Effect of Flow and Steel Microstructure on the Formation of Iron Carbonate M. Di Bonaventura, 1 B. Brown, M. Singer and S. Nesic Institute for Corrosion and Multiphase Technology Department of Chemical and Biomolecular Engineering Ohio University, Athens, OH 45701, USA ABSTRACT Iron carbonate (FeCO 3 ) is a protective layer that can form on the surface of the steel as a by-product of CO 2 corrosion. This layer acts as a barrier, thus slowing down further corrosion. Temperature, CO 2 partial pressure and pH are the main environmental parameters controlling FeCO 3 properties. However, the combined effects of flow and material microstructure on the formation of FeCO 3 have not been well documented. In this research, two materials were used to determine the effect of microstructure on the formation of iron carbonate: an annealed low alloy carbon steel (0.05 wt.% C), formally API (†) 5L X65, with ferrite and iron carbide (Fe 3 C) precipitates microstructure and UNS (‡) G10180 (0.18 wt.% C) with either a ferritic-pearlitic or a tempered martensitic microstructure. FeCO 3 formation and retention of Fe 3 C on the steel surface were investigated in experiments in a three-electrode glass cell under controlled water chemistry conditions. Experiments were performed using two different flow rotational speeds and lasted five days. SEM, EDS and XRD analyses of specimen surface after exposure provided corrosion product characterization. The experimental results clearly indicated an existence of a critical shear stress, above which FeCO 3 could not nucleate on the steel surface. In addition, the steel microstructure, rather the carbon content, had a strong effect on the results with the ferritic-pearlitic steel clearly favoring FeCO 3 precipitation. Key words: CO 2 corrosion, flow, iron carbide, iron carbonate, microstructure, shear stress INTRODUCTION FeCO 3 is the commonest corrosion product that can form on the surface of mild steel as a by-product of the CO 2 corrosion process. This FeCO 3 layer slows down further corrosion by acting as a diffusion barrier, preventing corrosive species from reaching the steel surface and by blocking the steel surface (1) Current affiliation: BP America, 501 Westlake Park Blvd, Houston, TX 77079, USA (†) American Petroleum Institute (API), 1220 L St. NW, Washington, DC 20005. (‡) Unified Numbering System for Metals and Alloys (UNS). UNS numbers are listed in Metals & Alloys in the Unified Numbering System, 10th ed. (Warrendale, PA: SAE International and West Conshohocken, PA: ASTM International, 2004). 1 Paper No. 11179 ©2018 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACE International, Publications Division, 15835 Park Ten Place, Houston, Texas 77084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.