The carbon dioxide corrosion electrochemistry of mild steel has been studied in the presence of high CO2 partial pressures and acetic acid (HAc). Potentiodynamic sweeps, linear polarization resistance (LPR) and weight loss (WL) experiments have been conducted to investigate the effects of flow velocity, CO2 partial pressure, and acetic acid concentration on the corrosion rate of mild steel. Electrochemical impedance spectroscopy measurements have also been conducted to help elucidate the corrosion mechanisms under the test conditions.
Due to the high cost and poor repeatability, very little experimental work on CO2 corrosion at high pressures and temperatures has been performed in large-scale flow facilities1. No systematic electrochemical studies at such conditions have ever been done to shed light on the corrosion mechanism using potentiodynamic sweeps and/or transient techniques such as EIS. As a result, there is a lack of reliable electrochemical data and little understanding of the mechanism of CO2 corrosion at high pressure and temperature. Therefore most of the mechanistic and semi-empirical models presently used in industry for predicting CO2 corrosion are based on low-pressure (typically glass cell) experiments and extrapolate to the high pressure and temperature conditions found in the field.
The present electrochemical study of CO2 corrosion of mild steel at high CO2 partial pressures and in the presence of HAc has been completed to fill in the gap and focuses on investigating the main parameters that affect the corrosion rate such as: flow velocity, CO2 pressure and HAc concentration. Using the experimental data which were generated, an electrochemical model,1,2 based predominantly on low-pressure glass cell work, has been tested and calibrated for corrosion rate prediction under these test conditions. The comparison between the electrochemical model prediction and experimental data is presented separately.3 Furthermore, the experimental results have been also used to calibrate the advanced mechanistic corrosion model4,5,6 built into the new 2003 version of Ohio University Corrosion in Multiphase Flow V3.0 software package.7
EXPERIMENTAL SETUP AND PROCEDURE
The experiments were carried out under in a 0.106 m I.D. inclinable stainless steel multiphase flow loop. A schematic sketch of the system is shown in Figure 1. To begin an experiment, the system was filled with 1515 L of de-ionized water containing 1% (mass) sodium chloride. The solution was deaerated by purging CO2 gas through the solution and flashing (for approximately 3 days) until the concentration of dissolved oxygen in the system was measured to be below 20 ppb. Experiments were conducted at three CO2 partial pressures: 3 bar, 10 bar and 20 bar. The solution pH was maintained at 5.00 ± 0.05 using sodium bicarbonate and hydrochloric acid in all of the experimental work. For each CO2 partial pressure, three water velocities of 0.2 m/s, 1 m/s, and 2 m/s were tested (single-phase flow) and the temperature of the test section was maintained at 60 ± 1°C. In selected experiments, at 10 bar CO2 partial pressure, the effect of HAc was investigated, by varying the total HAc concentration from 0 to 1000 ppm.
The corrosion rate and mechanisms were studied using the electrochemical techniques of liner polarization resistance (LPR), potentiodynamic sweep and electrochemical impedance spectroscopy (EIS). Due to the configuration of the flow loop, two electrochemical probes could be inserted in the loop at the same time. This allowed for the measurement of a potentiodynamic sweep and an EIS scan to be conducted simultaneously and each sweep was done at least twice to ensure reproducibil