Research On High Temperature Corrosion Of Candidate Alloy Of 700 ? Ultra-Supercritical Boiler Superheater In Different Medium

Inconel 740 H and CCA 617 are excellent high-temperature nickel-based alloys with outstanding high temperature creep strength,microstructure stability and corrosion resistance.They are the most suitable candidate materials for 700 ? ultrasupercritical boiler superheater tubes.The superheater tube of the ultra-supercritical units were in a complicated condition with high temperature,corrosive atmosphere and easy to accumulate ash,proned to security risks,it is necessary to conduct systematic research about high-temperature corrosion of materials.In this study,two types of coal ash corrosive environments that furnace arch and synthetic ash were designed,plused a group of ashless samples as a comparison,the Inconel 740 H and CCA 617 were subjected to a high-temperature endurance test up to 5000 hours at 750 ?.The corrosion rate,surface topography,cross-sectional morphology,element distribution and migration of the samples were systematically investigated by plotting the corrosion kinetics curves,combined with scanning electron microscopy and X-ray diffractometry.The results were as follows:Inconel 740 H and CCA 617 in the corrosive environment will occured high temperature oxidation,internal oxidation and sulfide corrosion,S and O preferentially diffused along the grain boundaries,occurred grain boundary corrosion.In the early corrosion of alloys,the surface will generated a variety of oxides,the matrix occurred within the oxidation and sulfide corrosion,resulted in holes.With the progress of corrosion,the complete Cr2O3 protective film hindered the outward diffusion of other element,and the chromium-poor layer appeared.At this time,the corrosion products of the outer corrosion layer were mainly Cr2O3,and the grain boundary corrosion products were mainly Al2O3.As a result of the solid-phase reaction,the oxide film appeared double-layer structure,the outer layer was a loose spinel structure with corrosion products,the inner layer was dense Cr2O3 protective layer.The corrosion of the two alloys in the furnace arch was the lightest,and the corrosion in synthetic ash was the most serious,which was mainly due to the low-temperature melting heat corrosion caused by the alkali metal sulfate,and the corrosion entered the acceleration phase.Since the PBR of the oxides were generally greater than one,the pores may be repaired or become larger due to internal oxidation,the oxide layer will be cracked resulted from the expansion of the growth stress,until the stratification and shedding.Corrosion resistance of Inconel 740 H is stronger than CCA 617,mainly occurred the outer oxide layer of shedding,and fall off slowly.