Corrosion Behavior Of Ultra-pure Ferritic Stainless Steel S44660 In Cl^- And H₂S Enviroment

In the harsh fields condition,the corrosion problem of gathering and transporting pipeline caused by Cl-and H2S are becoming increasingly prominent.Ultra-pure ferrite stainless steel S44660 is endowed with the features of excellent resistance to Cl-pitting and stress corrosion,so that it is supposed to be used under the complicated environment.However,Current research are focused on the corrosion environment of Cl-,while H2S and Cl--H2S coexist environments are scarcely mentioned.For further application in complex oil and gas fields with high Cl-and H2S,it is necessary to conduct sufficient experiment and theoretical research on S44660.Therefore,a comprehensive study of the S44660 corrosion behavior and regularity in Cl-and H2S environments is of great practical and theoretical value.In the present thesis,the corrosion behavior and mechanism of S44660 under Cl-,H2S and C--H2S co-existent environments were systematically investigated by electrochemical test,slow strain rate test,C-ring test and empirical electron theory of solid and molecules.The main contents and conclusiongs are summarized as follows:(1)Compared with 316L,2205,the corrosion resistance of S44660 under the simulated produced water of Luo Jia Zhai gas field which contains high concentrations of Cl-and H2S was speculated by immersion test,electrochemical test and slow tensile stress-strain test.The results showed the pitting corrosion resistance of S44660 was superior to 2205 and 316L,and the resistance to stress corrosion of S44660 was equivalent to 2205,but better than 316L.It is feasible for using S44660 in similar circumstances.(2)The electrochemical corrosion behavior and passivation properties of S44660 under 5%NaCl,saturated H2S,NACE standard solution(5%NaCl+saturated H2S+0.5%glacial acetic acid)were studied by electrochemical methods and SEM,XPS.The results indicated that the corrosion resistance of S44660 in NACE solution was the worst.Firstly,Ecorr,Eb of S44660 were the lowest while the Icorro and Ipass were the largest under NACE solution.Secondly,with the interaction of Cl-and S2-,the donor current density became larger,Moreover,a porous sulfide layer formed by the influence of H2S,The combination of the corrosion product layer and matrix was poor so that the protective properties was greatly weakened.(3)The effects of different concentrations of H2S and NaCl on the electrochemical corrosion behavior of S44660 were discussed by electrochemical test when Cl--H2S coexisted.In the low concentration of NaCl solution,as the synergy,increasing H2S would promote the corrosion of S44660.However,in high concentration of H2S solution,when Cl-increased to 2.0%.Cl-was preferentially combined with metal ions,which prevents S2-from participating the electrode reaction.Furthermore,it could premote the adsorption of oxygen on surface.In a result,the corrosion current dropped,the impedance increased,and corrosion was suppressed.(4)The stress corrosion behavior of S44660 in Cl-,H2S environment was analyzed by means of C-ring soaking and slow tensile strain test.After soaking in NACE solution for 720h,cracks were formed on the sample surface,but the cracks expansion was dispersed,it would not cause fracture.It indicates that S44660 had good resistance to stress corrosion in NACE solution.However,compared with NaCl condition,according to the results of slow tensile test,S44660 was more sensitivity to stress corrosion when it was in H2S and NACE,that was because that the fracture mode is ductile and brittle mixed fracture and the Is were more than 25%under H2S and NACE condition.(5)Calculation models of construction units in S44660 with or without H and the construction units in product films were established based on solid and molecular empirical electron theory(EET),corrosion mechanism in Cl-,H2S and Cl--H2S environment was explored using this Calculation models.According to the analysis results,in Cl-,H2S environment,the corrosion products on surface contained FeS,Cr2O3,Fe2O3.FeS has zigzag network structures composed by six covalent bonds with large nA values,which helped to resist the destruction of corrosive medium.In addition,for the interface between matrix and the passive film Cr2O3,Fe2O3,the higher content Cr,the smaller interfacial electron density difference ?? and the the more atomic configuration number a,which means the interface was more continuous,and tightly bond,so S44660 has excellent comprehensive corrosion resistance.Besides,in H2S,Cl--H2S medium,H atoms penetrated into S44660 matrix and caused the difference of bond energy on the different dissociation crystal surfaces.Cr,Mo,Ti,Nb could alleviate anisotropy,therefore,S44660 has good resistance to H2S stress corrosion.Moreover,the interfacial electron density between matrix and FeS was discontinuous and the interfacial stress was large,so it tended to occur the brittle fracture in H2S,Cl--H2S solution.