Korrosionsverhalten von Metallen in Verbindung mit Holz

Abstract

Im Holz vorhandene Essigsaureester konnen in Gegenwart von Wasser zu freier Essigsaure hydrolisieren. Diese Essigsaure wirkt auf verschiedene Metaile bei direktem Kontakt mit Holz korrosiv. Sie kann weiterhin wegen ihrer Fluchtigkeit auch Metalle im Darnpfraum in der Nahe von Holz beeinflussen. Der Gehalt an Essigsaureestern, die Bildungs-und Freisetzungsraten von Essigsaure sind holzartspezifisch. Femer ist die Bildungsrate als hydrolytischer Vorgang stark feuchtigkeits-und temperaturabhangig. Je nach Behandlung, Vorgeschichte und Art bleibt die Aggressivitat eines Holzes iiber Jahre erhalten. Holzschutzchemikalien, insbesondere in Form wafhiger Salzlosungen, konnen in gewissem Umfang die Aggressivitat eines Holzes erhohen. Das Korrosionsnsiko in der Praxis fur Metallteile in direktem Kontakt mit Holz hang stark von den Einsatzbedingungen, d.h. von Luft-und Holzfeuchtigkeit ab. Es ist in geschlossenen Gebauden gering. Bei hohem Feuchtigkeitsangebot sind korrosionsbestandigere Werkstoffe, wie z.B. Chrom-und Chrom-Nickel-Stahle zu venvenden.Wood contains acetic acid ester, which hydrolyses in the presence of water to give free acetic acid. This acetic acid acts as a corrodent to several metals in contact with wood and in the vapour space near wood surfaces because of the volatility of the acetic acid. The quantity of acetic acid ester, the rate of formation and emission of acetic acid is specific for the species of wood. Moreover the rate of formation, as a hydrolytical reaction, is strongly effected by moisture and temperature. According to treatment and species, wood may retain its corrosivity for many years. Wood preservatives, especially salt solutions, are able to increase the corrosivity to a certain degree. The risk of corrosion of metals in practical services depends on environmental conditions, i.e. relative humidity and moisture content of wood. In covered buildings under indoor conditions the risk is relatively small. If the humidity and the moisture content of wood are high, corrosion resisting metals, e.g. ferritic or austenitic stainless steel, should be used.

Durability of galvanised fasteners in timber construction

“…[9] Strong metal corrosion only occurs with high water contents in the timber of >30 wt.%. [3,6] Above the fibre saturation moisture content of 30 wt.%, timber absorbs additional water in the cell cavities if there is an appropriate supply. The water stored here is called free water.…”

“…Acetic acid formation is timberspecific. Aqueous extracts have pH values depending on the species [6] : e.g., between 3.3 and 3.9 (oak) or between 4.3 and 5.3 (coniferous woods such as larch and spruce). In combination with moisture, oak, for example, has a more corrosive effect on metals than spruce timber.…”

“…• The water content required for metal corrosion in contact with timber shall be at least 16-18 wt.%. [6] In nonirrigated spruce timber, such humidity occurs at a temperature of 20°C and a relative humidity of 80-85% ( Figure 6). • In contact with spruce timber exposed to weathering, the water content of which can then exceed 20 wt.%, the zinc corrosion rate, according to our findings, is about 1 µm/year in a comparable order of magnitude to that of weathered (irrigated) zinc surfaces in a normal urban atmosphere.…”