Novel corrosion experiments using the wire beam electrode. (I) Studying electrochemical noise signatures from localised corrosion processes

This series of papers presents four novel experiments that were designed to study localised corrosion phenomena using an electrochemically integrated multi-electrode array namely the wire beam electrode (WBE). This present paper reports a WBE based experimental method that has been employed, for the first time, to study electrochemical noise patterns (called noise signatures) from localised corrosion processes. The objective of this work is to demonstrate the applicability of the WBE for investigating the origin of spontaneous electrode potential/current fluctuations and their effects on electrochemical processes. The key strategy of this work is to apply the WBE in a novel experimental set-up to simultaneously measure electrode potential noise and WBE current distribution maps-an approach that allows the direct comparison and correlation of electrochemical noise and corrosion events. Preliminary experiments have been carried out using a classic pitting corrosion system: stainless steel in a solution containing FeCl3. A large number of anodic sites were found to exist on WBE surface at the very beginning of its exposure to the corrosion environment. Correlation between characteristic patterns in electrode potential noise and corrosion behaviour has been observed. More specifically, the characteristic sharp peaks in potential noise data (called noise signature I) were found to correlate with the sudden disappearance of single unstable anode in WBE current distribution maps. The characteristic noise pattern of quick potential changes followed by partial or no recovery (called noise signature II) was found to correspond with the massive disappearance of minor anodes in WBE current distribution maps. This result suggests that, in the corrosion system under study, electrode noise activities were associated with the disappearance of minor anodic sites, which lead to the eventual disappearance of most anodic sites. Localised corrosion was the result of the accelerated anodic dissolution of a small number of remaining anodic sites. The characteristics features in electrochemical noise and in WBE maps were reproducible. (C) 2005 Elsevier Ltd. All rights reserved.