Sodium Sulfate-Induced Corrosion of Pure Nickel and Superalloy Udimet 700 in a High Velocity Burner Rig at 900 C

Sodium sulfate-induced corrosion of pure nickel and a commercial nickel-base superalloy, Udimet 700 (U-700), were studied at 900 C in a Mach 0.3 burner rig with different Na levels in the combustor. The corrosion rate of Ni was independent of the Na level in the combustor and considerably lower than that measured in laboratory salt spray tests. The lower rates are associated with the deposition of only a small amount of Na₂SO₄ on the surface of the NiO scale. Corrosion of U-700 was observed to occur in two stages. During the first stage, the corrosion proceeds by reaction of Cr₂O₃ scale with the Na₂SO₄ and evaporation of the Na₂CrO₄ reaction product from the surface of the corroding sample. Cr depletion in the alloy occurs and small sulfide particles are formed in the Cr depletion zone. Extensive sulfidation occurs during the second stage of corrosion, and a thick scale forms. The relationship between the corrosion rate of U-700 and the Na level in the combustor gives a good correlation in the range of 0.3 to 1.5 parts per million by weight (wppm) Na. Very low levels of Na in the combustor cause accelerated oxidation of U-700 without producing the typical hot corrosion morphology.