Hydrogen evolution in aqueous solutions containing dissolved CO 2 : Quantitative contribution of the buffering effect E. Remita a,b,c, * , B. Tribollet c , E. Sutter c,d , V. Vivier c , F. Ropital b , J. Kittel b a IFP, 1 avenue de Bois Pre ´au, 92852 Rueil Malmaison, France b IFP, BP No. 3, 69390 Vernaison, France c Laboratoire Interfaces et Syste `mes Electrochimiques, UPR15 du CNRS, Universite ´ Pierre et Marie Curie, case 133, 4 place Jussieu, 75252 Paris Cedex 05, France d Laboratoire de Ge ´nie des Proce ´de ´s Plasma et Traitement de Surface, 11 rue Pierre et Marie Curie, 75005 Paris, France Received 26 November 2007; accepted 12 December 2007 Available online 26 January 2008 Abstract The hydrogen evolution reaction (HER) occurring on steel in an oxygen-free aqueous solution containing dissolved CO 2 was investigated. This reaction was modelled by taking into account the two dissociation reactions of dissolved CO 2 . The mathematical prob- lem was solved numerically using a finite element method (FEM). A fair agreement between the measurements performed on a steel rotating disc electrode and the theoretical calculations was obtained. Thus, the cathodic behaviour of steel in CO 2 -containing solutions can be fully explained by the buffering effect induced by the presence of CO 2 . Some interfacial pH measurements performed on a gold electrode also support this conclusion. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: A. Steel; B. Polarization; B. Modelling studies; C. Acid corrosion 1. Introduction The rate of the hydrogen evolution reaction (HER) on steel is enhanced in oxygen-free aqueous solutions contain- ing dissolved CO 2 , in comparison with strong acid solu- tions at the same pH [1–3]. This specificity is well known in the oil and gas industries [4] since it is generally used to explain the high corrosivity of the solutions containing dissolved CO 2 [1–7]. Thus, the understanding of the HER mechanism in the presence of dissolved CO 2 is of great importance in order to develop reliable corrosion models. However, up to now, this mechanism still remains poorly understood in spite of the large number of studies devoted to its investigation [1–11]. In aqueous solutions containing dissolved CO 2 , the most common assumption found in the literature consists in con- sidering that direct electrochemical deprotonation of car- bonic species (H 2 CO 3 or HCO  3 ) occurs at the steel surface during the HER. Such hypothesis was first pro- posed in 1975 by De Waard and Milliams [1] who assumed that the reduction of carbonic acid occurs on steel via the following sequence: H 2 CO 3ðadsÞ þ e  ! H ðadsÞ þ HCO  3ðadsÞ ðrate-determining stepÞ ð1Þ HCO  3ðadsÞ þ H þ () H 2 CO 3ðadsÞ ð2Þ 2H ðadsÞ ! H 2 ð3Þ Thereafter, a large number of alternative mechanisms involving a direct electrochemical deprotonation of car- bonic species were proposed [2,3,9–11]. The subject is still extremely controversial and the nature of the reactive car- bonic species has been hardly discussed in the past. 0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2007.12.007 * Corresponding author. Present address: Technip Flexifrance, Rue Jean Hure ´, 76580 Le Trait, France. Tel.: +33 235055296; fax: +33 235055018. E-mail address: eremita@technip.com (E. Remita). www.elsevier.com/locate/corsci Available online at www.sciencedirect.com Corrosion Science 50 (2008) 1433–1440