Electrochimica Acta 55 (2010) 3454–3463 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta Understanding aluminum behaviour in aqueous alkaline solution using coupled techniques Part II: Acoustic emission study M. Boinet 1 , J. Bernard 2 , M. Chatenet ∗ , F. Dalard, S. Maximovitch Laboratoire d’Electrochimie et de Physico-chimie des Matériaux et des Interfaces, UMR 5631, CNRS/Grenoble-INP/UJF, BP 75, 38402 Saint Martin d’Hères Cedex, France article info Article history: Received 18 December 2008 Received in revised form 11 June 2009 Accepted 13 June 2009 Available online 23 June 2009 Keywords: Aluminum Potassium hydroxide solution Acoustic emission Hydrogen evolution Hydrogen-assisted aluminum exfoliation corrosion abstract This work focuses on the behaviour of pure aluminum in alkaline media, by coupling both acoustic emis- sion (AE) and direct hydrogen voltammetry to electrochemistry. We notably monitored, recorded and analyzed the acoustic emission activity generated by the aluminum electrode as a function of its polar- ization during a linear sweep voltammetry (from anodic to cathodic potentials) on pure aluminum in 4 M aqueous potassium hydroxide solution. Such in situ coupling of electrochemistry and acoustic emission shows a perfect correlation between the two signals. After careful analysis of the AE signal using a statisti- cal treatment, and based on five relevant AE parameters (rise time, duration, amplitude, absolute energy, maximum frequency), we could separate various groups of AE signals occurring at the aluminum elec- trode. We further linked them to the different (and possibly concomitant) electrochemical phenomena, which are taking place upon polarization of the aluminum electrode in strong alkaline medium. First, we confirmed that hydrogen evolution initiates for potentials positive to aluminum open cir- cuit potential in 4M potassium hydroxide solution; such small but non-negligible hydrogen production occurs in parallel to aluminum oxidation. Second, aluminum oxides are present only around the open circuit potential; whereas they are eroded for high aluminum oxidation potentials, they are flaked off at high hydrogen evolution potentials. Such latter process is probably accelerated by the hydrogen evolution- induced alkalization of the electrolyte. Third, two modes of hydrogen evolution are recorded: one on the oxide, the other one on bare aluminum, the latter being the most efficient. This strong hydrogen evolu- tion at very low electrode potential probably assists the removal of the brittle residual oxide/passive film present on aluminum (which we denote as hydrogen-assisted aluminum exfoliation corrosion), there- fore causing the rapid erosion of the aluminum electrode. As a result, aluminum is never in immunity conditions in strong alkaline medium. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction Understanding the behaviour of pure aluminum in alkaline solu- tion is of great interest, for example in view of its use as anode material in metal–air batteries [1]. Unfortunately, the processes occurring on aluminum in alkaline media are numerous and com- plex [2,3], which render any basic electrochemical study difficult and probably not comprehensive. From this prospect, using in situ coupled techniques might enable unveiling some of the uncertain- ∗ Corresponding author. E-mail address: Marian.Chatenet@phelma.grenoble-inp.fr (M. Chatenet). 1 Present address: Europhysical Acoustics SA, 27 rue Magellan, 94370 Sucy en Brie, France. 2 Present address: Institut Franc ¸ ais du Pétrole (IFP), Rond-point de l’échangeur de Solaize, BP 3, 69360 Solaize, France. ties in the phenomena occurring at aluminum in such electrolyte solutions. In a first approach [4], we studied aluminum behaviour in 4M potassium hydroxide solution using the rotating disk elec- trode (RDE) and rotating ring-disk electrode (RRDE) techniques. Using the RRDE, we clarified some of the features observed on the quasi-steady state voltammogram plotted at 5 mV s -1 from anodic potential (E i = -0.7 V vs. NHE) to cathodic potential (E f = -2.5 V vs. NHE). The very reproducible voltammogram showed a regular cur- rent decrease down to a potential around -1.55 V vs. NHE, at which the curve did flatten. In the cathodic part of the voltammogram, a rupture (hereafter denoted as “break”) in the (i vs. E) slope was observed at -2.05 V vs. NHE, where the current absolute value started to increase more strongly, and became linear. Our main con- clusions were the following. Above aluminum equilibrium potential (ocp), which is located around -1.9 V vs. NHE for 5N or 5N5 alu- minum samples, the main process occurring at the electrode is 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.06.038