Stainless steel plays a pivotal role in the construction of street furniture, primarily owing to its remarkable resistance to corrosion and rust. This sets it apart from mild or carbon steel, which tends to corrode rapidly when exposed to the elements.
Why is Stainless Steel Corrosion-resistant?
All grades of stainless steel contain a minimum of 11% chromium, a critical element responsible for its corrosion resistance. Chromium forms an ultra-thin oxide layer, approximately 5 nanometers thick, on the steel’s surface when exposed to oxygen. This thin layer acts as a shield, protecting the underlying steel from rusting.
However, it’s important to note that stainless steel doesn’t automatically guarantee immunity from corrosion and rust. Selecting the appropriate stainless steel grade is crucial. Using an unsuitable grade can result in corrosion nearly as rapid as unprotected mild steel. For a deeper dive into the science behind stainless steel corrosion, you can [read more here].
Stainless Steel Grades
While there are more than 150 stainless steel grades available, only two (304 and 316) are typically relevant for external architectural metalwork.
Every grade, including the duplex grades used in industries such as oil and gas, has limitations regarding the types of corrosive elements and chemicals they can withstand. Cheaper grades, like 304, offer lower corrosion resistance. For instance, at room temperature, 304 can resist up to 3% sulfuric acid, whereas type 316 can resist up to 20%.
Types of Corrosion
In the realm of external architectural fittings, coastal corrosion or tea staining is the most prevalent form of corrosion. Tea staining results in a brown discoloration on the surface of stainless steel. While it doesn’t jeopardize the steel’s structural integrity, it is aesthetically displeasing.
A rule of thumb is that tea staining is a concern within 5 km of the coast. Measures to reduce this risk include using higher-grade steel, like 316, avoiding high polish finishes, and considering the installation location. For instance, a stainless-steel bench armrest on a coastal promenade should be 316 grade due to extreme exposure. Conversely, a stainless-steel cycle rack under a shelter may suffice with 304 grade steel, as it’s shielded from the elements.
An illustrative example of tea staining occurred in Newcastle when high polish litter bins made from 304-grade stainless steel were installed. Though Newcastle isn’t directly on the coast, its proximity makes tea staining a risk. Using 316 grade wouldn’t have eliminated the problem entirely but would have made it less noticeable.
As mentioned earlier, there are over 150 stainless steel grades categorized as Martensitic, Ferritic, Austenitic, and Duplex. Each group possesses unique properties, making them suitable for different applications. This article primarily focuses on the grades pertinent to external furniture rather than providing an exhaustive guide to all stainless-steel grades.
Martensitic Stainless Steels
Martensitic stainless steels, with their low chromium content (12–17%), aren’t as corrosion-resistant as other groups. They can be hardened through heat treatment, offering exceptional durability.
Ferritic Stainless Steels
Ferritic stainless steels are more corrosion-resistant than martensitic varieties but less so than austenitic ones. They contain lower amounts of nickel, making them cost-effective.
Austenitic Stainless Steels
Austenitic stainless steels, constituting the majority of stainless steel production, offer excellent corrosion resistance and mechanical properties. They contain higher levels of chromium and nickel.
Duplex Stainless Steels
Duplex stainless steels, characterized by high chromium and molybdenum content, have a molecular structure combining elements of austenitic and ferritic structures. However, they are more suitable for industries requiring strength rather than enhanced corrosion resistance, such as oil and gas.
In conclusion, when specifying stainless steel for external furniture or planters, consider the project’s proximity to the coast. Within 10 km of the coast, 316 grade stainless steel is advisable for bare applications. For locations within 5 km of the coast, opt for 316 grade without a high polish finish. If a high polish finish is desired, explore alternatives like duplex grade 2205 or higher austenitic grades like 317. Beyond 10 km from the coast, 304 grade stainless steel should suffice for most applications.