If you pay any attention at all to design and architecture, you may have heard the term Cor-Ten thrown around quite a bit. Cor-Ten is a registered trademark name for a type of weathering steel alloys, owned by the US Steel Corporation. It was first developed in the 1930’s for rail cars for coal transportation, and premiered in architectural applications in 1964, when Eero Saarinen used it for his design of the John Deere World Headquarters in Illinois. Since then, its popularity has skyrocketed for uses in architecture, landscape installations, and outdoor sculpture. Specifying it will land you at a price tag of double of what the regular A36 (that’s carbon steel, or mild steel to you and me) will cost. This made me ponder at what point it is actually worth specifying Cor-Ten as opposed to its humbler, more commonplace cousin. I had fun researching this, so bear with me when I start from the beginning.
First, a little primer: All metals can be divided into two basic groups – pure metals (= basic elements), and alloys. The pure metals are down to their basic atomic structure and can’t be refined any further, but they can be combined with each other, in which case they become alloys. The alloys can then be categorized into two major types – ferrous (from the Latin word for iron) and non-ferrous. Since Cor-Ten is a ferrous alloy, lets start with a historical glimpse at iron. As a fun aside – to hear a stunning rendition of the Periodic Table, click here! 🙂
By adding coke and limestone to iron ore in a blast furnace, the iron is separated from its ore. In this state, it is still not pure – it contains about 5% carbon and smaller amounts of other elements. Back in the day, the molten iron from the blast furnace would be collected in a channel to which long molds were attached, like piglets suckling from a sow. The basic bars from these molds were called ‘pig iron’. Pretty fun, huh? From pig iron, two kinds of iron were made; cast iron and wrought iron. Cast iron is more brittle, whereas wrought iron lends itself well to shaping, but neither is as strong as steel. But, fast forward to now… The overwhelming majority of raw pig iron produced today, is headed for the steel mills where it is turned into thousands of different types of steel, depending on which, and how much of other chemical elements are added to the mixture. Generally, steel can be sorted into three different classification: carbon steel, alloy steel, and tool steel.
Carbon steel is iron with less than 1% carbon and is the most widely used. Tool steel is a fine-grade steel, often made with only iron and carbon. Tool steel is the most expensive of all steel types. Alloy steels contain some carbon, but also other elements. From the excellent book ‘Direct Metal Sculpture’ by Meilach and Seiden: “For example, the addition of chromium to the carbon steel results in stainless steel – an alloy that is tough, durable and rust-resistant – properties that make it a favorite among sculptors. The addition of nickel increases toughness and resistance to heat and acids; manganese increases strength and resistance to heat; tungsten retains hardness at high temperatures; vanadium increases strength and resiliency or springiness. There are many other elements and materials which can be used for various purposes, but these are the most common.”
Cor-Ten is a high-strength, low alloy steel with the prime objective to obviate the need for protective coatings. Like any untreated carbon steel, its surface rusts when exposed to the elements, but its alloy metals allow it to develop a patina consisting of a protective oxidized layer that slows down any further atmospheric corrosion. According to this Cor-Ten Q & A, “Laboratory analysis of the rust film have shown that the alloying elements in the steel, particularly copper, chromium, and nickel produced insoluble compounds that clogged the pores at the rust/steel interface, thereby ending the regeneration.” On a microscopic level, the oxidized layer is smoother than the surface of carbon steel, and thus holds less water. The rate of corrosion in weathered steel slows after 2-5 years, but it will never cease to corrode completely.
The use of weathering steels is not recommended in overly wet environments – it needs distinct wet and dry periods to develop its patina. Per this source, it also is not recommended for use closer to marine environments than 2 kilometers. The reason is that salt in the coastal air retains moisture, and would maintain a constantly moist environment on the surface – keeping the essential dry periods at bay. (This surprised me, as – per the same article – it is the material shipping containers are made of. Mind you, they are usually painted, but still… I read somewhere that Cor-Ten should not be painted until it has weathered enough to have developed a complete patina, as the prematurely added coating would hinder the development of its protective layer. On the other hand, if painted after it had had a chance to weather properly, any damage to the finish would not result in the kind of rust that would ensue had the material underneath been mild steel. Still though – I have to wonder if weathering steel wouldn’t last longer than A36 at the coast. Sure, it might not last as long as it would in a more inland location, but my guess is that those copper, chromium and nickel compounds would slow down the deterioration process – at least somewhat. Per the guys at Delia which I wrote about in my last post, using Cor-Ten for planter boxes is overkill, unless you are on the coast, or in really exposed environments.
Anyway – to answer the title question – nope, it is very likely NOT necessary to pay double for Cor-Ten planters. You will get the same rusty look with A36, and with a thicker gauge – say 10 or 12 – it will last for many years. Remember – you’re not supposed to leave moist organic material like soil or leaves against the Cor-Ten surface anyway, as it will cause it to deteriorate in the same manner as “regular” steel. And, you probably would need to water your plants at least sometimes…. right? Also, to be picky – Cor-Ten isn’t really any one type of steel – there are variations: … There are basically two types of Cor-Ten that are most prevalent, Cor-Ten A (generally up to 12mm thick) and Cor-Ten B (generally 15mm thick and above). The comparison of Cor-Ten to the ASTM grades is loosely stated as Cor-Ten A is equivalent to ASTM A242 and Cor-Ten B is equivalent to ASTM A588 Grade A. Cor-Ten A and B both meet and/or exceed the requirements of ASTM A606 Type 4. [A606 is the thin sheet version.]
Regardless of which steel you choose, you are going to have to plan for rust runoff – because it WILL happen. Unless you specifically want rust stains on surrounding surfaces, you should create some kind of way for the rusty water to collect and disperse, where the resulting orange discoloration won’t bother you. Or choose to have the steel sit on a surface of a color that blends well with the rusty water.
In the case of mild steel, I suppose you could clear coat it, once it’s reached the level of rustiness you desire. (With Cor-Ten you obviously wouldn’t have to.) Or, even better, you could incorporate the staining into your design like in the design below. I really love how this makes the inevitable part of the design! Good design really IS in the details! 🙂 Now go out there folks, and get yourself some steel! It’s such a cool material!
Besides the websites I linked to, my research for this post came from my trusted old second edition copy of Edward Allen’s ‘Fundamentals of Building Construction, Materials and Methods’, the wonderful ‘Direct Metal Sculpture – Creative Techniques and Appreciation’ by Meilach and Seiden, and Rob Thompson’s excellent ‘Manufacturing Processes for Design Professionals’.