle and channel sections from weathering steel plate