Corten Steel

Corten Steel

High-Quality Special Steel for Metal Garden Products

In Germany the name CORTEN steel*) stands for weatherproof construction steel that continuously develops a barrier layer as part of a weathering process. This layer protects the elements from further corrosion and results in a characteristic patina in earthy and warm color shades. Corten steel has got a service life over average. It does not need to be varnished and is recyclable. We only use especially top-quality grade S355J2W+N, material number 1.8965, that can be bended and – with the right filler material – can also be welded.

*) The name COR-TEN (steel) consists of two parts. The first part COR stands for CORrosion Resistance, the second part TEN stands for TENsile strength. The patent for COR-TEN-Stahl was applied for in the USA in 1932. Worldwide there are a lot of different sorts of weatherproof construction steel.

Vorgerosteter Wetterfester Baustahl von Gartenmetall

Pre-Oxidized Weatherproof Construction Steel

Corten Steel - Field of Application

Corten steel can be used in many different contexts and surroundings: e.g. for bridge constructions, facades or crash barriers. A popular example are the crash barriers on Italian highways. In the last years corten steel has increasingly been used for metal garden objects. The characteristic patina develops a certain independent existence and keeps changing its structure and color. This property among others makes corten steel the ideal material for modern garden, landscape and urban design.

Delivery Condition "rolled blank"

When corten steel metal sheets are produced the rollers form a so-called “roller skin”. It consists of a tightly adhering, blue-black layer out of very hard iron oxides. The “roller skin” is brittle and delays the wanted development of the patina barrier layer. In the first months of weathering, however, this “roller skin” is continuously stripped away and reveals the “real” blank steel surface.
As the stripping is not wanted and mostly pre-oxidized objects are ordered, Gartenmetall® – as one of the top users of corten steel in Germany – realized the project “corten steel without roller skin” in 2016. The result was enormous. The steel manufacturers have started to produce top-quality corten steel metal sheets without roller skin exclusively for Gartenmetall®. This improved pre-product has meant a quantum leap not only regarding the surface but also in regard to the processing qualities, i.e. bending and welding.

How the Patina is Developed

When the crude steel plates are produced and processed the surface has not yet developed a patina. This patina only develops under natural weathering conditions. The development of a good-looking patina starts after 2 to 3 weeks and is only complete after 1.5 to 3 years. Important for the development of the barrier layer is the alternation between humid and dry weather. If there are surface parts, that are not exposed to weathering, they are excluded from the process of patina development.

Accelerating the Patina Development

Walzblanker Cortenstahl im Lieferzustand

"Rolled blank" corten steel in delivery condition
 

Angerosteter Cortenstahl nach 24-h Behandlung mit SUPER ROST-Verfahren

Slightly rusty corten steel after using our SUPER ROST-procedure

Cortenstahl nach mehr als 6 Monaten unter natürlicher Bewitterung

Corten steel after 6 months (natural weathering)

Customers often wish to get metal garden objects that have already got a precious-looking patina surface. As the natural development of the patina layer needs up to 3 years, we have developed a special procedure so that the oxidizing process starts within the first 24 hours. For this purpose use our SUPER ROST-PADs.
In the first step the surface should be cleaned with a fat-dissolving power cleaner and then rubbed evenly with SUPER ROST-PADs. One pad is enough for 1 – 2 m² (32.81 – 65.62 ft²). After 3 to 12 hours exposure time the surface has to be rinsed thoroughly, using water. In order to carry on with the oxidizing process and to finally build the protective layer, the surface is continuously moistened with water sprayers. Before every new wetting the surface must have dried up completely.

SUPER-ROST-Pad zur schnellen Ausbildung der Patina

SUPER-ROST-Pads: Accelerated Development of Patina

Pre-Oxidizing: Color Differences

It is hard to believe but it is just as it seems: The same material from the same production lot develops a different first rust coloring, depending on the weathering and the environmental effects.
The rust coloring is mainly influenced by the temperature and the relative humidity. With lower temperatures and higher humidity the color turns darker. Different colorings after the first rusting are leveled out after the progressive weathering of the objects. These differences in color are therefore no defect and thus no reason for rejection.

Farbunterschiede nach dem ersten Berosten

Color differences after the first oxidizing

Black Stains on Corten Steel

In the case of air exclusion it may happen that the fresh, slightly rusted surface turns black, e.g. if the material was packed in a tight-fitting foil for transport. This coloring tapers off after a few days of natural weathering. So it is no defect and thus no reason for rejection.

Sichtschutzwand, auf der sich unter Folie schwarze Stellen gebildet haben

Decorative screen wall with black parts, developed under foil

It can also temporarily come to a black color change, when the corten steel surface is touched by dead leaves or plantings over a longer period of time.
Remember: After removing the leaves the weathering process is started anew and the patina surface returns.

Sichtschutzwand
schwarzen Verfärbungen an der Oberfläche

Imprints from vacuum cup on corten steel

When processing the raw plates they are automatically seized at several manufacturing stations and then passed on, using vacuum cups. After the first rusting these cups can leave imprints.
Among experts these imprints are called “crop circles”. They occur when the surface is disturbed by minimal rest particles of SUPER ROST, which adhere to the cups. The imprints only turn up 12 to 24 hours after the first auburn patina. Under the effect of natural weathering the imprint disappears. If this disappearance has to be fastened, the surface can be mechanically roughened and rusted again.
Note: Imprints caused by vacuum cups on the pre-rusted surface are no reason for rejection.

Saugerabdrücke (= Kornkreise) nach dem ersten Berosten

Imprints from vacuum cups (=”crop circles”) after the first oxidizing

Corten steel classified as ecological material

Weatherproof construction steel can be classified as ecological material. It has an over-average product life span, does not need to be varnished and is completely recyclable. Corten steel contains minimal amounts (less than 1%) of high-grade copper and chrome, which make it nearly indefinitely durable and avoids rusting through.

Is corten steel or construction steel in the garden a risk to your health?
Every now and again we are asked whether weatherproof construction steel is injurious to one’s health due to its alloying components copper and nickel. If you look at how corten steel works, you will realize that the concerns are without any reason. Corten steel does not oxidize and forms a barrier layer together with its alloying components. These do not end up in the surroundings. That is why corten steel is as unproblematic as stainless steel which is used in the kitchen and in hospitals.
Problems can only occur when normal or cheap construction steel is used in the garden, which is often used for decoration objects from DIY-stores. This kind of construction steel slowly dissolves and sets free alloy components like copper and nickel which might be ingested by insects, pets or humans. The dissolved oxidation products of steel (=rust), however, are harmless and work as a fertilizer.

Cortenstahl

No-go: Normal steel (sometimes with the adding “patina“) is often used for small deco-objects. It can rust through after only a few months in the garden and is a threat to insects and pets.

Removing deposits from terrace tiles and natural stone

Auburn deposits, e.g. as from discharged water, can – more or less – be removed successfully with a suitable rust remover. The cleaning success is mainly determined by:

  • Basic material
  • Quality and quantity of cleaning
  • Suitability of cleaning agents

For acid-resistant floor coverings cleaners on the basis of phosphoric acid are most likely to be successful. In case of little and light stains household remedies are ideal, like table salt in combination with ascorbic acid and water.

No acidic cleaners may be used with concrete bound floor coverings. Only cleaners on the basis of tannin have a chance of achieving a sufficiently good cleaning result. As the treated areas turn red-purple during the treatment, know-how is essential. It is important to pay attention to the professional application.
Note: As a precaution please try on a hidden part first.

Sealing Corten Steel

In order to reduce auburn deposits and to protect the corten steel, it is possible to seal the rusty surface, as soon as the desired patina surface with its corresponding barrier layer has developed.

The seal works like a classic clear varnish and has to be renewed as required (e.g. on a yearly basis). For more information on sealing your surfaces see ROST-PROTECT®

Versiegelter Cortenstahl bei einer Wasserschale

Sealed corten steel – example: water bowl AQUA BOWL

Corten Steel as a Snail and Slug Barrier

Did you know that corten steel is an effective snail and slug barrier? The patina contains small amounts of copper (<1%) which stops snails and slugs from creeping and makes them turn away. To ensure that snails and slugs do not undercut this barrier it has to be at least 10 cm (≈ 39.37) inside the soil.

Cortenstahl Schneckenbarriere

Effective snail and slug barrier: Snails and slugs avoid corten steel.

Suggestion for Tender Template

“Rolled flat steel. Quality standard weatherproof construction steel corten steel Corten B ASTM A 588, quality S355J2W+N. material number 1.8965 according to EN 10025-5 with acceptance test certificate, surface pre-oxidized on all visible sides.”

Metal Sheets and Cutting

If you need cuttings, sheets or plates of plain or pre-oxidized corten steel we can offer them to you at a very good price. We can do so because we always buy big quantities ourselves. Even small steel traders and many locksmiths purchase their corten steel from us.

Also ask for corten steel without “roller skin”. This material is produced by big steel manufacturers exclusively for Gartenmetall®. For a price list and further information see SHEET-METAL-CUTTING

reference (in excerpts):
Werkstoffblätter ThyssenKrupp Steel, Nr. 1.8965
Merkblatt 434 „Wetterfester Baustahl“, Stahl-Informations-Zentrum
Wikipedia „COR-TEN“