Essential Metal Knowledge

Corten vs Powder Coated vs Stainless Steel

When designing an outdoor space, if external architectural steel is part of the project there are usually 3 choices – the increasingly ubiquitous Corten (Weathering) Steel, the familiar world of Powder Coating and the seldom used Stainless Steel.

It is crucial to understand the considerable differences between these finishes, not just in terms of aesthetics but also the myriad other factors that will make or break the success of a project. Here’s why.

Corten Overview

Before we begin, its important you know how Corten is related to Weathering Steel. Read our insight here to find out more, but essentially Corten is a generic brand name that refers to Weathering Steel. In practise, the terms are used interchangeably.

Corten is an exceptionally strong steel grade first used in heavy industry. Over time, it has become beloved by the architectural world for the rust-coloured finish it develops, thanks to the copper content. This finish can take anywhere from 6 months to 3 years to fully develop, but once it has it protects the steel underneath from rust which is why Corten is described as maintenance free.

Corten is great when used in suitable environments, but there many pitfalls to be aware of – for external architectural use, make sure Corten Grade A is specified and research the environmental conditions where the Corten will be located – it could save significant problems down the line!

Polyester Powder Coated (PPC) Overview

Powder Coating is simply an advanced method of painting. The materials that comprise conventional paint are simply ground into a flour like powder and sprayed onto the surface via a spray gun. This finish is far superior to conventional paint as there is a smooth finish free from running or dripping of paint and the process is more environmentally friendly. For external architectural purposes, the application surface is usually steel.

The application process is three part – firstly, the steel must be pre-treated as the coating will not apply properly if there is dust or grease on the surface. If the finished component is going to be placed outside, it is vital to properly protect it from corrosion and rust at this stage. The best way to do this is through hot dip galvanising the steel where the zinc becomes ingrained into the mild steel so the zinc protection can never be scratched off. The other option is with the use of zinc primer (known as cold galvanising) considered an inferior technique. Essentially the primer is just a paint so if you scratch the primer off, the mild steel is left open to the elements and will rust.

Secondly, the powder is applied to the surface via spray gun which gives it a negative charge. The surface is grounded which results in zero powder wastage as the powder is attracted to the surface.

Finally, the powder coated steel is placed in a curing oven which hardens the coating through chemical reactions in the powder known as crosslinking.

Stainless Steel Overview

Stainless Steel is a metal alloy that is highly resistant to corrosion and rusting through the presence of Chromium. All grades of Stainless Steel have a minimum of 11% Chromium content. The resistance to corrosion is what makes stainless steel popular for outdoor uses.

When researching steel, you may hear the term ‘steel grades’ frequently used. This simply refers to the chemical composition of the steel. For external architectural uses, the most common grades are Grade 304 and Grade 316. Grade 304 is the most common of all grades, featuring 18% Chromium content. Grade 316 is alloyed with Molybdenum which provides greater resistance to chloride corrosion. Therefore Grade 316 is commonly used in marine environments.

Aesthetically speaking, Stainless Steel is available in numerous different finishes. The 5 most popular finishes used in outdoor architectural environments are summarised in the table below. Typically, low numbered finishes correspond with coarser, ‘factory finish’ aesthetics.

ASTM Designation	Name	Description
01	Mill Finish	Matt Factory Finish, Industrial Aesthetic
04	Brushed Finish	Most common finish, features brushed lines in a parallel pattern. Caused by grit on the finishing belt at the steel mill. Not too reflective
06	Satin Finish	Very similar to brushed finish, but the grit used on the finishing belt is finer resulting in a smoother surface
07, 08	High Polish	Also known as a mirror finish, these are highly reflective and shiny. Extremely fine grit used
09	Bead Blasted	Small Balls are fired at the Steel at high velocity to create a matt, slightly pitted finish

Comparison

Corten Pros

Extremely damage resistant. Most external steels begin to date after years of exposure due to scratches and discolouration. Not so with Corten – any scratches will just blend in with the overall surface as soon as the weathering process is complete for the scratched area, and there is no discolouration since the rust colour is natural.
Adds visual interest. No sheet of Corten ever rusts in the same pattern, and the finish is never stops evolving. Therefore, Corten can add another dimension to a project through its unique ever-changing aesthetic.
Environmentally Friendly. Corten is the most environmentally friendly of these three finishes. There is no maintenance required, no paint chemicals used and fewer energy hungry stainless manufacturing processes.

Corten Cons

Lack of Aesthetic Choice. If you don’t like the distressed Corten look, or just think it looks rusty, there is no way to change it.
Common Corten Mishaps. If placing in the wrong atmospheric environments, eg too high humidity levels, Corten will either just rust through or not weather at all. This creates big problems, because either the structure will collapse after 20 years or so, or it will need replacing at a high cost.
Rust Coloured Runoff. Unfortunately, during the weathering process rust coloured run off will happen. Although this can be relatively easily cleaned when the weathering process is complete, it will look unsightly and will attract criticism when surrounding pavements are edged with a murky brown.

PPC Pros

Vast Visual Choice. Powder coating can be done in all 1625 RAL colours or different textures as desired. This offers huge flexibility when designing a project – a flexibility no other finish can come close to matching.
Value for Money. Powder Coating enjoys the benefit of having lowest price tag of all three options in this article. Even if specified in an eye-watering colour or unusual textured finish, the price will not rise above Corten or Stainless.

PPC Cons

Can be (relatively easily) scratched. Although much more scratch resistant than ordinary paint (hence why it is used externally), a powder coated finish will not resist say a scooter deck being dragged along the side of a planter. This will expose the bare silver steel underneath which looks unsightly and is permanent.
Lack of Corrosion Resistance. If the steel has been hot dip galvanised pre powder coating, this is not a problem. However, if the steel has only been primed with a zinc paint or no protection at all, if the finish is scratched the steel will be exposed and will corrode.

Stainless Pros

Good Corrosion Resistance. Stainless Steel has the best resistance to corrosion out of all three options. Even if placed in a humid environment, architectural grade Stainless Steel will resist corrosion unlike Corten.
Highly Durable. Because Stainless Steel is the same colour throughout, Stainless Steel will not fade due to UV exposure.
No Galvanising Necessary. Unlike Powder Coating, with Stainless there is no galvanising or pre-treatment work necessary due to the inherent corrosion resistance.

Stainless Cons

Expensive. Irrelevant of the finish, Stainless steel is expensive.
Lack of Visual Choice. Like Corten, although there are a few different finishes available, if you do not like the look of Stainless steel there is nothing much you can do in the way of different colours or textures.

Conclusion

There is no right or wrong conclusion here. It is a case of looking at the project and then deciding which option is most suitable. It’s heavily dependent on the surrounding area – for post-industrial settings, such as a refurbished warehouse, Corten looks great. For most schemes, such as a new build residential block, a bright powder coated finish may look more in keeping with the overall aesthetic.

There is a school of thought which argues that Corten has become overused in recent years. From a purely personal point of view, I cannot decide if I love or hate it but there is no denying that on certain projects it looks fantastic.

Intelligent readers may have realised there is an ideal option here, which is to powder coat stainless steel. This is possible and has been done, but it is prohibitively expensive – not only is there the high upfront cost of stainless steel, but also the added costs of powder coating.

There is a reason Powder Coating is the favoured method of finishing external architectural steel – there are no inherent risks using it (unlike Corten), its good value for money and looks great in all environments if a suitable colour is chosen. However, do not disregard Corten or Stainless Steel, because if the budget is large enough, they can add prestige to a project.

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