All low alloy steels have a tendency to rust in the presence of moisture and air. This rust is a porous oxide layer which can hold moisture and oxygen and promote further corrosion. The rate of rust formation depends on the access of oxygen, moisture and atmospheric contaminants to the metal surface. As the rusting process progresses, the rust layer forms a barrier to the ingress of oxygen, moisture and contaminants, and the rate of rusting slows down. The rust layers formed on most conventional structural steels detach from the metal surface after a certain time and in the process exposes the surface once again to rusting and thus commencing the corrosion cycle again. The rate of rust formation progresses as a series of incremental curves approximating to a straight line. The slope of this straight depends on the aggressiveness of the environment.
Weathering steels are weather resistant steels which work by controlling the rate at which oxygen in the atmosphere can react with the surface of the metal. These steels are high strength low alloy steels which can provide corrosion protection without additional coating. Increase in alloying elements, mainly copper, provides an arresting mechanism to atmospheric corrosion in the steel itself. The alloying elements in the steel produce a stable and durable rust layer that adheres to the base metal. This rust ‘patina’ develops under conditions of alternate wetting and drying to produce a protective barrier, which impedes further access of oxygen and moisture. This patina acts as a skin to protect the steel substrate. Section loss on the order of 100 mils (2.54 mm) may be expected before the patina sets up, but this is negligible to the structural performance. Comparison of rate of rusting in low alloy steel and weathering steel is at Fig.1.
Fig 1 Comparison of rate of rusting in low alloy steel and weathering steel
The material was developed in the 1930s for use in coal hopper train cars to resist the corrosive effects of the sulfur in coal and exposure to long periods of rain. The civil engineering applications for these steels appeared in the early 1960s.
Presently weathering steel isbest known as COR-TEN steel or Corten steel. United States Steel (USS) Corporation holds this registered trade mark. The original COR-TEN received the standard designation A 242 (COR-TEN A) from the ASTM International standards group. Newer ASTM grades are A 588 (COR-TEN B) and A 606 for thin sheet. All these three weathering steel grades are in production and use. The chemical composition of Corten grades is given below.
Unit Corten A Corten B
Carbon % 0.12 max 0.16 max
Silicon % 0.25-0.75 0.30-0.50
Manganese % 0.20-0.50 0.80-1.25
Phosphorus % 0.07-0.15 0.03 max
Sulphur % 0.03 max 0.03 max
Chromium % 0.50-1.25 0.40-0.65
Copper % 0.25-0.55 0.25-0.40
Vanadium % 0.02-0.10
Nickel % 0.65 0.40
Special features of weathering steels
- Weathering steels have very good weather resistance and hence can be used without any coating. It also provides an economic solution in terms of reduction in cost of recoating.
- It gives an aesthetic effect to the steel structure due to the sedate colour of the protective rust.
- Welding is easy using any method of welding such as manual welding or submerged arc welding etc. Spot welding is also possible for thin materials.
- Weathering steels can be subjected to cold working, hot working, and gas cutting in a similar way as ordinary steel with the same strength level.
- During ageing weathering steels exhibit a yellowish appearance in the initial stage of use. The color gradually changes to brown. Then the color changes to a sedate blackish brown in one to two years and the color changes very little but only to a deep brown (Fig 2)
Fig 2 Change of colour of weathering steel with time
During the design of the structures with weathering steels the following issues are important
- The actual corrosion loss varies with the
environment. For long-life, corrosion allowance must be considered.
- Crevices and water/dirt traps should be avoided
- Rust stains may run to adjacent surfaces and cause staining
- Fasteners should be made of weathering steel
- Specific low alloy welding rods should be used
- For an even weathering result, surface blasting may be necessary
- Weathering steels are unsuitable for use in marine and aggressive industrial environments
An example of steel structure made from weathering steel is given in Fig 3.
Fig 3 Example of structure made from weathering steel
Advantages of Weathering Steels
Weathering steel has atmospheric corrosion resistance and this enables it to be used without paint for many structural / architectural applications. That would include structures like bridges, some open-frame buildings, transmission poles and sculptures. Weathering steel also has high temperature advantages which make it a good choice of material for many flues, chimneys and high temperature ducting. Weathering steel’s corrosion resistance gives it major advantages over other metals for structures that are exposed to the outside environment. These are given below.
- Low maintenance –Weathering steel is ideal for bridges and other structures where access is difficult or dangerous, and where future disruption needs to be minimized. Inspection and cleaning should be the only maintenance required to ensure the structure continues to perform well.
- Start up cost benefits – The saving of not needing to use any protective coating / paints compensates for the incremental material cost of weathering steel. As an example, weathering steels cost is approximately five per cent lower than conventional painted steel alternatives in bridges.
- Project life cost benefits – Nominal maintenance needs of weathering steel structures significantly reduce the costs of maintenance operations and the potential indirect costs of traffic delays in case of bridges.
- Construction Speed – Since weathering steels do not need paint both on site as well as in the fabrication shop, the construction activity gets streamlined.
- Aesthetic appeal – The attractive appearance of mature weathering steel often blends pleasingly with the environment. Its appearance changes and improves with age.
- Environmental benefits – Use of weathering steel eliminates the need for blast cleaning and VOC laden paints.
- High temperature benefits – Steel can suffer oxidation at temperatures above 400 deg C. This can be decreased by using weathering steels. At temps above 400 deg C weathering steels form a protective patina. A typical improvement would be an increase of 50 deg C over equivalent loss in carbon manganese steels. Weathering steels are not suitable for use in significant load bearing members above 450 deg C.