The concept of floating electrode for contact-less electrochemical measurements: Application to reinforcing steel-bar corrosion in concrete M. Keddam a , X.R. Nóvoa b , V. Vivier a, * a LISE, UPR 15 du CNRS, Université Pierre et Marie Curie, 4 place Jussieu, 75252 Paris Cedex 05, France b E.T.S.E.I, Universidade de Vigo, Campus Universitario, 36310 Vigo, Spain article info Article history: Received 31 December 2008 Accepted 2 May 2009 Available online 18 May 2009 Keywords: A. Concrete B. Electrochemical calculation B. Electrochemical impedance spectroscopy (EIS) abstract The concept of floating electrode is introduced for defining the common electrochemical behaviour of any non-connected, electronically conducting, body immersed in an electrolytic medium. The emphasis is put on both its own polarisation features and its influence on the d.c. and a.c. current and potential across the cell, hence the feasibility, among others, of contact-less electrochemical measurements on floating elec- trodes. Application to reinforcing steel bars in concrete is investigated by numerical computation of the a.c. current and potential fields in a broad range of concrete resistivity, interfacial resistance and capac- itance. Impedance defined in a 4-electrode configuration, when rationalised against the concrete resistiv- ity, is shown to provide, within a realistic range of parameters, a practical mean to access the properties of the bar–concrete interface. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction The measurement of polarisation resistance (R p ) is a useful and widely used technique for the evaluation of corrosion rate of steel in concrete. It consists in linearly varying the potential within a po- tential range in the vicinity of the corrosion potential, allowing the corrosion current to be determined from the Stern–Geary equation. Following a general trend in corrosion studies, electrochemical impedance spectroscopy (EIS) measurements are increasingly applied; R p being simply the low frequency limit of the impedance (after correction of the Ohmic contribution) whilst the frequency dependence allows separating various contributions to the corro- sion behaviour of the steel bar and hence getting a better insight into the corrosion mechanisms. Besides the difficulties of interpre- tation, these approaches are not easily applicable to real structures since an electrical connection of the reinforcing structure to the measuring equipment is necessary, and uncertainty exists on the estimation of the active surface. In order to overcome those problems it has been intended to evaluate the polarisation resistance of the bar from its contribution to the electrical response of the whole concrete sample. It has been previously observed, under various experimental conditions, that the reinforcement bar can be polarised by application of an exter- nal polarisation to the concrete sample [1]. However, the true nature of the phenomenon observed was not clearly understood. The problem was then analysed by impedance measurements associated with computation of the frequency dependence of the current distribution across the concrete phase and the metal. This was performed initially assuming that the electrochemical cell can be described by a 2-D geometry [1] and then extended to a 3-D space [2]. It was shown that, as already taken into consideration in Ref. [3], the distribution of current density and potential along the bar is an overwhelming effect even though, as a first approxi- mation, the behaviour can be roughly interpreted by connecting in parallel the bar–concrete interface (represented by a R–C equiv- alent circuit) and the concrete Ohmic resistance. Very recently [4] this model was validated in d.c. condition for R p measurement in a non-contact configuration mode. Traditional problems and devices in electrochemistry are deal- ing with electrodes connected electronically to an external source of electrical energy or short circuited in a galvanic cell. The charac- terisation of the electrode processes is based essentially on the relationship between the applied potential versus a reference elec- trode, and the current density crossing the working electrode either in steady-state or non-steady-state conditions. Nevertheless, in a number of situations with a strong electrochemical concern, a piece of metal is actually in contact with an electrolytic medium but is not connected to any external circuit or, equivalently, is con- nected to an infinite impedance source like an ideal voltmeter or an electrometer. In both cases, no net current is allowed to flow at the interface between this so-called Electrically Floating Elec- trode (EFE) and the embedding electrolyte medium. Besides the very trivial case of the open-circuit potentials (true thermody- namic equilibrium or mixed potential), interesting situations arise when the EFE lies in region of the electrolyte where a current is 0010-938X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2009.05.006 * Corresponding author. Tel.: +33 1 4427 4158; fax: +33 1 4427 4074. E-mail address: vincent.vivier@upmc.fr (V. Vivier). Corrosion Science 51 (2009) 1795–1801 Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci