Abstract

Corrosion is a major concern in transmission pipelines that transport captured CO2. While dry CO2 is noncorrosive, significant corrosion has been reported in dense phase CO2 with trace amounts of water and impurities such as O2, H2S, SOx, and NOx. The aim of this work is to improve our understanding of the physicochemical aspects on the corrosion of carbon steels in the high-pressure environments associated with CO2 transmissions pipelines. The effect of flow on the corrosion of X65 carbon steel was investigated in a series of autoclave tests with different combinations of impurity concentrations in supercritical CO2 condition (8 MPa and 35°C). The corrosion rate of samples was determined by weight loss measurements. The surface morphology and the composition of the corrosion product layers were analyzed by using surface analytical techniques (SEM and EDS). Localized corrosion was measured via surface profilometry after corrosion products were removed. Results showed that no corrosion was observed in the supercritical CO2 with 650 ppm of water, 50 ppm SO2, and 100 ppm NO, but corrosion occurred when SO2 concentration was increased to 4500 ppm and 40,000 ppm of O2 was added to the system. The presence of flow significantly accelerated the corrosion of carbon steel. Furthermore, localized corrosion was observed in the presence of both O2 and flow.

Introduction

It is well known that dry CO2 does not corrode carbon steels, and negligible corrosion occurred at water-unsaturated conditions (below solubility level) in dense phase CO2 (liquid and supercritical).1-5 However, it has been reported that noticeable, and potentially severe, corrosion occurs at water-unsaturated conditions in dense phase CO2 with the presence of impurities, such as O2, H2S, SO2, NO2, etc. due to synergisms between chemical species.6-14

Hua, et al.,8 reported that general corrosion rates ranged from 0 to 0.012 mm/y with water contents varying from 300 ppm to 2800 ppm in the supercritical CO2 with 1000 ppm O2. They also found that the general corrosion rates of X65 steel increased from 0.01 mm/y to 0.06 mm/y with water concentration increasing from 300 ppm to 1770 ppm with 100 ppm SO2 and 20 ppm O2 at 35°C and 80 bar.9 Xu, et al.,10 reported that the general corrosion rates of X70 steel varied from 0.03 mm/y to 1.78 mm/y at relative humidities ranging from 45% to 100% at 10 MPa CO2 and 50°C with 2% SO2 and 1% O2. Dugstad, et al.,11 investigated the corrosion behavior of carbon steel exposed to liquid CO2 flow for 10 days at 10 MPa and 25°C. The results showed that there was no corrosion in the liquid CO2 flow with 500 ppmw H2O, while adding 500 ppmw SO2 induced corrosion at a rate of 0.02 mm/year, and adding 500 ppmw NO2 caused severe corrosion with rates reaching 1.6 mm/year. Choi, et al.,11 investigated the effect of H2S on the corrosion behavior of pipeline steels in high pressure CO2 systems. It was found that the general corrosion rates of the carbon steel and 1Cr steel tested were below 0.01 mm/year with the presence of 100 ppm H2O and 200 ppm H2S in the liquid (25°C, 12 MPa) and supercritical (80°C, 12 MPa) CO2 phases.

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