Carbon capture and sequestration (CCS) is gaining greater importance in the industry transition to meet climate goals reducing carbon intensity. As the source of CO2 captured for sequestration widens to many more applications from oil and gas productions, power plants, refineries, chemical plants, steel manufacturing and other industries, the composition of the captured gas stream for sequestration sees various components. Some of the common impurities particularly from combustion processes are carbon monoxide, oxygen, SOx, NOx and others. Understanding the effect of presence of the various chemical species on material limits and maximum tolerance limits are crucial for safe operations of the assets. In one of the recent projects, there was a presence of CO in the CO2 stream, and it was imperative to identify the effect of this on the existing carbon steel pipeline. The pipeline was of X65 steel and fabricated for natural gas transmission. Wet CO-CO2 cracking is a reported degradation mode particularly as the CO concentration increases beyond 200 ppm in the gas phase or at partial pressures above 0.3 bar. This study is a combination of literature review of current state of art with respect to CO-CO2 cracking in carbon steel in presence of impurities and laboratory testing of welded X65 specimens exposed to CO concentration in dense (liquid) phase up to 1000 ppm and also in the presence of small concentrations of oxygen that also replicates some of the expected conditions in the project. The tests clearly indicate that the risk of CO-CO2 cracking is not significant up to the concentrations tested.