Highlights • CO2 with various combinations and concentrations of potentially reactive impurities (H2O, NO2, SO2, H2S, and O2). • Certain impurity combinations are none reactive, while other combinations resulted in formation of corrosive phases. • The concentration limit for each impurity should be below 20 ppmv if NO2, SO2, H2S, O2 are present together (100 bar/25°C). • The limits may be different at other temperatures and pressures.

Abstract Material choice has a large impact on the total cost of the CO2 transport system for carbon capture and storage (CCS). Cost-considerations make carbon steel the preferred candidate for long pipelines. As carbon steel will corrode if aqueous phases are present, it is important to control the CO2 composition and operate the system such that formation of water containing phases is avoided. There are many suggested specifications and recommendations for the type and concentration of impurities to be allowed in the CO2 stream. The impurity limits were often set from a health, safety, and environmental point of view and are due to lack of knowledge not so much based on material integrity issues. This gap of knowledge makes it difficult to define a specification that will ensure safe operation and long-term integrity. The present paper summarises the results of a large research project that systematically tested CO2 with various combinations and concentrations of potentially reactive impurities (H2O, NO2, SO2, H2S, O2). It was clearly shown that many impurity combinations were basically inert, while other resulted in chemical reactions and some combinations even resulted in the formation of a separate aqueous phase that contained high concentrations of sulfuric and nitric acid as well as elemental sulphur. This aqueous phase was corrosive to carbon steel. Corrosion was also observed in certain situations even when a separate aqueous phase was not observed visually. It is important to avoid precipitation of solid products since it may cause problems at the injection point and in the reservoir. The present work demonstrated that the types and combination of impurities that are present are important for the maximum impurity concentration that can be allowed before chemical reactions and corrosion occur. For the investigated conditions, 100 bar and 25 °C, the concentration limit for each impurity should be below 20 ppmv if NO2, SO2, H2S, O2 are present together. The limits may be different at other temperatures and pressures.