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The objective of the present study was to evaluate the corrosion properties of carbon steel in supercritical carbon dioxide (CO 2 )/brine mixtures related to the deep water oil production development. Corrosion tests were performed in 25 wt% sodium chloride (NaCl) solution under different CO 2 partial pressures (4,8, 12 MPa) and temperatures (65°C, 90°C). Corrosion behavior of carbon steel was evaluated using electrochemical methods (linear polarization resistance [LPR] and electrochemical impedance spectroscopy [EIS]), weight-loss measurements, and surface analytical techniques (scanning electron microscopy [SEM], energy-dispersive x-ray spectroscopy [EDS], x-ray diffraction [XRD], and infinite focus microscopy [IFM]). The corrosion rates measured at 65°C showed a high corrosion rate (~10 mm/y) and a slight difference with pressure. Under these conditions, the sample surface was locally covered by iron carbide (Fe 3 C), which is porous and non-protective. However, the corrosion rates measured at 90°C increased with time at the initial period of the test and decreased to a very low value (~0.05 mm/y) due to the formation of protective iron carbonate (FeCO 3 ) layer regardless the CO 2 partial pressure.

Depending on the capture process, liquid or supercritical carbon dioxide (CO 2) can contain impurities like water (H 2 O) and sulfur dioxide (SO 2), increasing the likelihood of corrosion of carbon steel pipelines even if there is no free water in the system. To understand the corrosion risk for such pipelines, the present study focused on the effects of CO 2 phase change, water, and SO 2. Different CO 2 phases (liquid and supercritical) and concentrations of SO 2 (<1%) were simulated in an autoclave-based study. The corrosion rate of steel samples was determined with weight-loss (WL) measurements. The surface morphology and the composition of the corrosion product layers were analyzed using surface analytical techniques (scanning electron microscopy [SEM], energy-dispersive spectroscopy [EDS], and infinite focus microscopy [IFM]). Results showed that the corrosion rate decreased with decreasing SO 2 content in the supercritical CO 2 phase containing 650 ppmv of water with no localized attack. In contrast, in high-pressure liquid CO 2 , significant localized corrosion was observed.

Top-of-the-line corrosion (TLC) has the potential to lead to rapid loss of pipeline integrity. TLC also presents unique challenges in relation to its mitigation. Amine compounds are traditionally used as corrosion inhibitors in packages for mitigating CO 2 corrosion of carbon steel. They are also claimed to be effective volatile corrosion inhibitors also the mechanism involved in less clear. The principal objective of this work is to investigate and understand the TLC inhibition mechanism in the presence of diethylamine (DEA) and morpholine. In order to determine possible interactions between the tested amines and the steel surface, the surface charge was investigated by determining the potential of zero charge (PZC). The PZC was measured by means of electrochemical impedance spectroscopy (EIS) in a 1 wt.% NaCl solution at different pH values. The possible inhibitive properties of DEA and morpholine were first tested at the bottom of the line conditions by linear polarization resistance (LPR) and EIS, followed by TLC tests. The weight loss method was used to measure the corrosion rate at the top-of-the-line. After the experiments, the steel surface was characterized by scanning electronic microscopy (SEM). The results were interpreted based on the surface charge and provided key information about the possible mechanisms of adsorption amines (DEA and morpholine) on the steel surface. Results showed that the effect of the selected amines on the corrosion rate could be directly linked to the change in solution pH, both at the top and bottom of the line. Considering that the amines were almost fully protonated in the range of pH tested, their vapor pressures and consequently their concentrations in the condensed water were also very low, limiting further their effect on the corrosion at the top of the line. In addition, DEA and morpholine did not seem to have significant filming properties.