The Study Of The Elemental Sulfur Corrosion Mechanism Of X65 Pipeline Steel Under The Suspended Sulfur Environment

As the gradual reduction of oil and gas resources,increased the research and development of the high sulfurous oil and gas fields by domestic and foreign.During the processes of exploitation and transportation of high sulfur oil and gas,as the change of temperature and pressure,precipitation of a large number of elemental sulfur and deposited on the pipeline surface or suspended in the collection solution,resulting in pipeline clogging and corrosion,causing catastrophic accidents.Therefore,the keys of reducing the corrosion of materials were scientific understanding the corrosion behavior and mechanism of elemental sulfur,developing reasonable precautions.In this paper,for the above problems,the changes of pH value,S2-and SO42-contents with elemental sulfur hydrolysis solutions at different conditions were researched by ion chromatography and titration methods,the behavior and mechanism were studied and the model was established of elemental sulfcur hydrolysis,The average corrosion rate,the microstructures of substrate surface before and after removing the corrosion products,the composition and electrochemical properties of corrosion scales with X65 steel in suspended sulfur medium at different conditions were investigated by weight loss tests,Scanning Electron Microscopy(SEM),Energy Dispersive Spectrometer(EDS),X-Ray Diffraction(XRD),Self-corrosion Potential-time Curves,Polarization Curves(Tafel)and Electrochemical Impedance Spectroscopy(EIS)methods,respectively.Analyzed the sulfur corrosion mechanism of X65 steel in suspension elemental sulfur environment,the main contents are as follows:1.The hydrolysis behavior and mechanism of elemental sulfur were studied by measuring the change of pH values and S2-and SO42-concentrations of the hydrolysis solution.The results showed that the hydrolysis of elemental sulfur had an induction period and the temperature played a decisive role,but the effect of sulfur content is relatively small.The key of the elemental sulfur hydrolysis are the formation of hydrogen ions "Carrier" and its autocatalysis.2.The effect of elemental sulfur and NaCl content on the corrosion behavior of X65 pipeline steel and the electrochemical performance of corrosion products were studied.The results showed that the corrosion rate of X65 steel increased with the increase of sulfur and the presence of NaCl.Corrosion products loose and break off easily,mainly composed of C,O,S,Fe and other elements.Cl" could migrate along the defective voids to matrix and formation of soluble chloride,and promoting the sulfur corrosion.The change of elemental sulfur content mainly affects the cathodic reaction of X65 steel,the corrosion of suspended sulfur solution containing 3.5%NaCl is mainly controlled by diffusion processes,the sulfur solution of without 3.5%NaCl is mainly controlled by activation.3.The effects of temperature,corrosion time and corrosion medium on the corrosion rate,microstructure and electrochemical properties of X65 steel were studied.The results show that the weight loss corrosion rate of X65 steel increases first and then decreases with the increase of time,decreases first and then increases with the increase of temperature,and the diffusion effect of the membrane increases with the increasing of corrosion time.When the corrosive medium containing elemental sulfur,the electrochemical corrosion rate more pure NaCl medium big an order of magnitude;The temperature can change the structure of the corrosion product film and FeS2 content to affect X65 sulfur corrosion mechanism of steel,is given priority to with oxygen corrosion as anode control at low temperature is given priority to with sulfur corrosion at high temperature for the cathode control.4.Through the experimental results,the hydrolysis-catalyzed corrosion mechanism of suspended sulfur is proposed,which mainly includes two processes:first,the hydrolysis of the elemental sulfur and the generating of corrosion products FeS2,for corrosion control steps;Second,Corrosion products of FeS2 catalytic substrate further corrosion,it is the beginning of the severe corrosion.