COR-TEN STEEL USAGE AND RUSTING PROCESS
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 used extensively for ISO shipping 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. 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.
Cor-Ten exhibits superior corrosion resistance over regular carbon steel as a result of the development of a protective oxide film on the metals surface that slows down further corrosion. Their yield strength allows cost reduction through the ability to design lighter sections into structures. These steels were designed, primarily to be used in unpainted applications where a reduction in maintenance costs, such as painting, were desired. Weathering steels are now being used in a variety of applications, including bridges, rail cars, transmission towers, chimneys and shipbuilding. It is also becoming increasingly popular with sculptors and as an architectural feature.
Cor-Ten is the primary brand name for corrosion resistant products that were developed by United States Steel Corp. Cor-Ten has subsequently been licensed to be produced by other steel producers. There are basically two types of Cor-Ten that are most prevalent, Cor-Ten A (generally up to 12mm thick) and Cor-Ten B (generally 15mm thick and above).
The comparison of Cor-Ten to the ASTM grades is loosely stated as Cor-Ten A is equivalent to ASTM A242 and Cor-Ten B is equivalent to ASTM A588 Grade A. Cor-Ten A and B both meet and/or exceed the requirements of ASTM A606 Type 4.
Considerations for use of Cor-Ten and weathering steels:
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, (hence 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.
Cor-Ten A and Cor-Ten B 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 (from 1.0mm up to 12mm) and has applications in cladding and ductwork. The second type is more commonly produced as plate (15mm up to 50mm).
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 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 -IOOC. Weather-resisting grades of steel typically exhibit an improvement in the region 50C. In practice, this means that where surface loss due to oxidation in normal steels might be 1 mm per year, the temperature to achieve the same loss in weather-resisting grades would be that much higher. Load bearing capacity can be maintained up to temperatures of about 450C. Improved abrasion resistance (as in the coal wagons) is another feature.
Welding
If its less than 10mm thick and the weld is a single pass (a fillet) you can weld it with mild steel. If its MIG ER70s-6 / SG2 / G3Si1. You will get enough dilution from the plate to weatherise the weld.
If its over 10mm or if its multi-pass you need either a similar composition (nominally 1%Ni 0.5%Cu) usually classified as ER80S-G or ER80S-W alternatively you can use a 2.5% Nickel steel ER80S-Ni2
Corten "A" is a weathering steel that has a higher than normal copper content, this forms a rust preventative oxide on the surface that prevents "weathering" you can get a specific wire for corten but the general concensus is to treat it like s355 type material.
Wetting-drying cycle
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 and failure of a weather-resisting steel structure. There are some environments where special care must be exercised.
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 position 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 necessary.
The rules that apply regarding the electrochemical series of metals should be observed. If dissimilar metals are to 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 metal, 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 (see page 4).
Restrictions
The hygroscopic nature of salt adversely affects the 'patina' as it maintains a continuously damp environment on the metal surface. Consequently, as a general rule, unprotected weathering steel should not be used within 2km of the coastline
The texture of weathering steel is influenced by the orientation of the structure and the degree of shelter it provides. Surfaces facing south and west, and those subject to frequent wet and dry cycles, develop a smoother fine-grained texture. Sheltered structures, and surfaces facing north and east (slower drying), tend to develop a coarse granular texture.
Concrete, stone and unglazed brick may suffer from oxide staining when in contact with weathering steel. Connections to dissimilar materials, such as zinc or cadmium plated bolts, should be avoided.
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.
Weathering steels are high strength, low alloy, weldable structural steels that possess good weather resistance in many atmospheric conditions without the need for protective coatings. They contain up to 2.5% alloying elements, e.g. chromium, copper and nickel. On exposure to air, a protective rust patina forms that adheres to the surface of the steel. This layer causes the rate of corrosion to slow so that after 2-5 years, corrosion almost ceases. Requirement for the formation of the protective corrosion product layer is regular wetting and curing of the surface. Long wet periods may prevent the formation of the protective layer.
Wet environments, immersed or buried conditions are unsuitable for weathering steels.