Synchrotron Based X-ray Fluorescence Microscopy Confirms Copper in the Corrosion Products of Metals in Contact With Treated Wood

摘要

Copper based waterborne wood preservatives are frequently used to extend the service life of wood products when subjected to frequent moisture exposure. While these copper based treatments protect the wood from fungal decay and insect attack, they increase the corrosion of metals embedded or in contact with the treated wood. Previous research has shown the most plausible corrosion mechanism involves the migration of copper ions from the wood treatment through the wood to the metal surface, where they are then reduced. Despite this, under almost all conditions, copper has not been detected in the corrosion products as the proposed mechanism would imply. Recently, synchrotron based X-ray fluorescence microscopy (XFM) was used to examine the wood that had been in direct contact with metal fasteners in a corrosion test. These measurements showed a copper depleted region in the wood directly adjacent to the metal fastener. Based on the size of the region and the copper concentration, the amount of copper in the corrosion products was calculated to be on the order of 500 parts per million. This low concentration explains why previous attempts to find copper in the corrosion products using scanning electron microscopy, energy dispersive X-ray spectroscopy, and powder X-ray diffraction were unsuccessful. Here, we present XFM maps of corrosion products removed from corroded fasteners that had been in contact with preservative treated wood. The XFM maps of the corrosion products clearly show the presence of copper. These measurements definitively confirm the mechanism of corrosion in treated wood and give further insights into where and how the cathodic reaction takes place.