Copper distribution and oxidation states near corroded fasteners in treated wood

Abstract

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.

Corrosiveness of Thermally Modified Wood

“…The sample geometry and location of replicates closely followed ASTM standard G198 [43]. Previous work has shown that the area of interaction of the fastener with the wood is localized to a region less than 1 mm from the fastener surface [44,45].…”

Oxidation states of copper in preservative treated wood as studied by X-ray absorption near edge spectroscopy (XANES)

“…Recent measurements have cast doubt upon the amount of Cu ++ in treated wood. Zelinka et al [ 26 ] examined treated wood adjacent to corroded fasteners to better understand the corrosion mechanism in treated wood. They used both X-ray Fluorescence Microscopy (XFM) to map out the copper concentrations within the first 200 µm of the fastener surface and µXANES to measure the oxidization states in these areas.…”

“…The XFM map shows a purple region near the fastener surface caused by iron corrosion products diffusing into the wood. Zelinka et al [ 26 ] designated this region the corrosion affected zone and compared these measurements against those taken farther (hundreds of micrometers) from the fastener surface (labeled control in Zelinka et al [ 26 ] and Fig 1 ). However, both measurements showed that more than 2/3rds of copper was in the Cu + oxidation state [ 27 ] (bottom of Fig 1 ).…”

“…The measurements of Zelinka et al [ 26 ] are in contrast to the majority of the literature on wood preservation. Not only are they the only XANES measurements on copper in treated wood, they are also the only measurements of the copper oxidation states taken a long time after the preservative treatments.…”

“…Differences in the amount of Cu ++ in the wood may arise from differences in the conditions of the experiments, such as the long exposure duration and proximity to corroding fasteners in the work of Zelinka et al [ 26 ]. It is also possible that differences in experimental techniques led to the discrepancy in the literature.…”