COR-TEN® & OTHER HIGH STRENGTH WEATHERING STEELS

One group of carbon steels of special interest to both artists and designers are the HIGH STRENGTH WEATHERING STEELS. Weathering steels are primarily used in art, design and architecture to produce constructed steel fabrications with naturally weathered (rust) PATINA finishes.

The best known of these steels is COR-TEN®, an alloy originally developed by the American USX Corporation. Cor-ten is stocked in either an A or B grade version – alloyed primarily with additions of carbon, silicon, manganese, phosphorus, sulphur, chromium, nickel and copper (vanadium and soluble aluminium are added to the ‘B’ grade) [ref]. A product specific trade name, Cor-ten has become a something of a generic term for all weathering steels, when there is a fair number of comprable alloys available (designated under BS 4360, ASTM A242 (T1), NZ AS/NZ 3678-WR350 and various other trade names and specifications). Specialist founders can also usually produce a closely related alloy suitable for casting projects.

In common with other ferrous based products, Cor-ten and other weathering steels react with oxygen to produce a rust oxide on untreated surfaces. In normal carbon steels the oxidisation process can eventually lead to an oxide conversion of the entire metallic fabric – potentially leading to a structural collapse. The alloying elements added into weathering steels (in particular the copper and chromium components), encourage a very dense oxide layer to develop on the exposed surfaces; this exceptional density effectively inhibits any deeper corrosive penetration into the fabric of the metal. Dependent upon the ambient atmosphere and conditions the steel is exposed to, a rich rust oxide patina naturally develops without unduly compromising the longevity of the art or design work.

The weathered patina effect relies on a continuous cycle of WETTING followed by DRYING; however, weathering steels will deteriorate if a surface is prevented from drying, or if exposed to saline or other corrosive environments. To encourage an even development of a ‘rust patina’, sculptures constructed in weathering steels can be lightly shot-blasted prior to installation. The surface of the metal may also be treated with a mild SPIRIT OF SALTS (a weak solution of hydrochloric acid) to accelerate oxide development, though excessive treatment can potentially induce acid pitting (an ambient rusting process is recommended where possible).

Weathering steels are only usually available as COLD REDUCED or HOT ROLLED sheets and plates. Cor-ten can be supplied in thicknesses of up to 1 inch (25mm), other versions vary from about 3 - 20mm. Unfortunately this means that, the extensive ranges of cross sectional shaped stock (angles, hollow sections, beams etc) associated with standard grades of steel, are not available in weathering steels. Available stock for customer selection may also be limited outside of the North American & European market regions.

Welding procedures for weathering steels are broadly the same as for other carbon steels, though some of the techniques used to weld high strength (LAS) steels with LOW HYDROGEN rods can apply. Standard steel filler rods, wire and electrodes can be used for welding weathering steels (with dilution helping to retain the steel's properties), though matched fillers specific to weathering steels are preferred for welding thicker sections – this ensures metallurgical compatibility. For TIG and MIG processes an ER80S-G type filler wire provides a suitable match, and for MMA welding, a nickel/copper bearing mild steel low hydrogen (BASIC) electrode will normally be recommended (refer to individual manufacturer data sheets).

With the exception of weathering steels, nearly all carbon steel sculptures intended for long term exposure out of doors are subjected to some form of protective surface treatment. Suitable treatments include PAINTING, STOVE ENAMELLING and HOT DIP GALVANISING to name a few. These various coatings prevent the premature corrosion of the work and inhibit rust seepage. Weathering steels for art and design works typically do not have a protective coating applied, this can have implications, for example rust contaminated run-off water can stain surrounding environments. Likewise, if a weathering steel base is being used to mount an artwork constructed in a non-ferrous material (bronze for example), measures should be taken to insulate the non-ferrous artwork from ELECTROGALVANIC CORROSION and cross contamination.

INFO: Selected tradenames of weathering steel products: Cor-Ten (A/B), T1, Spartan (I-V), Duracorr, Xlerplate.

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