Study On Fireside Corrosion Of Ultra-Supercritical (USC) Steel

Since steel HR3C and Super304H have persistent high-intensity, good structure stability, excellent anti-steaming oxidation ability and superior corrosion resistance under high temperature performance, meeting the high steam pressure and temperature need in supercritical boiler pipe material, they have been the primary choose of boiler superheater and reheater high-temperature-part materials. The high-temperature corrosion and abrasion could happen on the pipe material mentioned above, with co-functioning of corrosive gas, melt-sulfate and coal particles near fireside, which would bring great risk on life safety and economic development.In this article, we performed a series fireside corrosion experiments on the two materials above in the environment closed to boiler. We analyzed the changing of corrosion rate by making dynamic curve in a way of weighing. By using XRD,SEM (EDS), we analyzed the corrosion products、sectional morphology during those two materials corrosion., in order to understand the corrosion behavior and mechanism of HR3C and Super304H in static atmosphere under different temperature.During the fireside corrosion period of HR3C steel, two processes including high temperature oxidation and sulfate-corrosion existed under various temperatures. High temperature oxidation would produce oxide coating which has protecting function, main components were Cr2O3、Fe2O3, causing ascending trend for the dynamic curves; as corrosion time continuing or corrosion temperature rising, the action between sulfate and Cr2O3became more and more acute, the volatilization of corrosion product Na4(CrO4)(SO4) caused the dynamic curves descend, thus destroyed the structure of oxide coating, sulfate could cause inter sulfuration and inter oxidation on the matrix, which could also cause the damages on matrix.Super304H steel has the similar corrosion process with HR3C steel. During the earlier period, corrosion was mainly high temperature oxide, which formed iron oxide and chromium oxide coating. The dynamic curve showed rising trend as a result of the oxidation, showed ascending trend for the volatilized of Na4(CrO4)(SO4). The densification of oxide coating became lower as time continued and temperature rising. At the same time, the matrix show inter sulfuration and inter oxidation as well, even secondary oxidation happened at the temperature above750℃. The second kind of low-temperature corrosion happened in experimental condition: low-melting temperature eutectic Na2O·Na2SO4formed in experimental temperatures, it melted and reacted with the oxide produced in the early stage of corrosion. The reaction mentioned above produced volatile production Na4(CrO4)(SO4). As the extension of time and with the increase of temperature, the oxide layer became more and more loosen. Besides, sulfate had a further contact with matrix, causing inter-sulfuration and inter-oxidation, bringing cracks and holes, the matrix deteriorated.Comparing the result of corrosion experiment performing on two materials, we find that Cr is the most important element related to fireside corrosion resistance. Compared with Super304H (20%wt Cr contained), HR3C(25%wt Cr contained) has slower oxidation rate in the same condition. Reactions between sulfate and oxide products、sulfate and matrix are more weaker on HR3C. In short, the sulfate-corrosion resistance performance of HR3C is better than that of Super304H.