Effects Of Water Vapor On Oxidation Behavior Of Ni-25Cr Alloy At High Temperature

Ni-Cr alloy has many advantages,such as high melting point,high hardness,strong corrosion resistance,good wear resistance and so on.It has a wide application prospect in modern energy,aerospace and other industrial fields,and become the most potential high temperature alloy material.Ni-Cr alloy inevitably suffers from high temperature oxidation in the process of service.A lot of researches have been performed for the high temperature oxidation of Ni-Cr alloys,but most of the researches focus on the oxidation behavior of Ni-Cr alloys in high-p O₂atmosphere?for example:air,oxygen or inert gas/oxygen mixtures?,while the research on Ni-Cr alloys in low-p O₂atmosphere?for example:inert gas and water vapor as the main gases?is relatively rare.However,Ni-Cr alloys are mainly used as the components of spacecraft,power plant boilers,and gasification systems,where the service environments of these components usually contains a large amount of hydrogen and water vapor with low equilibrium oxygen partial pressure.At present,it is controversial about whether water vapor promotes or retards the oxidation of the alloy,how water vapor affects the oxidation behavior of Ni-Cr alloys is unclear.In the present work,the oxidation behavior of Ni–25wt.%Cr at 950?in Ar-20%O₂and Ar-20%H₂O are compared using some characterization methods including ICP-AES,SEM,EDS,FIB and TEM,combine with first-principles based on density functional theory.The main contents are as follows:?1?The composition of the original alloy is determined by ICP-AES and the main composition was Ni-25wt%Cr.The microstructure of the samples after oxidation in Ar-20%O₂and Ar-20%H₂O are characterized using SEM,EDS,FIB and TEM.The results show that a layer of chromia scale forms on the surface of the alloy in oxygen or water vapor with some voids in the chromia scale,the chromia formed on the surface of the alloy is granular,but the grain size of chromia in water vapor environments is smaller and there exist more grain boundaries,and there are more voids in water vapor environment than in oxygen environment.Voids also forms at the metal/scales interface and new oxides appear in the voids.The new oxides display as alternating layers of Cr₂O₃and Ni Cr O₄.Moreover,we also find that the thickness of the scale in water vapor is thicker than that in oxygen at the same time,which means that the oxidation rate is higher in water vapor.?2?The first-principles calculation is used to explain the mechanism of water vapor accelerating alloy oxidation,The calculation results show that the presence of hydrogen protons in water vapor make it easier to form and aggregate vacancies in the scale,leading to the formation of more voids in the water vapor environment.In addition,the grain size of chromia is smaller and there exist more grain boundaries in water vapor than those in oxygen,thus the metal ions in the alloy are easier to pass through the chromia scale to form oxides on the surface of the alloy,resulting in higher oxidation rate.