Corten Steel – The Facts and The Unique Benefits

Corten Steel is a ‘Marmite’ material, and it is also widely misunderstood. In this article, we seek to separate the fact from the fiction; and to explain exactly why Corten Steel is a material to embrace when designing and specifying landscape projects

EXECUTIVE SUMMARY

If you just need the headlines, without the detail, then the key points are:

• Corten Steel encompasses a group of steels which possess exceptional corrosion resistance and tensile strength. Corten Steel is also known as a ‘weathering’ steel; as the steel forms a protective layer on its surface under the influence of the weather, and it does not need to be painted.
   
• The Key Benefits of Corten Steel are:
   
  • A characteristic aesthetic. Corten develops a variable patina of bronze, rust and copper hues – colours which act as the perfect foil for all manner of plantings schemes, both naturalistic and architectural.
     
  • Extreme longevity. Material loss from unprotected Corten Steel takes place with glacial slowness [in Central London, 0.8mm over 40 years and 2.0 mm over 120 years].
     
  • Low cost. For anything over a 10-year lifespan expectation, Corten Steel offers by far the best value-for-money return of all sheet metals, largely because it never needs to be painted / repainted.
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  • Essentially zero maintenance.  Any maintenance that is deemed necessary would be for purely aesthetic – rather than functional and/or structural – considerations.
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• Specification Considerations include:
   
  • Suitability. At the margin, the longevity of Corten Steel will be affected by the environment, and the aesthetic may vary slightly over time. However, for all practical landscape product purposes, Corten Steel is suitable for ALL environments, including industrial / coastal.
     
  • Thickness:
     
    • Corten Steel thicknesses of modest specification [2.0mm or 3.0mm] are perfectly fit-for-purpose for + 25 years longevity, in most environments / applications.
       
    • For + 40 years longevity, an additional 1.0mm thickness should be added, to mitigate the forecast material loss.
       
    • Corten Steel thicknesses =/> 6.0mm work out extremely expensive, and can only be commercially justified if the brief genuinely mandates + 120-year longevity.
       
  • Run-off. Concerns about run-off from Corten Steel are often over-stated. In the vast majority of cases, run-off does not create an issue, so long as the metal has been professionally pre-weathered before supply [as is a standard service from IOTA].
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  • Finishing
     
    • Corten Steel should be left unfinished – not painted, not waxed, not lacquered. Any such surface finish is [i] not needed, [ii] will not be permanent, and [iii] negates the distinct USP of Corten Steel [its extraordinary corrosion resistance in its natural, exposed state].
       
    • In applications where the Corten Steel cannot maintain ideal wet / dry cycles to establish its protective surface oxide [such as the inside of planters], there is an argument to seal inside / unseen Corten Steel with a bitumen damp proof membrane. However, this is ONLY cost-justifiable where longevity expectations are genuinely + 40 years.
       

DETAILED DISCUSSION

If you want to also understand the detail, then the full article below is structured as follows:

A. What is Corten Steel?
B. The Science
C. The Key Benefits of Corten Steel and Specification Considerations

A. WHAT IS CORTEN STEEL?

‘Corten Steel’ is used here to cover both the original product COR-TEN, and other generic grades of ‘weathering’ steels. For all practical purposes, for most landscape industry applications, this group of similar steels offer the same benefits. 

The original Corten Steel is sold under the registered trademark COR-TEN, which refers to the two distinguishing properties of this type of steel: corrosion resistance and tensile strength. Corten’s special combination of alloys, and its production methodology, mean that the steel retains its strength, and does not need painting because it is corrosion resistant.

The registered trademark for COR-TEN is held by US Steel, and the steel is also produced under licence by SSAB [Swedish Steel]. The so-called CR material [2.5mm and below] is rolled in Sweden, whilst the HR material [3mm to 30mm] is rolled in Finland – these are the specific materials which IOTA uses, where COR-TEN Steel is specified.

There are also other grades of ‘weathering’ steels available [such as S355JOW and S355J2W]. These are more recent, generic, versions of COR-TEN, they are cheaper, and they definitely have their place. Specifically, landscape products are rarely required to meet the most stringent technical performance standards [for example, they are rarely safety-critical], and cost is often a primary consideration. So, for many landscape projects, an alternative grade of ‘weathering’ steel may well be the most appropriate specification, and IOTA also offers this option.

B. THE SCIENCE

The basic metallurgical difference between Corten Steel and ordinary structural steel is the addition of chromium, copper and nickel alloying elements, which give the Corten its enhanced resistance to corrosion. Corten Steel is also known as a ‘weathering steel’, as the steel forms a protective layer on its surface under the influence of the weather; and it is this protective layer that gives Corten far superior resistance to atmospheric corrosion, compared to other steels.

The layer protecting the surface develops and regenerates continuously when subjected to the influence of the weather. The same process allows Corten to ‘heal itself’. So if, say, the surface is scratched, then the exposed area will regenerate its protective layer; and, over time, this area will blend in with surrounding areas, and be essentially invisible.

Counter-intuitively, Corten Steel doesn’t actually like being constantly dry; and nor does it like being constantly wet. In fact, what Corten Steel likes best of all is to be wet, then dry, then wet, then dry – and so on. In other words, the degree to which Corten’s surface weathers [and thus its ultimate longevity] is positively improved by being subjected to repeat wet-dry cycles. Of course, this makes Corten Steel particularly suitable for the great British climate…!

This weathering process is not really a ‘rusting’, as the word would be commonly understood – but it is more akin to the processes of curing and patination that also protect other long-lasting metals, such as Zinc, Lead, Bronze and Copper. Although Corten’s surface layer visually resembles ‘rust’, it is in fact a controlled oxidation which is stable – and which forms a corrective, rather than a damaging, layer across the metal.

Unlike the rusting of Mild Steel, which is progressive, Corten’s controlled oxidation is fundamentally non-progressive. However, over time, some material is inevitably lost from the surface of Corten Steel, as it heals itself from wear and tear, and atmospheric pollution. However, with Corten Steel, this material loss takes place with glacial slowness.
 

C. THE KEY BENEFITS OF CORTEN STEEL and SPECIFICATION CONSIDERATIONS

C.1 The Aesthetic

To deal first, briefly, with Corten Steel’s characteristic aesthetic, this is either a key benefit – or not – depending on whether you like Marmite…?! But it needs to go on the list, for consideration.

To its advocates, Corten develops over time a variable patina of bronze, rust and copper hues – colours which act as the perfect foil for all manner of plantings schemes, both naturalistic and architectural.  And it also offers a host of other benefits – as below.
 

C.2. Longevity

Key Benefit
Even amongst those who have yet to embrace Corten Steel, longevity is widely appreciated as being one of its key benefits.

According to the generally-accepted classification of EXTERIOR environments, as described in ISO12944: 2018, the vast majority of landscaping schemes within the UK would be designated C3 [urban and industrial atmospheres, moderate sulphur dioxide pollution, or coastal areas with low salinity]; and the rest would be best-case C2 [atmospheres with low level of pollution: mostly rural areas], and worst-case C4 [industrial areas and coastal areas with moderate salinity].

In most landscaping products [say, a planter], material loss will take place from both sides of the sheet steel; and the weathering process is not linear, with approx. 80% of the material loss occurring in the first years of life.

Allowing for these considerations, over a 40-year lifespan, one can expect the following material loss from Corten Steel in each environment:

C2     0.8mm     
C3     1.0mm  
C4     1.4mm         

Over a 120-year lifespan, one can expect the following material loss from Corten Steel in each environment:

C2     2.0mm
C3     2.0mm
C4     3.0mm

A material loss in a C3 environment [say, Central London] of 0.8mm over 40 years, or 2.0mm after 120 years – these are impressive figures. To give some points of comparison, Corten Steel loses its surface material roughly 25 times slower than unprotected low-carbon structural steels, and roughly 200 times slower than the cheapest Mild Steels.

Specification Considerations
The following key considerations arise in relation to longevity of Corten Steel in C3 environments – the most common:

• Even at 2.0mm Corten Steel thickness, IOTA’s shrub planters will definitely be fine for + 25 years, which is the maximum longevity that is generally demanded.
   
• For + 25 years longevity, we would normally recommend 3.0mm thickness for tree planters, simply because of the additional loadings.
   
• For + 25 years longevity, it is hard to make a commercial argument for thicknesses greater than 2.0mm or 3.0mm.
   
• For + 40 years longevity, we recommend to add an additional 1.0mm to each of the above thicknesses, to mitigate the forecast material loss [i.e. + 40 years shrub planters should be 3.0mm, and + 40 years tree planters should be 4.0mm].  
   
• IOTA frequently receives RFQs specifying extreme Corten Steel thicknesses of anywhere between 6.0mm and 10.0mm, for structures with no exceptional loadings. This works out eye-wateringly expensive, as cost rises near-exponentially as a function of thickness. As can be seen from the above, these kinds of thicknesses can only be commercially justified if the brief mandates + 120-year lifespan – and how many of those landscaping briefs genuinely exist…?

Finally, there are two other specification considerations linked to the issue of longevity:

• In applications where the Corten Steel cannot maintain the ideal wet / dry cycles to establish the protective oxide [such as the inside of planters], there is an argument to seal inside/unseen steel with a bitumen damp proof membrane. However, we very rarely get asked to do this, as most longevity expectations do not warrant the additional costs. But where the budget allows, and the longevity expectations are genuinely 40-120 years, then it is worth doing.
   
• Corten Steel does not particularly like sea water, as the chlorides inhibit the development of the protective layer. C2 or C3 environments are fine – but, for C4, we recommend that the landscaping products should be hosed down after every heavy winter storm. And Corten Steel, unpainted, is not officially recommended for C5 environments [industrial areas with high humidity and aggressive atmosphere, and coastal areas with high salinity].

So, for all practical landscaping purposes, Corten Steel can be used anywhere; and it can be specified with confidence, in terms of its longevity, so long as the above guidelines are adhered to.  

C.3. Cost

Key Benefit
There is a very simple message on the issue of cost:

“For anything over a 10-year lifespan expectation, Corten Steel offers by far the best value-for-money of all metals”.

The reason for this is because:

• Corten Steel is the cheapest of those sheet metals which do not need to be painted [which includes 304 / 316-grade Stainless Steels with a brushed / grained finish – which are a lot more expensive].
   
• Corten Steel is only around 15-20% more expensive than those sheet metals that do need to be painted [Galvanised Mild Steels, such as Zintec Steel, 1.4003-grade Stainless Steel, or Aluminium].
   
• For those painted metals, the protective paint layer must be completely refurbished, typically every 10-12 years, with an on-site wet spray application – and those costs will progressively negate, over time, the initial price discount vs. Corten Steel.

In simple terms: after the first repaint, the Galvanised Mild Steels etc. will have cost, in total, about the same as the original cost of Corten Steel; and after their second or third repaint, the Galvanised Mild Steels etc. will have ended up costing a lot more.

While most Corten planters we produce are large custom sizes, we do offer a range of standard sizes and shapes which is the most cost-effective way to buy.  The delivery time is around 8 weeks on these items, which can be found here.

Specification Considerations
The following key considerations arise in relation to the total cost of Corten Steel, over time:

• In terms of minimising initial expenditure [and assuming a 10-year lifespan expectation], then the cheapest sheet metal option will always be Mild Steel, ‘galvanised’ in some way with a protective Zinc layer [such as Zintec Steel], and then with one or more layers of Polyester Powder Coat [PPC] on top. So, if the longevity expectation is 10 years, or less, then Zintec Steel PPC is hard to beat on cost grounds.
   
• However, Corten Steel is only around 20% more expensive than Zintec Steel PPC – which is not a huge amount. And, for lifespan expectations greater than 10 years, there is a positive cost advantage to specifying Corten Steel, vs. the other metals that need to be periodically repainted [Galvanised Mild Steels, such as Zintec Steel, 1.4003-grade Stainless Steel, or Aluminium].
   
• Other steels that also do not need to be painted [such 304 / 316-grade Stainless Steels with a brushed/grained finish] are a lot more expensive. For a typical IOTA tree planter, for example, 304 / 316-grade Stainless Steels will be around 30-40% more expensive than Corten Steel.
   
• Metals other than steels that also do not need to be painted [such as Lead, Copper, Brass / Bronze or Zinc] are hugely more expensive [something like 200% more expensive than Corten Steel.

So, in fact, for typical landscape products, Corten Steel is actually very good value for money, especially since it also has almost no maintenance overhead, as below.  

C.4. Maintenance

Key Benefit
For all practical landscaping purposes, in most environments, the maintenance requirements for Corten Steel are either ‘nothing’ or ‘vanishingly little’. And any maintenance that is deemed necessary would be for purely aesthetic – rather than functional and/or structural – considerations.

There are, however, a couple of key considerations.

Specification Considerations

Run-Off
Concerns about run-off from Corten Steel are often over-stated.  It does occur – as a matter of fact, and of science. But it does not have to be an issue, in practical terms, if a few sensible steps are taken – both at the point of specification and commissioning, and as it relates to maintenance.

At IOTA, ALL Corten Steel products [such as planters, benches, pergolas, tree guards and grilles etc.] are always supplied by us pre-weathered. This means that they are stored outside at our manufacturing facility for a period of time, whilst we apply weathering treatments to the Corten Steel to accelerate its natural curing and patination. This means that the products arrive on site already looking rust-coloured [uncured Corten is a rather dull silver-grey]; and, as importantly, pre-weathering also means that the majority of the run-off has already occurred. Any staining that does occur subsequently can be cleaned from the FFL either frequently during the first month or two, or in one go after the metal has fully oxidised and stabilised.

And, if the project programme allows, then we are happy to hold and pre-weather the Corten for longer – and, after 2-3 months, the steel is stable and run-off is essentially non-existent. In fact, after 2-3 months, and even when soaking wet, you have to rub really pretty hard to get anything at all to come off the Corten.

The only caveat to the above is that we would NOT recommend that Corten Steel be placed onto a porous FFL substrate. So, for example, if the FFL is Travertine, Sandstone / Limestone or Marble, or expensive Hardwood Decking, then it would NOT be wise to put [say] Corten Steel planters on top. In those situations, it would be better to place the planters onto the insulation layer underneath [either on Wallbarn mini-pedestals or a drainage mat, to spread loadings], and then to pave / deck up to the sides of the planters, ideally leaving a 5-10mm gap. 

Graffiti
Graffiti is unwelcome wherever it occurs, and no less so on Corten Steel.

Corten Steel will weather under the graffiti at a different rate to the surrounding metal; so, even if the graffiti is carefully removed through solvent dissolution, there will typically be the ‘ghost’ of the graffiti still visible – at least for some time. So if the graffiti is old [and thus the ‘ghosts’ of the graffiti are highly visible after removal], then it is often better to sand the entire piece back to a uniform pattern, and then let the Corten ‘self-heal’ in the normal way.

Finally, a footnote on the subject of maintenance:

We are often asked why the Corten Steel cannot be ‘sealed’ in some fashion. And some landscape product suppliers do indeed offer that option, so it is ultimately a matter of professional opinion.  

In IOTA’s opinion, Corten Steel should be left unfinished – not painted, not waxed, not lacquered – as any such surface finish: [i] is not needed, [ii] will not be permanent, and [iii] negates the distinct USP of Corten Steel [its extraordinary corrosion resistance, and extreme longevity, in its natural, exposed state]. Also, as we have already discussed, any such surface finish will have to be periodically renewed in future, at an ongoing cost – whereas raw Corten, left to its own devices, is an exceptionally low-cost material, for lifespan expectations in excess of 10 years.