Current Transients from Passive Iron Surface during Micro-indentation

Abstract

Micro-indentation test of passive iron surface in pH 8.4 borate buffer solution was carried out to analyze current transients corresponding to the rupture and repair of passive film. During driving a conical diamond micro- indenter downward to the passive surface and driving upward from the surface, a couple of anodic current peaks were observed. The first current peak spiked during loading was responsible for partial exposure of iron substrate to the solution due to rupture of the passive film when the indenter tip deformed the substrate and penetrated. The second peak emerged during unloading when elastic deformation of the substrate recovered, which was ascribed to repair at the ruptured sites. The second peak current was larger than the first one. Both peaks were sensitively influenced by electrode potential or concentration of sulfate ions containing in solution. The model for a series of rupture and repair processes of the passive film by micro- indentation was proposed to discuss the current transients.

Current Transients from Passive Iron Surface during Micro-indentation

“…However the contact area in nano-indentation is so small (10 -14 to 10 -12 m 2 ) that it is difficult to monitor the transients of current (10 -15 to 10 -13 A) although the current response is indispensable to investigate the mechanoelectrochemical reaction in detail. Recently we have developed micro-indentation [4] to measure current transients in the range larger than 10 -10 A which are caused by mechanical breakdown and repair of the passive film. In this study, current transients during micro-indentation of passive surface on a polycrystalline iron in pH 8.4 borate buffer solution are discussed in detail.…”

“…The experimental procedures have been described elsewhere [4]. A polycrystalline iron plate with a purity of above 99.99%, having a mean grain size of ca.…”