Stainless steel basics
Stainless steel is a steel alloy containing a minimum of 10.5% chromium content, making it highly corrosion resistant. The chromium reacts with surrounding oxygen to form a thin oxide layer on top of the steel. This forms a protective layer that prevents steel from rusting through contact with its surroundings.
Stainless steel is low maintenance, and its resistance to oxidation and staining make it an ideal material for many applications. It also boasts aesthetic appeal due to its shiny luster. Stainless steels can be rolled into sheets, bars, wires, and tubing for major appliances, building materials, industrial equipment, and surgical instruments.
There are four main types of stainless steels: austenitic, ferritic, martensitic, and duplex. Austenitic stainless steel is dominant in industry and comprises over 70% of total stainless steel production. Its properties include a maximum of 0.15% carbon and a minimum of 16% chromium, yielding very strong protection against rust. Ferritic stainless steel has reduced corrosion resistance compared to austenitic grades, but fares better than martensitic stainless steel. Duplex stainless steels have high resistance to localized corrosion particularly pitting, crevice corrosion, and stress corrosion cracking.
Does stainless steel rust?
Stainless steel is armed with built-in corrosion resistance but it can still rust in certain conditions—although not as quickly or severely as conventional steels. Stainless steels can corrode when exposed to damaging chemicals, saline, grease, moisture, or heat for prolonged periods of time.
Stainless steel’s protection against corrosion is largely dependent on the amount of chromium present. If there is not enough chromium content near the surface of the stainless steel, a new chromium oxide layer cannot be formed when the top layer is scratched off. This leaves the material very vulnerable to several types of corrosion.
Types of stainless steel corrosion
There are six common types of stainless steel corrosion:
- Stress corrosion cracking
General corrosion is considered the safest form of corrosion as it is predictable, manageable, and often preventable. Localized corrosion, such as pitting and crevice corrosion, are harder to detect due to the smaller surface area affected, but can still have damaging effects. Stress corrosion cracking is also concerning as the cracks may not be detected until the application fails.
(or uniform corrosion)
- Uniform loss of metal over entire surface
- Stainless steel with pH value of less than 1 are more prone to general corrosion
(or bimetallic corrosion)
- Electrochemical process where one metal corrodes preferentially compared to another in the presence of electrolyte
- Corrosion where boundaries of crystallites are more likely to corrode than inside surfaces
- Occurs after heating austenitic stainless steel at approximately 842–1562°F
- Localized corrosion resulting in cavities or holes
- Occurs when stainless steel is exposed to environments containing chlorides
- Localized corrosion at the crevice between two joining surfaces
- Formed between two metals or between metal and non-metal
Stress corrosion cracking
- Growth of crack formation in corrosive setting
- Tensile stresses in combination with corrosive environmental conditions lead to cracking
Stainless steel rust prevention
Steps to prevent stainless steel corrosion should be undertaken throughout the stainless steel lifecycle. Best practices during the design and production stage, as well as routine maintenance, will prolong the performance and appearance of the metal.
Taking a proactive stance with stainless steel design pays off in the long term. Proper planning in the design stage of stainless steel applications will minimize water penetration and reduce the potential for surface damage. Drainage holes for water should be used when possible, and cavities and crevices limited. Air flow is critical, and the design should encourage air to circulate freely throughout the application.
During the fabrication stage, it is important to prevent stainless steel from contacting iron or ordinary steel. This requires vigilance in surveying the surrounding environment including work tables, tools, storage units, steel turning rolls, and chains. Any carbon steel dust particles settling onto the stainless steel during fabrication can contaminate its surface, increasing the potential for rust formation. Furthermore, cleaning and grinding tools that have been used with carbon or low alloy steel must be kept separate from stainless steels.
Regular maintenance plays a key role in stainless steel rust prevention, as well as limiting the progression of existing rust. It is essential to remove any rust that has formed using mechanical or chemical means. The resulting grime can then be cleared away using warm water and soap. After cleaning, a rust-resistant coating should be applied.
Stainless steel for outdoor site furnishings
Outdoor site furnishings are meant to create dynamic city spaces that are efficient, safe, and enhance social interaction. Benches, tables, planters, bike racks, and bollards are examples of furnishings that work to establish order on city streets by preventing clutter and protecting pedestrians from heavy traffic. Outdoor site furnishings are repeatedly exposed to mechanical damage and weathering. Therefore, stainless steel is a highly sought-after base material because of its rust resistance—built in as a chromium-oxide layer. This significantly reduces rust in most furnishings when exposed to harsh elements.
Cleaning stainless steel for ongoing maintenance is generally easy and suitable for outdoor environments. Minor surface cleaning can be done with soap and water or a diluted ammonia solution, and dried with a soft cloth or air blower. If rust formation is showing, commercial solutions can be used to passivate the surface of the stainless steel which removes surface oxidation.
Unlike industrial applications, urban furnishings are expected to maintain an attractive appearance and complement surrounding infrastructure. When taking on this dual role of functionality and aesthetics, stainless steel is a reliable material of choice due to its durable yet refined presence.