Electrochemical Study Of Fe-Cr Allovs Under Carbon Dioxide Envir-Onment

In the exploitation,gathering and transportation processes of oil and gas field,casing and tubing are confronted with the severe corrosion environment.Particularly,the material may be in direct contact with carbon dioxide(CO2),hydrogen sulfide(H2S)and corrosive anions of produced-free water,which will inevitably cause various degrees of corrosion and huge economic losses,and even catastrophic failure.Simultaneously,Fe-Cr alloys are as a kind of material with strong corrosion resistance and it remains to be investigated whether the excellent corrosion resistance can be exhibited under the complex corrosion environments.In addition,the corrosion behaviors and mechanisms are worth researching.In this work,the corrosion behaviors of various Fe-Cr alloys were investigated by using weight loss and in-situ electrochemical measurements,such as open circuit potential(OCP),potentiodynamic polarization and electrochemical impedance spectroscopy(EIS).In addition,the morphologies and the compositions of corrosion products were characterized by the ex-situ surface analysis techniques,such as scanning electron microscopy(SEM)and energy disperse spectroscopy(EDS)in the high temperature and high pressure environments including CO2-Na2SO4 and CO2-H2S-Cl-.Ultimately,the electrochemical corrosion mechanisms of Fe-Cr alloys were systematically researched and the effects of different corrosion factors were demonstrated.The specific conclusions are as follows:(1)In the CO2-Na2SO4 system,the anodic polarization regions of 3Cr and 13Cr steels principally included two passivation zones and one transpassivation zone.The passivation zone 1 was mainly contributed by FeCr2O4,Cr(OH)3 and Cr2O3,while the major contribution of the passivation zone 2 stemmed from iron oxides and high-valence chromium oxides.Besides,the transpassivation region was mainly caused by the dissolution of various oxides.Both temperature and partial pressure of CO2 could affect the corrosion behaviors of Fe-Cr alloys by changing the properties of the corrosive electrolyte and/or corrosion products.Temperature would initially exacerbate and then inhibit the corrosion of 13Cr steel with the maximum of corrosion rate at 85?.However,the influence on the partial pressure of CO2 was antipodal and the maximum impedance value was obtained at 0.5 MPa.Furthermore,the passive film was more protective and the corrosion resistance was enhanced with the prolonging immersion time.(2)In the corrosion system of CO2-H2S-H2O-Cl-,L80-1 steel was dominated by activat,on corrosion,while the passivation characteristics was exhibited on 3Cr and other steels regardless of the ratios of CO2/H2S and the concentrations of Cl-ion.At the same time,the pr-mary and secondary orders of 5 corrosion factors for L80-1 steel were water content,partial pressure of CO2,partial pressure of H2S,temperature and concentration of chloride ion,successively.Moreover,the most severe corrosion combination was 1.5 MPa CO2,112S-absence,80?,100000 mg/L Cl-and 90%water content.L80-1 steel mainly displayed the uniform corrosion morphologies,but the local corrosion might occur when the water content was no exceeding 30%.The uniform corrosion products were mainly carbonates,whilst the sulfides would enhance the inhomogeneity of corrosion products and even induce local corrosion.(3)In the CO2-H2S-H2O-Cl-system,the corrosion of L80-1 steel was aggravated with increasing pCO2 and dominated by the cathodic process for CO2 corrosion.As H2S concentration increased,the corrosion was retarded previously and followed by accelerated.H2S corrosion was conjointly controlled by the anodic and cathodic processes.At higher concentration,an increasing concentration of Cl-ion could suppress the corrosion of L80-1 steel,which was also mainly a hybrid control of the anode and cathode.Remarkably,a small amount of carbon dioxide and tiny bits of hydrogen sulfide could act synergistically to palliate the corrosion of L80-1 steel in an acidic chloride environment.Moreover,the corrosion of L80-1 steel was dominated by CO2 corrosion when the concentration of H2S was inferior to 195 mg/L,whilst that was controlled by H2S corrosion exceeded 195 mg/L in the coexistence of CO2 and H2S.(4)Chromium would diminish the general corrosion tendency as well as the pitting susceptibility and heighten the passivation by the formation of iron oxides and chromium oxides whether in CO2-Na2SO4 or in CO2-H2S-H2O-Cl-system.Besides,when chromium content surpassed 3 wt.%,the spontaneous passivation would occur and the corrosion rate was reduced dramatically in CO2-H2S-H2O-Cl-system.However,the chromium effect would be affected by the temperature in CO2-Na2SO4 system.In addition,the intricate influences on temperature,partial pressures of CO2 or H2S and chromium content,remain to the further research.