| At present,the fossil energy is dominant in the energy consumption structure of our country,and the per capita possession is insufficient.There is a great contradiction between supply and demand,which brings great pressure to the environment.The production of hydrogen from coal gasification technology is one of the effective approaches to alleviate energy and environmental pressure in the future.However,the corrosion issues for alloy materials still is a significant factor restricting the commercial application of supercritical water gasification technology.During SCWG process,harsh reaction conditions(high temperature and pressure)and the heteroatoms in reaction medium,such as sulfur and chlorine,could contribute to corrosion of equipment materials.This study selected stainless steel 316SS and nickel-based alloy(Incoloy800,Incoloy825,Incone1625,HastelloyC276)as research object,using the supercritical batch reactor to simulate the working environment of SCWG for coal,and execute corrosion experiments under different sulfur concentration and temperature.The scanning electron microscope(SEM)was employed to observe the film surface morphology,the line scanning analysis film structure,while the X ray diffraction(XRD)was used to analyze the film phase composition.According to the reaction kinetics and thermodynamic data for oxide and sulfide of Ni,Fe,Cr,Mo,the isothermal equilibrium state diagram was established.Finally,the corrosion mechanism for stainless steel and nickel-based alloy in supercritical water containing sulfur is disscussed.The main conclusions are as follows:(1)The corrosion degree of each alloy increases with increasing temperature or sulfur concentration.Electrochemical corrosion occurs at 350℃,while chemical corrosion occurs at 400℃ and 520℃.(2)HastelloyC276 occurred pitting corrosion under 350℃ and sulfur concentration of 5000ppm.Incoloy825.For Incoloy825,Incone1625 and HastelloyC276,the crack corrosion occurred on surfaces of samples,with no phase was detected by XRD.It indicates no complete film was formed,but the metal dissolution corrosion was liable to occur under subcritical temperature,following the mechanism of metal dissolution and ion diffusion corrosion.(3)Under supercritical conditions(400℃ and 520℃),the surface of the alloy formed the corrosion film which can be detected by XRD.Moreover,for HastelloyC276,the pitting corrosion degree at 400℃ was slightly lighter than that at 350℃with sulfur concentration of 100ppm.(4)As the temperature rose up to 520℃,the alloy corrosion was aggravated obviously.The corrosion products of 316SS and Incoloy800 were mainly Fe oxide and spinel phases,which could prevent anions from invading into the substrate.The corrosion products for Incoloy825,Incone1625 and HastelloyC276were similar,including Ni sulfide(NiS,Ni3S2),Cr sulfide(CrS,Cr2S3),Cr2O4,NiFe2O4 and FeCr2S4.Owing to the corrosion film characteristic that sulfide channels existed in oxide products,providing the transport and diffusion route for ions,the corrosion for HastelloyC276 was the most severe.(5)At 400℃ and 520℃,the dense corrosion film derived from the precipitation of oxide and sulfide products contributed to the corrosion resistance of alloys.(6)The metal element in alloys were an important influence factor for the corrosion resistance property of alloys.And the corrosion resistance was better with higher contents of Fe and Cr,or lower Ni/Cr ratio.Correspondingly,the corrosion resistance of 316 SS and Incoloy800 were superior to Incoloy825,Incone1625 and HastelloyC276.Above all,the research conclusion in this study would provide the beneficial reference on alloy selection for hydrogen production system by supercritical water gasification of coal. |
