Weathering steel

Weathering Steel, or Cor-Ten, sculpture, c. 1987

Weathering steel, known also as COR-TEN or Corten steel, is a steel alloy that was created in an attempt to reduce the need for paint. Weathering steel forms a stable rust-colored patina after being exposed to the weather for an extended period. The original COR-TEN has the standard designation A 242, or COR-TEN A, given to it by ASTM International Standards Group. More recent grades are given the name A 588 (COR-TEN B) and A606. Weathering steel is one of the first six current classes of high-strength, low alloy steels. It has less carbon than low-carbon steels and has other alloy elements. Weathering steels are significantly stronger and more resistant to atmospheric corrosion that plain low-carbon steels. Weathering steel is different from most plain low-carbon steels because it naturally develops a protective, rust-colored patina. The patina is different in composition than most other types of rust. Weathering steel, when exposed to wet/dry cycles and normal atmospheric qualities, will develop a bright orange rust, that will turn into a reddish color than eventual fades to a darker, rich brown. It is more likely to see darker browns in urban areas than rural because of the varying quality of air and the pollutants in the air. Weathering steel should be capable of being left outdoors unpainted without causing serious damage to its structural integrity.

History

In the early 20th century, metallurgists found that copper steels (with 25% of more of copper) tended to be more corrosion resistant than copper free steels. In 1933 U.S Steel introduced its low-alloy, high strength steel product known as Cor-Ten A, which was later followed by Cor-Ten B. Weathering steel soon became interchangeable with the USS Cor-Ten name. In 1937, a book promoting Cor-Ten steel states that weathering steel was exceptionally strong, required little or no maintenance, and it had an aesthetically pleasing well-developed rust surface. Weathering steel proves to be economically beneficial because they require minimum maintenance while providing exceptional strength. With a strong contemporary movement, the natural rust color proved compatible. If the natural brown rust color was not the color desired, weathering steel also proved to hold paint color significantly longer than most other steels on the market then and today.

Metallurgy

High-carbon iron is obtained from iron ore in a blast furnace and converted into useful steels through a similar process that most other steels endure. Excess carbon is removed from molten metal by a reaction caused by the oxygen, which forms unstable carbon monoxide. Oxygen can be added by injecting gas or by dissolving the iron oxide into the melt. Alloy elements are then added under a vacuum in melting conditions and then cooled. The steel solidifies and then I mill rolled hot while cooling under “controlled rolling” to produce a sheet or billets. Like many other steels, weathering steel is made into plates, bars, and other shapes up to 8 inches thick. The composition of the alloys will affect the steels strength, corrosion resistance, and weathering characteristics (which in turn will alter its color). Copper is necessary to prevent corrosion but high-phosphorous weathering steels may have twice the corrosion resistance of low-phosphorus weathering steels.

Uses and Installation

Weathering steel has several purposes and is used extensively in architecture, industrial engineering, and art. The design, construction, and fabrication as well as environment are elements that will affect the weathering steels structure. In order for weathering steel to be successful, regular cycles of wetting and complete drying are of the utmost importance, especially if the structure is unpainted. If the drainage around the weathering steel is poor, it is likely to develop little or no protective rust and deteriorates quickly. Popular applications of weathering steel in the architectural industry include: I-beams, sheet and plate surfacing shapes, columns, ledges, and shaped components for windows and are usually welded or bolted into place. The welds or bolts must have a similar steel base in order to flourish in conditions that promote strong weathering steel. Weathering steel now has special welding electrodes available that promote the rust growth instead of hindering it. Generally, high-strength bolts, nuts, and washers are readily available in weathering steel and should be sealed to prevent crevice corrosion. Weathering steel is efficiently used as flat sheet siding, corrugated siding, roofing, and insulated panels. It is suggested that weathering steel be tested on the field to see how its patina will withstand the conditions as several investigations have shown extensive damages at one job site and a flouring weathering steel just miles away. Michigan employed the strength of the weathering steel when it built several of its bridges. Most bridges are still in good shape, though some have been plagued by salt deposits and nondraining water.

Conservation

Weathering steel is affected by three main factors: the composition of the weathering steel, the environment surrounding it, and its design and detailing. Before attempting to apply conservation measures, it is important to investigate the cause of deterioration and understand it completely.

Identification

• Simple methods

visual examination,spot tests,specific gravity

• Scientific methods

xrf,xrd,pixe,libs,electrochemical techniques,metallography

Deterioration

There are multiple layers of protective rust on the weathering steel. These layers each serve a separate and important purpose. The first layer, the inner layer, is created by amorphous and crystallize ferric hydroxides. This layer helps keep oxygen and corrosive materials from the metal and prevents the ferrous ions from dissolving. When the rust becomes wet, the copper and phosphorus dissolve at cracks in the inner layer and form additional goethite, which dry to fill in crevices on the primary layer. This process can be affected by the metal in solution and by the solution pH. The outer region of the inner layer is slowly turned into oxide and hydroxides of iron which forms crystalline layers below the rusted surface. The protective rust is formed and maintained by the cycles of wetting and complete drying. Constant moisture or dryness slows this process, corrosion. As the cycle breaks down, the protective rust begins to flake and the sheets of rust separate from the surface. When extensive corrosion has been endured, the steel erodes quickly and will fail. If the weathering steel is in a tightly fitted surface, the steel will push with great force against opposing structures, causing them to become deformed.

Examining the structure with mechanical devices or visually is essential. Measuring the depth of the corrosion and pits are made, and fatigue-sensitive elements should be examined for cracks or breaks. Some may require laboratory analysis. If a surface remains undrained, the weathering steel will corrode quickly. It is common to have damage caused by water infiltration and corrosion at closely fitted joints. If glass is installed near the weathering steel, it is likely to see staining on the glass from rust runoff in times of excessive moisture. A mildly abrasive soap should remove the stain but the steel should be rinsed after cleaning to ensure there will be no discoloration. Glass breakage is also a common problem because of the expansion of the steel in times of extreme moisture. If weathering steel as been created as a skin panel and been placed over masonry or has been created as an industrial curtain, it is possible that there will be extensive moisture condensation behind the steel and could cause corrosion in the space between the steel and whatever may be behind it. It is imperative that there be a vent or drain to wick away moisture. Some urethane foam insulation that contain chlorides can corrode weathering panels due to the foams chlorides (which become soluble when exposed to moisture). While the patina is forming, the outer millimeters of the steel surface corrode, which will be washed away onto surrounding materials. Stains are common during this time and could become permanent, especially on: concrete, stucco, porcelain enamels, stones, unglazed brick, and unsealed (sometimes sealed) woods. Foundations must be designed to reduce runoff from weathering steel and to protect weathering steel from the soil. Weathering steel has a propensity to corrode quickly if buried under the ground, and may corrode faster than carbon steel.

Conservation Techniques

Weathering steel structures survive best when the structure has little pockets and crevices for moisture collection. Periodic clean-up programs are suggested for structure in which these pockets could not be eliminated. Trash, leave, an exfoliated rust should be removed to ensure unnatural corrosion. A dry broom or brush is the most suggested way to care for weathering steel. Other ways of cleaning off these structures may include: water blasting with low pressure, mopping with a diluted detergent solution, or organic solvents. While weathering steel may be capable of being washed with a high-pressure water blasting, there is a chance that the weathered patina will also be disturbed. Baking soda and a low-force pressure washing is the ideal way to expose white metal, while sand blasting should be avoided by all cost. Stabilizing and improving the condition of the weathering steel is capable in several ways. Faying surfaces should be sealed to prevent water infiltration. The most durable repair is to weld the edges of the faying surface. Welding is not always practical because of surrounding materials, in which case a sealant can be applied, but will require maintenance. Creating a better drainage system is the easiest way to remedy water pooling. Creating weep holes is a simple, effective way to improve drainage, though proper sizing and location are of the highest importance. Sealing may be required if the area is not capable of being drained but this does not assure the end of corrosion since the sealant will need attention for upkeep. Some structures are in locations where drainage is impossible or pollutants (i.e. graffiti) are all too common, to address this, a barrier may be the ideal route to take. After conservation, it may only be required to seal the threatened area. It is nearly impossible to indentify and repair all deteriorated surfaces when corrosion occurs from the inside out. Recaldding or overcladding may be the only solution when the main cause of corrosion is unidentifiable. Glass installations must be retrofitted so that the glass is not subject to expansion forces from the steel.

Replacement

Reinforcement or replacement of deteriorated parts often requires refabrication. It is important to determine the specific location from which the weathering steel originated in order to match the original structure. If sheet material is used to replace materials, it should be at least ¼ of an inch thick. Coating the underside and the connections is strongly recommended to ensure water shed. Mill scale has to be removed before installing replacement pieces to ensure the proper development of the patina. Arc welding is acceptable to repair joints except in unwelded structures. Mild-steel covered electrodes are used in a single pass motion for welding joints and generally ensure proper composition enrichment from adjacent steel. Newer developments of homogeneous rust on weathering steel is usually created with abrasive blast cleaning. Though the cleaned weathering steel appears significantly brighter than matured patinas, the colors should eventually match after the patina has time to mature. Abrasive blasting is not always advisable, especially to untrained individuals.

References

• Scott, John C., and Carolyn L. Searls. "Weathering Steel." Twentieth-century Building Materials: History and Conservation. By Thomas C. Jester. New York: McGraw-Hill, 1995. 72-77. Print

Further reading

Selwyn,L. Metals and Corrosion - A Handbook for Conservation Professional,Ottawa 2004.

External Links

• Weathering steel

• A Primer on Weathering Steel: a white paper from the National Steel Bridge Alliance

• Corten FAQ from the American Institute of Steel Construction

Examples

Ahava statue (Jerusalem, Isreal, c.1977).   Weathering steel used for bridge support.