Wood is a corrosive substance by nature and it can be made more corrosive by treatment given to it. Unlike most other corrosive substances, one of the corrosive chemicals in it, acetic acid, is volatile, and in an ill-ventilated space, wood can cause corrosion of metal nearby but not actually in contact. Where there is contact in atmospheric conditions, corrosion can occur by the usual micro-electrolytic mechanisms, and in immersed conditions, large-sized electrolytic cells can form.

• inside wooden containers, by vapour corrosion without contact;
• at contacts in land-based structures, through attack by wood acids and wood treatment chemicals;
• at contacts in immerse structures, where macro-galvanic mechanisms predominate.

Sources of corrodents
a. NATURAL CORRODENTS
b. EFFECT OF FUNGI
c. EFFECT OF KILN DRYING
d. ADVENTITIOUS CORRODENTS
e. TREATMENTS FOR WOOD THAT INTRODUCE CORROSIVE SUBSTANCES

a. NATURAL CORRODENTS
The principle constituent of wood is cellulose, which is a polysaccharide, i.e. a polymer made of sugar molecules joined in long chains. Each sugar unit contains mildly basic hydroxyl radicals, a proportion of which is combined with acetic acid radicals (acetylated) in the form of ester (organic salt) groupings. These groupings can combine with water (hydrolyse) to give free hydroxyl radicals and acetic acid.

The chemical equation is:  

• X - 0.CO.CH3  + H2O <--> X - OH + CH3COOH
  acetylated group    water       free radical    acetic acid

where X is the sugar unit in the chain.

It is an equilibrium reaction, which causes the moisture in wood to be always acid, but because the acetic acid is volatile and can escape, the reaction move slowly to the right hand side all the time. The acetyl radical constitutes about 1 to 6% by weight of dry wood, more in hardwoods than in softwoods, and this figure determines the total quantity of acetic acid than can be formed. The rate of emission of acetic acid depends on the species, and a wood of lower acetyl content can liberate acetic acid faster under given conditions than another wood of higher content. In a given wood, the rate of formation of acetic acid depends on the temperature and the moisture content of the wood, and the rate of its escape to the atmosphere depends on the geometry of the piece of wood in question. Besides acetic acid, small quantities of formic, propionic and butyric acids are present in wood, but their effects can be neglected in comparison with those of acetic acid.

Wood contains from 0.2 to 4% of mineral ash, which consists largely of calcium, potassium and magnesium as carbonate, phosphate, silicate, and chloride; aluminium, iron and sodium are also present. Sulphate contributes 1 to 10% of the ash by weight, and chloride 0.1 to 5%, and these two radicals augment the corrosive action of the acetic acid.

b. EFFECT OF FUNGI
In experiments on the development of acidity in wood during storage, and its liberation to the atmosphere, it has been found that wood preservatives which inhibit the growth of fungi and other biological actions do not affect the formation of acetic acid, showing that the mechanism is purely chemical and not biological. On the other hand, free acid does not appear when the wood is infected by certain fungi. These fungi are of the thermophilic type, which are able to grow well at relatively high temperatures, but are still active at warm atmospheric temperatures. The mechanism by which the fungi prevent the accumulation of acid is not certain, but it appears likely that they act by neutralising or decomposing the acid as fast as it is produced and not by preventing its formation. The idea of using fungi to inhibit the development of corrosive acid may appear attractive, but these fungi will only grow when the wood is damp and therefore open to attack by wood-rotting fungi. In extreme cases of fungal attack within enclosed spaces, volatile products from the fungi themselves, mainly carbon dioxide but probably with traces of other volatile acids, can attack metals within the enclosure. 

c. EFFECT OF KILN DRYING
Kiln drying accelerates the production of free acetic acid in wood, but most of the acid does not have time to escape. Kiln dried wood is more acid and more immediately corrosive than air dried wood, though it contains less combined acid that can be set free in later years. 

d. ADVENTITIOUS CORRODENTS
The natural chloride content of wood is low (0.1 to 5% of the ash, which is 0.2 to 4% of the dry weight of the wood), but cases have occurred of severe corrosion of metals in contact with woods that were not particularly acid, but which were found to contain appreciable quantities of salt, up to 0.8% by weight. Wood can absorb salt; the likely sources of adventitious salt are twofold:

• (a) salt spray and mist near the coast;
• (b) the floating of logs in seawater.

The following are examples of corrosion in wood found to contain substantial quantities of chloride

• (i) corrosion of plain steel nails in tile battens in a church in a south coast town resulting
• in tiles falling from the roof; ascribed to sea spray ;
• (ii) corrosion of steel cables wound on soft wood drums;
• (iii) bimetallic corrosion of nickel plated steel covers to coffin handles made of nyatch, a timber from south-east Asia;
• (iv) corrosion of coppered iron tacks fixing canvas to the ramin frames of hospital beds. The bimetallic corrosion produced enough alkali to degrade and weaken the canvas.
  

In examples (ii) to (iv) the source of chloride was probably the floating of logs in seawater. Timber is often floated downstream to the coast, and may float in seawater for some time before it is picked up for further transhipment.

e. TREATMENTS FOR WOOD THAT INTRODUCE CORROSIVE SUBSTANCES
i) Salt seasoning
In Section d(ADVENTITIOUS CORRODENTS) above, the accidental contamination of wood by salt was mentioned. Even larger quantities of salt, up to 4% by weight, can be introduced by a slat seasoning process used in some parts of the world for drying certain hardwoods, including maple. In this process the green timber is close piled with layers of salt between the boards. Salt absorbed by the wood lowers the vapour pressure of water in it, so that when the boards are subsequently dried, the rate of evaporation at the surface, and the extent of surface checking, is reduced.

Salt seasoning was thought to be the cause of corrosion of the screws, brackets and other metallic components in contact with maple in certain pianos, especially some stored in humid conditions, for example, in tropical climates. The maple was found to contain up to 4% salt. Other metals parts in the same pianos in contact with other woods not containing salt did not corrode.

ii) Flame retardants
The salts most commonly used in the UK are mono- and di-ammonium phosphate, ammonium sulphate, boric acid and borax. Most proprietary materials are mixtures of such substances. Formulations may include copper-chrome-arsenic fungicide to give a dual purpose material, and corrosion inhibitors may be added. The concentration of flame retardant in the surface of the wood can be quite high. Of these salts, ammonium sulphate is considerably corrosive. The ammonium phosphates are less corrosive, but not negligibly so. Boric acid is not appreciably corrosive, and borax is a mild inhibitor. All the salts increase the moisture content of the wood in contact with air of given relative humidity. The ammonium compounds can reduce the mechanical properties of the wood, especially if the wood is kiln dried after impregnation. Corrosive flame retardants can contribute markedly to the corrosion of metal in contact with wood, but not, except in the unusual circumstances described in Section 5 below, to vapour corrosion.

iii) Preservatives
Treatments to prevent attack by fungi and wood-boring insects fall into following main types

• Creosote or tar oil This substances have little corrosive action, except towards lead; in so far as they waterproof the wood, the effect may be protective.
• Copper-chrome-arsenic (CCA) This is widely used. The chromium salt constituent may have a small protective effect, and the arsenate radical a small corrosive one, but other salts formed during the process, notably sodium sulphate, remain soluble and are corrosive. In addition, the copper itself is potentially corrosive, for copper-based preservatives can leach soluble copper compounds to the extent of parts, or tenths of parts per million, and this copper can then plate out as metal on to iron, zinc and aluminium, forming galvanic cells that accelerate the corrosion of the substrate metal. The leaching is much greater from freshly treated wood, and it is recommended that preserved wood be allowed to age for seven days before fasteners are inserted in it, thereby giving time for the preservative to become fixed in the wood.
• Naphthenates Copper naphthenate has the same potential hazard by formation of soluble copper as copper-chrome-arsenic. Zinc naphthenate does not.
• Boron compounds These have a negligible corrosive effect on fasteners but prolonged exposure to damp causes the formation of alkali which might degrade the wood.
• Organic solvent types Examples are pentachlorphenol and lauryl pentachlorphenate, sometimes containing water repellents. They have a negligible effect on corrosion unless alkali produced by other corrosive action decomposes them to form soluble chloride. Any water repulsion they confer will be beneficial.