Applied Surface Science 264 (2013) 605–610 Contents lists available at SciVerse ScienceDirect Applied Surface Science j our nal ho me p age: www.elsevier.com/loc ate/apsusc Nanoporous alumina formed by self-organized two-step anodization of Ni 3 Al intermetallic alloy in citric acid Wojciech J. St˛ epniowski ∗ , Grzegorz Cie´ slak, Małgorzata Norek, Krzysztof Karczewski, Marta Michalska-Doma ´ nska, Dariusz Zasada, Wojciech Polkowski, Paweł Jó´ zwik, Zbigniew Bojar Department of Advanced Materials and Technology, Faculty of New Technologies and Chemistry, Military University of Technology, Kaliskiego 2 Str., 00-908 Warszawa, Poland a r t i c l e i n f o Article history: Received 8 July 2012 Received in revised form 3 September 2012 Accepted 12 October 2012 Available online 23 October 2012 Keywords: Nanopores Anodization Self-organization Anodic oxide Electrochemical impedance spectroscopy a b s t r a c t Formation of the nanoporous alumina on the surface of Ni 3 Al intermetallic alloy has been studied in details and compared with anodization of aluminum. Successful self-organized anodization of this alloy was performed in 0.3 M citric acid at voltages ranging from 2.0 to 12.0 V using a typical two-electrode cell. Current density records revealed different mechanism of the porous oxide growth when compared to the mechanism pertinent for the anodization of aluminum. Electrochemical impedance spectroscopy experiments confirmed the differences in anodic oxide growth. Surface and cross-sections of the Ni 3 Al intermetallic alloy with anodic oxide were observed with field-emission scanning electron microscope and characterized with appropriate software. Nanoporous oxide growth rate was estimated from cross- sectional FE-SEM images. The lowest growth rate of 0.14 m/h was found for the anodization at 0 ◦ C and 2.0 V. The highest one – 2.29 m/h – was noticed for 10.0 V and 30 ◦ C. Pore diameter was ranging from 18.9 nm (2.0 V, 0 ◦ C) to 32.0 nm (12.0 V, 0 ◦ C). Interpore distance of the nanoporous alumina was ranging from 56.6 nm (2.0 V, 0 ◦ C) to 177.9 nm (12.0 V, 30 ◦ C). Pore density (number of pore occupying given area) was decreasing with anodizing voltage increase from 394.5 pores/m 2 (2.0 V, 0 ◦ C) to 94.9 pores/m 2 (12.0 V, 0 ◦ C). All the geometrical features of the anodic alumina formed by two-step self- organized anodization of Ni 3 Al intermetallic alloy are depending on the operating conditions. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Recently, nanoporous oxides attract attention of researches because of their numerous potential applications. The most fre- quently applied nanoporous oxide is anodic aluminum oxide (AAO), very often used as a template for nanofabrication [1]. Pore diameter, interpore distance and thickness of nanoporous aluminum oxide are fully controlled by operating conditions, including anodizing voltage, type and temperature of electrolyte and duration of the process [2–12]. Formation of nanoporous and nanotubular titania is also being developed recently [13–22]. Moreover, the anodiza- tion of elements like iron [23], magnesium [24], niobium [25,26], silicon [27], tin [28] and zirconium [22,29] are researched to obtain regular porous oxides of these metals. Joint Pourbaix diagram for nickel and aluminum shows a pos- sibility of selective oxidation of aluminum with simultaneous immunity of nickel at certain pH range [30]. Moreover, Wood has reported growth of alumina on the surface of aluminum–zinc alloy [31]. However, this oxide was composed of few layers: pure ∗ Corresponding author. Tel.: +48 22 683 94 46; fax: +48 22 683 94 45. E-mail address: wstepniowski@wat.edu.pl (W.J. St˛ epniowski). alumina, alumina enriched with zinc and metallic pillar struc- tures made of the unoxidized zinc [31]. Also aluminum-containing intermetallic alloys were successfully anodized and nanostruc- tures were formed. For example anodization of purple gold (AuAl 2 ) resulted in a formation of gold nanoparticles with average diameter of about ∼30 nm [32]. Anodization of titanium–aluminum inter- metallics was also done [33,34]. TiAl 3 , TiAl and Ti 3 Al were anodized and nanoporous anodic oxides were formed. Here, the anodization process of a single phase Ni 3 Al inter- metallic is investigated. The formation of nanoporous alumina on the surface of the alloy in a simple two-electrode system, like in the case of aluminum anodization, is studied [4,5]. Moreover, the influence of operating conditions, especially anodizing voltage, on the geometry of nanopores including pore diameter and interpore distance, is analyzed. 2. Experimental Ni 3 Al (Zr, B) intermetallic alloy was cast from elemental powders and subsequently rolled and recrystallized. Chemical composition of the alloy is given in Table 1. To provide single-phase structure of the alloy, the material was annealed at 1100 ◦ C for 100 h in Ar atmosphere. As prepared material was made of  ′ phase. Next, 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.10.074