Study On Corrosion Behavior Of Nickel-based Alloys In High Sulfur Environment

Nickel-based alloy is widely used in the harsh corrosive environment of the petroleum industry due to its excellent corrosion resistance.However,in the development of acid oil and gas fields,the deposition of the elemental sulfur causes serious local corrosion,which also makes the Nickel-based alloy in the course of service to undergo a serious test.Therefore,it is necessary to study the corrosion mechanism of sulfur-based alloys and study the root cause of the corrosion of sulfur deposition.The correct understanding of the local corrosion mechanism caused by elemental sulftur is a necessary problem to be solved in the development of high sulfur gas field.In this study,the corrosion behavior and the local corrosion mechanism of the nickel-based alloy under different sulfur conditions were studied with reference to the results of sulfur corrosion at home and abroad.In order to study the corrosion and protection of high sulfur-containing oil and gas production provide theoretical reference.In this paper,the corrosion behavior and corrosion law of Ni-based alloy 825 under simulated high temperature H2S/CO2 and different conditions were studied.The results show that the temperature and sulfur state are important factors affecting the corrosion behavior of Nickel-based alloy 825.Under the condition of low temperature,the corrosion of Nickel-based alloy 825 shows the characteristics of uniform corrosion,and the corrosion rate rise slightly with the change of sulfur state.The corrosion rate of Nickel-based alloy 825 is increased when the temperature is raised to 90 ?,and the pitting phenomenon is obvious.The corrosion rate of the Ni-based alloy increases with the increase of the etching time.It is especially obvious that the alloy is deposited under the sulfur condition,and the intergranular corrosion cracking occurs.Under the condition of increasing temperature,the activity of sulfur in the corrosive medium increases,and the stability of the Nickel-based alloy 825 passivation film decreases which makes the corrosion rate increase gradually.X-ray diffraction(XRD)analysis shows that the product film is mainly Cr2O3 under the condition of blank and molten sulfur,and under the condition of suspended sulfur the sulfide appears on the surface of the substrate due to the sulfur deposition.When the temperature is raised to 90?,the product film is still Cr2O3 but the peak height is reduced under the blank condition,the stability of the passivation film is decreased.The structure of the Nickel-based alloy 825 corrosion product film under suspended sulfur and molten sulfur is the sulfide and oxide composition,the change of the structure of the film from the oxide to the sulfide is the main reason for the deepening of the corrosion of the Nickel-based alloy 825.The Nickel-based alloy 825 corrosion especially in the case of molten sulfur under the room temperature uniform corrosion changes into localized corrosion.Secondly,the polarization curves,AC impedance tests and scanning electrochemical microscopy tests were carried out on the Nickel-based alloy 825 which had been corroded under different conditions of H2S/CO2 and different sulfur stated conditions.The results were in agreement with those obtained by macroscopic corrosion The polarization curves results show that the self-corrosion current density of the Nickel-based alloy 825 increases with the increase of temperature and the change of the sulfur state,and the passivation interval is narrowed and gradually changed from passive dissolution to active dissolution The stability of the passive film decreased,the protective performance deteriorated.The fitting results of the AC impedance curves also show that the electric double layer capacitance tends to increase in the case of temperature rise and sulfur change,so that the polarization resistance decreases and the resistance of the active ions through the passivation film and the protective properties of the passive film are reduced,and the addition of sulfur makes the surface stability of the alloy obviously weakened.The results of scanning electron microscopy(SECM)show that the activity of the surface of the Nickel-based alloy 825 substrate is enhanced under the condition of sulfur state change and temperature rise,the current peak height and peak frequency increase on the surface of the substrate,the pitting susceptibility increases,corrosion ability to drop.Finally,the Nickel-based alloy 825 was subjected to grain boundary engineering treatment.The results showed that the ratio of grain boundary at Nickel-based alloy 825 was significantly improved by electron backscatter diffraction(EBSD)analysis.The special grain boundary ?3,?9,?27 and other random distribution in the general large-angle grain boundaries constitute the grain boundary network,can destroy the original grain boundary network connectivity,can effectively block the grain along the grain boundary corrosion cracking and other failure forms of expansion,so that Nickel-based alloy 825 in high sulfur environment,enhanced corrosion resistance.The results show that the Nickel-based alloy 825 after grain boundary engineering treatment has better corrosion resistance and lower pitting sensitivity by polarization curve,AC impedance curve and scanning electrochemical microscopy.