Building Envelope blog posts
Always interesting Cor-Ten®
Anssi Lehmonen | Jun. 23, 2020
COVID19 has had an effect in construction industry, but not as severe as for service industry. Façade contractors are still having strong workload and summer looks busy in that sense. Summer is the peak season for the industry and contracts are made now for the season.
I have received plenty of phone calls & inquiries related to different raw materials, their behavior and of course price level of those. It has been interesting that materials like Titanzink, Copper, Anodized aluminum and
Cor-Ten® has reach the decision makers, and they have also approved these unique materials for different projects. Sustainability factor could be behind this trend. Many of mentioned metal materials are having more than 100 years lifetime expectation.
If I will pick up one material from the list…its Cor-Ten®. It has been the most preferred material so far.
Cor-Ten® itself has been in our portfolio for a long time and we have long history of producing it. BUT how many of us know the background and details of this mystical patinated wonder?
It was innovated and patented by US steel at 1933 with exceptional mechanical resistance, primarily for use in railroad hopper cars, for the handling of heavy bulk loads . The controlled corrosion for which this material is now best known was a welcome benefit discovered soon after, prompting USS to apply the trademarked name Cor-Ten® . As Finn I’am proud that one of the first famous facades made from Cor-ten® was designed by Finnish architect Eero Saarinen. Project was built in sixties for John Deere head office in Illinois, USA. It’s also good example from sustainability point of view because its still company’s head with original Cor-Ten® façade!
Lifetime of the building is coming from the good detailed planning and installation made with correct way. You have to take couple features into account when designing or installing Cor-Ten®. I listed some facts frequently asked from me.
About Cor-Ten® steel
Cor-Ten® steel has an increased ability to resist atmospheric corrosion. This feature relates to a carefully selected and developed steel composition, of a certain steel grade and a chemical composition of alloying elements like copper, chromium, nickel and phosphorus. The result is an oxide layer on the steel surface, i.e. patina. The patina layer in normal atmospheric conditions remains a fully tight and durable protective layer, which prevents further corrosion and thus limits losses on steel thickness.
Based on 32year long study made by VTT Technical Research Centre of Finland  Cor-ten® material loss has been only 70µm (meaning 0,07mm)!
Special attention needed when overlapping Cor-Ten® cassettes/steel! In facade water and moisture penetrates easily between the overlapping steel sheets. In the gap, corrosion rapidly consumes oxygen dissolved in water. This is causing low oxygen corrosion environment and creating crevice corrosion. Crevice corrosion is local form of corrosion with much higher corrosion rate than general corrosion. Based on laboratory studies of Cor-Ten®, crevice corrosion rate was 10 times higher than general corrosion. In practice meaning 1,2 mm material loss in 60-years lifespan and generally material thickness used in Cor-Ten® facades is 1,0-1,5 mm. This is why you need to separate Cor-Ten® steel from other materials e.g. with EPDM rubber gaskets (see picture below).
Picture illustrating overlapping in horizontal joint of cassette with and without EPDM gaskets.
Below is a picture from a project, were the cladding had to be taken down and re-installed after 3 years. The reason was incorrect installation. There was lack of EPDM rubber band preventing the oxide surface of the weathering steel to peel-off against the battens and there was no EPDM-packing’s in the holes for the fasteners. It is not enough with just rubber on the screws washer…there should first of all be a big enough hole allowing the goods to move during temperature differences. We supply all needed accessories and may produce the goods with pre-punched holes in correct dimension and location, and of course fasteners with correct size washer. If the cladding is installed correctly the protective oxide layer will stay intact, and minimum rust-stains will appear under the cladding.
Cor-Ten® is leaving as raw and covered with a temporary protective water based oil/wax. The purpose is to protect material during the delivery and it will disappear during rain. However if the layer instead will be cleaned at site before installation, the oxidation process will start much quicker and the Cor-Ten® will obtain its smooth, tight and an even surface will appear quicker. Another benefit of cleaning product and removing the protective oil/wax is to minimize the level of rust particles, which otherwise follow the oil/wax downwards the façade when it is exposed for rain.
Patination takes time 12-24 months to reach final patina layer . Time for patination is depended on location and weather situation. Patination needs cycles that steel is getting wet and drying.
If Cor-Ten® should be used indoors it either needs to be cleaned from the temporary protective layer and exposed outside for a while, or treated to accelerate the oxidation process un-natural.
There are various types of weathering steels on the market. Our owner, SSAB, produces the original weathering steel Cor-Ten® on a European license, and in our production we mark the goods accordingly. Other types of weathering steel, for example the whole series of S355W, is NOT Cor-Ten®, hence do not have the marking.
Early stages of steel patination creation, rust is dissolved in the drainage water, which can stain other materials. It is recommended to channel rusty drainage water away from other parts of façade. Stains can be cleaned from Aluminium but from concrete and other porous materials, it is hard to get it off.
Constant contact with water is also speeding up the material loss. So rainwater systems from Cor-Ten® are not recommended.
Declaration of Performance (CE marking) is not existing for cold rolled(thickness up to 4mm) Cor-Ten®steel due to the fact there’s no harmonized EN standard.
 = Wikipedia, History of Cor-Ten® steel
 = VTT report: VTT-CR-05247-14, Säänkestävien teräslevyjen korroosiotutkimus v. 2014, Leena Carpen