The effect of localized electric fields on the detection of dissolved sulfur species from Type 304 stainless steel using scanning electrochemical microscopy

This paper discusses a method for imaging localized sulfur concentrations dissolved from sulfide inclusions in Type 304 stainless steels. The method involves amperometric detection using the I-/I-3(-) redox couple as a mediator for sulfide oxidation. The microelectrode current for I- oxidation is amplified by chemical reduction of I-3(-) to I- by dissolved thiosulfate and/or H2S. While galvanostatically inducing corrosion, an unexpected current inversion at the microelectrode was observed at higher sample current densities in close proximity to the area where sulfur was detected at lower sample current. In the article first describing this method, J. Electrochem. Soc. 147 (2000) 4120 did not observe this feature. It was found that the inverted feature resulted from the localized increase in the electric field at the corrosion site, shifting the potential of the microelectrode when positioned over the site. The electric field measurement is analogous to measurements performed using a scanning vibrating electrode technique (SVET). This paper presents the first known combination of localized electric fields and chemical detection for determining localized electrochemical activity at a corroding surface. (C) 2003 Elsevier Science Ltd. All rights reserved.