Stainless Steel
In very simple terms stainless steels are carbon steels with added chromium. The chromium allows the steel to form a chromium-rich oxide film which inhibits corrosion and is self repairing. This film is chemically stable provided sufficient oxygen is available to the surface. The detailed metallurgy is more complex.
Stainless steel breaks down into three main generic groups, Ferritic, Martensitic and Austenetic. These three groups are each tuned to optimise particular metallurgical properties. Ferritic stainless steels are magnetic, have a low carbon content and contain chromium as the main alloying element (usually between 13 and 17%). Typically they are used in situations which are less demanding in respect of corrosion resistance. Marstensitic stainless steels are magnetic; they have a medium carbon content and typically contain 12%, chromium. They are used for cutlery, aerospace and general engineering. Austenitic stainless steels are non-magnetic and in addition to chromium at 18%, they contain nickel and molybdenum which increase their corrosion resistance. Typically they are used in architectural, marine/off shore applications, structural engineering and chemical and processing plants. There are further generic types such as Duplex and High Alloy, however these are limited in availability and highly specialised. Duplex stainless steels are alloys designed to have improved localised corrosion resistance, specifically to stress corrosion cracking, crevice and pitting corrosion. “Super” austenitic or “super” duplex grades are also available and have better pitting and crevice corrosion resistance compared with the ordinary austenitic or duplex types due to additions of chromium, molybdenum and nitrogen.
Corrosion attacks the surface of a material and understanding and specifying suitable surface finishes is important. Clean and uncontaminated surfaces are a prerequisite and can be achieved though good workshop practice (particularly avoiding non-stainless steel contact from tools and dust) and by surface treartment, such as shotpeening. This ensures that corrosion resistance provided by additional chromium, nickel, molybdenum etc. can be fully exploited.
Points to watch
The surface finish of stainless steel has a considerable effect on durability and resistance to staining discolouration. This should be carefully researched. More information on finishes can be found in SCI publication No P.179.
Stainless steel is easily distorted by the heat from welding. It is essential to use the correct welding procedures and jigs to hold the material.
Welding of stainless steel is expensive and therefore best avoided if economy is the aim.
Only use fabricators which have a proven track record with stainless steel. Look at examples of their work.
Bi-metallic corrosion of less noble materials than stainless steel or between different grades of stainless steel is a potential problem. Fixings and background materials require careful research.
Crevice corrosion is a potential problem with stainless steel. This requires careful research to ensure that details avoid this.
The stainless steel industry is highly organised for recycling material of all grades. New material generally incorporates a high level of recycled material in its production. This should be verified with the mill of origin when making environmental/energy statements.
© Gordon Talbot – Ian Ritchie Architects 1999; © Gordon Talbot + Ian Ritchie Ian Ritchie Architects Ltd. 2006
LA VILLETTE BIOCLIMATIC FACADES
Stainless steel primary and secondary structure, cable trusses and glass support fittings. The tubes were centrifugal castings, the connecting nodes were investment castings. Other components were combinations of machined plate and investment castings. Stainless steel was selected to accentuate slenderness of members, light reflectance and long term durability.
BERMONDSEY STATION and MID-LINE VENTS
Stainless steel plate cladding to roofs, passenger services accommodation, ticket office, blade wall ends and platforms. Stainless steel equipment supports, services distribution and station fittings. Stainless steel standing seam roof, stainless steel acoustic cladding. All stainless steel used in connection with Bermondsey Station and the Mid-Line vents is from the 300 series. The majority are grade 316 with grade 304 being used in a small number of locations. The selection of grade 316 is for reasons of long term durability, resistance to airborne pollutants, low maintenance, resistance to damage and the quality of the surface appearance.
Stainless steel external perforated solar louvres; stainless steel plant room louvres and glass support fittings. Stainless steel selected for the solar louvres for strength (ability to span when a curve induced), light reflectance and low maintenance. The plantroom louvres were selected for low maintenance and light reflectance.
REINA SOFIA MUSEUM OF MODERM ART
Stainless steel glass fixings, support brackets and suspension rods. The suspension rods were formed in a duplex material due to its low coefficient of thermal expansion enabling the thermal movement of the suspension of the rods to be kept within manageable dimensions.
Stainless steel plate selected for long term durability, resistance to mechanical fatigue, resistance to staining (effect of coastal and industrial environment) and surface response to artificial and natural light. Plate grade 316L, plate thickness 10mm-35mm rolled into conical spire. A self-cleaning and corrosion resistant surface will be achieved by treating the surface with stainless steel and glass beads.
