Models for Calculating Corrosion Rates in Water-Saturated and Under Saturated CO2 Systems & Water Solubility in CO2 Systems at Supercritical Conditions

Accurately predicting internal corrosion rates in high-pressure CO₂ environments is critical to the design and operation of pipelines used for Carbon Capture and Storage (CCS) systems. In water-containing dense phase CO₂ environments, corrosion rate increases in conjunction with humidity, typically reaching a critical humidity, beyond which corrosion rates become particularly high. The presence of impurities in the CO₂ stream influence the solubility of water and hence the humidity and corrosivity of the CO₂ stream. If the CO₂ stream is under saturated and below a particular humidity level, then there can be significant cost saving on CCS projects, as this enables carbon steel pipelines to be implemented with confidence. In this work, we discuss the development of a model to calculate corrosion rates in CO₂ systems, both pure as well as those containing impurities, with under-saturated water content at supercritical conditions. For CO₂ systems with water-saturated conditions, we present corrosion rates calculated from existing in-house models. We also present experimental data used in validating the corrosion rates calculated from these models.

The model developed to calculate corrosion rates at under-saturated water conditions uses the solubility of water in the CO₂ rich phase at saturated conditions. A modified Peng-Robinson equation of state, E-PPR78, available in literature is modified to match water solubility in the CO₂ phase for CO₂ systems in the presence and absence of impurities at both supercritical conditions and on both sides of the two phase-region. We present results from the newly developed model, validating calculated water solubility in the CO₂ rich phase against experimental data available in literature over a wide range of pressures and temperatures. For pure CO₂ with water system, the error in the calculated water solubility in CO₂ rich phase is less than 4% in sub-critical region and is less than 7% in the super-critical region.