Investigation of chloride-induced pitting processes of iron in the H 2 SO 4 solution by the digital holography Chao Wang a,b, * , Shenhao Chen b,c , Xuegeng Yang c , Liang Li a,c a Department of Chemistry, Xuzhou Normal University, Xuzhou 221116, China b State Key Laboratory for Corrosion and Protection, Shenyang 110015, China c Department of Chemistry, Shandong University, Jinan 250100, China Received 28 June 2004; received in revised form 31 July 2004; accepted 31 July 2004 Available online 26 August 2004 Abstract The digital holography is a new technique applied in electrochemistry. It is used for the first time to observe the dynamic pitting processes of iron in 0.5 mol dm 3 sulfuric acid caused by chloride ions. Such processes as passivation, initiation and propagation of pitting corrosion were investigated real-time by the method. Results are shown here to demonstrate the use of the technique in corrosion science. Ó 2004 Published by Elsevier B.V. Keywords: In-line digital holography; Pitting; Fe/H 2 SO 4 interface; Passive film; Interferometric holography 1. Introduction Localized corrosion is a big problem in industries be- cause it often causes catastrophic failures in engineering structures. Corrosion behavior of metallic materials de- pends heavily on the passive films on their surfaces. Localized corrosion normally results from the break- down of passive films in aggressive environments, espe- cially in the media containing chloride ions [1]. Although a lot of effort has been made to understand its mechanisms and to prevent its occurrence [2–8], local- ized corrosion is still a common cause of failures in seawater desalination units, gas and oil refining equip- ment, water-cooling systems and pulp and paper manu- facturing machineries [9]. Up to now, it is still not clear whether the chloride ions just adsorb on the surface of the electrode or penetrate through the passive film. Sazou and coworkers [1–3,10] studied the oscillatory behavior of the Fe/H 2 SO 4 system affected by chloride ions, and they proposed that general and pitting corro- sion can be deduced from different modes of current oscillations. But how the chloride ions affect the passive film formed on the electrode surface is still not com- pletely understood, still less is known about the initia- tion of localized corrosion. The non-linear dynamic response during the elec- trodissolution of metals is very sensitive to the changes of the chemical environment at the elec- trode/electrolyte interface. The electrochemical re- sponse to the perturbation of the interface chemical environment provides us with important information about the passivation film formation and dissolution processes. A useful method to study the effects of dif- ferent ions on the passive films on metals is the inject- ing experiment [11,12], which can change the chemical environment at the electrode/electrolyte interface as wished by injecting a certain amount of solution into the vicinity of the electrode surface. 1388-2481/$ - see front matter Ó 2004 Published by Elsevier B.V. doi:10.1016/j.elecom.2004.07.020 * Corresponding author. Tel.: +86 516 3403021/3403320; fax: +86 516 3403320. E-mail address: wangc@xznu.edu.cn (C. Wang). www.elsevier.com/locate/elecom Electrochemistry Communications 6 (2004) 1009–1015