High-grade [304- and 316-grade] stainless steels are expensive. However, they do not need painting, are durable and highly corrosion-resistant; and they have a distinctive metropolitan style, when left with a raw, unpainted, natural metal finish.
The only sheet metals that can definitively guarantee a 25+ year lifespan are Corten steel and 316-grade stainless steel. And 316 is aka 'Marine-grade' for a reason – in exposed coastal locations, it really is the only option.
High-grade stainless steels [such as 316-grade, aka ‘Marine-grade] are ‘austenitic’ stainless steels, which contain significant quantities of expensive alloying elements, such as nickel and chromium. This contrasts with lower-grade utility or ‘ferritic’ stainless steels, such as 1.4003-grade stainless steel, which lack these alloys.
These expensive alloys have, in fact, a relatively limited impact on the structural performance of the metals over time, compared with utility stainless steels. Instead, these alloys serve to reduce the occurrence of non-structural surface discolouration [aka ‘tea staining’], when the metals are used raw and uncoated – as in the ‘brushed’ finish, commonly specified.
The two most commonly-specified high-grade stainless steels are 304-grade and 316-grade [aka ‘Marine-grade’]. The essential difference is that 316-grade also includes molybdenum, which gives 316-grade superior resistance to chloride-related corrosion and pitting compared to standard outdoor 304-grade steel.
High-grade stainless steels deliver unparalleled corrosion resistance and longevity, even greater than Corten steel. Whereas Corten steel might be expected to have a lifespan of 40-120 years, 316-grade stainless steel’s lifespan can, for all practical purposes, be considered ‘virtually indefinite’ in relatively benign environments. Also 316-grade stainless steel can also be permanently buried in wet soil or concrete, in which conditions Corten steel is liable to premature failure.
High-grade stainless steels are stronger than mild steels or galvanised steels, such as Zintec steel, and are highly resistant to being ‘punched through’. This is a particular issue with aluminium planters, where – in public realm, for example – vehicular impact damage can breach the metal, and cause spillage of the soil / plants etc. All steels, and stainless steels in particular, tend to absorb such impacts and deform, rather than the metal splitting.
There are a number of reasons why a landscape architect may wish to specify a ‘raw’ metal – unpainted, unsealed and in its natural metal finish. These include:
In sheet metal, the only credible options are Corten steel and stainless steel. Each have their own individual, highly distinctive aesthetic – so specification is never a choice of ‘either-or’. But where one of these metals is the right look for the brief, then significant additional benefits accrue.
High-grade stainless steels deliver unparalleled corrosion resistance and longevity in the outdoor environment, even when uncoated / unpainted. As a result, the material is used in the landscape industry for myriad off-the-shelf products, and customisable product system solutions, such as street furniture [litter bins, cycle stands, bollards etc.]; handrails, balustrades, cable railings and structural posts; all manner of lighting applications; and drainage applications [linear slot drainage channels, floor gullies, gratings etc.] The list goes on.
However, IOTA does not supply off-the-shelf products, nor customisable product system solutions. Instead, we supply bespoke landscape products made from metal in sheet form – mostly planters, seating and architectural metalwork. For those kinds of products, the following are typical applications.
High-grade stainless steels are more expensive than other metal materials that can be used in public realm. However, where budgets allow, these metals do not need painting, are durable and corrosion-resistant, and have a distinct metropolitan style when left with a raw, unpainted, natural metal finish.
For most urban or inland landscape installations, 304-grade steel is perfectly adequate and more cost-effective than 316-grade.
316-grade stainless steel is aka ‘Marine-grade’ for a reason. It also contains molybdenum, which gives 316-grade superior resistance to chloride-related corrosion and pitting compared to standard outdoor 304-grade steel.
316-grade should be used in extreme environments such as:
On some projects, there may be contractual warranties demanding a 25+ year Design Life Performance under any-and-all circumstances. Only 316-grade stainless steel can deliver that assurance.
High-grade stainless steels do not need to be painted, and this removes a major maintenance hurdle.
However, it is not true to say that high-grade stainless steels are completely ‘zero maintenance’ – at least in situations where aesthetics matter. As these steels will ‘tea stain’.
Tea staining is a form of surface discoloration and corrosion that develops on stainless steel. It gets its name because the blemishes appear as light brown or yellow marks that closely resemble spilled tea. While unattractive, it is strictly a cosmetic issue that does not compromise the strength, structural integrity, or lifespan of the material.
Click here for A Guide to Tea Staining on Stainless Steel, covering what it is, how & why it happens, how to prevent it, and – crucially – how to fix it.
Galvanic corrosion between stainless steel and aluminium occurs when the two metals touch in the presence of an electrolyte [like moisture or salt], causing the aluminium to act as a sacrificial anode and corrode rapidly.
Click here for A Guide To Galvanic Corrosion. However the most important thing to note is that, when specifying these metals, one must make all efforts NOT to mix the two metals within the same product design. This error most commonly occurs when aluminium fastenings are used on stainless steel products [or vice versa]. The fastenings and the sheet metal must be the same material.
Speak to IOTA about the potential benefits of 304-grade and 316-grade stainless steels for your specific project.
