Corrosion Behavior Of N80 Carben Steel Under The Coupling Effect Of Hydrogen Permeation,Crevice And Stress

The screwed connecction is the weakest and most dangerous part of tubing string in oil/gas well.Serious corrosion problems often occur in the thread jionts of pipe string in complex corrosion environment,which are usually caused by the coupling effect between hydrogen permeation,crevice and stress.However,due to the lack of effective in-situ research methods and simulation devices,the corrosion behavior of the threaded joints under multi-factor coupling has not been reported.Therefore,in this paper,taking N80 steel as the subject investigated,electrochemical method and surface analysis method as the main methods,this paper studies the corrosion damage behavior of the threaded connection of tubing string in the oil/gas field.The main research contents are as follows:(1)An experimental device was designed and manufactured for the study of corrosion beha'vior under hydrogen permeation,crevice and stress coupling.The in-situ test of corrosion electrochemical behavior at atmospheric pressure and the simulation of corrosion environment of the threaded joints of oil well tubing in high pressure environment under multi-factors are realized,which provides a new method for studying the corrosion mechanism of the threaded joints of oil well tubing.Using the theoretical calculation and ANSYS software simulation,the stress distribution on the surface of the sample in the device is obtained,which provides basic parameters for subsequent experiments.(2)The corrosion behavior of N80 steel under the conditions when hydrogen permeation,crevice and stress existing alone,hydrogen permeation-stress coupling condition as well as hydrogen permeation-crevice coupling condition and hydrogen permeation-crevice-stress coupling condition are studied by means of electrochemical testing.The results show that the presence of hydrogen in N80 steel accelerates the corrosion of the metal,and has a great influence on the mechanical properties of the metal,which makes the metal more prone to occure brittle fracture in the same stress environment.The existence of crevice in N80 steel forms potential difference between outside and inside the crevice,and accelerates the metal corrosion inside the crevice.The stress in N80 steel increases the corrosion current density of the metal surface,which greatly promotes the corrosion of N80 steel.In the case of hydrogen permeation and stress coupling,the corrosion potential of N80 steel is lower and the corrosion current density is higher than that of stress applied alone,which further accelerates the metal corrosion.In the case of liydrogen permeation and crevice coupling,the impedance inside and outside the crack of N80 steel is slightly smaller than that of crevice existing alone.Under the condition of hydrogen permeation,crevice and stress coupling,the metal charge transfer resistance and open circuit potential inside and outside the crevice are much smaller than those inside and outside the crevice under the condition of crevice.The coupling effect of three factors leads to serious local corrosion of N80 steel.(3)The corrosion behavior of N80 steel under simulated conditions when hydrogen,crevice and stress existing alone,crevice and stress coupling,hydrogen permeation and stress coupling,hydrogen permeation and crevice coupling,hydrogen permeation,crevice and stress coupling,are studied by surface analysis methods.According to the results of mechanical simulation,the state of stress of the specimens after corrosion is studied,and the mechanical properties of the specimens after corrosion are also discussed.The corrosion rate of N80 steel increases remarkably under multi-factor coupling,especially when crevice exists,the local corrosion rate of N80 steel increases sharply.The results show that,under multi-factor coupling,the corrosion rate of N80 steel increases significantly,especially when crevice existing,the local corrosion rate of N80 steel increases sharply.The groove depth of the specimen under the coupling of hydrogen permeation,crevice and elastic deformation reaches 264.6?m at the experimental temperature of 70? and corrosion time of 24h,and the groove depth of the specimen under the coupling of hydrogen permeation,crevice and plastic deformation is about 331.0pm.This study reveals the corrosion behavior of the threaded connection of oil well,and provides theoretical reference for corrosion protection of oil well,service life evaluation and material selection.