Study On Corrosion Properties Of Domestic X70 Pipeline Steels And Welding Joints In Medium With Hydrogen Sulfide

Corrosion of domestic X70 pipeline steels and its' welding joints used in engineering of "Gas Transmitting From West to East" were carried out in medium with Hydrogen sulfide (H2S) in this paper. Sulfide Stress Corrosion Cracking (SSCC) of domestic X70 pipeline steels and its' welding joints were studied by Constant Load Test and Slow Strain Rate Test (SSRT). Hydrogen Induced Cracking (HIC) was also conducted by the standard of HIC testing ,the electrochemical permeation method of Devanathan. Finally electrochemical method was adopted to study the interactive behavior of H2S concentration,Cl- concentration ,pH values on corrosion properties of domestic X70 pipeline steels. In this study, not only numerical value analyses ,mathematical statistics and computer technology were adopted, but modern microscopic testing techniques such as SEM (Scanning Electron Microscopic ),TEM (Transmission Electron Microscopic) were also applied to further discuss the testing results. Main results were summarized as follows:For domestic X70 pipeline steels and its' welding joints, the influence of H2S concentration on SSCC was firstly studied by the method of SSRT. The testing results suggests that the increase of H2S concentration will raise the brittleness coefficient, leading to the decrease of corrosion resistance for either X70 base metals or welding joints. Meanwhile, the results reveals that effect of H2S concentration on X70 base metals and welding joints is different: the influence of H2S concentration on welding joints'SSCC is bigger than on base metals'SSCC. When H2S concentration is 50×10-4%, brittleness coefficient of base metal is 19.95% , brittleness coefficient of welding joint is 59.19%. As far as SSCC is considered, if it is assumed safe when brittleness coefficient is 25%, domestic X70 steel base metal is safe but it's welding joint is not in the medium with 50×10-4% H2S concentration.Electrochemical permeation hydrogen method was firstly adopted to study the influence of H2S partial pressure on HIC properties of domestic X70 pipeline steels. With the increase of H2S partial pressure (0.01 MPa～0.1MPa), hydrogen diffusivity in steels will increase and hydrogen concentration on the surface will decrease. When H2S partial pressure is 0.1 MPa～1.0MPa, with the increase of H2S partial pressure, hydrogen diffusivity in steels will decrease and hydrogen concentration on the surface will increase. From the above, hydrogen diffusion coefficient in X70 pipeline steels is maximum when H2S partial pressure is 0.1MPa, that is to say, under this condition , domestic X70 pipeline steels are extremely sensitive to HIC.Aiming at domestic X70 pipeline steel, interactive behavior of influence factors such as H2S concentration,Cl- concentration,pH value were firstly conducted by electrochemical method in this test. The quintic equation is gained by regression methode: Y=7.014×10-14 X14X3－3.386×10-5X2X32＋3.509×10-2It reflects more accurately the effect of interactive action of the three factors on corrosion current as one of potentiodynamic parameters. When H2S concentration is lower , with the increase of pH value, the corrosion current value will decrease, when H2S concentration is higher, with the increase of pH value, the corrosion current value will increase. The interactive behavior of two factors Cl- concentration,pH value also play key role in corrosion current, meanwhile, higher pH value, more obvious the influence of Cl-concentration on corrosion current. Whereas, the influence of Cl- concentration on corrosion current will slow down as a result of increase of H2S concentration. In a word, the study and understanding of domestic X70 pipeline steels have important effect on pipeline steels construction in the point of theory meaning,science and practical engineering meaning. Further more, all testing results and analyses will play important role in ecnomic and social meaning.