In this work, the focus was set on the corrosion process of condensate as drops on the surface of carbon steels (X52, X70), martensitic steel UNS S41500, and superaustenite UNS N08031 in CO2 atmosphere with impurities at 278 K (to simulate the transportation condition in a buried pipeline). Exposure tests were performed at both normal pressure and high pressure where CO2 is supercritical or in dense phase. The drop, 1 – 10 µL in volume, was prepared by dropping CO2 saturated ultra-pure water onto the surface of steel coupons in a one-liter-autoclave. The CO2 gas stream, simulating the oxyfuel flue gas with varying concentration of impurities (SO2 and O2), was then pumped into the autoclave to observe the condensation and corrosion impacts of impurities. Comparable exposure tests were carried out with the same gas mixture and the same volume of water as vapor to observe the drop formation and the corrosion process that follows. The wettability and stability of drops on the surface of steel coupons in CO2 supercritical/dense phase environment was evaluated additionally by contact angle measurement.
CCUS (Carbon Dioxide Capture, Utilization and Storage) technology is considered as a potential means of mitigating the contribution of fossil fuel emissions to global warming. According to EU directive 2009/31/EG, a CO2 stream shall contain overwhelmingly CO2 but may “contain incidental associated substances from the source, capture or injection process and trace substance added to assist in monitoring and verifying CO2 migration”.1 Concentrations of these “incidental associated substances” (named as impurities later on) are depended on the emission sources, post process and cleaning/purifying technologies.1-3
Emitted CO2 has to be compressed into a supercritical state to prevent pressure drop and to be transported mostly by pipeline to a site suitable for storage. When the transportation pipelines are buried at about 5 meter depth, the temperature of the pipeline is about 278 K, which increases the chance of dew formation followed by droplet corrosion from CO2 stream containing SOx, NOx, O2 and water as impurities. To date, it has been shown that the CO2 transport pipelines are highly in risk when exposing to these corrosive impurities. At 10 MPa and in the temperature range from 278 to 298 K the dissolution of water into CO2 is from 1900 to 3200 ppmv. However, the corrosion rate is significantly increased by the water concentration of 500 ppmv.4,5 Ruhl et al. reported that at the concentration of 1000 ppmv H2O and 278 K, normal pressure, the reactions and condensation of SO2 and NO2 with O2 and H2O resulted in acidic drops (pH ~ 0,5) on the surface of metal coupons and vessel.6,7 Following this study, carbon steels (X52, X70), alloys UNS N08031 and UNS S41500 were evaluated in bulk synthetic electrolytes of varied chemistries relevant to those real condensates.8-10 These studies principally revealed the mechanism as well as the role of each gas impurity on the corrosion process. However, most of the gaseous tests have been performed under normal pressure or high pressure but with the temperature of more than 278 K. The CO2 saturated synthetic bulk solution is not so closed to the real phenomenon, where only a drop or thin film might be formed under the transportation condition.