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How to Achieve Rusty Exterior Cladding


A recent trip to Minneapolis revealed a sky scraper with an exterior steel cladding that was rusted and had amazing color and patina. My recent home renovation, combined with my technical expertise in fabrication etc., could really use such a cool application of my favorite material.

How is this done? What alloy is capable of oxidizing to a degree and then relying on it's metallurgical properties to halt oxidization via the protective layer of the oxidization.

Your comments are appreciated.

Scott Kobryn
- Buffalo, New York

If rust is orange red you can try salt water or very diluted chlorhydric acid (you may combine it with hydrogen peroxide 3%, first acid, when dry peroxide, and so on until result is good). Surface must be grease free! Hope it helps and good luck!

Goran Budija
- Cerovski vrh Croatia


Hi Scott,
It's 'weathering steel,' an HSLA (high-strength, low-alloy) steel with improved atmospheric corrosion resistance. Produced to standards such as ASTM A242, A588, ASTM A606 Type 4 and A709. Commonly called Corten or COR-TEN®, the original US Steel version. Alloying with small amounts of Cu, Cr, Ni, Si and P results in a less porous rust, with a long-term rusting rate 1/4 to 1/8 that of ordinary steel. Initial rust is bright orange gamma-FeOOH (lepidocrocite) on both weathering steels and ordinary steel. However, on weathering steels, years of wet-dry cycling results in dense alternating layers of alpha-FeOOH (goethite) and lepidocrocite, with scattered patches of maghemite (gamma-Fe2O3, of magnetite structure) and/or magnetite Fe3O4. Takes about 15 years to develop the final rust structure in the US 'Rustbelt' and 15 years in Japan. From the study 'Structure of Protective Rust Layers Formed on Weathering Steels...' by M. Yamashita et al., ISIJ Int. (1998).

Lots of information; search for "weathering steel." Include microstructure for the scientific stuff.

The John Deere Co. Headquarters in Moline, Ill., designed by Eero Saarinen and completed in 1964, was the first architectural use of Cor-ten®. Not often used in residential, although a rather famous Weathering Steel House in Toronto was completed in 2001. letter #25532 gives some comments on initiating rusting, but note that while rusting decreases significantly after several years, it still continues. Due to rusty runoff stains, US Steel discourages residential use.

Another downside: It isn't readily available for consumers, unless you are willing to cut up used Sea-Land shipping containers. Some availability in 18 & 22 gauge corrugated sheet for siding and sloped roofs. About twice as expensive as ordinary steel (really cheap in 100 ton mill quantities, though). Fasteners should also be weathering steel (ASTM A563 C3 or DH3 nuts, ASTM A325 Type 3 bolts and F436-3 washers), but these are only available in sizes 1/2" and up. Ordinary steel or galvanized fasteners should not be used; stainless steel with non-metallic washers is OK (but doesn't color match).

Ken Vlach [deceased]
- Goleta, California
contributor of the year honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at, continue to benefit from.

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