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.
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.
|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|
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.