Direct-Current Methods for the Estimation of Corrosion Rates in Aqueous Timber Preservatives

As part of a program to assess the durability of metals in timber preservatives, direct-current electrochemical analyses have been carried out using mild steel, American Iron and Steel Institute 316 stainless steel and galvanized steel immersed in three liquid timber preservatives containing dissolved copper as a biocide. The aqueous preservatives examined were copper–chrome–arsenate (Cu–Cr–As), copper azole (Cu-Tebucanazole), and alkaline copper quaternary (Cu-didecylthyl ammonium chloride). These preservatives are known commercially as CCA, CuAz, and ACQ-B, respectively. Despite previous assumptions, neither polarization resistance- nor Tafel slope-based analyses are suitable methodologies for the universal derivation of absolute values of corrosion rate. An alternative approach is suggested based on anodic linear sweep voltammetry. The results show that the hot-dipped zinc coating was found to be the most active material in terms of the polarization behaviour, especially within the CCA-based electrolyte. There is a strong qualitative correlation between the electrochemical data and corrosion rates estimated using the American Wood Preservers’ Association E-17 standard test for aqueous treatment solutions.