Electrochimica Acta 49 (2004) 1663–1672 Pitting initiation and propagation of hypoeutectoid iron-based alloy with inclusions of martensite in chloride-containing nitrite solutions W.S. Li a,∗ , N. Cui b , J.L. Luo b a Department of Chemistry, South China Normal University, 510631 Guangzhou, China b Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alta., Canada T6G 2G6 Received 25 April 2003; received in revised form 18 September 2003; accepted 8 December 2003 Abstract The pitting initiation and the pitting propagation of a hypoeutectoid iron-based alloy with inclusions of martensite in nitrite solutions containing chloride ions were studied by various electrochemical techniques. It was shown, by scanning reference electrode technique (SRET), that pitting occurred only on the martensite phase rather than hypoeutectoid areas. Anodic polarization behaviors of martensite electrode in nitrite solution with and without chloride ions are similar to those of hypoeutectoid electrode: both of martensite and hypoeutectoid electrodes were passive and the passive films could be broken by chloride ion. However, the open circuit potential of martensite electrode is more negative than that of hypoeutectoid electrode, and the passive current density of martensite electrode in the solution containing chloride ions was larger than that of hypoeutectod electrode. It was shown, by Mott-Schottky analysis, that the passive film of martensite electrode in the solution containing chloride ions had a higher donor concentration than that of hypoeutectoid electrode. There was a galvanic interaction between martensite and hypoeutectoid electrodes and it was this interaction that induced and accelerated the pitting corrosion on martensite area. Current and potential fluctuations, which reflected pitting initiation, were observed when martensite and hypoeutectoid electrodes were coupled together. © 2003 Elsevier Ltd. All rights reserved. Keywords: Hypoeutectoid iron-based alloy; Pitting corrosion; Martensite; Nitrite; Chloride 1. Introduction Iron-based alloys are widely used in industry due to their excellent strength, good mechinability, and good weldabil- ity, especially due to their low cost. However, the industrial equipments made from iron-based alloys need to be pro- tected from corrosion, because iron is active in many envi- ronments. One important way to protect iron-based alloys from corrosion is to form a passive film on the alloys with oxidants or by anodic polarization if the alloys can be passi- vated in the environments. Unfortunately, pitting might oc- cur on the passivated metal due to local weakness of the passive film [1–3]. Pitting might cause catastrophe because its initiation and propagation is difficult to be predicted. Un- derstanding more about pitting initiation and propagation is necessary for the best application of iron-based alloys. ∗ Corresponding author. Tel.: +86-20-8521-1368; fax: +86-20-8521-0763. E-mail address: liwsh@scnu.edu.cn (W.S. Li). Many mechanisms have been proposed to explain pitting corrosion of metals [4–17]. It has been found that pitting corrosion of a metal or an alloy may be caused by the effect of second phase intermetallics and alloying inclusions [17]. An alloy with micro-alloyed elements and with 0.43–0.47% carbon content has been widely used for pipeline to transport different media such as water, oil, and their mixtures. This alloy has a hypoeutetoid base structure, which inevitably contains other microstructures such as ferrite and martensite due to the working process. In practice, pitting usually occurs favorably on some microstructure. The purpose of this paper is to understand the pitting mechanism of a hypoeutectoid ion-based alloy with inclusions of martensite. 2. Experimental 2.1. Specimen preparation All specimens were made from an iron-based alloy with a composition (wt.%) of 0.47 C, 1.20 Mn, 0.15 Si, 0.05 P, 0013-4686/$ – see front matter © 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2003.12.003