Corrosiveness of Thermally Modified Wood

Thermally modified wood is becoming commercially available in North America for use in outdoor applications. While there have been many studies on how thermal modification affects the dimensional stability, water vapor sorption, and biodeterioration of wood, little is known about whether thermally modified wood is corrosive to metal fasteners and hangers used to hold these members in place. As thermally modified wood is used in outdoor applications, it has the potential to become wet which may lead to corrosion of embedded fasteners. Here, we examine the corrosiveness of thermally modified ash and oak in an exposure test where stainless steel, hot-dip galvanized steel, and carbon steel nails are driven into wood and exposed to a nearly 100% relative humidity environment at 27 °C for one year. The corrosion rates were compared against control specimens of untreated and preservative-treated southern pine. Stainless steel fasteners did not corrode in any specimens regardless of the treatment. The thermal modification increased the corrosiveness of the ash and oak, however, an oil treatment that is commonly applied by the manufacturer to the wood after the heat treatment reduced the corrosiveness. The carbon steel fasteners exhibited higher corrosion rates in the thermally modified hardwoods than in the preservative-treated pine control. Corrosion rates of galvanized fasteners in the hardwoods were much lower than carbon steel fasteners. These data can be used to design for corrosion when building with thermally modified wood, and highlight differences between corrosion of metals embedded in wood products.