Combustion and Flame, Vol.159, No.5, 1987-1996, 2012
The chemical nature of combustion deposition and corrosion: The case of alkali chlorides
Quantitative deposition studies are reported involving flame gases containing traces of sodium and chlorine. Comparisons with previously published and unpublished data involving Na2SO4 and Na2CO3 deposition indicate a similar behavior and a generic type mechanism. A wide series of propane and hydrogen flames of varying equivalence ratio have been examined that contain sodium on a part per million by volume scale (ppmv) and chlorine either as a similar trace amount or in excess. An important aspect is that deposition is efficient and the product does not directly correlate to the gas-phase metal speciation. Whether sodium is present as gaseous NaCl, NaOH or Na in a flame is irrelevant to the composition of the deposit. Rates are controlled by the total amount of sodium reaching the surface. Once there, the flame becomes solely a provenience of preferred ingredients. Below 850 K, formation rates of NaCl, Na2CO3 or Na2SO4 are the same, and for Na/Cl containing flames the ranking is NaCl > Na2CO3 > NaOH as predicted by thermodynamics. Rates are first-order in alkali and zero-order for chlorine concentrations. They are independent of equivalence ratio, flame temperature, flow velocity, or temperature gradient across the surface boundary layer. Compositional analyses of deposits indicate that below 850 K, where NaCl vaporization is negligible, a several fold excess of chlorine over sodium is required to produce the theoretically predicted 100% deposit of NaCl. The data suggest slight kinetic constraints relating to the chlorine concentrations, which might imply conversions of transient hydroxide or carbonate. Deposition of NaCl continues to higher temperatures but rapidly becomes short-lived due to vaporization. Deposition processes are seen as a steady state balance between a perpetual influx and egress of materials at all temperatures. In this regard, steel corrosion by NaCl is reported above its 'dew point' with the formation of Na2CrO4. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
Keywords:Combustion;Alkali chlorides;Heterogeneous chemistry;Deposition rates;Compositions;Corrosion