Mechanism of oxidation-sulfation reactions of coo in the presence of na 2 so 4
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I.
INTRODUCTION
THEdetrimental effects of liquid Na2SO4 deposits on the lives of gas turbine components exposed to hot combustion gases have been known for over twenty-five years. These deposits form by condensation from hot combustion gases obtained by burning fuels containing sodium and sulfur impurities in air, which may also contain sodium. Although Na2SO4 melts at 884 ~ recently gas turbines operating in marine environments have experienced corrosion of cobalt base coatings at 700 to 750 ~ 1,2This has been attributed to the formation of an Na2SO4-CoSO4 liquid which forms from sulfation of cobalt oxides present on the surface of the coatings. Although a few studies have been conducted on the mechanism of sulfation of cobalt oxides, no work is reported in the literature on how sulfation can influence the life of gas turbine components in the presence of Na2SO4. Some relevant work available in the literature concerns the role of Na2SO4 in promoting the sulfation of oxides, an important step in the sulfation roasting of sulfide ores. 3'4 Fletcher and Shelef3 concluded from their studies on sulfation of NiO at 688 ~ that by forming Na2SO4 9 NiSO4 the Na2SO4 produced discontinuities in the sulfate layer. This led to faster transport of SO3 to the oxide/sulfate interface, which led to higher sulfation rates. Holtz and Ingraham 4 concluded from their studies on Co304 and NiO using DTA and X-ray diffraction that the sulfation rates were increased due to the formation of liquid Na2S207. This liquid acted as a carrier of sulfur trioxide and a solvent for CoSO4 and NiSO4. Both of the above studies were confined to temperatures below the eutectic temperatures of the Na2SO4-CoSO4 (565 ~ and Na2SO4-NiSO4 (685 ~ systems. S However, corrosion studies on cobalt base alloys from 600 to 900 ~ indicate that rapid corrosion rates are observed at about 650 to 750 ~ 6 The present study was, therefore, aimed at understanding the mechanism of sulfation of CoO in the presence of Na2SO4 at 600 to 900 ~ to provide a more
K. L. LUTHRA is Metallurgist, Corporate Research and Development, General Electric Company, Schenectady, NY 12301. Manuscript submitted August 11, 1981. METALLURGICALTRANSACTIONSA
detailed understanding of one of the processes involved in the hot corrosion of Co-base alloys.
II.
THERMODYNAMIC CONSIDERATIONS
Although pure Na~SO4 does not melt at temperatures below 884 ~ a liquid phase can form on cobalt oxides at lower temperatures due to formation of the low melting Na2SO4-CoSO4 eutectic. The sulfation reactions are: CO304(S ) "[- SO3(g) = C o S O 4 ( s , 1 ) -[- ~ O2(g) C o O ( s ) -b SO3(g) = C o S O 4 ( s , l )
[1]
[2]
where the underline indicates that COSO4 can exist in an Na2SO4-CoSO4 solution at an activity of less than one. Figure 1 shows as a function of temperature and Pso3 the stability of different phases that can form in the presence of cobalt oxides and Na2SO4. Details of the calculations that led to Figure 1 have been described elsewhere. 6 At Poz = 1 atm (0.1 MPa), Co304 is the stable cobalt oxid
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