High Temperature Corrosion of High Strength Materials for Boiler Applications in the Temperature Range from 550 C to 750 C

Abstract

Since more than 10 years boiler concepts up to 750 °C using ferritic-martensitic and austenitic steels as well as nickel base alloys above 650 °C have been developed to increase power plant efficiency and to reduce CO2- emissions by post combustion as well as oxyfuel processes. The paper summarizes the influence of temperature and Cr- concentration on the steam oxidation behavior of ferritic martensitic steels as well as of austenitic steels and their limits for long-term high temperature applications, which are mainly determined in conventional boiler applications by steam oxidation. For temperatures well above 650 °C nickel base alloys have to be used, which have good steam oxidation resistance, if Cr- contents are high enough. At temperatures above 670 °C nickel base alloys are prone to type II hot corrosion on the fireside of superheaters, if deposits with high NaSO4 and sulfur content are formed as a consequence of Na- and sulfur rich coal combustion. The maximum of hot corrosion attack is at 750 °C, in good agreement with similar results obtained from gas turbine experience. The susceptibility to hot corrosion can be calculated using Calphad based simulation, if coal composition and combustion conditions are known.Oxyfuel processes exhibit high CO2- concentrations between 60 to 70 % and approx. 30 % H2O. Due to recirculation of dust free exhaust gas to increase heat transfer within the boiler and superheater, impurities like SO2, NOX and HCl may result in increased oxidation and sulfidation rate. The high CO2- content result in extended internal carburization of ferritic martensitic steels, if water vapor contents are low. With increasing water vapor content, carburization rate decreases due to reduced carbon activity according to the water gas shift reaction. Formation of chromia scales on austenitic steels as well as nickel base alloys prevents carbon uptake in the base alloys. The higher SO2- contents in the oxyfuel flue gas power plants do not significantly accelerate corrosion rates of ferritic martensitic steels at temperatures below 600 °C, which are currently considered for oxyfuel power plants.