| The high temperature corrosion of cobalt-base alloys associated with deposits likely to condense on components when fuel derived from coal is used have been studied for a wide range of alloy and deposit compositions. The alloys include the commercial alloy FSX-414 as well as simpler laboratory alloys such as Co-Cr, Co-Cr-Ti, and Co-Cr-Al. The deposits were Na(,2)SO(,4), with and without additions of such contaminants as K(,2)SO(,4), SiO(,2), TiO(,2), C, and NaCl.; Simple Cr(,2)O(,3)-forming cobalt base alloys have good hot corrosion resistance to most of the deposits and are insensitive to changes in salt composition except the deposits containing considerable amounts of sodium chloride. Additions of nickel or titanium promote the formation of Cr(,2)O(,3) scales and have beneficial effects in oxidation and hot corrosion resistance. The influence of alloy composition, salt composition and temperature on the hot corrosion process is discussed.(,); The effect of SO(,3) pressure and NaCl vapor in the gas phase on Na(,2)SO(,4)-induced hot corrosion has been studied in the low temperature range 700-750(DEGREES)C. The alloys Co-27Cr and Co-18Cr-6Al were chosen to be representative of Cr(,2)O(,3) and Al(,2)O(,3)-forming alloys for this study. A Ni-20Cr alloy was also included for comparison. The degradation of the Na(,2)SO(,4)-coated Co-Cr and Ni-Cr alloys in So(,3) pressures of 10('-4) to 10('-2) atmospheres was found to be associated with the formation of liquid mixed sulfates (Na(,2)SO(,4)-CoSO(,4) or Na(,2)SO(,4)-NiSO(,4)) which provided a selective dissolution of the Co or Ni and a subsequent sulfidation/oxidation mode of attack which prevented the maintenance of a protective Cr(,2)O(,3) film. A clear mechanism was not developed for the degradation of Co-Cr-Al alloys. The sulfite model proposed by Pettit and coworkers or a modification of the sulfidation/oxidation mechanism are both capable of explaining the experimental results.; The NaCl vapor has a strong effect on Na(,2)SO(,4)-induced hot corrosion at low temperatures. The corrosion morphology is characterized by a porous zone consisting of sulfides and pores. Thermochemical calculations and experimental results indicate the pores are formed due to vapor transport of metal chlorides. AlCl(,3) or CrCl(,3) vapor removed the alloy element required to form a protective scale and are converted to metallic oxide (Al(,2)O(,3) or Cr(,2)O(,3)) near the matrix/scale interface where the oxygen pressure is higher. Chlorine gas is recycled to react with elements in the alloy. The possible combined effects and relative importance of SO(,3) pressure and NaCl vapor on hot corrosion of these alloys are discussed. |
