Weathering Steel - Advantages and Disadvantages - Research Paper Example

Weathering steel: Introduction: “Weathering steels or weather resistant (WR) steels are colloquial terms used to describe structural steels with improved atmospheric corrosion resistance.” (Corus, 2005). Weathering steel is indeed a group of steel alloys that when exposed to open air, oxidize to develop a rusty, dark brown protective layer that keeps the steel from oxidizing further, thus stabilizing the oxidation and obviating the need for the steel to be coated and protected with paint. It is essentially a low-alloy steel with high strength. Weathering steel is also known as Indaten or Cor-Ten (www.constructalia.com, 2010). Its mechanical properties are influenced by the percentage of different alloys constituent of the weathering steel. The most distinguishing characteristic feature of weathering steel is its anti-corrosive behavior exhibited through a self rusting procedure that is a byproduct of the unique nature of oxidization it undergoes, which is quite different to that experienced by unalloyed forms of steel. Although weathering steel experiences corrosion just like other steels in the beginning, the process is different once the rust layer is formed. Conventional steels undergo rusting to develop an unstable layer of rust, that would detach from the base metal with due course of time allowing for the process of rusting to continue. Thus the steel continues to corrode. It is the enhanced stability of the rust layer of weathering steel that distinguishes its rusting process from that observed in other steels. This paper evaluates the advantages and disadvantages of weathering steel, with special reference to its use as a construction material. Advantages: When the benefits of different metals are measured for use in structures and members exposed to weather, weathering steel shows a considerable edge over others on the basis of the following characteristic features. Beautiful natural color: Weathering steel attains the color of rust after some time of exposure to open air and under alternate cycles of drying and wetting. The beautiful rusty appearance of weathering steel is one of the reasons of its selection as a construction material because it is not possible to artificially produce the color and texture it owns naturally. Decorative uses: With its unique dark rusty purplish brown color, it appears very simple yet appealing to be used in various forms of art in general and the sculptures in particular. Street statues and monuments are commonly made of weathering steel. It is the same weathering steel, that lends the statues a rich texture and rusty shine. The intrinsically beautiful rusty color of weathering steel lends a touch of antiqueness to the sculptures, which adds to their value. The Chicago Picasso sculpture in Chicago is one such example of the use of weathering steel in outdoor sculptures. (www.worldlingo.com, 2010). Use as a construction material: In addition to that, the use of weathering steel in the construction of structures as important and large as the New River Gorge Bridge, the Newburgh-Beacon Bridge as well as the Australian Centre for Contemporary Art (ACCA) speaks of its popularity and usage in structures of both social and cultural significance. (www.worldlingo.com, 2010). Decorative use of weathering steel is widespread. Simple maintenance: Maintenance expenses associated with the weathering steel are considerably low as compared to other types of steel or unalloyed steel. The only maintenance required to keep weathering steel in proper form is general inspection and cleaning. This specialty of weathering steel makes it the best material for use in limited access structures like bridges and high rise buildings or towers in which access for maintenance can prove dangerous. Therefore, weathering steel is best-suited to places where future contact is essentially minimized given the simple and general nature of maintenance required. Corrosion resistance: The biggest advantage of weathering steel is that it offers strong resistance to corrosion without the help of a protective paint layer covering the steel. This eliminates the need for the steel to be painted. As the name implies, weathering steel “weathers” or “rusts” to form a protective layer that is corrosion retardant in nature. This becomes possible because of its chemical composition that necessarily is based upon steel alloys that are distributed and concentrated in the steel in such a way that favors the continuous rusting of the steel, when it is subjected to open air. Big structures, typically the bridges and open frame buildings that make use of steel other than weathering steel incur huge painting and maintenance costs every year. Due to its widely accepted usefulness in steel bridge construction, a research was conducted to compare the costs of weathering steel and other alternatives in bridge construction and maintenance. Owing to its corrosion resistant properties, the cost of weathering steel bridges was estimated to be 5 % lower as compared to conventional bridges using unalloyed painted steel that adopted a HA Type 2 paint system. (Townsend et al, 2000). Besides, costs cut because of elimination of paint outshine the costs incurred because of any additional material needed in case of weathering steel bridge construction. Increased life of structure: Because of corrosion resistant nature of weathering steel, it tends to maintain its strength over long periods of time. This eliminates the need for rehabilitation, retrofitting or demolition of old structures because of loss of strength. Hence, life of the structures in which weathering steel is used is elongated considerably. Durability: Weathering steel maintains its strength and durability when exposed to very high temperatures. This is evident from the findings of an eight-year long study based on the atmospheric corrosion tests of A 588B weathering steel which were executed by (Townsend et al, 2000) in different environments namely the industrial, marine, and rural environments employing the following two methods of heat treatment: 1. Quenching and tempering of the weathering steel to result in the development of a microstructure of tempered Martensite. 2. Normalization of the weathering steel to build a microstructure composed of Pearlite and Ferrite. (Townsend et al, 2000). The findings of this research suggested no relationship between the above mentioned heat treatment methods and the corrosion resistance properties of the weathering steel because of no observable alteration in its performance as a result of the tests. This also reaffirmed the concept that the corrosion resistant behavior of weathering steel is governed by its composition instead of the extent of heat it is exposed to. Ordinary steel starts to loose oxidation at temperatures higher than 400 degree centigrade. Use of weathering steel instead of unalloyed steel serves to reduce such a loss of oxidation even at temperatures above 400 degree centigrade. “At these temperatures a patina forms even in the absence of water. The precise performance improvement is dependent on the heating cycle and environmental conditions.” (www.corusgroup.com, 2004). Besides, the corrosion preventing shield that the base metal wears tends to enhance its life and reduce the life cycle costs by obviating the need for successive replacements of corroded steel. Sustainability: The material weathering steel is made up of is both recyclable and recycled in nature, which is the reason why it is thought of as sustainable. Disadvantages: Although weathering steel provides enough benefits to justify its selection as a construction material for use in massive steel structures, yet certain factors need to be considered while evaluating its utility. These factors include occurrence of uncontrolled corrosion in salt-laden air areas and under continuous contact with water, rust staining of adjacent buildings or structural members in contact with the weathering steel and others as discussed as below: Complicated welding match: Due to its unique properties, weathering steel can often be very difficult to suit a welding electrode. Choice of the right kind of welding electrode can be cumbersome. Before welding begins, it is imperative that the pre-heat levels for the welding are defined. These levels are influenced by the extent of Carbon in the steel. The Carbon Equivalent Value (CEV) of a weathering steel is generally much higher as compared to that of unalloyed steel and must be determined to locate the pre-heat level required for welding, which can be quite cumbersome. Differential weathering: The rate of weathering of the base steel and that of the weld points may not necessarily match. This may give rise to a difference in the strength of the base weathering steel and the weld points, which may be a cause of the development of unwanted stresses. Therefore, special welding techniques might be required to ensure that the base steel, fasteners and the weld points weather at the same rate in order to prevent the steel from experiencing a non-uniform weathering. Requirement of drainage: Although weathering steel is known for its anti-corrosive properties, yet the fact that it does undergo self-rusting should not be underestimated. Weathering steel can only develop the self protective rust layer when it is exposed to weather and rain. However, water should not be allowed to stay on the weathering steel, given the fact that it is not rust-proof. If water finds its way to the pockets, such areas may undergo increased corrosion, and the corrosion may continue to develop holes in the structure. Thus, accumulated water may cause harm to the integrity of the structure and cause the weathering steel to behave like conventional unalloyed steel. Therefore, structural members made of weathering steel should be designed in such a way that would ensure proper drainage of water. This requirement calls for a modification in the design. The design would need to be altered to provide more trenches and slopes. Unsuitable for salty areas: Certain places like sea-shores and near-sea areas have a salty environment. In such places, the air is also salt-laden. Weathering steel is not the right option for the construction of bridges or high rise steel structures in such areas. The salt-laden air may interfere with the protective rust layer as a result of which, it may loose its stability. Hence, in salty environments, the weathering steel may undergo continuous corrosion. This may lead to the development of holes in the structure interfering with the distribution of load among the structural members. No choice of color: Use of weathering steel in structures makes them look antique. Structures made of weathering steel are necessarily rusty in appearance, which may not always be appealing. The rusty appearance may make the structure look unfinished or corroded and weather-worn. An antique looking open frame steel building may look very odd with elegant painted modern buildings in the surroundings. This may call for a need to paint the structure. Rough surface for paint: Although use of weathering steel eliminates the need to paint the structure, yet this might be a compromise on the look of the structure. Even if there is a need to paint the structure to restore the beauty and elegance of the structure, the weathering steel members do not provide a very smooth surface for the paint because of the presence of the rust layer over the base steel and the final look and texture is always rough. Also, because of the roughness of the surface of the underlying weathering steel, the uneven coat of paint may start to peel off on its own with due course of time, thus projecting an even weathered image of the structure. Effects on nearby structures: Corrosion always leaves an impact on the surroundings wherever it takes place. Similarly, when the weathering steel initially undergoes corrosion in the course of formation of the protective rust layer, it leaves stains on nearby structures. Rusty stains may be observed on buildings or structural members in contact with the rusting weathering steel structure. In order to prevent structural members in contact with the weathering steel from rusting, drip pans must be provided at the interface. (McDad et al, 2000, p.52). High temperature limitations: At temperatures higher than 450°C, weathering steel no more remains eligible for employment in huge loaded structures. (www.corusgroup.com, 2004). Conclusion: Weathering steel offers a very healthy and cost effective solution for massive steel construction including bridges and open frame structures exposed to harsh weather conditions in terms of low maintenance costs, enhanced strength and elongated life of the structure. Works cited: Corus. “Weathering steel bridges”. Orchard Corporate Ltd. 2005. 26 June 2010. . “Indaten ®: weathering steel”. Constructalia. 2010. 27 June 2010. . McDad, B., et al. “Use of Weathering Steel in TxDOT Structures”. 2 June 2000. 27 June 2010. . Townsend, Herbert E. and Homer Research Labs. “Atmospheric Corrosion Performance of Quenched and Tempered, High Strength Weathering Steel”. March, 2000: 26 – 31. 27 June 2010. http://www.onepetro.org/mslib/servlet/onepetropreview?id=NACE-00447&soc=NACE&speAppNameCookie=ONEPETRO “Weathering steel”. Orchard Corporate Ltd. 2004. 27 June 2010. . “Weathering steel”. WorldLingo Translations LLC. 2010. 27 June 2010. < http://www.worldlingo.com/ma/enwiki/en/Weathering_steel>. Read More