Study On The Expansion And Evolution Of Sucker Rod Corrosion Pits In Multi-ions Mixed System

With the development of oil and gas field in our country,the high water cut and corrosive ions increase especially year by year,which makes the local corrosion of the sucker rod more and more serious.The existence of corrosion pits accelerates the initiation and propagation of cracks and shortens the service life of components.That study the effect and interaction of sucker rod pitting and transformation corrosion pit into stress corrosion cracking and the distribution of stress and strain under the environment of multi-ions,which will not only provide the theoretical significance of service safety of sucker rod,provide theoretical basis for the safe service,repair and reuse of component with corrosion pits.In this paper,the orthogonal experiment of 2 factors and 4 levels was designed by simulating the multi-ions environment of oil well fluid.The potentiodynamic polarization curves of 20CrMo in different concentrations of HCO₃^-and Cl^-solution were measured.The results show that HCO₃^-plays passive role in the multi-ions system.With the increase of the HCO₃^-concentration,the corrosion potential and pitting potential increase,while the reduction of the apparent current decreases,and the passivation effect increases.Cl^-has the property of erosion,which makes the passive film rupture and promotes pitting initiation.The electrochemical impedance spectroscopy and Mott-Schottky curve test results show that the passivation film is n type semiconductor and HCO₃^-will reduce the number of defects in the passivation film which makes the passive film stability and protection better.While Cl^-makes the passive film loose and the passivation film structure unstable.The erosion of Cl^-plays an important role in the experimental system of multi-ions interaction.By means of four point bending stress corrosion test under the environment of high temperature and high pressure,SEM,EDS,XRD analysis and finite element simulation,the results show that stress corrosion cracks originated at the bottom of secondary corrosion pits,and extended along the depth direction.The calculated values of stress and strain are located at the bottom of the corrosion pit,which is consistent with the stress corrosion cracking test.The criterion for the transformation of corrosion pit to stress corrosion crack is established.The threshold stress intensity factor(K_ISCC)of stress corrosion is derived,（?）which is related to the fracture toughness,yield12which is related to the fracture toughness,yield strength and the amount of metal consumed in the anodic dissolution process.Through the finite element software ABAQUS to simulate distribution of stress and strain of corrosion pits under different tensile stress,the calculation results show that stress concentration factor is maximum when the tensile stress is 50%σ_0.2,the value is smaller under70%σ_0.2,while the value is minimum under 90%σ_0.2.The stress concentration factor K and plastic strain become smaller with the increase of pit radius（c）.With the increase of pit depth（a）,the stress concentration factor K and plastic strain become larger.From the fitted curve,corrosion radius（c）is a linear function of the stress concentration factor K and the maximum plastic strain value.Corrosion pit depth is a exponential function of the stress concentration factor（K）and the maximum plastic strain value.