Rust, baby, rust
In the 1930s, the United States Steel Corporation developed Cor-Ten, primarily for use in railway coal wagons. The controlled corrosion that is a feature of the material was a welcome by-product of the need for a tough steel capable of withstanding the rigours of America's burgeoning marshalling yards and collieries. Because of its inherent toughness, weathering steel (the generic name for Cor-Ten, along with weather-resisting steel) is still used extensively for containers.
The civil engineering applications that appeared in the early 1960s made direct use of the improved resistance to corrosion, and it would not be long before the applications in architecture would become apparent.
Cor-Ten gets its properties from a careful manipulation of the alloying elements added to steels during the production process. All steel produced by the primary route (in other words, from iron ore as opposed to scrap) comes into being when the iron smelted in blast furnaces is reduced in a converter. The carbon content is lowered and the resultant iron, now steel, is less brittle and has a higher capacity for loading than before.
Other material is commonly added during the process. Stainless steel has added chromium and molybdenum, for example, and weathering steel has a combination of chromium, copper, silicon and phosphorus, the amounts depending on the exact attributes required.
Weather-resistant steel works by controlling the rate at which oxygen in the atmosphere can react with the surface of the metal.
Iron and steel both rust in the presence of air and water, resulting in the product of corrosion - rust, iron oxide. Non-weather-resisting steels have a relatively porous oxide layer, which can hold moisture and promote further corrosion. After a certain time (dependent on conditions), this rust layer will delaminate from the surface of the metal, exposing the surface and causing more damage. Rusting rates seen on a graph would appear as a series of curves approximating to a straight line.
Rate of corrosion
The oxide layer on weathering steel is not as porous because it adheres more firmly to the base metal. The curve of rate of corrosion initially progresses at the same rate as ordinary steel, but soon begins to level out.
The weathering process is dependent on the aggressiveness of the environment into which the steel is placed. As might be expected, rural sites fare the best and marine ones the worst when it comes to the eventual longevity of the material. Another factor to consider is the aspect of the weathering steel.
West- and south-facing surfaces weather at a more even rate and form a more even oxide layer. North- and east-facing surfaces tend to be wetter for longer periods of time and often have areas that are darker and more uneven in colouration. This is unavoidable, unfortunately, and is a feature of the material. In the same way that timber bleaching in red-cedar cladding is regarded as something mildly unpredictable, we should look upon the eventual appearance of the oxide layer in weather-resistant steel as an equally natural, and therefore serendipitous, process.
The wetting and drying cycle is important. Continuous dryness is obviously not a problem, pace those burned-out Second World War vehicles that litter North Africa and are destined to remain for some time because they don't rust. Continuous wetness can be problematic, however. Some time ago a series of bridges was constructed from weather-resistant steel for some forest roads.
The condition of the forest floor was typical, moist and mildly acidic. The bridges rusted in the same way as ordinary steel, with the oxide layer attacked by the corrosion products of leaves and the continual exposure to moisture.
Ideally, to weather in the expected fashion, weather-resistant steel needs wetting and drying cycles. This is because moisture activates the corrosion process but, with the drying, the oxide layer obtains its nonporous state. The more rapid the wet-dry cycle, the more even the oxide layer.
Another factor that can affect the finished appearance is size. One reason the Angel of the North exhibits an even orange layer of rust is because of its mass. The south- and west-facing aspects, which collect the majority of the sun's energy, absorb and transmit sufficient heat to limit the amount of condensation that can form on the rest of the statue. If the north and east aspects are borrowing the heat, they will tend to weather at more or less the same rate.
Try that one the next time someone complains about cold bridging.
Two types of weathering steel are commonly produced. These are sometimes referred to as Cor-Ten A and Cor-Ten B. The types differ primarily in the amounts of phosphorous alloyed into the mixture. Uses reflect the different properties imparted to the steel. The first type is typically produced as sheet or coil and has applications in cladding and ductwork. The second type is more commonly produced as plate, structural sections or tube.
Applications of weather-resisting steel vary widely but recently there has been a trend towards an appreciation of the finish in more elegant surroundings. The Royal Court Theatre (featured on page 4) is a good example of the gentrification process slowly happening to what has been regarded as one of the more muscular industrial products.
Another application is in high-temperature environments. Normal steel grades - that is, carbon or carbon/manganese steels - form an oxide layer in the absence of moisture at around 400infinityC. Weather-resisting grades of steel typically exhibit an improvement in the region of 50infinityC. In practice, this means that where surface loss due to oxidation in normal steels might be 1mm per year, the temperature to achieve the same loss in weather-resisting grades would be that much higher. Loadbearing capacity can be maintained up to temperatures of about 450infinityC. Improved abrasion resistance (as in the coal wagons) is another feature.
Designing in weathering steel is primarily concerned with ensuring the wetting-drying cycle, which forms the protective oxide layer, is allowed to happen. As in previous technical articles, the importance of detailing out pockets, crevices, upward-facing channels and so on cannot be over-emphasised. Where such a condition is unavoidable, say for structural reasons, then it is important to include drainage holes or to ensure sufficient ventilation. Anything that retains moisture should be discouraged, again preferably by design.
Leaves, moss and the proximity of trees can all affect the performance of the material adversely.
When viewed in conjunction with the intended environment, detailing can make the difference between success or failure of a weather-resisting steel structure. There are some environments where special care must be exercised. First, atmospheres where there is a high concentration of industrial fumes (thankfully increasingly rare). Second, submerging, or burying in the ground. If this is unavoidable other methods of protection can be employed such as concrete encasement or cathodic protection. Third, exposure to chloride ions, such as in a marine environment or close to a highway, where exposure to salt may pose a problem. Salt can affect the oxide layer because it is hygroscopic and will retain moisture.
Another detailing problem is that of runoff from the steel. It will be impossible, especially while the oxide layer is forming, to prevent the run off from staining susceptible materials unless the detailing of channels and the juxtaposition of such materials is considered carefully.
Organic coatings Non-porous materials are much better.
Glass, stainless steel, glazed bricks and tiles, washable organic coatings and paints, aluminium (anodised or non-anodised), polycarbonates and neoprene remain unaffected or can be cleaned if need be.
The rules that apply regarding the electrochemical series of metals should be observed. If dissimilar metals must be placed in proximity to weathering steel, then good detailing practice should ensure the elimination of traps for water and/or the separation of the metals with an inert material.
This will apply in some cases with fixing techniques. It is common to specify weathering-steel nuts and bolts in conjunction with the main structure. It is also possible to use stainless steel or even galvanised steel fixings, providing the latter are isolated from the surface of the weathering steel. Welding poses no problem. Most manufacturers of welding materials provide consumables suitable for the fabrication of weather-resisting steel.
It is possible to paint weather-resistant steel. The requirements of such a paint system do not differ from those required for normal grades of steel. One significant advantage that occurs when doing this (as is common in containerised storage) is that damage to the paint does not result in under-creep corrosion to the surrounding painted area.
In summary, then, the success of weatherresistant steel in a building or structure is highly dependent on the level of thought applied to three main areas: its immediate environment; the creation of the wettingdrying cycle; and its relationship with other materials. If these simple rules are followed, weather-resistant steel should do exactly what it says on the tin.