The degradation of wood by metal fastenings and fittings

IRG/WP 302

L C Pinion

As well as the hazards of biological decay, timber used in boat building is subject to the effects of chemical decay associated with the corrosion of metallic fastenings. The title has been deliberately chosen to emphasize that in wooden construction the troubles are not just those of corrosion of the fastenings, but also the destructive secondary effects on the wood caused by the products of the corrosion processes (Plate I). For centuries the world has lived with these troubles commonly referred to as "nail sickness". In contrast to biological decay the chemical decay associated with corrosion is confined to the wood adjacent to corroded fastenings and it is regrettable that in repair work so much wood has to be replaced because of a small percentage degrade in vital spots. Before methods can be devised to overcome these troubles, it is necessary that a better understanding is reached of the mechanisms operating in the degradation process, and recent work at FPRL has thrown some light in this area. The general problem will now be considered in some detail followed by suggestions on possible preventative and remedial measures. Corrosion is the result of an electrochemical process in which the corrosive effect is proportional to the current which flows between areas of potential difference. There are a number of causes of this electrochemical effect such as dissimilar metals in contact, differences in concentration of some chemical factor such as the electrolyte or oxygen availability, stress etc. In all cases areas of different polarity are produced and corrosion only proceeds when the electrical circuit is completed by an electrolyte bridge eg sea water, and a conductor between the anodic and cathodic areas. Figure 1 shows a simple diagram of electrochemical corrosion.


Conference: 72-10-26/28 Berlin (West), Germany (FRG)

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