Corrosion Behavrior Of N80 Carbon Steel Under The Synergistic Effect Of Stress And Crevice

Downhole tubings are connected by casing collar through thread in the exploitation of oil and gas.However,the aggressive medium transported in the tubings could enter the crevice in the connected thread and cause crevice corrosion.Meanwhile,the thread bears great stress produced by the weight of tubings,which could result in stress corrosion cracking?SCC?of the tubings.Therefore,the coexistence of crevice and stress would cause serious corrosion of the thread in the tubings and casing collar.Once the sealing surface was destroyed by the corrosion,leaking would be caused at casing collar.Due to corrosion,the thread would fail to bear huge stress,resulting in serious accident.Therefore,it is of great significance to study the corrosion mechanism of casing collar under stress and crevice.In this work,weight loss,electrochemical measurements and surface analysis were used to investigate the crevice corrosion behavior of N80 carbon steel in CO2-saturated NaCl-HAc solution.The influencial factors of crevice corrosion,the inhibition effect of imidazolone inhibitor on the crevice corrosion of N80 carbon steel,as well as the corrosion behavior of N80 carbon steel under stress and crevice were also determined.1.A setup was designed for in situ electrchemcial measurements inside and outside crevice.It is demonstrated that crevice corrosion of N80 carbon steel could be initiated in the solution containing HAc,but not in the solution without HAc.The crevice corrosion of N80 carbon steel is triggered by the galvanic corrosion effect between the parts of steel inside and outside crevice.The cathodic reactions inside crevice are inhibited due to the decrease of the cathodic reaction species concentration,which results in the negative shift of the potential of the steel inside crevice,and then a galvanic effect is built between the parts of steel inside and outside crevice.The steel outside crevice is hardly corroded,while the steel inside crevice suffers from severe corrosion.Furthermore,deep corrosion groove is formed at crevice mouth.The galvanic effect between the parts of steel inside and outside crevice is enhanced with increasing concentration of HAc and decreasing opening dimension of crevice,and then the crevice corrosion is promoted.2.The effect of concentration difference of undissociated HAc and H⁺ inside and outside crevice on the crevice corrosion was studied by in situ electrochemical measurements.It is demonstrated that the crevice corrosion can be initiated by the concentration difference of either undissociated HAc or H⁺ inside and outside crevice.The equilibrium potential of HAc reduction is more positive than that of H⁺.Therefore,the reduction of HAc is thermodynamically dominant cathodic reaction in the solution containing HAc.The presence of HAc could cause larger galvanic effect and then severer crevice corrosion.OHparticipates in the anodic dissolution of Fe.Therefore,the anodic reaction inside crevice is promoted due to the increasing OH-concentration inside crevice.The corrosion rate of the steel inside crevice is higher than that outside crevice under coupled condition.The sum corrosion rate of the steels inside and outside crevice is smaller when they are connected together than the sum corrosion rate of the steels inside and outside crevice when they are not connected.3.The inhibition effect of imidazolone inhibitor on the crevice corrosion of N80 carbon steel in the solution with different HAc concentrations was studied when the imidazolone inhibitor was added at different times.It is demonstrated that the imidazolone inhibitor belongs to anodic type inhibitor in the solution containing 1000 mg/L HAc and it belongs to cathodic type inhibitor in the solution containing 3000 mg/L HAc.The inhibitor can diffuse into crevice and substantially increase the reaction resistance.Therefore,the crevice corrosion can be inhibited in the solution containing 1000 mg/L HAc or 3000 mg/L HAc when the inhibitor is added at the beginning.However,the diffusion of inhibitor into crevice is hindered due to the accumlated corrosion product at crevice mouth when the inhibitor is added after corrosion for a period of time.The potential difference between the steels inside and outside crevice increases,resulting in severer crevice corrosion in the solution with 1000 mg/L HAc.However,the potential difference between the steels inside and outside crevice decreases and then the crevice corrosion is inhibited in the solution with 3000 mg/L HAc,but the inhibition effecticency of inhibitor decreases significantly in the solution with 3000 mg/L HAc.A cathodic inhibitor has a better inhibition effect than an anodic inhibitor on the crevice corrosion of N80 carbon steel in the CO₂-saturated NaCl-HAc solution solution.4.An electrochemical test device was designed to investigate the corrosion behavior of N80 carbon steel under stress and crevice.It is demonstrated that the corrosion potential becomes more negative and the corrosion rate is enhanced with the increase of stress for the corrosion of N80 carbon steel in the CO2-saturated solution containing HAc.In the coexistence of crevice and stress,the stress concentration appears at the corrosion groove which is formed due to crevice corrosion.Slow strain rate testing?SSRT?shows the yield strength,ultimate tensile strength and elongation of fracture decrease when there is crevice on the specimen surface,which suggests an increasing stress corrosion cracking susceptibility.Meanwhile,the corrosion potential at corrosion groove shifts to negative direction due to the stress concentration,which enlarges the galvanic corrosion driving force,and then results in severer crevice corrosion.Therefore,the stress and cevice have synergistic effect to enhance the corrosion of N80 carbon steel.