Influences Of H₂S On Stress Corrosion Cracking And Hydrogen Permeation Behaviour Of Oil Pipelines

Nowadays,most of oil and natural gas fileds confronted with worse situation that they contained more water and corrosive H2S gas.This situation would enhanced the possibility that oil pipelines arised sulfide stress corrosion cracking (SSCC) phenomenon.While X60,X70 grade line pipe steel were used frequently in exploring and transporting oils and natural gases,so studying of SSCC phenomena of this kind of materials was of great importance in exploring and transporting oils and natural gases safely and helped us take some measures to avoid disaster events.The effects of concentration and potential on susceptibility to cracking of X70 grade line pipe steels in 3.5%NaCl aqueous solutions containing H2S have been investigated by potentiodynamic polarization curves, slow strain rate test and hydrogen permeation test.The results indicated that the presence of H2S would increase susceptibility to cracking.And with increasing H2S concentration,the fracture mode changed to cleavage. No cracking was observed at an applied anodic potential.When cathodically polarized weakly,the elongation a little prolonged in comparison with that at free corrosion potential and the susceptibility to cracking went down,while cathodically polarized greatly, the elongation shortened sharply and the susceptibility to cracking went up remarkably. At the same time,the results showed that the steady state permeation current P∞ didn't increase monotonously with increasing the content of H2S, and while the permeated hydrogen atoms N didn't increase monotonously within a definite period of time.The hydrogen entry and permeation depended strongly upon the cathodic potential.The more negative the potential was, the higher the steady state permeation current P∞ was.The steady state permeation current P∞ and the permeated hydrogen atoms N are linear with cathodic potential over a limited range of potential.So we suggested that the mechanism of SSCC for X70 grade line pipe steel is synergism of anodic dissolution and hydrogen embrittlement.