Stainless steels are defined as iron alloys with a minimum of 10.5% chromium.
Other alloying elements are added to enhance their structure and properties, but fundamentally, stainless steels are considered for selection as steels with corrosion resistant properties.
In economic terms they can compete with higher cost engineering metals and alloys based on nickel or titanium, whilst offering a range of corrosion resisting properties suitable for a wide range of applications. They have better strength than most polymer products, (GRP), are readily repairable, and are ‘recyclable’ at the end of their useful life.
When considering stainless the most important features are: –
The basic approach is to select a grade with as low a cost as possible, but with the required corrosion resistance. Other considerations such as strength and hardenability are secondary.
Chromium, (Cr), content sets stainless steels apart from other steels.
The unique protective, self-repairing ‘passive’ surface layer on the steel is due to the chromium.
Commercially available grades have around 11% chromium as a minimum. In those stainless steels without a deliberate nickel alloying addition, these can be either ferritic or martensitic, depending on carbon range control.
Increasing chromium enhances corrosion and oxidation resistance, so a 17% Cr 430, (1.4016), ferritic would be expected to be an improvement in these aspects over the lower chromium, ‘410S’, (1.4000), types.
Similarly martensitic 431, (1.4057), at 15% Cr can be expected to have better corrosion resistance than the 12% Cr 420, (1.4021 / 1.4028), types.
Chromium levels over 20% provide improved ‘aqueous’ corrosion resistance for the duplex and higher alloyed austenitics and also forms the basis of the good elevated temperature oxidation resistance of ferritic and austenitic heat resisting grades, such as the quite rare ferritic 446, (25% Cr), or the more widely used 25 % Cr, 20% nickel, (Ni), austenitic 310, (1.4845), grade.
In addition to this basic ‘rule’, nickel, (Ni), widens the scope of environments that stainless steels can ‘handle’.
The 2% Ni addition to the 431, (1.4057), martensitic type improves corrosion resistance marginally, but its main purpose is to improve the impact toughness of the steel. Additions of between about 4.5% and 6.5% Ni are made to create the duplex types. The austenitics have ranges from about 7% to over 20%.
The corrosion resistance is not simply related to nickel level however. It would be wrong to assume that a 304, (1.4301), with its 8% Ni therefore has better corrosion resistance that a 1.4462 duplex with only 5% Ni.
More specific alloy additions are also made with the specific aim of enhancing corrosion resistance.
These include molybdenum, (Mo), and nitrogen, (N), for pitting and crevice corrosion resistance. The 316 types are the main Mo bearing austenitics. Many of the currently available duplex grades contain additions of both Mo and N.
Copper is also used to enhance corrosion resistance in some ‘common’, but hazardous, environments such as ‘intermediate’ concentration ranges of sulphuric acid. Grades containing copper include the austenitic 904L, (1.4539), type and some 25% Cr “superduplex” steels such as 1.4501 and 1.4507.
Basic mechanical strength increases with alloy additions, but the atomic structure differences of the various groups of stainless steels has a more important effect.
Only the martensitic stainless steels are hardenable by heat treatment, like other alloy steels. Precipitation hardening stainless steels are strengthened by heat treatment, but use a different mechanism to the martensitic types. Very small particles are formed by the appropriate heat treatment and act as the strengthening agent in the steel matrix.
The ferritic, austenitic and duplex types cannot be strengthened or hardened by heat treatment, but respond to varying degrees to cold working as a strengthening mechanism.
Ferritic types have useful mechanical properties at ambient temperatures, but have limited ductility, compared to the austenitics. They are not suitable for cryogenic applications due to loss of impact toughness and lose strength at elevated temperatures over about 600°C, although have been used for applications such as automotive exhaust systems very successfully.
Austenitic types, with their characteristic face centred cube, ‘fcc’, atomic arrangement, have quite distinct properties. Mechanically they are more ductile and impact tough at cryogenic temperatures.
The main physical property difference from the other types of stainless steel is that they are ‘non-magnetic’, i.e. have low relative magnetic permeability, provided they are fully softened. They also have lower thermal conductivity and higher thermal expansion rates than the other stainless steel types.
Duplex types, which have a ‘mixed’ structure of austenite and ferrite, share some of the properties of those types, but, fundamentally are mechanically stronger than either ferritic or austenitic types.
Depending on their type and heat-treated condition, wrought stainless steels are formable and machinable. Stainless steels can also be cast or forged into shape.
Most of the available types and grades can be joined by use of appropriate ‘thermal’ methods including soldering, brazing and welding.
Austenitics are suitable for a wide range of applications involving flat product forming, (pressing, drawing, stretch forming, spinning etc).
Although ferritics and duplex types are also formable by these methods, the excellent ductility and work hardening characteristic of the austenitics often make them a better choice.
Formability of the austenitic types is controlled through the nickel level.
The 301, (1.4310), grade which has a ‘low’ nickel content, around 7% and so work hardens more rapidly when cold worked, enabling it to be used for pressed ‘stiffening’ panels.
In contrast nickel levels of around 8.0% make the steel ideally suited to stretch forming operations, for example in the manufacture of stainless steel sinks. Higher nickel levels around 9.0% are required for deep drawing.
Martensitics are not readily formable, but are used extensively for blanking in the manufacture of cutting blades.
Most stainless steel types can be machined by conventional methods, provided allowance is made for their strength and work hardening characteristics.
Techniques involving control of feed and speed to undercut work hardening layers with good lubrication and cooling systems are usually sufficient.
Where high production volume systems are employed, machining enhanced grades may be needed.
In this respect, stainless steels are treated in similar ways to other alloy steels, sulphur additions being the traditional approach in grades like 303, (1.4305). Controlled cleanness types are now also available for enhanced machinability.
Most stainless steels can be soldered or brazed, provided care is taken in surface preparation and fluxes are selected to avoid the natural surface oxidising properties being a problem in these thermal processes.
The strength and corrosion resistance of such joints does not match the full potential of the stainless steel being joined, however.
To optimise joint strength and corrosion resistance, most stainless steels can be welded using a wide range of techniques.
The weldablity of the ferritic and duplex types is good, whilst the austenitic types are classed as excellent for welding. The lower carbon martensitics can be welded with care, but grades such as the 17% Cr, 1% carbon, 440 types, (1.4125), are not suitable for welding.
There is a wide range of stainless steel types.
Special grades with enhanced compositions have been developed and are available that minimise the short comings of any particular type.
These include: –
← Back to previous
↑ Top
Stainless steels are selected for architectural applications, as with most other applications, for their corrosion resistance.
This is usually the prime consideration.
Environmental factors such as temperature and humidity need to be taken into account, but the location of the proposed site is the initial consideration.
The Nickel Institute’s ‘Stainless Steels in Architecture, Building and Construction Guidelines for Corrosion Prevention’ publication categorises sites as either: –
Rural sites are defined as unpolluted, inland sites away from industrial atmospheres or discharges.
Urban sites are defined as residential, commercial or light industrial areas with non-aggressive airborne pollution, typically from road traffic, (exhaust fume and winter road salt spray may be issues).
Industrial sites are typified by airborne pollution such as sulphur dioxide or gases released from chemical process plants, which can form potentially dangerous acid condensates.
Marine sites are defined as areas where wind borne sea spray or mist may be present. These contain chlorides which can also concentrate in condensates or as surface moisture evaporates.
The environment cannot usually be defined precisely in these terms and it also important to bear in mind that environmental changes may occur during the design life of a proposed building, i.e. is the environment getting more polluted or cleaner, for any given location?
Additionally ‘micro-climates’ can influence the general categorisations and may be worth investigating for any proposed site before a final stainless steel grade selection is made. Micro-climates can exist in coastal locations or near chemical plant chimneys, where unexpected acid condensates can form.
Sub-divisions of the ‘site-types’ should also be considered.
Low temperatures and low humidity reduce the risks of corrosion and can mean that a steel grade perhaps not thought suitable for a particular site may be worth considering.
Selection guidelines are summarised in the table.
Only the ‘common’ 304, (1.4301), and 316, (1.4401), stainless steel types are considered as candidates for most UK sites.
The ‘local’ conditions are defined as:
Conditions L Least corrosive conditions e.g. low humidity and low temperatures M Typical atmospheric conditions for the site type H Harsh atmospheres, typified by persistent high humidity, high temperatures or high levels of pollutionThe performance ratings are defined as:
Performance Rating 3 Probably over-specified, for corrosion resistance requirements and cost 2 Probably the best choice for corrosion resistance and cost 1 Worthy of consideration if precautions are taken (i.e. good standard of surface finish and regular cleaning specified) X Likely to suffer severe corrosionThis shows that the 304, (1.4301) type can be considered for most sites, except either heavily polluted industrial sites or most marine sites. In these cases the 316, (1.4401) type should be the preferred choice.
Life expectancy for stainless steels in external environments
Natural rain washing of the items should be considered an advantage, as the corrosion risk from pollutants or condensates is reduced.
Similarly, exposed sections are less likely to hold condensation due to the improved natural ‘ventilation’ available to the steel surfaces.
Other important factors in stainless steel selection are: –
As a general rule, the smoother the finish, the better is the corrosion resistance. The more corrosive the environment, the more critical is the surface finish selection. In all marine environments, the default choice is 316 mirror polished. In no circumstances should a standard 240 alumina brushed finish be used in a marine environment, even with grade 316. This combination has proved to be disappointing in many situations.
Selection of polished surface finishes often requires a considerable amount of work before a final agreement is reached. This may involve having swatch samples prepared and agreed by the specifying parties.
Polished finish 1K/2K of BS EN 10088-2 is noted in the standard, Table 6, as being intended for external architectural applications, but is only one of many options. This is also known as a 240 silicon carbide finish. It has a maximum surface roughness Ra of 0.5 micron, and can be considered for marine environments where mirror polished cannot be used.
Extract from EN 10088-2 Table 6
Surface Finish Symbol Type of process route Surface finish Notes 1G or 2G Groundd See footnote e Grade of grit or surface roughness can be specified. Unidirectional texture, not very reflective 1J or 2J Brushedd or dull polishedd Smoother than ground. See footnote e Grade of brush or polishing belt or surface roughness can be specified. Unidirectional texture, not very reflective. 1K or 2K Satin polishd See footnote e Additional specific requirements to a “J” type finish, in order to achieve adequate corrosion resistance for marine and external architectural applications. Transverse Ra < 0.5 micron with clean cut surface finish Footnotesd One surface only, unless specifically agreed at time of enquiry and order
e Within each finish description the surface characteristics can vary and more specific requirements may need to be agreed between manufacturer and purchaser (e.g. grade of grit or surface roughness).
Highly reflective finishes may not be advisable especially for roofs, as this could be a hazard to air traffic on buildings near airports or on flight paths.
Alternative dull finishes have been developed for such applications.
Reflective finishes can be used to advantage however to reflect light into dark, enclosed courtyard areas of buildings.
Patterned finishes are better for hiding scratches and fingermarks in ‘high traffic’ areas.
Coloured finishes are also available for special aesthetic affects.
Crevices must be avoided, as these can be sites for localised corrosion.
Fabrication methods that avoid crevices should be considered.
Mechanical fixings can introduce crevices both at the fastener and at the lapped metal joint. Aluminium fasteners, (e.g. rivets), should be avoided for securing stainless steel panels, as galvanic corrosion to the aluminium can be a problem in harsh environments. Avoid moisture traps at any mechanically fastened joints.
Contact with lead or copper should not result in galvanic corrosion, but staining to stainless steel parts from the patina may be visible if rain water drains over the stainless steel.
Sealants can be considered to avoid such problems. Adhesive bonding, if mechanically strong enough, usually eliminates such problems.
Welds should be full seam welds, rather than intermittent fillet welds.
Compatible welding consumables should be specified with full penetration weld designs, where possible.
Iron contamination during storage and erection MUST be avoided. This is a common cause of unnecessary rust staining and attendant remedial post hand-over costs.
Mortar cleaning, (hydrochloric), acids must not be allowed to come into contact with stainless steels.
Periodic cleaning is advisable on stainless steel, as with most building exterior materials.
The frequency will depend on local conditions and the ‘visibility’ of the steelwork. Where cleaning and maintenance is difficult or costly, e.g. on the outside of high rise buildings, then a more resistant grade selection than suggested by the tables may be appropriate.
The mechanical properties of the commonly used 304 and 316 types of austenitic stainless steel do not usually present a cause for concern.
The thermal expansion rates of these grades however is about a third as much again as most steels.
i.e. around 16 x 10-6 /C compared to around 12.2 x 10-6/C for carbon steels.
Expansion joint allowances must account for this to avoid thermal buckling problems and any sealants used must be compatible.
← Back to previous
↑ Top