Research On High Temperature Corrosion Of 700℃ Ultra Supercritical Boiler Superheater

Under simulated conditions of coal/gas, nickel-based superalloy Inconel 740H for 700℃ ultra-supercritical boiler superheater pipe was studied in this paper. The experimental conditions were two kinds of coal-ash medium and a flue gas. After 3000 hours high temperature corrosion in 750℃ and 780℃, the surface corrosion products, corrosion layer thickness, cross section morphology characteristics and elements distribution of alloy were analyzed by XRD, SEM, EDS and other equipments. The purpose was to explore Inconel 740H alloy’s corrosion behavior and corrosion mechanism in the laboratory simulated corrosion environment. The results were as follows:Under different temperature and corrosion medium conditions, Cr2O3 oxide layer were formed on alloy’s surface after 500h corrosion. With the time increasing the thickness of the oxide layer thickening. The corrosion product of specimen were Cr2O3 and Fe2O3. Alkali metal sulfate in synthetic ash were reacted on alloy’s surface. Undr high temperature the reaction product compound sulfate melted on alloy’s surface. It cause the alloy specimen weight gained significantly.During the corrosion process, sulfur would penetrate into the matrix inside. It would formed sulfide layer in which pore and crack produced. Aluminum became Al2O3 by internal oxidation in the matrix. Al2O3 were mainly distributed in pore and crack which were on the grain boundary. With time increasing, pores and cracks will grow and spread to the interior of the matrix. It will cause damage between the protective Cr2O3 layer and matrix, and even cause the Cr2O3 layer peeling. At the same temperature, synthetic ash with higher sulfur content will lead to more pore and crack formed in sulfide layer, even deeper matrix. It prove that synthetic ash has more serious corrosion behavior on alloy than the ash of actual condition.When brushed same ash, the chromium in matrix under the temperature of 780℃ move to the surface faster than 750℃. The Cr2O3 oxidation layer formed faster and the corrosion weight gain significantly when the temperature was 780℃. But the Cr2O3 layer were more easily dropped in 780℃. In the actual working environment, the corrosion of Inconel 740H’s flue gas side is more serious when temperature exceeding 30℃ and their service life will be shorted.