CORROSION SCIENCE SECTION 598 CORROSION—JULY 2006 Submitted for publication June 2005; in revised form, December 2005. Part 1 of this manuscript appears in CORROSION 62, 5 (2006), p. 419-432. ‡ Corresponding author. E-mail: v.ruzic@uq.edu.au. * Department of Mechanical Engineering, The University of Queensland, Brisbane Queensland 4072, Australia. ** Institute for Corrosion and Multiphase Flow Technology, Chemi- cal Engineering Department, Ohio University, 340 ½ W. State St., Stocker Center, Athens, OH 45701. Protective Iron Carbonate Films— Part 2: Chemical Removal by Dissolution in Single-Phase Aqueous Flow V. Ruzic, ‡, * M. Veidt,* and S. Nes ˘ic ´** ABSTRACT The dissolution of deposited, protective iron carbonate films in oil and gas transportation pipelines may drastically enhance corrosive processes on steel surfaces, and thus, seriously affect the longevity of the equipment in use. An investiga- tion had been carried out to get a better understanding of the kinetics and underlying mechanism of film dissolution, as well as to provide baseline data for further study of the possible synergistic effect between chemical and mechanical film removal. To address this goal, a series of iron carbonate film dissolution experiments was conducted covering a wide range of pH values (pH 3 to pH 6.1) and Reynolds numbers (Re = 3.68 × 10 3 to 1.84 × 10 5 ) using a rotating cylinder con- figuration. The polarization resistance technique was used for implicit quantification of film removal kinetics via corrosion rate monitoring, whereas scanning electron microscopy was utilized for the purpose of residual film characterization. The results suggested that chemical film dissolution was governed by mass transfer and showed the strong dependence of film removal kinetics on the level of saturation in the solution (pH value) and fluid velocity. In addition, the physical mechanism of chemical film removal has been explained and discussed in light of the obtained results. KEY WORDS: carbon dioxide corrosion, film dissolution, mass-transfer control, protective iron carbonate film, rotating cylinder electrode, turbulent single-phase flow INTRODUCTION Iron carbonate (FeCO 3 ) films formed on mild steel equipment, encountered in the oil and gas production and transportation industry, generally provide satis- factory protection from potentially destructive acts of internal pipeline corrosion. 1-2 It is commonly known that they can be damaged chemically by dissolution and/or mechanically by hydrodynamic forces. While the mechanical removal of FeCO 3 scales has been previously reported by Ruzic, et al., 3 virtually no infor- mation is available in the open literature on the disso- lution of these films. The FeCO 3 dissolution/precipitation reaction can be written as: FeCO s Fe CO 3 2 3 2 () – ⇔ + + (1) When the forward rate is equal to the backward rate, the FeCO 3 film is in equilibrium with the species in solution. Hence, supersaturation (S) is defined as: S Fe CO K sp = + [ ][ ] – 2 3 2 (2) where [ ] denotes concentration of the species and K sp is the solubility product (equilibrium constant). For equilibrium (saturation), S = 1. It is known that the 0010-9312/06/000115/$5.00+$0.50/0 © 2006, NACE International