Selective dissolution of the c phase in a binary Ni(c)/Ni 3 Al(c 0 ) two-phase alloy Hye-Youn Lee a,b,⇑ , Masahiko Demura b , Ya Xu b , Dang-Moon Wee a , Toshiyuki Hirano b a Department of Materials Science and Engineering, KAIST, Daejeon 305-701, Republic of Korea b National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan article info Article history: Received 27 May 2010 Accepted 29 July 2010 Available online 6 August 2010 Keywords: A. Nickel A. Aluminium B. Potentiostatic C. Selective oxidation abstract Selective dissolution of Ni(c) in a binary Ni(c)/Ni 3 Al(c 0 ) two-phase alloy was performed in an aqueous electrolyte including 1 wt.% (NH 4 ) 2 SO 4 and 1 wt.% citric acid to obtain a rough, c 0 -terminated surface. The electrochemical potential for the selective dissolution was determined from the polarization curves of the c and c 0 single-phase alloys. The selective dissolution tests proved that c was precisely removed above 1.7 V SCE , resulting in the formation of a rough, c 0 -terminated surface. Surface analyses revealed that a passive AlO x , which retarded the dissolution, was preferentially formed on c 0 , resulting in a successful selective dissolution. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Ni 3 Al (c 0 ) intermetallic is well known for its use as a high-tem- perature structural material due to its excellent high-temperature strength and good oxidation/corrosion resistance [1]. Recently, its potential catalytic activity has been revealed for hydrogen produc- tion reactions such as methanol decomposition [2–5] and methane steam reforming [6,7]. Interestingly, this high activity is clearly found not only in the form of a fine powder, but also in the form of a flat foil [2,3]. The c 0 foil is a thin sheet that has been success- fully fabricated using a metallurgical process: cold rolling [8,9]. With these two beneficial functions, i.e., high-temperature struc- tural and catalytic properties, the foil can be used as a plate-type catalyst in a micro-channel reactor for hydrogen production. For practical use, the activity of the c 0 foil must be further in- creased. As is well known, catalytic activity can be improved by increasing the surface area of the catalyst. In this study, an attempt is made to improve the activity using a binary Ni–Al two-phase al- loy that consists of c 0 and a Ni solid solution (c). With fine cuboidal c 0 particles in the c matrix, the c/c 0 two-phase structure is known as a fundamental structure of Ni-base superalloys [10]. This study attempts to selectively remove c from the alloy surface with the expectation of producing a rough, c 0 -enriched surface. The advan- tage of the c/c 0 alloy is that foil fabrication is possible, i.e., the c/c 0 alloy can be cold rolled into thin foils more easily than the c 0 sin- gle-phase alloy [11,12]; this advantage fits with the current study’s development plan of a plate-type catalyst. The validity of this ap- proach has been previously demonstrated in a NiAl (b 0 )/c 0 two- phase powder [6]. The b 0 phase was selectively dissolved in an acid solution, leaving a porous c 0 surface behind. As a result, the powder surface was mostly c 0 -terminated with a large surface area and the catalytic activity for methane steam reforming was significantly improved. The purpose of this paper is to determine a condition that selec- tively removes c from the c/c 0 two-phase alloy to enhance the cata- lytic activity using an electrochemical method. The electrochemical method has advantages over ordinary chemical methods because the optimum condition can be explored by selecting an electrolyte and an applied potential. This method, known as electrochemical extraction, has been used previously for separating the c 0 particles from the c matrix of Ni-base superalloys in order to analyze their crystal structure, composition, volume fraction, and so on [13,14]. Recently, Mukherji et al. reported an interesting application of this method in commercial Ni-base superalloys [15,16]: they success- fully fabricated two different types of nano-porous materials by selective dissolution of either c 0 or c. However, thus far, most elec- trochemical studies have been undertaken for Ni- and Fe-base heat-resistant alloys containing ternary and other elements. For the binary c/c 0 two-phase alloy, there is only a brief report in the re- view by Mukherji et al. [16]: c 0 was selectively dissolved and porous c was obtained. The selective dissolution of c has not yet been stud- ied and its selective dissolution behaviour has not yet been clarified. In this study, the electrochemical behaviour of two single-phase alloys, c and c 0 , were investigated first using potentiodynamic polarization tests. Based on the results, selective dissolution of c for the binary, c/c 0 two-phase alloy was undertaken. Furthermore, a surface characterization of the c/c 0 two-phase alloy was per- formed after the selective dissolution and the mechanism of the selective dissolution was discussed. 0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.07.036 ⇑ Corresponding author at: Department of Materials Science and Engineering, KAIST, Daejeon 305-701, Republic of Korea. Tel.: +82 42 350 4255; fax: +82 42 350 4275. E-mail address: dishy@kaist.ac.kr (H.-Y. Lee). Corrosion Science 52 (2010) 3820–3825 Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci