Comparison of Performance of NiCr2O4 and Cr2O3 Formed on the Ni-Based Superalloy RR1000 Under Corrosive Conditions

Samples of the Ni-based superalloy, RR1000, were exposed to 98% Na₂SO₄/2% NaCl salts at 700 °C with a flux of 1.5 µg cm⁻² h⁻¹ in flowing air + 300 ppm SO₂ for a total of 250 h. Three pre-exposure conditions were studied: a bare reference alloy; fast heating to the test temperature followed by a 100 h hold; heating at a rate of 5 °C min⁻¹ to the test temperature following by a 100 h hold. The surface oxide formed under the latter two conditions were Cr₂O₃ or NiCr₂O₄, respectively. The results show corrosion pit formation on the surface of the base, reference sample, and no pits present on the sample with the preformed Cr₂O₃. Some protection was found for the sample heated at 5 °C min⁻¹ with a delay in the progression to accelerated corrosion attack. Additional testing under moisture containing air was also conducted. This showed no obvious difference in surface oxide morphology under the two tested heating rates for the short-term exposures examined but a difference was noted to be dependent on the moisture content of the air.