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ADVANTAGES of COR-TEN STEELS for CARGO CONTAINERS
COR-TEN steels are high-strenght low-alloy weathering steels which have improved resistance to denting, puncture and atmospheric corrosion as compared to structural carbon steels-longer paint life is also obtained. Because of these advantages, they are finding increased usage in cargo containers and other applications.
Due to poor economic conditions in the early 1930s, designers in the USA were very cost conscious and searched for low-cost steels with higher strenght than standard structural carbon steel. Such steels would allow lightweight construction and reduce costs for many products and structures. Also, the designers wanted a steel with improved atmospheric corrosion resistance, so that when lighter sections were used the life of the structure would not be diminished. Constructional nickel steels (2.0 to 3.5 per cent nickel) and structural silicon steels were the only high-strenght steels being produced in plates and structural shapes, but these steels were too costly for general use and were hampered by their limited weldability and formability.
In view of the large potential market for a new type of steel meeting the above criteria, United States Steel Corporation (now US Steel, a division of USX Corporation) launched a lengthy investigation in which hundreds of steel compositions were evaluated for mechanical properties and atmospheric corrosion resistance. The results of these tests culminated in the development ofCOR-TEN steel, which was first introduced in 1993.
COR-TEN steels are part of a group of ?igh-strenght, low-alloy?steels. ?igh-strength?because they have minimum yield strengths of 345 to 480 MPa (50 to 70 ksi) and ?ow-ally?because they generally contain only 2 to 3 per cent total alloying elements. In addition, these steels are part of a group called ?eathering steels? because they provide improved atmospheric corrosion resistance as compared to that of plain carbon steel. There are several grades of COR-TEN steel presently available for different mill products and end uses. The most commonly used grades are COR-TEN A (the original COR-TEN composition) and COR-TEN A-F (for better formability) for thicknesses up to 0.5in (12.7 mm), and COR-TEN B for greater thicknesses. The compositions of COR-TEN A and COR-TEN B steels are shown in Table 1. COR-TEN A-F is essentially the same composition as that of COR-TEN A, except that it is produced with low sulphur and inclusion shape control, which provides the improved formability.
CORROSION TEST DATA
While numerous atmospheric corrosion tests had previously been made with COR-TEN steel and its superiority over structural carbon steel had been well established, these tests were conducted primarily in the USA. Therefore, in the early 1970s US Steel and several of its COR-TEN steel licensees initiated an extensive testing programme to demonstrate the atmospheric corrosion performance of their steels on a worldwide basis. In this programme test panels of COR-TEN A and COR-TEN B steels, along with panels of copperbearing steel and carbon steel, were exposed for time periods of up to 16 years at test sites in 10 different countries. The sites included urban-industrial, rural, and moderate-marine atmospheres. The results of the test programme are summarised below.
The average corrosion rates for the 8 to 16 year period are shown in Figure 1 (urban-industrial sites), Figure 2 (rural sites), and Figure 3 (moderate-marine sites). With few exceptions, COR-TEN A showed the lowest corrosion rate of the steels tested, and COR-TEN B the next lowest rate. Average corrosion rates for COR-TEN A ranged from 0.1 to 11.2 um/year (0.01 to 0.44 mils/year) in urban-industrial sites, 0.3 to 6.6 um/year (0.01 to 0.26 mils/year) in rural sites, and 2.3 to 13.09 um/year (0.09 to 0.55 mils/year) in moderate-marine sites. For COR-TEN B the corresponding rates were 0.3 to 17.2 um/year (0.01 to 0.68 mils/year) in urban-industrial sites, 0.8 to 10.7 um/year (0;03 to 0.42 mils/year) in rural sites, and 3.8 to 14.7 um/year (0.15 to 0.58 mils/year) in moderate-marine sites. These rates indicate that COR-TEN steels are suitable for use in most environments.
The greatest differences in corrosivity of the various test sites were noted in the industrial environments, where much higher corrosion rates were exibited in the European countries as compared to those in the USA, South Africa and Japan. This is believed to be an effect of latitude; at the higher latitude locations in Europe greater wetness time would be expected.
PAINT LIFE ON COR-TEN STEELS
It has been found that in most cases the paint life on COR-TEN steels is about 1.5 to 2 times that of carbon steels. The appearance of painted carbon steel and COR-TEN steel samples after 15 years?exposure in a severe marine atmosphere is shown in Figure 4. The reason for the improved paint life is believed to be that, at any breaks or discontinuities in the paint film, the voluminous corrosion products that form on carbon steel tend to undercut the paint. On COR-TEN steels the corrosion products are more dense and less voluminous and therefore do not undercut the paint as severely. This effect is evident in Figure 5 on painted and scribed panels of carbon steel and COR-TEN A exposed to a harsh chemical atmosphere for about eight months.
COR-TEN steel first became known to the container industry 30 years ago when US Steel furnished an all COR-TEN container to Matson Navigation for in-service testing.
This box was monitored for some 18 years and still had a useful working life remaining. From this successful use of COR-TEN in reducing maintenance cost the market expanded.
While first used exclusively for container roofs, Interpool and ICS (now) Transamerica) in the mid-1970s started specifying COR-TEN for the balance of the dry bulk van containers.
In the mid-1980s US Steel participated in several container conferences discussing the advantages of using COR-TEN steel. From this time on many of the major leasing and marine container purchasers started their own testing programmes.
Companies such as Genstar, Cronos, Orient Overseas Container Line along with Transamerica Leasing and Interpool have proven in their tests the advantages of using COR-TEN steel. All of the major players in the container leasing field are now specifying COR-TEN steel for their dry bulk and more recently the speciality containers, such as tank and reefer containers.
These containers are subject to considerable phisical abuse during loading and unloading, and also to severely corrosive industrial atmospheres and ocean salt spray. For this application the greater strength and dent resistance of COR-TEN steel (40 per cent greater than that of structural carbon steel), plus its good corrosion resistance and superior paint life, result in substantial cost savings. One large user of these containers has found that maintenance and repair costs can be cut by up to 80 per cent by using COR-TEN in place of carbon steel.
COST EFFECTIVENESS OF USING
There are also cost advantages to the use of bare COR-TEN steel for structures. They include no initial painting, no periodic blast cleaning and repainting, and no environmental control costs associated with painting and blast cleaning of painted structures. Over the life of the structure, these advantages far outweigh the higher base price of the COR-TEN steel as compared to that of carbon steel. Examples of structures in which use of bare COR-TEN steel has shown significant cost advantages include bridges, highway sign structures, electrical transmission towers and chemical plant structures. Some specific examples are described below.
Bridges. The example used here is that of a two-lane plate-girder bridge, and is taken from data reported by Aulthouse2, with cost figures updated to October 1993.
Costs for an 886-ton, 1233ft (376 m) span bridge were calculated using (a) bare COR-TEN B steel for all components, and (b) substituting painted non-weathering steels (ASTM A572 grade 50 and ASTM A36) for the COR-TEN B steel. In case (b), A572 steel was substitued for all the girder flanges and web plates over the piers, where a minimum yiel strength of 345 Mpa (50 ksi) was required, and A36 steel was substitued for all other areas of the bridge.
To determine life-cycle cost comparisons, it is necessary to estimate the maintenance painting requirements for the non-weathering steels over the life of the bridge. Aulthouse used a 50-year life and estimated that, although the zinc primer and intermediate epoxy coats would not require maintenance painting, the urethane topcoat would require repainting at 15-year intervals. Using 1993 prices and annual inflation rate of 5 per cent, total maintenance painting costs over the 50-year life were estimated at $1 302222. Adding the savings for the original cost of bare COR-TEN B versus painted non-weathering steel ($208542) the total savings over the life of the structure is ? 510764.
Chemical plant structures. Schmitt and Mathay have published data on comparative costs for COR-TEN steel and carbon steel structures based on tests in 10 different types of chemical plants. Using a theoretical example for a plant structure requiring 1000 tons of structural steel, they calculated a discounted cash-flow rate of return on investment of 23.8 per cent for the additional initial cost of COR-TEN steel. Calculation of the uniform annual cost using the method described by Dillon also showed a decrease of about 24 per cent through use of COR-TEN.
Taking into account interest rates, taxes and depreciation schedules, the calculated rate of return on investment was 45 per cent.
COR-TEN steel is available world-wide through a network of steel producers who are licensed by US Steel to produce the COR-TEN sheets and other products required in the containers. The major sources are Nippon Steel (Japan), ISCOR (South Africa), Krakatu (Indonesia), ILUA (Italy), Thyssen Stahl (Germany), Rautaruukki Oy (Finland), British Steel (UK), US Steel and five licensees in the USA.
Source: World Freight Technology