Corrosion

Corrosion Science 47 (2005) 3034–3052 www.elsevier.com/locate/corsci

Evaluation of corrosion rate from polarisation curves not exhibiting a Tafel region
Harvey J. Flitt, D. Paul Schweinsberg
*
School of Physical and Chemical Sciences, Queensland University of Technology, GPO Box 2434, Brisbane, Queensland 4001, Australia Available online 25 August 2005

Abstract Evaluation of corrosionrate by Tafel extrapolation is often impossible, simply because an experimental polarisation curve does not exhibit linear Tafel regions. This paper shows how such curves for the Fe/H2O/H+/O2 corrosion system can be accurately deconstructed to furnish both kinetic and thermodynamic parameters for the anodic and cathodic reactions. The curved anodic branch (due to film formation) is then amenable tocorrection for IR voltage drop and the resulting Tafel slope and other parameters are then substituted in the Tafel equation to accurately determine icorr. An alternative method to obtain the anodic Tafel slope has been used to validate the above approach. Polarisation curves describing the inhibition of mild steel in industrial cooling water were scanned/digitised from the literature. Ó 2005Elsevier Ltd. All rights reserved.
Keywords: Carbon steel; Corrosion rate; Polarisation curve deconstruction; Tafel behaviour

1. Introduction The modern theory of aqueous metallic corrosion is now based firmly on electrode kinetics, and for this we are indebted to the work of Tafel [1], Wagner and Traud [2], Butler [3] and Erdey-Gruz and Volmer [4]. The story can be said to have started one
*Corresponding author. Tel.: +61 73 864 2111; fax: +61 73 864 1804/1508. E-mail address: p.schweinsberg@qut.edu.au (D.P. Schweinsberg).

0010-938X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2005.06.014

H.J. Flitt, D.P. Schweinsberg / Corrosion Science 47 (2005) 3034–3052

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hundred years ago when Tafel [1], following the work of Caspari [5],showed that for hydrogen evolution at a metal electrode the overpotential, g, (where g = Eapplied À Ereversible) increased logarithmically with the applied current density, jij, g ¼ a þ b log jij ð1Þ

At the time the importance of this ‘‘Tafel equation’’ (1) to corrosion was not apparent and it was only later, following the introduction of mixed potential theory [2] and electrode kinetics [3,4] tothe corrosion process, when it was realised that (1) was a special case (when g P 50 mV) of the more general Butler–Volmer relationship. Thus (1) can be rewritten gact;c ¼ bc logðic =i0 Þ ð2Þ

where gact,c = activation overpotential; bc = cathodic Tafel slope = À2.303RT/anF; a = transfer coefficient; n = number of electrons involved in the reaction; F = FaradayÕs constant (96,500 C molÀ1); R =8.314 J KÀ1 molÀ1; T = temperature in Kelvin. (A similar relationship to (2) holds for the anodic process.) The Tafel relationship is routinely invoked in the measurement of a metalÕs corrosion rate (icorr), and the theoretical basis of the technique can be found in most corrosion textbooks (e.g., [6,7]). Thus, for Zn actively corroding to Zn2+ in aqueous oxygen-free H2SO4 [6] the corrosion potential(Ecorr) is established and the metal is polarised in the negative direction and then in the positive direction from Ecorr. At each applied potential (the ‘‘overpotential’’ is now Eapplied À Ecorr) the recorded current is the sum of the anodic and cathodic components of the corrosion reaction, and the overall polarisation curve is the sum of two ‘‘true’’ curves, one describing oxidation of Zn andthe other reduction of H+ ions. Polarisation curves are commonly plotted as potential versus log current density and for potentials not greatly removed from Ecorr the shape of the anodic and cathodic branches will differ from that exhibited by each true curve. For potentials further from Ecorr the shape of each branch eventually becomes an accurate representation of the kinetics of the anodic and...