The characteristics of stainless steel, notably its corrosion resistance, aesthetic appearance and mechanical properties, make it ideally suited for many architectural applications. Optimum performance is achieved by taking into account these characteristics when designing in stainless steel.
What is Stainless steel?
Stainless steel is not a single specific material; it is the name given to a group of corrosion resistant steel alloys which contain a minimum of 10.5% chromium.
The chromium in stainless steel reacts with oxygen in the air to produce a very thin, inert, chromium rich oxide film on the surface of the steel. It is the presence of this film which provides the corrosion resistance of stainless steel. This passive film is unlike coatings such as paint or galvanising in one very important way. If it is damaged by abrasion or mechanical means such as cutting, it re-forms and continues to protect the steel.
Factors in Material Selection for Stainless Steel in Architectural Applications
Stainless steel is largely chosen for its corrosion resistance and aesthetic appearance. It is important to understand the factors which are involved in making a selection of both grade and surface finish. This will ensure that the performance of the product will match the expectation.
The influence of grade on the performance of stainless steel is fairly well known. However, it is not so well known that surface finish has an equally important role in determining corrosion resistance. Poor quality polished finishes can lead to disappointing performance of stainless steel. The results of the laboratory tests below are mirrored in actual situations:
Accelerated laboratory tests
Rough surface (Ra >1.0 micron)
Smooth surface (Ra approx 0.3 micron)
In addition to the polished surfaces there is a huge range of patterned and/or coloured surfaces which are available.
The surface finish of stainless steel can vary from a matt descaled finish used for construction fixings to a bright highly polished finish.
Hot rolled, softened and descaled (normally used on thicknesses of 4mm, 5mm, 6mm, 8mm and 10mm)
Cold rolled and lightly rolled on polishing rolls
Structural Design of Stainless Steel
Stainless steel is being used increasingly as a structural material. It is important that its distinctive properties as compared to standard carbon and alloy structural steels are understood and built in to the design of structural components.
Stainless steel does not readily corrode, rust or stain with water as ordinary steel does, but despite the name it is not fully stain-proof, most notably under low-oxygen, high-salinity, or poor-circulation environments. There are different grades and surface finishes of stainless steel to suit the environment the alloy must endure. Stainless steel is used where both the properties of steel and corrosion resistance are required.
Stainless steel differs from carbon steel by the amount of chromium present. Unprotected carbon steel rusts readily when exposed to air and moisture. The rust is active and accelerates corrosion by forming more iron oxide, and due to the greater volume of the iron oxide this tends to flake and fall away. Stainless steels contain sufficient chromium to form a passive film of chromium oxide, which prevents further surface corrosion by blocking oxygen diffusion to the steel surface and blocks corrosion from spreading into the metal’s internal structure, and due to the similar size of the steel and oxide ions they bond very strongly and remain attached to the surface.
Stainless steel is used for buildings for both practical and aesthetic reasons. Stainless steel was in vogue during the art deco period. The most famous example of this is the upper portion of the Chrysler Building (pictured). Some diners and fast-food restaurants use large ornamental panels and stainless fixtures and furniture. Because of the durability of the material, many of these buildings retain their original appearance.
Maintenance of stainless steel
If treated or stored incorrectly, any grade of stainless steel may discolour or stain. To maintain optimum appearance, the surface should be cared for regularly.
Maintenance during installation
The quality of installation affects the durability and lifespan of stainless steel. Therefore it is important to make sure stainless steel is in good condition before installation. Normally, giving it a quick clean is enough prior to installation. However, if surface contamination is present, more attention is required. In fields such as aerospace, pharmaceuticals and food handling, an extremely high standard of cleanliness may be required so extra care should be taken.
Maintenance is required to maintain the quality and appearance of steel. Depending on the environment, it is carried out between one and ten times per year. A proper maintenance routine significantly prolongs the life of stainless steel.
Why use Stainless Steel?
Stainless steel has been used with success by the construction industry throughout the UK and Europe for over seventy years. Its use has increased rapidly in recent times as the benefits of stainless steel over traditional materials have become more widely recognised.
Stainless steel offers many advantages to the specifier:
• Excellent corrosion resistance
• High ductility and strength
• Non-magnetic (Austenitic only)
• Excellent high and low temperature properties
• Resistance to unsightly staining
• Life-cycle costing benefits
• Aesthetic surface finish
• 60% recycled content
• 100% recyclable
Using stainless steel means no costly remedial or refurbishment measures are required during the life of the structure.