Moisture and acetic acid are the biggest threat to metals being used with timber. Richard White of BM TRADA explains the importance of correct specification
Metals that are used correctly in conjunction with timber will last the lifetime of a structure under dry conditions. But metals may be at risk from corrosion in wet or damp conditions. This risk can be minimised through correct specification, design, storage, construction and maintenance.
The key agents of corrosion of metals by timber are moisture and acetic acid, a naturally occurring compound in all timber. Moisture may be present from rain, snow, condensation or vapour. Moisture content levels of 20% and over present a risk of metal corrosion.
While acetic acid in timber can corrode metal, timber itself has good resistance to acids but is degraded by alkalis, which may form as a by-product of the corrosion of metal by timber. The combination of acidic corrosion of metal and alkaline degradation of timber can cause iron and mild steel fasteners and fixings to loosen, with a condition sometimes described as “nail sickness” developing.
Salts in seawater and some salts found in wood preservatives and fire-retardant treatments will accelerate the corrosion of metals in the presence of moisture. The effects of corrosion of metals by timber are most pronounced where the two materials are in contact under one or more of these circumstances:
- Where the metal and timber are exposed to the weather, to a damp environment or to intermittent wetting.
- Where the timber has been treated with a wood preservative or a fire retardant in the presence of moisture.
- Where the timber is exposed to seawater or seawater spray.
Specifiers should consider whether these situations are likely to arise and make appropriate choices of materials. Different species of timber vary in acidity, so it’s important to specify metals with appropriate corrosion resistance and/or provide separating membranes or barriers between metal and timber, and/or specify a less acidic timber.
Wood preservatives and fire retardants
Preservatives containing metals as active ingredients carry a risk of corroding metal fixings. Manufacturers and suppliers of preservatives usually advise leaving the treated timber for at least 14 days before fixing to allow it to dry out.
Some fire-retardant salts also promote the corrosion of metals and can raise the moisture content of treated timber. The potential of preservatives and fire retardants to corrode metals increases as the moisture content of the timber rises. The manufacturer of the wood treatment product must be consulted about the appropriate fixings to be used. There are other fire retardants with active ingredients that are not corrosive to metal fixings.
Seawater
The potential for corrosion by timber of metals increases in the presence of salt water and salt water spray. For example, to counter this effect, austenitic stainless steel fixings for roof battens may be used in locations close to the coast, while galvanised steel fixings may be suitable for low-risk locations away from the coast.
Swimming pool halls
Provided the appropriate measures have been taken, the moisture content of timbers in swimming pools will not generally be high enough to be the direct cause of corrosion in metal fixings. However, occasional but repeated wetting, and/or condensation on glazing and chemicals in the atmosphere, could cause corrosion problems. All metal fixings should therefore be of appropriate corrosion resistance and should be compatible with the timber species being used and with any preservative treatment specified.
Iron stain
Iron stain is the result of a chemical reaction between iron and tannins or tannin-like materials in the timber that contribute to form iron tannin compounds. Commonly described as iron tannate, these compounds form a blue/black stain that can develop in damp timber in contact with ferrous metals such as iron and mild steel. Iron stain is most commonly associated with oak, but also with sweet chestnut, makore, idigbo and kapur. Softwood species prone to iron stain include Douglas fir and western red cedar.
Iron stain can arise where ironworks have been carried out near to the affected timber. It can be avoided by keeping ironworks and timberworks separate and/or by carrying them out at different times in the building process.
Acidity of timbers
Some timbers are significantly more acidic than others. pH values range across a scale of 0 to 14, where a value of 7.0 is neutral, less than 7.0 is acidic and more than 7.0 is alkaline. The pH scale is logarithmic, so that a pH value of 4.0, for example, is 10 times more acidic than a value of 5.0 and 100 times more acidic than a pH value of 6.0. In general, timbers with a value of 5.0 or more offer the least risk of corrosion to metals. The nominal pH value should only be used as a guide, however, as it can vary by two pH units, being influenced by factors such as soil conditions during growth and by the age of the timber.
Metals
Metals that are less able to form chemical compounds – described as more noble – resist oxidisation and are therefore less corrodible. Those that form compounds more easily with other chemicals – known as less noble or base metals – oxidise and corrode more easily.
Metals can be ‘blended’ to form metal alloys or can be coated to increase their resistance to corrosion. Austenitic stainless steel generally offers a reliable solution with a good range of products readily available.
Often, however, the product may also be available in other metals, in metal alloys or in coated materials. Galvanised products, for example, may be preferred to austenitic stainless steel for shorter desired service lives for cladding fixings. More precisely engineered products such as nuts, bolts and other connectors may be available in more than one material.
Use classes
Use classes classify timber according to the risk of fungal decay or insect attack resulting from wetting in service and are defined in BS EN 335:2013 Durability of Wood and Wood-Based Products. Use classes: definitions, application to solid wood and wood-based products. These classes provide a useful reference for specifying metals and timber to be used together.
Conclusion
In summary, there is no reason why metals and timber cannot work in harmony in dry conditions, so long as materials are specified correctly and that the right precautions are taken during design, storage, construction and maintenance. If there is any doubt, the manufacturers of metal components should be consulted when specifying metals for use with timber.
