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

doi:10.1016/S0013-4686(03)00228-7

Electrochimica Acta

Volume 48, Issue 17, 15 July 2003, Pages 2371-2378

https://doi.org/10.1016/S0013-4686(03)00228-7 Get rights and content

Abstract

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⁻/I₃⁻ redox couple as a mediator for sulfide oxidation. The microelectrode current for I⁻ oxidation is amplified by chemical reduction of I₃⁻ 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.

Introduction

Decades of study have been devoted to the study of corrosion mechanisms involving stainless steels in chloride environments because of its technological importance. In more recent times, research has focussed on understanding the problem from a surface science perspective to determine the localized chemistry involved in pitting corrosion processes. Detailed mapping of the surface has revealed that pitting tends to occur at or in close proximity to sulfide inclusions in Cl media [1], [2], a mechanism that had been proposed early on [3]. Other studies have mapped the surface topography of stainless steel at high resolution during corrosion using in-situ scanning probe techniques such as scanning tunneling microscopy (STM) [4] and atomic force microscopy (AFM) [5], [6], [7]. Recently, a chemically specific technique, scanning electrochemical microscopy (SECM), has been used to map electrochemically active sites at the surface of a metal, revealing the localized nature of electron transfer and its correlation with pitting events [8], [9], [10], [11], [12]. Another method, the scanning vibrating electrode technique (SVET) measures localized electric fields allowing in-situ measurement of pitting dynamics [13], [14]. Combined, these methods provide a spectrum of research tools for determining the mechanisms involved in corrosion of metal surfaces.

The link between sulfide inclusions in stainless steel and its corrosion susceptibility may be connected to changes in local chemistry in proximal regions surrounding the inclusion. Chemical mapping after corrosion indicated high S and Cl concentrations near the corrosion pits [1], [2]. Other studies have shown that addition of reduced sulfur species to an electrolyte contacting the steel surface substantially increases its rate of corrosion [15], [16], [17]. In a recent paper, SECM was used to detect localized concentrations of dissolving sulfur species (HS⁻ or S₂O₃²⁻) near corrosion pits formed on Type 303 and 304 stainless steels [18]. The detection scheme uses an I⁻/I₃⁻ redox couple as a mediator: I₃⁻, generated at the SECM tip, is reduced to I⁻ by either HS⁻ or S₂O₃²⁻. Thus, current at the SECM microelectrode is amplified in the presence of the dissolved sulfur species due to an increase of I⁻ at the microelectrode. The sulfur species were presumed to dissolve from the inclusions upon poising the potential in the breakdown region. In this paper, we extended the use of SECM for detecting the dissolution of sulfur species at local corroding stainless steel sites to situations at higher current densities, where an interesting signal inversion occurs in close proximity to the position where S species were measured at low current.

Section snippets

Experimental

The SECM was a homebuilt instrument constructed of both commercially obtained devices and homebuilt apparatus. Electrochemical control of the sample and probe was performed using a bi-potentiostat designed for electrochemical STM/AFM/SECM obtained from a Topometrix Explorer 2000 scanning probe microscope. This potentiostat has a provision for performing galvanostatic polarization at the sample while allowing potentiostatic control of the microelectrode tip. The three dimensional position of the

Results and discussion

Recent work in our laboratory has focussed on developing a probe to detect S species at stainless steel samples due to production by sulfate reducing bacteria (SRB) which are implicated in biocorrosion of iron alloys [21]. Using SECM, the detection of localized dissolved sulfide could be correlated with SRB activity thus providing a tool for the study of what is thought to be the most common type of steel biocorrosion. Two probes that were considered in the detection of sulfide are: (a) a I⁻/I₃⁻

Conclusions

Previous research in the corrosion of 300 series stainless steels concluded that initiation of pitting corrosion occurred at sulfide containing inclusions. Very recently, an SECM method for in-situ detection of localized S (HS⁻ or S₂O₃²⁻) concentrations dissolved from Type 304 and 303 stainless steels was developed [18]. The results presented above confirm the presence of these localized concentrations and additionally uncovered an artifact that may show future promise for studying corrosion

Acknowledgements

We gratefully acknowledge the help of Professor Henry S. White, and his research group at the University of Utah, for help bringing our SECM on-line, supplying the carbon fiber microelectrodes, and for insightful discussions involving our results. This work was supported by the United States Department of Energy (DOE), Assistant Secretary for Environmental Management, under DOE Idaho Operations Office Contract DE-AC07-99ID13727.

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

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgements

Corros. Sci.

Appl. Surf. Sci.

Mater. Sci. Eng.

Colloids Surf.

Corros. Sci.

J. Electrochem. Soc.

J. Electrochem. Soc.

Br. Corros. J.

Mater. Corros.

J. Electrochem. Soc.

J. Electrochem. Soc.

J. Electrochem. Soc.