Best practices in rust prevention from design to maintenance
What is stainless steel?
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 protects the 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 has 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 steel is known to have a high resistance to localized corrosion—particularly pitting and crevice corrosion, and stress corrosion cracking.
Does stainless steel rust?
Stainless steel is highly resistant to corrosion, but it can rust. If there is not enough chromium present near the surface of the stainless steel, a new chromium oxide layer cannot be formed when the top layer is scratched off—leaving the material vulnerable to several types of corrosion.
Types of stainless steel corrosion
General corrosion (or uniform corrosion) General corrosion results in the uniform loss of metal over an entire surface. Stainless steel with a pH value of less than 1 are more prone to general corrosion.
Galvanic corrosion (or bimetallic corrosion) Galvanic corrosion is an electrochemical process where one metal corrodes preferentially compared to another in the presence of electrolyte. An example of this is when stainless steel comes into contact with rain, causing it to corrode.
Intergranular corrosion Intergranular corrosion occurs where the boundaries of crystallites are more likely to corrode than the inside surfaces. This can happen after heating austenitic stainless steel at approximately 842–1562°F as the carbon in the steel converts to grain boundaries, leading to corrosion.
Pitting corrosion Pitting corrosion is a localized corrosion resulting in cavities or holes in the material, especially when stainless steel is exposed to environments containing chlorides. Pitting is more serious than uniform corrosion as it can be difficult to detect and protect against.
Crevice corrosion Crevice corrosion is a localized corrosion that occurs on a metal surface at the crevice between two joining surfaces. This gap may be formed between two metals or between a metal and non-metal. This is typically not of high concern unless the stainless steel is kept in a stagnant solution, leading to the accumulation of chlorides.
Stress corrosion cracking Stress corrosion cracking is the growth of crack formation in a corrosive setting. Tensile stresses in combination with corrosive environmental conditions can lead to cracking of stainless steel.
Overall, 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 affected surface area being smaller, but the effect more damaging. Stress corrosion cracking is also concerning as the cracks may not be detected until the application fails.
Stainless steel rust prevention
Steps to prevent stainless steel corrosion should be undertaken throughout the stainless steel lifecycle. Best practices in rust prevention during production and use will prolong both the performance and appearance of the metal.
Taking a proactive stance with 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
Stainless steel is highly sought after for its corrosion-fighting capabilities, built into it in the form of a chromium-oxide layer. This significantly reduces rust in most applications, especially those that are 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. For furnishings with rust formation showing, commercial solutions can be used to passivate the surface of the stainless steel, removing surface oxidation.
Outdoor site furnishing such as bike racks and bollards are repeatedly exposed to mechanical damage and weathering—but unlike industrial settings, these urban furnishings are expected to maintain an attractive appearance and enhance streetscapes. When site furnishings are meant to complement surrounding infrastructure, it is particularly beneficial to consider stainless steel and its combination of durability and aesthetics.
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