Corrosion tests were carried out to study the corrosive behaviour of medium carbon steels with a ferrite–pearlite microstructure during exposure to a synthetic flowback facturing fluid saturated with carbon dioxide. Metallographic investigations, screening tests, surface morphology investigations and identification of surface products were used for this purpose. The results obtained indicate that the corrosion of N80‐1 carbon steel was significantly lower than that of K55 carbon steel. The layer of corrosion products consisted of an outer layer of carbonates and an inner layer with cementite lamellas. Higher grain refinement of N80‐1 steel compared to that of K55 steel caused the formation of a denser, more homogenous layer of corrosion products, which significantly reduced the corrosion of the base material.