Flow microcapillary plasma mass spectrometry-based investigation of new Al–Cr–Fe complex metallic alloy passivation N. Ott a,c,n , A. Beni b , A. Ulrich a,1 , C. Ludwig c,d , P. Schmutz b a Laboratory for Analytical Chemistry, EMPA – Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland b Laboratory for Joining Technologies and Corrosion, EMPA – Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland c EPFL – École Polytechnique Fédérale de Lausanne, ENAC-IIE, 1015 Lausanne, Switzerland d PSI – Paul Scherrer Institute, ENE-LBK, 5232 Villigen PSI, Switzerland article info Article history: Received 25 July 2013 Received in revised form 26 November 2013 Accepted 30 November 2013 Available online 6 December 2013 Keywords: Al–Cr–Fe complex metallic alloys ICPMS analysis Flow cell Microcapillary Potentiostatic polarization Passivity abstract Al–Cr–Fe complex metallic alloys are new intermetallic phases with low surface energy, low friction, and high corrosion resistance down to very low pH values (0–2). Flow microcapillary plasma mass spectrometry under potentiostatic control was used to characterize the dynamic aspect of passivation of an Al–Cr–Fe gamma phase in acidic electrolytes, allowing a better insight on the parameters inducing chemical stability at the oxyhydroxide–solution interface. In sulfuric acid pH 0, low element dissolution rates (in the mg cm À2 range after 60 min) evidenced the passive state of the Al–Cr–Fe gamma phase with a preferential over-stoichiometric dissolution of Al and Fe cations. Longer air-aging was found to be beneficial for stabilizing the passive film. In chloride-containing electrolytes, ten times higher Al dissolution rates were detected at open-circuit potential (OCP), indicating that the spontaneously formed passive film becomes unstable. However, electrochemical polarization at low passive potentials induces electrical field generated oxide film modification, increasing chemical stability at the oxyhydr- oxide–solution interface. In the high potential passive region, localized attack is initiated with subsequent active metal dissolution. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Complex metallic alloys (CMAs) are a new class of crystalline intermetallic phases characterized by a complex crystallographic structure. They are organized in large unit cells, containing hundreds of atoms arranged in highly symmetric clusters, and often considered as stable approximants of quasicrystals [1, 2]. Within the CMAs family, Al-based CMAs are promising for future industrial application [2, 3]. They are developed to be used as functionalized coatings [4–6] to enhance the aluminum corrosion resistance. Al-based CMAs exhibit a unique combination of surface properties [2, 3, 7, 8], such as low surface energy, low friction coefficient and high stability in a broad pH range [9, 10], which depend on their chemical composition rather than on their structural complexity [9, 11, 12]. Consequently, many of these properties are tightly related to the oxyhydroxide layer, which sponta- neously forms on the surface [13]. This nm-thick oxyhydroxide layer, also known as passive layer, continuously undergoes dynamic pro- cesses associated with film growth at the metal –oxyhydroxide inter- face and chemical dissolution at the oxyhydroxide–solution interface. A comprehensive understanding of these processes is therefore necessary to guarantee the material's properties in operando conditions for longer service times. Surface analytical techniques provide useful information about the surface chemical composition, structure, topography and mechan- ical and electronic properties of passive films [14]. However, they are mainly ex situ methods and consequently no mechanistic informa- tion about film dynamics can be retrieved. They are usually used in combination with electrochemical methods to determine kinetic data [14]. However, none of these techniques allow full character- ization of the dynamic of passivation (including chemical dissolu- tion) at the oxyhydroxide–solution interface. The electrochemical quartz crystal microbalance (EQCM) proved to be suitable to monitor changes in the passive film induced by electrochemical film growth and chemical dissolution processes at the oxyhydroxide–solution interface with a sub-monolayer mass change sensitivity [15–17]. It could be determined that an increase in the polarization potential leads to an increase of the passive film thickness associated with a significant cation dissolution from the passive film surface. The obtained information remains nevertheless averaged mass changes. Therefore, this method, despite its extremely high sensitivity, only suits for model homogeneous materials. Over the past years analytical chemistry methods have gained increasing interest in corrosion science. They can provide quantita- tive information about the element-specific releases even at early corrosion stages, allowing better characterization of the passivation Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/talanta Talanta 0039-9140/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.talanta.2013.11.091 n Correspondence to: EMPA, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Analytical Chemistry, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland. Tel.: þ41 58 765 4845; fax: þ41 58 765 6915. E-mail addresses: noem.ott@gmail.com (N. Ott), patrik.schmutz@empa.ch (P. Schmutz). 1 Deceased on March 12th 2013 (A. Ulrich). Talanta 120 (2014) 230–238