Research On High Temperature Oxidation And Corrosion Behavior Of New Type Of Austenitic Heat-Resistant Steel C-HRA-5

China's power industry mainly relies on traditional coal-fired thermal power generation.At present,the energy shortages are becoming more and more serious.In order to reduce environmental pollution and improve the energy crisis,utilizing coal and other non-renewable resources rationally,as much as possible to increase the parameters of coal-fired power industry to increase units'thermal efficiency,which is one of the most effective methods.However,as the improvement of temperature,the ultra-supercritical unit with higher working parameters and more harsh working conditions require the material need to have better performance.Superheater and reheater tubes as an important part of the ultra-supercritical unit.In the long-term service process,the severe steam oxidation inside the tube and the harsh environment of coal ash and corrosion gas outside the tube will affect the safe operation and power generation efficiency seriously.Therefore,it is value to study the anti-oxidation and corrosion resistance properties of heat-resistant steel materials for the ultra-supercritical unit boiler tubes in service environment.New type of austenitic heat-resistant steel C-HRA-5 is based on the traditional Fe-22Cr-25Ni stainless steel and adding about 3 wt.%Cu and a small amount of W,Co,Cu,Nb,Mo,N and other elements.Due to its high Cr content,which the material would form a protective oxide film Cr₂O₃ with changes in temperature and environment,making its good oxidation resistance and corrosion resistance,and becomes one of the candidate materials for reheaters and superheaters of ultra-supercritical units.However,long-term exposure to harsh high-temperature environment will cause the material to face intergranular corrosion,steam oxidation,and coal ash and gas corrosion.Therefore,in this paper the intergranular corrosion,high temperature oxidation and hot corrosion of austenitic heat-resistant steel C-HRA-5 were studied by simulates the service environment.After sensitization treatment at 650?,750?,and 850?for 2 h.Using of the double loop electrochemical potentiokinetic reactivation(DL-EPR)method to study the intergranular corrosion behavior of C-HRA-5 steel.The optimized test condition is in a solution of 0.5 M H₂SO₄+1.5 M HCl+0.01 M KSCN at 30?with a scan rate of 1.667 m V/s.The result show that the precipitation amount of Cr₂₃C₆ second phase near the grain boundary of C-HRA-5 steel increased significantly,and the susceptibility to intergranular corrosion increased with the increase of sensitizing temperature.The reason for intergranular corrosion is that the Cr-rich carbides along the grain boundary precipitation and a significant Cr-depleted zone be formed near the grain boundary with the increase of sensitizing temperature,making the austenitic heat-resistant steel C-HRA-5 to the susceptibility to intergranular corrosion increased.According to the precipitation,Cr-depleted zone and the intergranular corrosion morphology,a theoretical model of the entire intergranular corrosion process of austenitic heat-resistant steel C-HRA-5 was proposed.In the static air at 650?,700?and 750?for 1000 h was studied the high oxidation behavior of C-HRA-5 steel,through oxide weight change to establish the relationship of oxidation weight gain with the temperature and time,and through SEM/EDS,XRD analyzed the morphology,composition and phase composition of the oxide film.The results show that the oxidation kinetics curve of C-HRA-5 steel follows the parabolic law at each experimental temperature.As the temperature of the experiment increases,the oxidation rate of C-HRA-5 steel increases,and the oxide film gradually grows and transforms from the original small lump particles into strips and needles,and was covered entire substrate surface with the extension of the reaction time.The more dispersed oxide particles are formed at 650?,while the double-layer structure oxide film formed at 700?and 750?,and the outer layer is a bulk particle oxide,mainly containing Fe,Cr,Mn and O,the inner layer is a dense spinel oxide FeCr₂O₄.In order to simulate the gas corrosion environment,the experiment selected at 700?in Na₂SO₄ and Na₂SO₄?-?Na Cl medium to study the high temperature hot corrosion behavior of C-HRA-5 steel.The corrosion rate is the fastest in the Na₂SO₄?-?Na Cl medium.This is due to selective oxidation in the early corrosion stage,forming an oxide film.With the change of the medium,the melting of the composite salt destroys the integrity of the oxide film,appearing small holes and microcracks with the extension of the corrosion time,occurring oxidation-chlorination and internal sulfide cycle,resulting accelerate material hot corrosion behavior.