Advanced Metal Knowledge

Which grade of stainless steel should you specify?

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 rapidly 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 discolouration 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 planter tub on a coastal promenade should be 316 grade due to extreme exposure. Conversely, an inland, public realm space with stainless-steel planter edging may suffice with 304-grade steel.

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.

410 Grade: A general-purpose martensitic stainless steel, cost-effective but not ideal for outdoor furniture.
416 Grade: Similar to 410, easy to machine due to increased sulfur content.
420 Grade: Commonly known as ‘Cutlery Grade,’ suitable for internal use due to low corrosion resistance but high wear resistance.

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.

404 Grade: Occasionally used in street furniture with the recommendation of a powder-coated finish for protection.
430 Grade: A common ferritic grade with balanced properties, unsuitable for outdoor furniture due to low corrosion resistance.

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.

303 Grade: Easy to machine due to added sulfur and phosphorus, known as A1 in ISO grades.
304 Grade: The most common stainless steel grade, known as 18/8 due to its 18% Chromium and 8% Nickel composition, referred to as A2 in ISO grades.
316 Grade: Recommended for external architectural purposes, labeled ‘Marine Grade’ for its exceptional corrosion resistance in marine environments. Referred to as A4 in ISO grades. The inclusion of 2% molybdenum enhances its corrosion resistance against chloride ions.
317 Grade: Offers even greater corrosion resistance than 316 due to over 3% molybdenum content.

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.

2205 Grade: Extremely strong, ideal for high-pressure environments.

Conclusion

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.