Corrosion Behaviour of Zinc Alloy Coated Steel Fasteners in Timber

Abstract

The corrosion behaviour of a wide range of zinc and zinc alloy coated steels (hot-dip galvanised and electrodeposited), with or without a conversion coating treatment, in timber environments has been investigated using electrochemical measurement and SEM/EDXA analysis. The corrosion tests were carried out on specimens immersed in copper/chromium/arsenic(CCA) "salt" and "oxide" formulated solutions for rapid preliminary evaluation of corrosion performance. The polarisation resistance was measured in CCA "salt" solution as a function of time, while the corrosion behaviour in CCA "oxide" solution was determined by potentiodynamic scan. A corrosion sensor was devised to assess the corrosion performance of coated steels in CCA treated timber by measuring corrosion potential and polarisation resistance. The effect of moisture content of timber on the corrosion of zinc coated steels was also carried out using the sensor. It was found that the sensor worked well, even at low moisture content. The sensor is simple, easy to construct and has the potential to be used in field studies. The ranking of corrosion tendency of zinc and zinc alloy coated steels in CCA solution and CCA treated timber has been given based on electrochemical measurements and SEM/EDAX analysis. It was found that the corrosion resistance of zinc coated steels is significantly improved by the addition of alloying element and conversion coating. The Zn-55%A1 and Zn-12.5%Ni coated steels were found to give the best corrosion performance. Surface observation and analysis of the wood and coated steel samples were conducted using SEM/EDAX. Evidence of cracking and pits were observed on a number of coated steels and general corrosion products found were Zn(OH)2 and ZnSO4. The corrosion mechanism and service life prediction have been discussed.