ABSTRACT
A similar basic fluxing mode of attack is also observed for protective oxides such as Cr20 3 and Ah03 (usually formed on high-temperature alloys) as in the hot corrosion of Ni-8% Cr-6% AI alloy [58]. Initial formation of Cr20 3 and Ah03 depletes the molten salt of oxygen and lowers the oxygen potential, creating an oxygen gradient across the melt. As a result of this gradient in oxygen pressure, the sulfur activity increases and nickel sulfide is formed at the surface of the alloy. The low sulfur and oxygen potentials of the salt due to formation of oxides and sulfides lead to an increase in the oxide ion or Na20 activity in the melt, reaching values at which Ah03 and CrP3 can dissolve. A process is therefore developed whereby sulfate ions diffuse toward the alloy, and as a region of lower oxygen pressure is approached, these ions sulfidize nickel whereupon oxide ions are produced. These in tum react with Ah03 and Cr20 3 to form soluble products in oxide ion-enriched Na~04•
Figure 6.39 Superimposed stability diagrams for Ni-S-0 and Na-S-0 systems showing schematic reaction path during hot corrosion of pure Ni [16].
