Available online at www.sciencedirect.com Electrochimica Acta 53 (2008) 4740–4747 Corrosion of steel under the defected coating studied by localized electrochemical impedance spectroscopy C. Zhong a,b , X. Tang a,c , Y.F. Cheng a,∗ a Department of Mechanical and Manufacturing Engineering, University of Calgary, Calgary, Alta., T2N 1N4 Canada b Department of Materials Science, Fudan University, 220 Handan Road, Shanghai 200433, China c College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao, Shandong 266100, China Received 4 December 2007; received in revised form 4 February 2008; accepted 5 February 2008 Available online 12 February 2008 Abstract Corrosion of steel under the defected coating in near-neutral pH solution was investigated by localized electrochemical impedance spectroscopy (LEIS) measurements. The LEIS response is dependent on the size of the defect. For small defects, e.g., less than 200 m in diameter, localized corrosion process and mechanism of steel, as indicated by the measured LEIS plots, change with time. The diffusion process dominates the interfacial corrosion reaction, which is due to the block effect of the deposited corrosion product combined with the geometrical factor of a large coating thickness/defect width ratio. In the presence of a big defect, e.g., up to 1000 m, the LEIS responses measured at the defect are always featured by a coating impedance in the high-frequency range and an interfacial corrosion reaction in the low-frequency range. The block effect of corrosion product does not apply due to the relatively open geometry. Conventional EIS measurements on a macroscopic-coated electrode reflect the “averaged” impedance results from both coating and defect. The information of the localized electrochemical corrosion processes and mechanisms at the small defect is lost, and the coating impedance information is “averaged” out when a big defect is contained. LEIS measurement provides an essential technique to characterize microscopically the local electrochemical corrosion reaction of steel under the defected coating. © 2008 Elsevier Ltd. All rights reserved. Keywords: Corrosion; X-65 pipeline steel; Defect; Coating; Localized electrochemical impedance spectroscopy 1. Introduction Electrochemical impedance spectroscopy (EIS) technique has been used extensively to characterize the coating perfor- mance and the corrosion of steel under the coating [1–3]. Mans- feld and co-workers [1,2,4–6] have developed typical impedance criteria to use EIS characteristics and parameters for determina- tion of corrosion mechanism and rate of steel under coating. Characterization of coating performance using conventional EIS measurement is associated with a major shortcoming, i.e., the measured impedance result is attributed to the electrochem- ical response of the whole electrode, reflecting an “averaged” behaviour of the macroscopic electrode. As a consequence, local electrochemical process occurring at micro-defect such as pin- hole in the coating is “averaged” out. Therefore, analysis of EIS data is incapable of revealing the process and mechanism of ∗ Corresponding author. Tel.: +1 403 220 3693; fax: +1 403 282 8406. E-mail address: fcheng@ucalgary.ca (Y.F. Cheng). localized electrochemical corrosion reaction of steel under coat- ing with micro-scaled defects. Without such information, it will be impossible to develop a complete understanding of localized coating degradation and the resultant corrosion of the steel. Localized EIS (LEIS) measurements provide a promising alternative to investigate microscopically the coating degrada- tion and localized corrosion of steel under coating. Zou and Thierry [7] used LEIS technique to study the degradation of organic coatings on metals. It was found that LEIS could be used to detect the onset of coating blistering and give a measure of the rate of water diffusion into the organic coating at a microscopic scale. The technique has also been applied to study the propaga- tion of pits on two different iron–chromium alloys in chloride- containing environments [8], demonstrating that LEIS can give valuable information on the mechanism controlling pit growth. High performance composite coating (HPCC) is being planned to use for maintenance of pipeline integrity in the Arctic and sub-Arctic areas [9,10]. To date, there has been very limited work to characterize electrochemically the coat- ing property for prevention of corrosion of steel, especially 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.02.014