Effect Of W Element On High Temperature Corrosion Behavior Of As-cast GH3535 Alloy

With the acceleration of China’s urbanization process,the problem of urban waste disposal has become prominent.Waste incineration is not only widely applicable and will not cause land and air pollution.In addition,the heat generated by incineration can also be used as energy generation.But the key heating surface in the waste incineration system including the water wall in the furnace,superheater,boiler tube,economizer,air preheater and some heat transfer equipment often received serious corrosion,affect the efficiency of the incinerator and the life safety of workers,so the corrosion resistance of waste incinerator structure material is very important to waste incinerator.The main corrosion of waste incinerator at high temperature is high temperature oxidation corrosion and the corrosion of molten salt produced in the process of waste incineration.Ni metal in molten salt has excellent corrosion resistance,usually nickel-based high temperature alloy is the preferred structure of the molten salt environment,the Hastelloy N in the mechanical properties and corrosion resistance are excellent,due to the political factors in the United States banned Hastelloy N exports to our country,then the Chinese Academy of Sciences metal by generic Hastelloy N named GH3535 alloy.The main component is 8 Cr-16Mo-4 Fairt%.The remaining elements are Ni elements,and the alloy salt is not good for the corrosion resistance of molten chlorine salt.In order to adapt to the corrosion environment of waste incineration,it is necessary to be modified.Element optimization method is one of the common means to improve the corrosion resistance of the alloy,mainly to improve the performance of the alloy by adjusting the existing elements or adding new elements.Through consulting relevant data,it is found that the compound Gibbs formed by W element and Cl element has low free energy,and has better corrosion resistance in chloride salt.The relatively large solid radius of W element will produce large lattice distortion in Ni matrix,and the solid solution of W in Ni matrix is large,and its solid solution strengthening effect will be better.At present,the mechanism of action of W element in nickel-based alloy system is not very clear,and no systematic corrosion study has been conducted in chloride molten salt and sulfate,so the most suitable W element composition needs to be further determined.This paper mainly studies the influence of different W content（0wt.%,3wt.%,6 wt.%,9 wt.%,12 wt.%,15 wt.%）on the corrosion resistance of cast GH3535 alloy under high temperature oxidation and molten salt corrosion environment.First by calculating the majorγphase decrease found with the W content in GH3535 alloy,μphase content gradually increased,and W content gradually added to 10%,precipitate the new phase P phase,P phase,P phase in increasing also reduce theμphase content,with the W element content in GH3535 alloy,XRD mapγphase peak gradually offset to the low Angle,and the greater the content offset,that the W element is good solid solution into the alloy matrix,W element caused the lattice distortion in the alloy expanded the crystal surface spacing.The temperature oxidation corrosion behavior of cast GH3535 alloy at 1000℃air atmosphere was studied with different W content.The corrosion dynamics curves of NMC-0,NMC-3W,NMC-6W and NMC-9W showed a parabolic trend,while NMC-12W and NMC-15W showed a straight rise.Its corrosion resistance was NMC-6W>NMC-0>NMC-3W>NMC-9W>NMC-12W>NMC-15W,and the average oxidation rate of NMC-6W was 0.04（mg/cm²·h）to reach the full antioxidant level.The main corrosion products on the surface are Ni Cr O₄,Ni WO₄,Mo O₂,and Cr₂O₃.Because the existence of W element makes the alloy form the structure of double-layer corrosion layer on the surface,the corrosion resistance of the alloy is enhanced.When the W element content of alloy is more than 6%,the oxide film on the surface will be destroyed and the corrosion resistance of the alloy is also reduced due to the reduction ofγphase.Thermal corrosion of six samples with different W contents for 80h with50%Na Cl-50%Na₂SO₄ as corrosion medium at 700℃,Its molten salt corrosion resistance is successively NMC-0>NMC-6W>NMC-3W>NMC-9W>NMC-12W>NMC-15W,The corrosion dynamics curves of NMC-12W and NMC-15W are straight upward,The remaining samples were parabola,The average corrosion rates of samples NMC-0 and NMC-3W were 3.03（mg/cm²·h）and3.56（mg/cm²·h）were significantly better than the other samples,However,the maximum corrosion rates of NMC-15W and NMC-12W were 18.75（mg/cm²·）and 17.9（mg/cm²·h）.The main corrosion products on the surface are Ni₂O₃,Cr₃S₄,Ni₃S₂,Mo Cl₄,and WCl₄.Sample NMC-0 due to the highestγphase content so resistant molten salt corrosion ability,but Cl element diffusion into the matrix inside the S and O along the diffusion path of Cl to corrosion inside the matrix,and sample NMC-6W because W elements on the surface and Cl,prevent Cl to the erosion of the matrix,make other elements inside the matrix corrosion rate reduced,higher content of W element sample due to the content ofγphase continues to reduce,molten salt corrosion performance also weakened.