The Effect of Superimposed Stress on High Temperature Degradation of Single Crystal Superalloys during Exposure to Various Sulphur and Sea Salt Environments
Alternative fuels and novel cycles are increasingly used to supply the world growing demand in energy whilst reducing the carbon emission. Such cycles include the integrated gasification combined cycle (IGCC) with carbon capture, which uses coal as fuel. In such situations however, sulphur and sodium chloride, contained in the coal, are known to form sodium sulphate that, if deposited at the root of the single crystal turbine blades, can initiate hot corrosion. Low temperature Type II corrosion consists of the formation of a fused salt melt, which interacts with the combustion gases and causes other constituents of the alloy to dissolve. The resulting melt prevents the oxide to be protective. Simultaneously, the large centrifugal stresses in the blade attachment area can cause the substrate to crack. If undetected, such cracks will grow and result in catastrophic failure. The current work consisted in the construction and commissioning of a customised test bench reproducing the environments experienced at the root of the turbine blades. The investigation compared a first-generation Ni-based superalloy with 12.2 wt% Cr (SCRY-83) commonly referred to as “Cr2O3-former” in regard to its oxide composition, to a second generation one with 6.5 wt% Cr (SCRY-4) also referred to as “Al2O3-former”. A mixture of NaCl and Na2SO4 is often used in the literature to reproduce hot corrosion. The effect of each deposits was first analysed independently. It was found that NaCl caused scale damage, via the formation of high vapor pressure compounds for the Al2O3-former, or via the transport of Cr to the gas interface for the SCRY-83. In contrast, Na2SO4 formed a liquid phase at the surface of both alloys, but initiated damage only when in contact with the Al2O3-former. Sulphur ingress was the largest on sample coated with NaCl and exposed to a mixture of air and SO2/SO3, affecting particularly the Al2O3-former. The simultaneous application of force in these environments always resulted in a larger reaction layer, and was thought to originate from the flow of liquid phases (NaCl-NiCl2 when coated with NaCl, or Ni3S2 when exposed to air+SO2/SO3, or Na2SO4-NiSO4 when coated with NaCl and exposed to air+SO2/SO3). The type II environment with applied force was the most aggressive and it initiated two cracks on the Al2O3-former, and a 40 µm thick damaged region was detected on the Cr2O3-former. Proven and novel methods of damage characterisation were assessed throughout this work, and their suitability was found to depend on the harshness of the environment. The high Cr alloy always outperform the Al2O3-former or exhibited similar depth of attack (when coated with NaCl).
Hot corrosion, single crystal, sulphidation, type II, superalloys, salts, sea salts, sodium sulphate
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