Causes of metal corrosion in timber fixings

Why do metal fixings corrode in timber? The single most important factor in determining the incidence of corrosion is moisture. (The threshold moisture content below which no appreciable corrosion of embedded metal occurs is the same as the safety limit for wood-rotting fungi, i.e about 20%.) Above this level, wood is always acidic due to the breakdown by water of wood cellulose to acetic acid, through natural ageing processes.

No wood species can be regarded as 'innocuous' but some are more aggressive, notably the heartwood of oak, sweet chestnut, western red cedar and Douglas fir.

Acetic acid is volatile and, in poorly ventilated areas, can cause corrosion at sites away from the main wetted regions. Any soluble salts tend to increase the conductivity of moisture in the wood and thus the potential for corrosion. Wood naturally contains varying levels of mineral salts of which sulphate (1-10% of the ash) and chloride (0.1-5%) increase the corrosion hazard of the acetic acid.

If the timber has been in prolonged contact with seawater, or if the structure is exposed to coastal air, the chloride content and corrosivity of the wood can be increased. Where timber is immersed in seawater, a number of corrosion cells can be set up. The shaft of a fastener can become starved of oxygen and corrode whilst alkaline conditions produced at the head may degrade the wood. Incidentally, cells may also occur where fasteners connect two different woods.

Timber treatments can be potential corrosion hazards. Salt seasoning, used for drying certain hardwoods (e.g. maple), and flame-retardant salt treatments, such as ammonium phosphate and sulphate and boric acid and borax can be a corrosion hazard.

Treatments to prevent attack by fungi and wood-boring insects vary in their influence on corrosion. Creosote and tar oil can be beneficial because of their waterproofing action. Boron compounds and organic solvent preservatives, such as pentachlorophenol and lauryl pentachlorophenate, present few problems unless there are alkaline conditions which can result in reactions liberating soluble chlorides which can then attack stainless steel.