Selection of Stainless Steel for Building External Application

Stainless steel 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 Steel in Architecture, Building and Construction Guidelines for Corrosion Prevention' publication categorizes sites as either: Rural,Urban,Industrial,Marine

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 windborne 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 ie. 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. Microclimates 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:

The performance ratings are defined as:

This 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.

Additional factors for consideration

Other important factors in stainless steel selection are: -

Surface finish

As a general rule, the smoother the finish, the better the corrosion resistance.
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 K 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.

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.

Design

Crevices must be avoided, as these can be sites for localised corrosion.

Fabrication methods and corrosion hazards

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.

Accessibility for cleaning and maintenance

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.

Mechanical and physical properties of stainless steels

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.

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