This work was undertaken to investigate the pitting corrosion mechanism of pipeline steel in an impure supercritical CO2 environment by executing experiments with different exposure times and varied rotation speeds. The results revealed that in a water-saturated supercritical CO2-SO2-NO2-O-2 environment, the average maximum pitting depth stabilized after three days, which indicated that pits were unstable and cannot rapidly propagate with time while also implying that a local acidification process might not exist. The product accumulation in the pit cavities can eventually prevent the diffusion of the corrosive media and water from the product surface to the metal substrate. The effect of flow on the propagation of pits was marginal, revealing the weak influence of flow on the mass transfer of species through the product scales. The formation of spherical products was possibly attributed to the influence of NO2 on the contact angle of water droplets on the steel surface.