(Invited) Liquid Phase Ion-Gun Technique for Degradation and Evaluation of Oxide Films

Abstract

Halide ions in environments destabilize oxide films on materials and lead to a local corrosion of the material. In order to investigate a quantitative effect of halide ions on oxide degradation, an electrochemical technique with a concentrating method of halide ions on the oxide in an experimental setup is necessary. Liquid phase ion-gun (LPIG), in which silver halide layer on a silver microelectrode is reduced in liquid phase, is one of the beneficial techniques to concentrate halide ions the vicinity of the material surface. Fushimi and Seo developed an LPIG as a chloride ion-generator and successfully applied in boric acid-borate buffers to degrade passive film on pure iron locally and to induce the local corrosion [1, 2]. Degradation reaction of the film was classified into three processes; depassivation induction, film depassivation, and corrosion propagation or repassivation, from electric charges consumed by both LPIG microelectrode and specimen electrode. Since an electric charge consumed on the microelectrode for the generation of chloride ions during the induction process corresponds to the amount of chloride ions needed for the degradation of passive film, the stability of passive films on different crystallographic grains were also evaluated. LPIG was also combined with electrochemical quartz crystal microbalance and used to investigate local corrosion of copper [3]. Lee and Fushimi have developed another type of LPIG, in which silver sulfide layer on the silver microelectrode is reduced to generate sulfur species, and applied to sulfide surface of silver electrode [4] and type 316L [5], 304, 430 and 443 [6] stainless steels. In this paper, the beneficial features of LPIG technique for investigating the initiation of local corrosion on iron-based materials are reviewed. The quantitative analyses for the halide ions generated from LPIG microelectrode will also be discussed.