Mechanism for hydrogen evolution reaction on pipeline steel in near-neutral pH solution Y.F. Cheng a, * , L. Niu a,b a Department of Mechanical and Manufacturing Engineering, University of Calgary, Calgary, Alta., Canada T2N 1N4 b School of Chemistry and Chemical Engineering, Shandong University, Jinan, Shandong 250100, China Received 3 October 2006; received in revised form 17 October 2006; accepted 19 October 2006 Available online 16 November 2006 Abstract Cyclic voltammetry, hydrogen permeation tests and electrochemical impedance spectroscopy measurements were combined to study the mechanism for hydrogen evolution reaction on X-70 pipe steel in near-neutral pH solution. It is found that hydrogen evolution reac- tion is dominated by the reduction of water molecules, followed by either an electrochemical hydrogen recombination reaction or a hydrogen absorption reaction. The near-neutral pH environment is capable of generating catalytic surface effect on hydrogen evolution on the pipe steel. The increasing dissolution of the cathodically pre-polarized steel could be due to the enhanced activation of the steel, rather than the increasing amount of hydrogen atoms in the steel. These results provide mechanistic information to understand the near- neutral pH stress corrosion cracking of pipelines. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Hydrogen evolution reaction; Near-neutral pH solution; Pipeline steel; Hydrogen recombination; Hydrogen absorption 1. Introduction The occurrence of near-neutral pH stress corrosion cracking (SCC) in natural gas pipelines is always associated with electrolyte characterized with anaerobic, dilute solu- tions (of the order of 0.01 M bicarbonate ions) with pH in the range of 6 to 7.5, and the stress corrosion crack has a transgranular, quasi-cleavage crack morphology with very little branching [1–3]. Compared with the high pH SCC that has been studied extensively to develop the well-accepted mechanism, i.e., high pH SCC being attrib- uted to anodic dissolution at the grain boundaries and repeated rupture of passive films that form over the crack-tip [1,3,4], there has been relatively limited under- standing of near-neutral pH SCC. Cheng’s previous work [5] showed that hydrogen could involve in near-neutral pH SCC in pipeline steels. The experimental results indicated that the electrolytes extracted from soils with previous SCC history were always associated with high hydrogen permeation currents and sub-surface hydrogen concentrations. Cheng developed a thermodynamic model to illustrate the interactions of stress, hydrogen and anodic dissolution at the crack-tip [6,7]. It is shown that the growth rate of stress corrosion cracks under near-neutral pH condition depends on the synergistic effect of hydrogen and stress on the anodic dis- solution rate of steel at crack-tip, and the effect of the con- centration difference of hydrogen atoms between the stressed steel and unstressed steel on the anodic dissolution reaction. There is a considerable amount of evidence [8–10] that hydrogen plays a critical role in the near-neutral pH SCC of pipelines. However, the precise mechanism has not been identified. Since the hydrogen absorption and permeation is part of the hydrogen evolution reaction, it is essential to investigate the mechanism for hydrogen evolution reac- tion on pipeline steel in order to understand the fundamen- tals of the hydrogen involvement in near-neutral pH SCC. 1388-2481/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.10.035 * Corresponding author. Tel.: +1 403 220 3693; fax: +1 403 282 8406. E-mail address: fcheng@ucalgary.ca (Y.F. Cheng). www.elsevier.com/locate/elecom Electrochemistry Communications 9 (2007) 558–562