Study On High Temperature Corrosion Resistance Of Sintered Alloy Containing Tellurium Co-based Powder

Cobalt-based super alloys has lower coefficient of thermal expansion,better resistance to high temperature oxidation and corrosion and higher thermal conductivity than iron-based and nickel-based alloys,which is commonly used to manufacture various gas turbine hot end parts of the engine of cars and ships.However,Turbine blades and turbine discs,which are used as a modern driving force device applied in the field of aerospace,aviation and ship,and heavy gas engine valve and valve seat has must suffered from long-term high temperature oxidation and gas residue heat corrosion,resulting in epidermal oxidation off.These reasons are becoming one of the major failure forms of hot-end components.Therefore,in order to prevent the failure of key components,to extend the service life of high-temperature hot parts and improve the quality of engine equipment,it is great significance to develop high-performance high-temperature corrosion-resistant alloy material.Tellurium is a dilute element.Very small amount of tellurium can improve the self-fluxing of alloy powder.Te not only plays the role of exhaust gas cleaning alloy and weaken the adverse effects of harmful elements,but also significantly improve its overall performance.Tellurium is also one of the indispensable elements in contemporary high-tech materials.Therefore,it is of great practical value to study the cobalt-based alloy with trace element tellurium,in order to improve the hot-corrosion resistance in molten salt and the quality of its high-temperature hot-end parts.From the research situation of domestic and foreign superalloy preparation technology,the powder metallurgy products have stable organization,uniform composition and no obvious macroscopic segregation,which is one of the effective methods to improve the oxidation resistance of high temperature alloy.Therefore,in this study,powder metallurgy technology is applied to sintering the Co-Te superalloy sample.Then,cobalt-based alloy with different tellurium content?0,0.4wt.%,0.8wt.%,1.6wt.%,2.4w.t%?was subjected to a thermal corrosion test in 800?75%Na₂SO₄ +25%NaCl environment.The thermal corrosion and high temperature oxidation corrosion behavior of the alloy were studied by means of analytical balance,metallographic microscope,XRD diffractometer and scanning electron microscope.The results shows that:?1?The addition of Te element in cobalt-based alloy can improve the density,continuity and stability of the oxide film under the environment of molten salt corrosion with 800 ± 5? 75%Na₂SO₄+25%Na Cl.The corrosion rate of the alloy decreases with the increase of the mass fraction of Te element.When the Te element have been added into the alloy,the Cr-rich phase was uniformly precipitated at the grain boundary of the alloy and new phases such as CoTeO₃,Fe₂TeO₅ and CrTe are distributed in the alloy matrix.In the process of high temperature molten salt corrosion,Cr-rich phase forms Cr₂O₃ oxidation protective film while the new phase containing Te compounds can inhibit the diffusion of Co atoms to the grain boundary,to avoid the alloy matrix Co element dissolved,to prevent deep corrosion of molten salt,to improve the hot-corrosion performance.?2?In the initial stage of high temperature oxidation at 800?,the corrosion products increased rapidly and the whole process is consistent with Wagner's parabolic oxidation theory.At the later stage of oxidation,the cobalt-based alloy forms oxide agglomerates such as Cr₂O₃ and SiO₂ at the grain boundary.The compactness of the oxide film is poor,which is liable to fall off and causes porosity on the surface of the alloy oxide film.Cobalt-based alloy with 1.6wt.%Te forms compound phase containing tellurium,on the one hand which promotes the selective oxidation of cobalt-based alloy sample surface to form a continuous dense Cr₂O₃ oxide film,on the other hand plays the pinning role on oxide film?to prevent the oxide film burst off?.The formation of SiO₂,WO,Fe₂O₃ and other transition layer oxide film improve the high temperature oxidation resistance of the alloy.