Chapter 4 - Carbon dioxide (CO2) corrosion

Abstract

This chapter provides general information about carbon dioxide including physical properties and corrosiveness. The main focus of this case study is on the carbon dioxide corrosion on carbon and low alloy steels. In industrial applications, CO2 or sweet corrosion of carbon and low-alloy steels is a well-known problem that was first observed in the US oil and gas industry. Thus, it is important to understand the mechanism of CO2 corrosion. Carbon dioxide forms carbonic acid from reaction with water. Various corrosion calculation models have been developed by research organizations and oil companies. Although CO2 corrosion is complex, and therefore, no completely accurate model exists for its prediction, a carbon dioxide prediction model is created and proposed in this chapter. The main assumption in the proposed model and other models developed to address CO2 is that CO2 is considered a cause of uniform corrosion. This section discusses selection of corrosion allowance as an additional thickness for carbon and low alloy steels to mitigate carbon dioxide corrosion. To calculate the corrosion allowance, process parameters such as the operating pressure and temperature, pH of the fluid, design life of the valve, and information about glycol and corrosion inhibitors should be taken into account. The NORSOK standard proposes standardizing the corrosion allowance for carbon steel piping and valves to 3mm. A corrosion allowance of 6mm on carbon steel should be avoided as it would increase the piping and valve wall thickness and make them heavy. If the calculated final corrosion rate exceeds 3mm in offshore or 6mm in onshore units, then various solutions can apply like using corrosion-resistant alloys (CRAs) or injection of corrosion inhibitors.