| The present study was undertaken to understand the Na(,2)SO(,4) induced corrosion of alloys at temperatures that correspond to the low power operation of gas turbines (650-850(DEGREES)C). Studies have been made to understand the effect of sulfurous gases plus Na(,2)SO(,4) deposits, Na(,2)SO(,4)-NaCl deposits and HCl(g) in the atmosphere on the corrosion behavior of nickel base alloys containing Cr, Al and Co.; In the presence of sulfurous gases (SO(,2) + SO(,3)), corrosion is initiated by the formation of a low melting, mixed sulfate (e.g. Na(,2)SO(,4)-MSO(,4) or Na(,2)SO(,4)-M(,2)(SO(,4))(,3)). From the thermodynamic studies of the molten sulfate mixtures, it was concluded that at the P(,SO2) or P(,SO3) level, that is usually present in the gas turbine only Na(,2)SO(,4)-NiSO(,4) and Na(,2)SO(,4)-CoSO(,4) melts can form.; Corrosion experiments, in the presence of sulfurous gases and Na(,2)SO(,4) deposits showed the melt penetrating the outer oxide layer and reaching the metal or alloy surface. For pure nickel, the interaction of the metal with the melt generated large amount of nickel sulfides (Ni(,3)S(,2)), and this was responsible for rapid degradation of nickel. For the alloys, the transient stage of oxidation persisted for longer times and a protective oxide could not be formed. A pitting type of morphology was the most prominent feature of the alloy corrosion, and a model was developed to explain the development and growth of pits.; The presence of NaCl along, with Na(,2)SO(,4), caused preferential removal of Al and Cr from the alloy. This was due to the formation of Al and Cr chlorides at the scale-metal interface, which was explained based on the thermodynamics of the system. The corrosion behavior was dependent on the Na(,2)SO(,4)/NaCl ratio of the deposit. Sulfide formation inside the alloy was the prominent mode of degradation of Na(,2)SO(,4)-NaCl deposits of high Na(,2)SO(,4)/NaCl ratio. The presence of small amounts of HCl(g) in the gas also caused preferential removal of Al and Cr from the alloy. |
