Nickel Oxide Scales and Sodium Sulfate-Induced Hot Corrosion of Nickel

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Abstract
The protection that a nickel oxide scale gives nickel metal against sodium sulfate induced hot corrosion is investigated in this research. The failure of a scale to protect the metal is attributed to infiltration, rather than dissolution, of the by molten salt along cracks and pores. The addition of chromium, chromium sesquioxide, or chromate ions to the sodium sulfate could not be shown to inhibit corrosion in spite of the fact that the chromium content of nickel base alloys is known to enhance their hot corrosion resistance. These results are consistent with a pseudo scale model of sodium sulfate induced hot corrosion for nickel base alloys rather than an acid base model. The addition of molybdenum, tungsten, or their oxidation products to sodium sulfate was found to increase the rate of corrosion. This increase is ascribed to an increase in the rate of infiltration of the scale by the molten salt as the result of an increase in the size or number of defects, which is also consistent with the pseudo scale model.

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