Copper distribution and oxidation states near corroded fasteners in treated wood

Metal fasteners are used to hold wood structures together. In outdoor applications, these fasteners are subject to corrosion when the wood is treated with certain preservative treatments. Typically, these treatments contain copper. Prior work has hypothesized that the mechanism of corrosion in treated wood involves reduction of copper ions from the wood treatments. However, copper was rarely detected in the corrosion products of metals embedded in treated wood, which contradicts the hypothesized mechanism. This present work utilizes synchrotron based X-ray fluorescence microscopy (XFM) and X-ray absorption near edge spectroscopy to examine the corrosion mechanism in treated wood by looking at the spatial distribution and oxidation states of copper in the treated wood near the fastener and in the corrosion products removed from the fastener surface. The samples were obtained after a 1-year corrosion test. In the wood cell walls, the oxidation state of the copper treatment did not change in the immediate vicinity of the fastener, although there was a depletion of copper near the fastener. Furthermore, copper was detected in the corrosion products in trace amounts using XFM. Together, these techniques confirm that the corrosion mechanism involves transport of the cupric ions to the fastener surface, where they are reduced and suggest that previous attempts to detect copper were unsuccessful because the concentration of copper in the corrosion products was below the level of detection of the previously used techniques.