Stainless Steel plays a key structural role in street furniture. At the heart of its appeal is the resistance it offers to corrosion and rusting. Compared to mild, or carbon steel which will corrode almost immediately if placed unprotected in an outdoor environment this is a major advantage. Structural seating and planter components must be manufactured to have a long lifespan so stainless steel is often a necessity for structural components.
Why is Stainless Steel corrosion resistant?
All grades of stainless steel have a minimum of 11% chromium content. Chromium is the ‘key ingredient’ in stainless steel because it forms an oxide layer on the surface of the steel. This layer is extremely thin, around 5 nanometres, and is only formed when the surface is exposed to oxygen. This layer is what protects the steel underneath from rusting.
If less than 11% chromium is used, the oxide layer will not form. Conversely, there is a direct correlation between increasing chromium content and corrosion resistance. This point highlights the nucleus of the question addressed by this blog – essentially different grades refer to the different levels of chromium alloyed in the steel.
Stainless steel does not automatically guarantee immunity from corrosion and rust. Therefore, choosing the correct stainless-steel grade is important – if a grade is used that is not suitable, the component will rust almost as quickly and unprotected mild steel.
Stainless Steel Grades
Stainless steel grades simply refer to the chemical composition of the steel, for example the chromium content or other alloys such as nickel or molybdenum. There are over 150 grades, but for external architectural metalwork purposes only 2 grades (304 and 316) are relevant.
All grades, even the duplex grades used in the oil and gas industry, have a limit to the number of corrosive elements and chemicals they can comfortably handle. Typically, cheaper grades (eg 304) are less resistant to corrosion. For example, at room temperature, 304 grade will resist up to 3% sulfuric acid whereas type 316 will resist up to 20%.
Types of Corrosion
For external architectural fittings, the main type of corrosion encountered is coastal corrosion, or tea staining. This is a brown discolouration on the surface. Although it does not pose any risk to the structural integrity of the steel, it is very unsightly. Tea staining is caused by a salt film on the surface which slowly corrodes the steel. Inland, this would not be a problem as rain would quickly wash away any salt deposits, such as grit, left on the steel. However, in coastal areas this is a major consideration because of constant salt content in the air.
A good guide is that tea staining will be a risk up to 5km from the coast – obviously the closer to the coast, the higher the risk will be. There are several measures to reduce the risk. The first would be to use a higher-grade steel, such as 316. Secondly, high polish finishes are much more susceptible than factory and mill finishes. Another consideration is the location. A stainless-steel bench armrest on a coastal promenade must be 316 grade because it is extremely exposed. However, for a stainless-steel cycle rack under a cycle shelter, 304 grade may be sufficient because the steel is sheltered from the elements.
A good (or bad!) example of tea staining is when several 304 grade high polish litter bins were installed in Newcastle. Although Newcastle is not on the coast, it is relatively close and tea staining is a risk. The litter bins were specified in a 304 grade stainless steel and within a few months the bins were starting to tea-stain. In this situation, using 316 grade would not have completely eliminated the problem but it would have helped to make the tea staining less noticeable.
As previously noted, there are over 150 grades of stainless steel. These grades are grouped into subcategories called Martensitic, Ferritic, Austenitic and Duplex groups. Each group is classed according to its molecular structure, which gives each group generic properties that distinguish them from each other. This article will only examine the grades relevant to external furniture rather than providing a full metallurgical guide to all stainless-steel grades.
Martensitic Stainless Steels: Not as corrosion resistant as other groups due to the low chromium content (12 – 17%). Can be hardened by heat treating. Their advantage lies in their toughness – martensitic steels are very hard wearing.
Ferritic Stainless Steels: More resistant to corrosion than martensitic but not as much as austenitic. Aside from the molecular structure, the main difference between ferritic and austenitic steels is that ferritic contains smaller amounts of nickel which typically makes it more cost effective.
Austenitic Stainless Steels: The most common group of stainless steels comprising two thirds of all stainless-steel production. Contains higher levels of chromium and nickel which provides excellent corrosion resistance and good mechanical properties.
Duplex Stainless Steels: Characterized by high (19%-30%) chromium and molybdenum (approx. 4%) content. At a molecular level, they are a combination austenitic and ferritic structures which is why they are known as duplex grades. However, they are something of an overkill for external furniture purposes – the main advantage they offer is increased strength rather than corrosion resistance. Heavily used in the oil and gas industry.
The conclusion is relatively simple. If you plan on specifying bare (ie non powder coated) stainless steel as part of external furniture or planters, you should specify 316 grade if the project location is within 10kms of the coast. If the project location is within 5kms, you should specify 316 grade without a high polish finish. If a high polish finish is desired, maybe consider a duplex grade such as 2205 or a higher austenitic grade such as 317. On the other hand, if the location is greater than 10kms from a coastline then 304 should be suitable for most applications.