Characterisation of the surface films formed on Nb during electrodissolution in aqueous alkali Claudia Baruffaldi a , Umberto Casellato b , Sandro Cattarin a , Marco Musiani a, * ,1 , Bernard Tribollet 1c , Barbara Vercelli a a IENI-CNR, Corso Stati Uniti 4, 35127 Padova, Italy b ICIS-CNR, Corso Stati Uniti 4, 35127 Padova, Italy c CNRS-UPR15, ‘Physique des Liquides et Electrochimie’, Universite ´ P. et M. Curie, CP 133, 4 Place Jussieu, 75252 Paris Cedex 05, France Received 14 February 2002; received in revised form 12 April 2002 Abstract Anodic polarisation of Nb in warm and concentrated aqueous alkali results in sustained electrodissolution. The process is investigated in NaOH by voltammetric, steady-state and impedance techniques. The j  /E steady-state curve is characterised by a current peak and a subsequent plateau extending up to the explored positive limit (8 V SCE ). Upon addition of tartrate to the electrolyte the dissolution current increases markedly. In some NaOH /Na tartrate electrolytes a dissolution  /precipitation mechanism sets in, with formation of a surface layer which causes a gradual current decrease; this layer is identified by XRD and elemental analysis as a mixed Na and Nb oxide hydrate. Larger currents may be sustained in KOH /K tartrate solutions without any precipitation process. Impedance diagrams recorded in the plateau region are analysed on the basis of the equivalent circuit resulting from the surface charge model; the oxide formation ratio and the main model parameters are estimated and their temperature dependence is discussed. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Anodic film; Formation ratio; Niobium oxide; Tartrate; Impedance 1. Introduction Anodic polarisation of Nb electrodes in most aqueous or water-containing electrolytes results in the growth of a compact, nonporous film of Nb 2 O 5 which may find use in dielectric applications [1 /3]. Several investiga- tions have been performed in acid and neutral media [4  / 16]. Studies devoted to alkaline media were performed in conditions (low temperature, low alkali concentration) in which oxide films are stable or subject to slow dissolution [10,17  /20]. A few investigations were de- voted to analyse the impedance response of the passive films [8,9,15,18,19]; in both acid and alkaline media this response, often measured in a narrow frequency range, corresponded to a RC parallel, the capacity term being sometimes replaced by a constant phase element (CPE) [15,19]. If anodisation is performed in media able to dissolve the oxide, like HF, the resulting process is electrodisso- lution mediated by film formation [3]. In a previous paper we have characterised Nb electrodissolution in acid fluoride medium by means of voltammetric, steady- state and impedance investigations [21]. The j  /E curve shows as main features a first dissolution/passivation peak, a current increase and, at more positive potentials, a large pseudo-plateau with a small negative slope. Impedance spectra recorded in the plateau region are characterised by a capacitive loop at high frequency, an inductive loop in the medium frequency range and a capacitive straight line at low frequency. Analysis of impedance data, performed on the basis of the surface charge model [22,23], allowed estimating the model parameters and the film formation ratio. In this paper we present an investigation performed in warm concentrated alkali containing or not Nb(V) complexing agents. Strongly basic solutions are expected to allow high dissolution rates of Nb, as Nb 2 O 5 is converted to soluble niobates and polyniobates at high * Corresponding author. Fax:  /39-049-8295853 E-mail address: m.musiani@ipelp.pd.cnr.it (M. Musiani). 1 ISE member. Electrochimica Acta 47 (2002) 2989  /2997 www.elsevier.com/locate/electacta 0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII:S0013-4686(02)00202-5