The effect of acetic acid on the CO 2 corrosion of grade X70 steel Mehdi Honarvar Nazari, Saeed Reza Allahkaram * Center of Excellence in High Performance Ultra Fine Materials, School of Metallurgy and Materials Engineering, University College of Engineering, University of Tehran, North Kargar, Tehran 11155-4563, Iran article info Article history: Received 11 January 2010 Accepted 2 April 2010 Available online 10 April 2010 Keywords: E. Corrosion G. X-ray analysis G. Scanning electron microscopy abstract The effect of acetic acid (HAc) on the CO 2 corrosion of grade X70 steel was investigated using X-ray dif- fraction (XRD), scanning electron microscopy (SEM), polarization tests and electrochemical impedance spectroscopy (EIS). In the absence of acetic acid, a fairly dense layer of iron carbonate (FeCO 3 /siderite) was formed. At 500 ppm HAc, FeCO 3 layer became more porous. In addition, anodic/cathodic polarization curves were activated with the more pronounced effect on the cathodic side. By adding 1000 ppm HAc, similar polarization behavior was obtained and FeCO 3 layer became yet more porous than previous con- ditions. At 2000 ppm HAc, FeCO 3 layer disappeared completely, while polarization behavior changed and the limiting diffusive current density was observed in the cathodic side. There were two major increases in the corrosion rate at 500 and 2000 ppm HAc. The EIS results reflected similar behavior for the speci- mens exposed to the solutions with 0–1000 ppm HAc. Under these conditions, a charge transfer con- trolled behavior due to the FeCO 3 layer was observed which was accelerated by increasing HAc concentration. At 2000 ppm HAc, the corrosion behavior changed considerably and the formation/ adsorption of corrosion product followed by the dissolution process was observed. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction The effect of acetic acid (HAc) on the corrosion rate of carbon steel in most of the oil and gas fields containing brine and CO 2 has been studied [1–5]. Acetic acid is the most common organic acid in multiphase systems containing brine. CO 2 corrosion in the presence of HAc is known as a major reason for premature failures in oil and gas pipelines, which are usually made of carbon steel [1,6]. In 1983, Crolet and Bonis [7] reported that the presence of HAc in brine will significantly increase the corrosion rate. He and his co- workers [8] suggested the direct reduction of HAc in the surface of steel. Hedges and McVeigh [9] reported that HAc dissociated incompletely, which could cause a reserve of H + ions more than the value defined using solution pH (log[H + ]). Oblonsky et al. [10] found solid Fe 3 O 4 and dissolved Fe 2+ as the corrosion products in the corrosion of Fe in the solution saturated with argon in the presence of acetate ion (in the absence of CO 2 corrosion). They found that acetate concentration had not any ef- fect on the chemical analysis of corrosion product layer. Joosten et al. [11] saw an increase in corrosion rate in the presence of HAc due to decreasing of pH. Garsany and co-workers [5,12,13] emphasized that due to the fast dissociation of HAc, it is impossible to distinguish the reduction of HAc from that of H + . Nafday and Nešic ´ [14] reported that HAc cannot cause any localized corrosion, has not any effect on the thickness of corrosion product iron carbonate (FeCO 3 /siderite) layer but affects layer mor- phology. However, Okafor and Nešic ´ [15] reported that acetic acid can cause localized corrosion by removing iron carbonate layer. George and Nešic ´ [6] reported that the presence of HAc strongly af- fects the cathodic limiting current. The anodic reaction (iron disso- lution) was unaffected or mildly retarded with increasing HAc concentration at room temperature. Gulbrandsen [3] reported undermining of corrosion product FeCO 3 layer in the presence of HAc. Liu et al. [16] investigated the effect of HAc using electrochem- ical impedance spectroscopy (EIS). They found that the surface chemical reactions of cathodic reduction were enhanced in the presence of HAc. They also found that HAc can remove FeCO 3 layer. Zhang and Cheng [17,18] reported similar results. In addition, they observed an increase in the current density of anodic reactions and they saw localized corrosion on the surface of steel. As it is seen, only Oblonsky et al. [10] investigated the chemical analysis of corrosion product layer in the presence of acetic acid and in the absence of dissolved CO 2 systematically. The important point is that in the CO 2 corrosion, under a special condition a pro- tective corrosion product layer can form on the surface of steel. Therefore, there seems to be a gap in the investigation of chemical analysis of corrosion product layer in CO 2 corrosion of carbon steels in the presence of acetic acid. In addition, most of the re- searches have focused on the effect of acetic acid or acetate on 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.04.002 * Corresponding author. Tel./fax: +98 2161114108. E-mail address: akaram@ut.ac.ir (S.R. Allahkaram). Materials and Design 31 (2010) 4290–4295 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes