A quantitative program has been completed concerning the detailed chemistry of alkali sulfate deposition onto surfaces immersed in hot flame gases. It has been irrefutably proven that the process is heterogeneous. It is the surface and the alkali that are the predominant controlling features. It is insensitive to the sulfur or any of the flame parameters. The nature of the surface is not important and it uses the flame gases solely as a source of ingredients. Alkali speciation in the flame gases is irrelevant. The chemistry is rapid and shows no activation energy. Formation of sulfate appears to be driven largely by thermodynamic stability considerations. The alkali deposit that results depends on what is available in the hot flame gases. A preferential ranking is observed showing the pronounced trend of sulfate>chloride>carbonate>hydroxide. Consequently, additives have been sought and found that the alkali prefers to sulfur to in this way modify and control the deposition chemistry. It has been determined that the only such candidate species originate in the first few columns of the transition elements. Of these, salts of niobium, tantalum, molybdenum and tungsten are successful. Molybdenum and tungsten salts are the more readily available. When these are added to the burned gases at a concentration twice that of the alkali, on an atomic basis, it is observed that no alkali sulfate or chloride is formed. Instead benign alkali polymolybdates or polytungstates are produced. It appears reasonable to expect similar behavior on thermal barrier coatings.

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