Austenitic stainless steel for timber fixings | Causes of metal corrosion in timber fixings
Austenitic stainless steel are considered by the UK National Corrosion Service to have Group 4 (excellent resistance) in the ranking of the resistance of common metals to corrosion by acetic acid emitted by wood.
Where CCA preservative treatments have been applied, the timber should be aged for at least seven days before fasteners are inserted. The UK National House-Building Council (NHBC) Standard for external timber framed walls recommends austenitic stainless steel as a suitable material for:
- staples and nails for fixing sheathing and breather membranes
- nails for fixing CCA treated wood
- wall ties and fixings.
Austenitic stainless steel is also a recommended material for external structure of woods such as cedar and permanent structures (painted or oiled).
Timber in immersed structures
Austenitic stainless steel fasteners are suitable for use in immersed timber structures. The choice of stainless steel grade will depend on water conditions such as:
- chloride level
- water flow rates
- temperature
- oxygen levels
- crevice conditions in the fastener system geometry.
Stainless steel grade 304 (1.4301) may be suitable for fresh water, but in higher chloride levels 316 (1.4401) is preferable. The conditions in tidal river estuaries and seawater where higher chlorides, low flow rates and low oxygen levels can predominate may require higher molybdenum grade stainless steels such as a 6% molybdenum grade (1.4547). Care must also be taken to avoid bimetallic (galvanic) corrosion if combinations of dissimilar metals are to be used.
Timber in swimming pool buildings
Particularly aggressive internal environments can also exist in swimming pool buildings, especially for roof fixings where applied loading can also give rise to stress corrosion cracking (SCC) failure hazards. For these applications SCC resistant grades such as 904L (1.4539) or a 6% molybdenum grade (1.4547) should be considered.