WITH major electrification projects on existing lines underway in two European countries, along with the electrification of new high-speed lines under construction, and existing electrification equipment requiring periodic maintenance and renewal, there is currently a growing demand for steel. The question is should railways continue to use galvanised steel, or are there cost and environmental benefits in changing to "weathering" steel?
Design work has started on the electrification of 550km of lines in Denmark, while in Britain Network Rail (NR) is electrifying around 800 route-km of the network, including the Great Western Main Line from Heathrow Airport Junctiom to Oxford, Basingstoke and Swansea, the Midland Mainline from Bedford to Sheffield, and the Trans-Pennine route linking York with Liverpool.
The efficiency and sustainability of railways is even more important in these times of fiscal constraint, while governments face the challenge of maintaining let alone increasing investment, so the pressure is on to secure value for money, increase operational performance, and be kinder to the environment.
Masts and structures for overhead line equipment (OLE) are traditionally manufactured from mild steel rolled sections which are then immersed in molten zinc to provide an anti-corrosive layer commonly known as hot dip galvanising. Yet it could be argued that if the advantages of using weathering steel were utilised, then the hot dip galvanising process could be eliminated.
The anti-corrosive properties of weather-resistant steels are better than those of other structural steels, and in many applications, there are strong economic reasons for using weathering steels, especially when the total life-cycle costs of the structures are taken into account at the project design stage.
The enhanced weather resistance of this type of steel is based on a "self-healing" oxide layer. This corrosion-retarding effect of the oxide layer is produced by the particular distribution and the product's concentration of alloying elements. The oxide layer protecting the surface develops and regenerates continuously when subjected to the influence of the weather. To put it simply, the steel is allowed to rust in order to form the protective oxide layer.
The use of uncoated weather-resistant steel in steel structures saves on expensive surface treatment processes. Currently it is primarily used in architecture, bridges and other civil engineering structures. A prime example is its use on highway widening schemes where weathering steel is used extensively for earth retaining walls in cuttings. It is evident that engineers are satisfied that the use of weathering steel on highways where highly concentrated salt spray in the winter, and inherent ground salts and leachates have to be contended, is up to the job.
Manufacturing structures from weathering steel is basically no different or more costly than using equivalent mild steel, and procedures related to selecting the correct welding consumables and techniques to match the parent metal are now well known throughout industry. Indeed, weathering-steel OLE masts and structures have been in operation on the Danish rail network since 1986.
For a typical model mast design, the approximate cost of hot dip galvanising accounts for around 20% of the total manufacturing cost. There is also an environmental disadvantage with the manufacturing process. Zinc ore has to be mined, the smelting of zinc ores produces large amounts of sulphur dioxide and cadmium vapour, and the smelter slag and other toxic residues of the process contain significant amounts of heavy metals.
Transport and energy costs involved with the conversion of the zinc ore into ingots, and the final hot dip galvanising process, use up non-renewable resources. These costs are easily overlooked and the environmental consequences are often unknown to the end user.
While switching to weathering steel masts and structures would reduce maintenance costs because it would avoid having to repair the deteriorated or damaged zinc coating on mild steel masts, there are a number of hurdles to overcome.
Steel stockholders say that weathering steel is currently only available in sheet form due to a lack of demand for rolled sections. This leads to the question: what would be required to make it commercially viable for steel producers to supply rolled sections? In addition, would there be a price mark up?
Fabrication of masts from plate must be avoided due to the cost outweighing the benefits of using rolled steel sections.
The standard rolled steel sections used for galvanised masts are actually far stronger than necessary, if the design was to consider a new range of rolled steel sections specifically designed for the expected and known loads imposed on masts and structures, it is likely further savings could made on material costs.
Danish example
Danish State Railways (DSB) and Banedanmark's specification for the OLE masts and structures required that they remain in-service and maintenance-free for 50 years. In addition, DSB insisted that the OLE design fit aesthetically into the environments in which it would be erected.
A solution was found through close collaboration between the architects and the design engineers. The neutral reddish brown oxide layer has proved to be an aesthetically pleasing design that fits perfectly into the natural landscape of Denmark. Indeed, DSB and its design team received the European Award for Steel Structures in 1987 due to its good design and the interplay with the surrounding environment.
Atmospheric conditions were also considered during development. Studies have indicated that weathering steel is sensitive to salt-laden environments where the oxide layer can become unstable leading to further corrosion. But is this not the case with other metals?
While more work will be needed to fully assess the costs and implications of using weathering steel for electrification masts and structures, it does show potential benefits.
