Assessment and influence of temperature, NaCl and H2S on CO2 corrosion behavior of different microstructures of API 5L X52 carbon steel in aqueous environments

In this study the effect of four types of microstructures, including annealed, normalized, normalized-tempered and quench-tempered on CO2 corrosion behavior of API 5L X52 steel were investigated. The CO2 corrosion behavior of these microstructures in different temperatures, NaCl salt concentration and H2S gas concentration were studied by employing electrochemical impedance spectroscopy and polarization techniques. The chemical composition of corrosion product and surface morphology were analyzed by the use of scanning electron microscopy equipped with energy dispersive spectroscopy. It was concluded that the CO2 corrosion behavior of the investigated material is dependent on its microstructure. It was observed that at low temperatures of 40 and 60 degrees C where the kinetics of formation of corrosion product is limited, the effect of micro galvanic cell between pre-eutectoid ferrite and cementite in pearlite phase is dominant thus normalized-tempered and quench-tempered microstructures which have more pearlite phase shows enhanced corrosion resistance. The influence of NaCl salt concentration on corrosion behavior of different microstructures was not significant. Furthermore, the presence of 100 ppm H2S concentration resulted in formation of FeS as corrosion product which inhibited the corrosion rate. The normalized-tempered and quench-tempered microstructures showed superior corrosion resistance in this state. However, at 200 ppm H2S concentration an increase of corrosion rate was observed due to occurrence of defects in corrosion product.