Study On The Corrosion Resistance Mechanism Of Titanium Alloy Tubes In Severe Environment With H₂S/CO₂

Based on domestic and international investigation of research and application of titanium alloy pipe,the corrosion resistance of Ti6Al4 V titanium alloy(hereinafter referred to as TC4)was studied in severe environment by corrosion tests.Combined with electrochemical test,the anticorrosion mechanism was analyzed.The results show TC4 has good resistance to uniform corrosion,pitting and crevice corrosion in low pH environment.In the high temperature and high pH completion fluid(into the CO2 gas),the stability of passivation film of TC4 is relatively inferior,and the corrosion rate is as high as 0.4247 mm/a,while TC4 has good resistance to SCC in severe environment.The diffusion of hydrogen in TC4 is an irreversible process,and the diffusion is the reaction control step in the whole electrode reaction.In the acidic and alkaline corrosion environment,the diffusion coefficients of hydrogen in TC4 are3.4377×10-14 cm2/s and 2.5703×10-7 cm2/s respectively,and the diffusion coefficient in the alkaline corrosion environment is larger,so corrosion resistance is relatively poor.The anodic polarization curves of TC4 has obvious passivation zone in neutral,CO2,H2 S,H2S/CO2,CO2/O2 and high pH environment,the corrosion reaction is controlled by the anodic reaction process,and the corrosion rate is kept at the low level,while the corrosion current density of TC4 is relatively higher in alkaline environment.The polarization curves in the positive scan and in the opposite scan intersect at the anodic polarization zone of TC4 in neutral,CO2 and H2 S corrosion environments,and the protective potential Ep and pitting potential Eb are higher,so TC4 has the good passivation and re-passivation performance.The polarization resistance of TC4 is very large in neutral,CO2,H2 S,H2S/CO2,which gives TC4 the good corrosion resistance.While in the alkaline environment,the polarization resistance is relatively small,and corrosion resistance is relatively poor.The passivation film of TC4 is n-type semiconductor in CO2,H2 S and H2S/CO2 environment,and the donor concentration of the passivation film increases with the increasing of temperature,thus the ion diffusion speed increases and the corrosion resistance of TC4 decreases.