Fastener Corrosion: A Result of Moisture Problems in the Building Envelope

This paper reviews recent literature on the corrosion of metals embedded in wood and highlights the link between moisture accumulation in wood and fastener corrosion. Mechanisms of fastener corrosion are described including dependence upon wood moisture content. These fundamental concepts are applied to practical examples by explaining how hygrothermal models can be used to predict fastener corrosion in wood and showing how fastener corrosion affects the strength of woodmetal connections. The goal of the paper is to familiarize professionals in the wood design community with corrosion. Introduction to Corrosion of Embedded Metals Metal fasteners embedded in wood are subject to corrosion from organic acids present in the wood. When present, fire-retardant or preservative treatments may increase the corrosiveness. Over the last 10 years, there has been an increased interest in fastener corrosion because of a shift in commercially available wood preservatives. In January 2004, chromated copper arsenate (CCA) was voluntarily withdrawn from service for residential applications and newer wood preservatives such as alkaline copper quaternary (ACQ) and copper azole (CA) were introduced. Several years later, so called “micronized” formulations were introduced to the market. In these formulations, soluble copper is not injected into the wood; rather solid copper, copper oxide, or copper carbonate is ground into submicron particles (“micronized”) and suspended in solution prior to injection. Not surprisingly, the newer wood preservatives have varying degrees of corrosivity, and the lack of previous data onthe wood preservatives has caused some confusion and concern regarding the durability of metal fasteners embedded in wood. The wood preservatives mentioned above use copper as a biocide, and for these treatments the corrosion mechanism of embedded fasteners involves the reduction of cupric ions introduced by the wood preservatives. The role of cupric ions in the corrosion mechanism was first hypothesized by Baker (1988) and later confirmed by work of Zelinka (Zelinka et al. 2010; Zelinka and Stone 2011) and Kear (Kear et al. 2008; Kear et al. 2009) through energy dispersive x-ray analysis, Pourbaix diagrams, and examinations of the role of cupric ion concentration and acidity. Several studies have also shown that as the copper concentration increases, so does the corrosion rate of embedded fasteners (Kear et al. 2009; Zelinka and Rammer 2011). Table 1 summarizes the composition and required retention for several wood preservatives and highlights the differences in copper concentration. CCA has the lowest copper concentration of the preservatives listed in Table 1 and is also the least corrosive. An important difference between the corrosion of fasteners embedded in wood and atmospheric corrosion involves passivation. Passivation refers to the process through which corroding metals form a protective oxide or hydroxide layer (patina). In atmospheric corrosion, the corrosion kinetics of steel and zinc are affected by passivation and the corrosion rate decreases with time (Legault and Preban 1975; Legault and Pearson 1978). However, for metals embedded in wood, the corrosion rate has been found in multiple studies to be constant with time (Baker 1992; Zelinka and Rammer 2009). Another difference between atmospheric corrosion and embedded metals involves the relative corrosion rates of different metals. In atmospheric corrosion, zinc forms a passivating zinc carbonate layer that greatly reduces the corrosion rate; however, Zelinka et al. (2010) have examined the corrosion products of steel and zinc fasten-