Passivation And Instability Behavior Of Super 13Cr Martensite Stainless Steel In O₂/CO₂ Environment

Super 13Cr martensitic stainless steel is widely used in oil field with CO₂ downhole environment due to its good mechanical properties,excellent CO₂ corrosion resistance and moderate price.With the application of new production technologies such as injection of multiple thermal fluid flooding,O₂ enters the wellbore environment and the corrosion environment of the oil tubing changes from pure CO₂ environment to O₂/CO₂ co-existence environment,leading to corrosion perforation or even fracture.At present,the passivation and instability behaviors of super 13Cr martensite stainless steel under O₂/CO₂environment are still lack of systematic and in-depth research,especially the passivation characteristics,pitting corrosion characteristics and cracking mechanism.In this study,electrochemical techniques such as potentiostatic polarization,Mott Schottky measurement and artificial pitting electrode were used,combined with slow strain rate tensile test,scanning electron microscope,energy spectrum analyzer,X-ray photoelectron spectroscopy and laser confocal microscopy,respectively,to study the effect of O₂ on the characteristics and evolution of passivation film,the mechanism of pitting growth dynamics,the stress corrosion cracking,the local instability and dissolution of the super 13Cr martensitic stainless steel under different conditions.The main conclusions are as follows:(1)In O₂/CO₂ room temperature environment,in the complete passivation stage of super13Cr martensitic stainless steel,the presence of O₂ increased the thickness of passivation film,enhanced the diffusion coefficient of point defects,reduced the donor concentration of point defects,and enhanced the stability of passivation film.In the incomplete passivation stage,O₂did not change the growth mode and growth rate of metastable pitting,but reduced the number of metastable pitting nucleation,shortened the metastable pitting life,correspondingly reduced the dynamic parameters and inhibited the occurrence and growth of metastable pitting.During the transition from metastable pitting to steady pitting,O₂ reduced the stable pitting product of metastable pitting,inhibited the transformation from metastable pitting to steady pitting.In the steady pitting stage,O₂ increased the limiting diffusion current density,weakened the repassivation capacity after the steady pitting occurs.(2)The addition of HCO₃^-reduced the number of corrosion active sites on the surface of super 13Cr martensitic stainless steel in Cl^-solution,and reduced the average peak value of metastable pitting current density,which had a significant inhibition on the nucleation and growth of metastable pitting.With the increase of HCO₃^-concentration,the passivation range became wider,the pitting potential shifted forward,and the steady-state pitting sensitivity decreased.The addition of HCO₃^-changed the semiconductor characteristics of passivation film from n type to n+p type.With the increase of HCO₃^-concentration in solution,the donor concentration and acceptor concentration in the passivation film decreased,and the stability of the passivation film improved.(3)Super 13Cr martensitic stainless steel had stress corrosion sensitivity in 10%O₂+90%CO₂and 50%O₂+50%CO₂environment.When the polarization potential of anode and cathode was applied,the sensitivity of stress corrosion increased with the increase of polarization potential.There were different stress corrosion cracking mechanisms at different applied potentials,which were anodic dissolution mechanism at 50 m V(vs.saturated Ag/Ag Cl,the same below)and 250 m V polarization potential,the anodic dissolution+hydrogen embrittlement mechanism at-300 m V and-475 m V polarization potential,and the hydrogen embrittlement mechanism at-800 m V and-1000 m V.(4)In high temperature(120?)and high CO₂ content(3MPa)environment,the addition of a small amount of O₂(<0.06MPa)increased the open circuit potential and pitting potential of super 13Cr,decreased the donor concentration,and enhanced the stability of the passivation film.With the increase of O₂ content(0.06MPa?1MPa),open circuit potential and pitting potential decreased,linear impedance decreased,and passivation film stability decreased.The high temperature Cr-Cl-H₂O-CO₂/O₂system E-p H diagram and X-ray photoelectron spectrum results both confirmed that O₂ did not change the phase composition of the passivated film(Cr₂O₃).When the O₂ content continued to increase(>1MPa),the local instability of super 13Cr was in the active dissolution state,the corrosion product film showed double-layer characteristics,with the outer membrane loose and many holes,and the main component was Fe oxide.The structure of the inner layer was dense,and it was mainly Cr oxide.(5)The surface of super 13Cr martensitic stainless steel was modified by laser melting.The cross section structure of super 13Cr martensitic stainless steel was divided into three layers:laser melting layer,transition layer and stainless steel matrix.The surface hardness was increased by 25%by laser melting.The increase of laser scanning speed improved the corrosion resistance of the melted layer in O₂/CO₂ environment.When the scanning speed was more than 10 mm/s,the pitting resistance of the fused layer was better than that of the matrix,and the transition layer always showed good corrosion resistance.