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.
The birth of weathering steel can be traced back to the development of steels containing copper (Cu), known as copper steels. In 1910 Buck observed that steel sheets with 0.07 % Cu produced by US
Steel and exposed in three environments of different corrositivities (rural, industrial, and marine) showed a 1.5 % to 2 % greater atmospheric corrosion resistance than the plain C (carbon) steel. Hence, in 1911, US Steel started to market steel sheets with a certain Cu content. During 1930s, weathering steel was being used in coal hopper train cars to resist the corrosive effects of the sulphur in coal and exposure to long periods of rain. The civil engineering applications for these steels appeared in the early 1960s. Weathering steel is also being known as COR-TEN steel.
Weathering steels are corrosion resistant steels which work by controlling the rate at which oxygen in the atmosphere can react with the surface of the metal. Weathering steels are low‐alloy steels with a carbon (C) content of less than 0.2 % to which mainly Cu, Chromium (Cr), nickel (Ni), phosphorus (P), silicon (Si, and manganese (Mn) are added as alloying elements to a total of no more than 3 % to 5 %. The enhanced corrosion resistance of weathering steel in relation to mild steel or plain C steel is due to the formation in low aggressive atmospheres of a compact and well‐adhering corrosion product layer known as patina. This definition of weathering steel, however, has not remained unchanged but has evolved as new weathering steel compositions have been developed to achieve improved mechanical properties and / or withstand increasingly aggressive atmospheric conditions from the corrosion point of view, especially in marine environments.
Weathering steels are high strength low alloy steels which can provide corrosion protection without additional coating. Increase in alloying elements, mainly Cu, provides an arresting mechanism to atmospheric corrosion in the steel itself. The alloying elements in the steel produce a stable and durable rust layer which adheres to the base steel. 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.
Further the patina on weathering steels not only offers greater corrosion resistance than on mild steel, but is also responsible for its attractive appearance and self‐healing abilities. The main applications for weathering steels include civil structures such as bridges and other load‐bearing structures, some open frame buildings, road installations, electric transmission poles, utility towers, guide rails, ornamental sculptures, façades, roofing, etc. Comparison of rate of rusting in low alloy steel and weathering steel is at Fig.1.
Fig 1 Comparison of rate of rusting in structural steel and weathering steel
Weathering means that due to their chemical compositions these steels, when used unprotected, shows increased resistance to atmospheric corrosion compared to plain C steels. This is because it forms a protective layer on its surface under the influence of the weather. The corrosion retarding effect of the protective layer is produced by the nature of its structure components and the particular distribution and concentration of the alloying elements in it. The layer protecting the surface develops and regenerates continuously when subjected to the influence of the weather. Formation, duration of development and protective effect of the covering layer on weathering steels depend largely upon the corrosive character of the atmosphere. Its influence varies and depends mainly upon the general weather conditions (e.g. continental), macro-climate (e.g. industrial, urban, maritime, or rural climate) and the orientation of the structure components (e.g. exposed to or shaded from the weather, vertical or horizontal position). The amount of aggressive agents in the air has to be taken into account. In general the covering layer offers protection against atmospheric corrosion in industrial, urban and rural climate. When using this steel in unprotected condition, it is upto the designer to take into account the expected loss of thickness due to the corrosion and as far as necessary, compensate for it by increasing the thickness of the material.
The requisites for the protective rust layer to form is that a wet/dry cycling is necessary to form a dense and adherent rust layer, with rainwater washing the steel surface well, accumulated moisture draining easily, and a fast drying action (absence of very long wetness times). Structures is to be free of interstices, crevices, cavities and other places where water can collect, as corrosion progresses without the formation of a protective patina. It is also not advisable to use bare weathering steels in continuously moist exposure conditions or in marine atmospheres where the protective patina does not form. Hence, the ability of weathering type steels to fully develop their anti-corrosive action is dependent on the climate and exposure conditions of the metallic surface. It is also to be taken into account that a truly protective oxide film never develops on certain areas, or that their evolution is excessively slow
In 1968 ASTM standard A‐242 presented two specifications for weathering steels, one with high P content ( less than 0.15 %) and the other with a lower phosphorus content (0.04 % maximum). The latter was ultimately replaced by ASTM standard A‐588 weathering steel (Tab 1). This steel possesses less resistance to atmospheric corrosion due to its lower P content, but for this same reason it has better weldability.
|Tab 1 Chemical composition of commonly used weathering steels|
|Elements / Steel type||Unit||ASTM A-242 (COR-TEN A)||ASTM A-588 Gr. A (COR-TEN B)|
|Typical concentration||Typical concentration|
|Carbon (C)||%||0.15 maximum||0.019 maximum|
|Manganese (Mn)||%||1.0 maximum||0.8-1.25|
|Phosphorus (P)||%||0.015 maximum||0.15 maximum||0.04 maximum||0.04 maximum|
|Sulphur (S)||%||Less than 0.05||Less than 0.05|
|Copper (Cu)||%||0.2 minimum||0.25-0.4||0.25-0.4||0.3-0.4|
|Nickel (Ni)||%||0.5-0.65||0.4 maximum||0.02-0.3|
The typical mechanical properties of weathering steels are given in Tab 2. The mechanical properties are dependent on the type of the processing of steels.
|Tab2 Typical mechanical properties of weathering steels|
|Steel grade||Thickness||Mechanical properties|
|Yield strength||Tensile strength||Elongation|
|mm||Newton/sq mm||Newton/sq mm||% minimum|
Special features of weathering steels
Weathering steels have very good weather resistance and hence can be used without any coating. It 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 show a yellowish appearance in the initial stage of use. The colour gradually changes to brown. Then the colour changes to a sedate blackish brown in one to two years and the colour changes very little subsequently 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 important issues are (i) the actual corrosion loss varies with the environment and hence for long-life, corrosion allowance is to be considered, (ii) crevices and water/dirt traps are to be avoided, (iii) rust stains run to adjacent surfaces and cause staining, (iv) fasteners are to be made of weathering steel, (iv) specific low alloy welding rods are to be used, (v) for an even weathering result, surface blasting is necessary, and (vi) weathering steels are not suitable 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. 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 also gives it major advantages over other metals for structures which are exposed to the outside environment. Some of the advantages of weathering steels 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 are the only maintenance required in case of weathering steels 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 around 5 % lower than conventional painted steel alternatives in bridges.
Project life cost benefits – Nominal maintenance needs of weathering steel structures considerably 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 volatile organic compound (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 temperatures above 400 deg C weathering steels form a protective patina. A typical improvement is 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.