ABSTRACT

Corrosion experiments were performed on X65 carbon steel in water-saturated supercritical CO2 (SC-CO2) and under-saturated SC-CO2 conditions at 80 bar and 35°C in the absence and presence of various combinations of NO2 and O2 ranging from 0-100 ppm and 0-1000 ppm, respectively. The purpose of the experiments was to understand the implications of the presence of these two species on both general and localized corrosion in likely conditions encountered during CO2 transport.

The presence of 100 ppm NO2 and 1000 ppm O2 in water-saturated SC-CO2 resulted in general corrosion rates reaching 0.3 mm/year after 48 hours and a localized corrosion rate of 6.8 mm/year, compared to 0.1 and 0.92 mm/year, respectively in the absence of both species. The final stages of testing considered corrosion rates in under-saturated SC-CO2 with 100 ppm NO2 and 1000 ppm O2 at water contents between 0 and 1770 ppm. Although no corrosion occurred in dry conditions, increasing water concentration from 300 to 1770 ppm resulted in the general corrosion rates rising from 0.05 to 0.68 mm/year. However, there was no significant difference in the extent of localized corrosion observed with changing water content over this range, with values ranging between 0.5 and 0.6 mm/year.

INTRODUCTION

The corrosion in carbon steel pipelines transporting anthropogenic CO2 from large sources has received considerable attention from many researchers.1, 2, 3, 4, 5, 6 The possible presence of contaminants such as O2, SOx and NOx in the CO2 stream can influence and potentially accelerate the corrosion process.1, 2, 3, 7, 8, 9, 10, 11, 12 Although carbon steels is a common and cost-effective pipeline material, it is highly susceptible to attack by corrosive impurities such as H2O, CO2, SO2 and O2.

With reference to literature within this subject area, most research efforts have been directed towards understanding the corrosion behaviour of X65 in dense phase CO2 containing combinations of water, SO2 or O2 as impurities. Studies considering the effect of NO2 individually or combined with O2 on the corrosion behaviour of carbon steel in dense phase environments are rare and results have only been considered by a limited number of researchers.2, 9, 13 The purpose of this work is to determine the susceptibility of UNS G15130 steels to water-containing supercritical CO2 environments in the presence of NO2 (50 or 100 ppm) and O2 (1000 ppm) by measuring the general corrosion rates through the implementation of the weight loss method and surface profilometry to quantify the localized corrosion behaviour. Focus is directed towards three key areas; synergistic effects of NO2 and O2 in water-saturated conditions, the effects of solution replenishment on the corrosion rates determined in autoclave experiments, and the influence of water content in undersaturated conditions. A combination of Scanning Electron Microscopy (SEM), Raman spectroscopy and X-Ray Diffraction (XRD) is also implemented to understand the morphological and compositional changes in the corrosion products produced in the presence of NO2 and O2.

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