A COMPARISON OF THE CORROSION PRODUCTS AND MECHANISMS OF VARIOUS FORMS OF DEPOSIT-INDUCED CORROSION AT 700oC

Tests were performed on selected steel and Ni-based alloys in simulated fireside corrosion conditions with synthetic coal ash deposits. The tests provide fireside corrosion information as well as insights on corrosion mechanisms for guiding materials development. General mechanisms for the different types of hot corrosion caused by Na2SO4 deposits were inferred; although, in some cases, specifics were unclear. Tests were also conducted on MCrAlY alloys and Ni-based disk alloys under simulated hot corrosion conditions at 700oC. The experimental conditions simulated fireside corrosion by varying deposit compositions, temperatures, and gas atmospheres until the most severe corrosion environment was found. This was determined to be at 700oC with a deposit consisting of K2SO4:Na2SO4:Fe2O3 in a 1.5:1.5:1.0 molar ratio in a gas atmosphere containing oxygen with 1000ppm SO2. Once this was determined, the effects of alloy composition on the resistance to corrosion were obtained, and a mechanism for the corrosion was deduced. An SO2 threshold value in the gas atmosphere for severe corrosion to occur was also observed. Each of the alloys suffered corrosion in the form of pits similar to those found during Type II hot corrosion. A variety of disk alloys typical of those used in gas turbine engines were exposed to aggressive corrosion conditions. The exposure conditions at 700oC included air oxidation, oxidation with Na2SO4 salt deposits in air, and O2 + 1000ppm SO2. The Mo-containing alloys suffered degradation with the salt deposits in air, without the presence of SO2. The attack was similar to alloy-induced acidic fluxing usually observed around 900oC. Each of the alloys was significantly degraded under the Type II hot corrosion conditions, with the Mo-containing alloys experiencing more corrosion than the other alloys. The corrosion products and mechanisms for the various forms of corrosion were compared. Fireside corrosion was found to be very similar to Type II hot corrosion. The Mo-containing disk alloys which were exposed in air with Na2SO4 salt deposits were found to undergo alloy-induced acidic fluxing even at 700oC. The corrosion of the disk alloys under Type II hot corrosion conditions was found to be similar to Type II hot corrosion of NiCrAlY