Kinetics and mechanism of corrosion of laboratory hot briquetted iron
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I. INTRODUCTION
THE models suggested in literature to describe the corrosion of direct reduced iron (DRI) were many and varied. However, the controlling mechanism for the corrosion rate of DRI at room temperature is most commonly recorded as diffusion. Parabolic and logarithmic models have been fitted to experimental data by various other authors.[1,2,3] Bandopadhyay et al.[1] found that at low temperatures (35 °C), the reoxidation behavior of moistened DRI in a pure oxygen environment for 1 hour indicated that the reaction was diffusion controlled, and resistance offered by the oxide surface layer played a vital role. Reoxidation at low temperatures was aided by the development of cracks and fissures resulting from physicochemical phenomena.[2] They also found that the rate of corrosion was initially high, followed by a very slow second stage that obeys a logarithmic rate law. This transition was attributed to the decrease in surface area available for oxidation, as well as changes in local void fraction of the particle; pores are blocked by the theoretical volume difference between iron and its oxides. Towhidi[3] proposed a two-stage mechanism for the oxidation of DRI. In the initial stage, the porosity of the sample decreased as the oxidation products filled the available reactive surface area. The mixed mechanisms of chemical reactions and diffusion controlled the kinetics of this stage. The second stage was diffusion controlled, and corresponded to a parabolic rate law. Briquetting decreases the porosity and induces an earlier transition from the first to second stage. Corrosion is an electrochemical phenomenon comprised of anodic and cathodic half-cell processes. The rate of the anodic dissolution of iron (i.e., the corrosion rate) and the JUANITA GRAY, formerly Postdoctoral Student, and VEENA SAHAJWALLA, Professor, are with the School of Materials Science and Engineering, The University of New South Wales, Sydney 2052, Australia. Contact e-mail: [email protected] or [email protected] RAJAKISHORE PARAMGURU, Deputy Director and Head of Hydro & Electrometallurgy Department, is with the Regional Research Laboratory, Bhubaneswar 751013, Orissa, India. Manuscript submitted June 22, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
corrosion potential depend on a balance of all the processes occurring at the corroding surface. This balance can be altered by a vast number of changes in the experimental conditions, e.g., pH, ions formed, stirring, sample preparation, and contaminants in solution or in the iron electrode.[4] A commonly used electrochemical method of studying corrosion is through polarization measurements. The main advantage of such a measurement is that it allows identification of the individual anodic and cathodic reactions taking place. In addition, the corrosion current (and hence the corrosion rate) can be determined by extrapolation of the cathodic or anodic Tafel lines on a polarization plot to the corrosion potential (Ecorr). This is then inserted into Faraday’s Law: Corrosion rate a
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