Comparing the Methodologies in ASTM G198: Is There an Easy Way Out?

ASTM G198, Standard test method for determining the relative corrosion performance of driven fasteners in contact with treated wood, was accepted by consensus and published in 2011. The method has two different exposure conditions for determining fastener corrosion performance in treated wood. The first method places the wood and embedded fasteners in a chamber at 95% relative humidity (RH), 32 °C for 120 days. The second method involves a cyclic fog, with 48 hours of fog followed by 72 h of dry for 120 days. The corrosion rate of embedded metals is an extremely sensitive function of the wood moisture content. While the wood moisture content is well defined, and fixed, in the constant RH test, the moisture content fluctuates and there are moisture gradients in the cyclic fog test. As a result, the fasteners in the cyclic fog test experience non-uniform and constantly changing conditions, which may be more or less challenging than the steady state test. This paper presents the results from a combined hygrothermal/corrosion model to explore how the moisture content and temperature at the fastener interface differs between these two methods and shows how these differences in moisture content affect corrosion.