The Study On Corrosion Behavior Of L245NCS Carbon Steel And316L Stainless Steel In CO₂/H₂S Environment

In this study, the corrosion performance of L245NCS and316L were studied toinvestigate the corrosion of carbon steel and stainless steel in CO₂/H₂S. The corrosion rateof the two metals under different circumstances （CO₂to H₂S partial pressure ratio,temperature, Cl-concentration and flow rate） were evaluated by means of weight lossmeasurement. The electrochemical behaviors of the two metals were investigated in hightemperature autoclave by electrochemical techniques in situ. Moreover, scanning electronmicroscopy （SEM） and EDS were also employed to investigate the corrosion morphologiesand structures as well as the corrosion mechanisms. Results showed the corrosion rate ofL245NCS in vapour is lower than that in liquid phase. The regression equation of thecorrosion rate from uniform design was: Rcorr=0.051+0.951PCO₂+1.315PH₂S-0.299CCl-0.3463T+0.0753V or Rcorr=0.586-0.2828PCO₂/H₂S-0.0057CCl+0.0613T-0.2491T2+0.0935V,which meant that the H₂S partial pressure had the most significant impact on the corrosionrate: The corrosion rate in liquid phase increased with the increasing partial pressure of H₂S,followed by the impact of CO₂partial pressure: the corrosion rate in liquid phase increasedwith the increasing partial pressure of CO₂. The corrosion rate in liquid phase decreasedwith increasing the temperature and showed the maximum value between60to100oC.Corrosion rate increased with increasing the rotating speed. Water vapor pressure increasedwhen temperature was elevated to100oC. More condensate on the sample surface resultedin an increased corrosion rate. The corrosion products mainly consisted of FeCO₃and Fe₃Cafter corrosion of L245NCS. With the increasing partial pressure ratio of H₂S/CO₂, thecorrosion products mainly consisted of sulfide and corrosion process was controlled by H₂S.EIS results showed the corrosion rate increased at the initial stage of the corrosion process.Ferrite of the L245NCS dissolved as the anode and Fe₃C remained on the surface andaccelerates the cathodic reaction. As the corrosion proceeded, the corrosion products tosome extent could protect the substrate from corrosion. EIS of L245NCS at high H₂S/CO₂partial pressure ratio and temperature showed two capacitance loops in which the highfrequency loop was related to the double-layer capacitance and charge transfer resistance, the low frequency loop was associated with the corrosion product film. As corrosionproceeded, the corrosion product film resistance increased and corrosion rate decreased.High flow rate was not beneficial for the formation of the protective corrosion product film.The316L showed a corrosion rate of two orders of magnitude lower than that of L245CNS.A denser and protective Cr containing film on the surface could protect the substrate. Thefilm resistance increased and corrosion rate decreased in which the corrosion processshowed a diffusion control.