Scanning Electrochemical Microscopy Detection of Dissolved Sulfur Species from Inclusions in Stainless Steel

Abstract

Sulfur-rich inclusions in stainless steel (SS), e.g., MnS inclusions , are well known to serve as sites where corrosion pits form. 1 However, yet to be resolved is the mechanism for the specific chemical aspects of inclusion dissolution and how it leads to acceleration of local metal dissolution, eventually to stable pitting corrosion. The goal of this paper is to measure the local distribution of dissolved sulfur species on a microscopic scale in the vicinity of a MnS inclusion during early stages of pit initiation. Pit initiation at the site of inclusions has been studied by Castle and Ke using Auger spectroscopy on SS. 2 Ke and Alkire 3 used Auger electron spectroscopy, scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDX) to study the onset of pitting on SS 304 in 0.1 M NaCl and reported that pits initiate only at MnS and mixed MnS/oxide inclusions with sizes above 0.7 m. High concentrations of S were found within the pit cavity as well as the surrounding area. Marcus et al. 4 suggested that the S species form a surface layer that prevents repassivation of the metal. The role of dissolved S species in 0.25 M NaCl solution on SS 304 was also studied by Newman. 5 The effects of S 2 O 3 2 , S 4 O 6 2 , H 2 S, SCN , and SO 3 2 added to the NaCl solution on pitting were observed, of which thiosulfate was reported to be the most aggressive species toward the alloy. Lott and Alkire 6 used UV spectrophotome-try to detect the presence of thiosulfate in a crevice of SS 304 and considered it to be generated by dissolution of sulfide inclusions. Wil-liams et al. 7 distinguished between thiosulfate that originated from inclusions at high anodic potentials and hydrogen sulfide that was generated at lower potentials by a reaction catalyzed by chloride. Brossia and Kelly et al. 8 used ion chromatography and capillary electrophoresis to identify aqueous sulfide from MnS inclusions as the only sulfur species present in the initiation of crevice corrosion under their conditions of study, which were at lower potentials. They also noted a constant chemical composition of the local site despite an increase in sulfur content in the alloy which shortened the incubation time of crevice corrosion. Thus, both thiosulfate, generated by electrochemical dissolution of sulfur-rich inclusions, and hydrogen sulfide, generated by a chemical dissolution, probably exist, but in relative amounts that vary as different potentials are applied. The present work was initiated to clarify the two different dissolved sulfur species at different stages of MnS dissolution and pitting of SS 304 alloy. Current densities on corroding SS surfaces have been measured and mapped by Isaacs. 9 Williams et al. 7 used scanning electrochem-ical microscopy (SECM) to measure and map the local current density for dissolution of MnS inclusions in SS 316 in NaCl and NaClO 4 solutions, which in some cases was higher than 1 A cm 2 , and also catalyzed by chloride. To pursue further, it is of interest to probe the local chemistry involved with such high current flow. In this work, I /I 3 redox mediator was used in the SECM technique 10-14 to detect the electroactivity over S-rich inclusions in SS alloys. The redox chemistry used to detect dissolved sulfur species by SECM is proposed in Fig. 1. At the inclusion, MnS undergoes chemical or electrochemical dissolution to produce either hydrosul-fide or thiosulfate, respectively 7 MnS 2Cl r MnCl 2 S 2 [1] S 2 H HS [2] or 2MnS 4Cl 3H 2 O r S 2 O 3 2 2MnCl 2 6H 8e [3] To detect HS or S 2 O 3 2 , the iodide/triiodide couple was used as a redox mediator. 15 At the tip, iodide is oxidized to produce triiodide which further reacts with the sulfur species