Choice of fastener material
Austenitic stainless steels are considered 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. Standard for external timber framed walls recommends austenitic stainless steel as a suitable material for:
- staples, screws, and nails for fixing sheathing and breather membranes
- nails, screws for fixing CCA treated wood
- wall ties and fixings.
Austenitic stainless steel is also a recommended material for external structures 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
- 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. BUMAX 316L HiMo (1.4436) is considered to be the strongest of 316L classifications and able to withstand corrosion longer. http://www.BUMAX.se/en 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:
1.4436 BUMAX www.BUMAX.se/en/
should be considered.