Corrosion of ferrous- and zinc-based materials in CCA, ACQ and CuAz timber preservative aqueous solutions

Timber construction relies on connections between members. Traditionally, metallic nails and bolts are used for this. Previous research has shown that these metallic components are subject to corrosion in certain treated timbers. The American Wood Preservers’ Association AWPA E17-99 standard test has been applied using mild steel, hot-dipped galvanised steel (HDG) and 316 stainless steel coupons to examine the feasibility of using aqueous media to model corrosion rates of fastener materials in contact with timber electrolytes. In order to achieve this, comparison has been made with previously published research dealing with corrosion rates in solid timber electrolytes. The results of the corrosion testing in aqueous copper-chrome-arsenate (CCA), copper azole (CuAz) and alkaline copper quaternary (ACQ) timber preservative solutions show that aqueous electrolytes cannot be used to accurately model the corrosion behaviour of mild steel and HDG mild steel timber fixing materials. In contrast to the results obtained in treated timber, HDG steel was found to be the most active material in terms of overall metallic losses during exposure to the aqueous solutions. In addition, CCA was considerably more aggressive than either the CuAz or the ACQ preservatives. The results are discussed in terms of electrolyte pH, metallic passivation and the thermodynamic behaviour of the anodic and cathodic reactions at the mixed (corrosion) potential.