Short Communication Probing the vertical profiles of potential in a thin layer of solution closed to electrode surface during localized corrosion of stainless steel Chen-Qing Ye a , Rong-Gang Hu a , Yan Li a , Chang-Jian Lin a,⇑ , Jin-Shan Pan b a State Key Laboratory for Physical Chemistry of Solid Surfaces, and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China b Div. of Surface and Corrosion Science, Dept. of Chemistry, Royal Institute of Technology, Drottning Kristinas väg. 51, SE-100 44 Stockholm, Sweden article info Article history: Received 1 November 2011 Accepted 10 April 2012 Available online 20 April 2012 Keywords: A. Stainless steel B. Potentiostatic C. Pitting corrosion abstract Based on a home-built integrated system of scanning reference electrode technique and scanning tunnel- ing microscope (SRET/STM), a method for in situ measurement of vertical potential in the thin layer of solution closed to electrode surface during localized corrosion of stainless steel has been successfully established in this work. An obviously different vertical profile of potential over corrosion active sites and passive sites were firstly observed. The reason for this different was discussed taking into account the distribution of electric field and concentration of Fe 3+ /Fe 2+ in a thin layer solution closed to electrode surface, due to the occurrence of localized corrosion. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Localized corrosion is one of the most crucial and frontier sub- jects in corrosion science, and various research methods have been developed for studying localized corrosion [1–4]. Unfortunately, traditional electrochemical techniques, such as polarization curve and electrochemical impedance spectroscopy (EIS) [5,6] are usually only able to provide general and average information for a local- ized corrosion system [7,8]. Especially, it is difficult to distinguish the contribution of a single localized corrosion event from the overall signal of corrosion. In the past decades, several scanning probe techniques have been developed and employed for in situ corrosion studies [9–15]. Among these methods, the scanning reference electrode technique (SRET) is a powerful tool to detect the active sites and monitor their development by continuously measuring the potential distribution on the metal surface without interrupting the process during local- ized corrosion [13]. However, it is difficult to control the distance between tip and substrate (d t–s ) exactly by manual adjustment un- der an optical microscope. Especially, there were few studies aiming at the vertical corrosion information over the active and passive sites during localized corrosion [15]. In this work, the vertical potential in the thin layer of solution closed to the metal surface were recorded by using a home-built SRET/STM system. A distinct difference of the vertical potential profiles over the corroded and passive sites of stainless steel in FeCl 3 solution was observed for the first time, and the reason for this difference on vertical corrosion information was discussed for further understanding localized corrosion. 2. Material and methods 2.1. Materials and reagents The 1Cr18Ni9Ti stainless steel (Taiyuan Iron & Steel Co., Ltd, PR China) discs (ø10 mm  3 mm) cut from a rod 13 mm in diam- eter were imbedded in epoxy resin with one circular face exposed as working surface. The chemical composition of the steel was as follows (wt.%): C, 0.12; Si, 1.00; P, 0.035; S, 0.30; Mn, 1.70; Ti, 0.98; Cr, 17.00–19.00; Ni, 8.00–11.00; Fe, balanced. Before the measurements, the specimens were mechanically ground with sil- icon carbide paper from 400 up to 1500 grit and polished with 1– 0.3 lm Al 2 O 3 powders, rinsed with acetone and alcohol for 5 min, respectively. The test solution was 10% (wt.%) FeCl 3 solution (pH = 0.7) which were prepared using double deionised water and analytical grade reagents. The probe was prepared by etching Pt–Ir wire (GRINM, PR China) in a mixture of saturated CaCl 2 and 4% (wt.%) HCl solution (CaCl 2 :HCl = 6:4 by volume) at an AC potential of 25 V for about 40 s, the diameter of the etched probe tip was less than 1 lm as shown in Fig. 1. The probe was then coated by Apiezon wax (Arrow Fastener Company, LLC, USA) except its very end. The detailed pro- cedure for probe preparation was previously described in Ref. [16]. 2.2. Instrumental The potential mapping was performed by a home-built inte- grated SRET/STM system [17,18] which was able to in situ image 0010-938X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2012.04.020 ⇑ Corresponding author. Tel.: +86 592 2189354; fax: +86 592 2186657. E-mail address: cjlin@xmu.edu.cn (C.-J. Lin). Corrosion Science 61 (2012) 242–245 Contents lists available at SciVerse ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci