Solid Deposit-Induced High Temperature Oxidation

The present study is aimed at investigating the high temperature oxidation induced by ash deposition from use of alternative fuels. The alloys and coatings being studied are typical of those used in current power generating gas turbines, as well as those that may be used in advanced systems. To achieve this objective, the alloys Rene„S N5, GTD 111, and IN 738 as well as these alloys coated with platinum aluminide and CoNiCrAlY were exposed to conditions relevant to corrosion induced by using alternative fuels. The test conditions representative of deposits from use of alternative fuels were selected based upon initial experiments that involved testing the alloy Rene„S N5 with a platinum aluminide coating at 750„aC, 950„aC, and 1150„aC in a variety of environments with deposits of CaO, CaSO4, and Na2SO4. Based upon the results from such tests, a temperature (950oC) and a deposit (CaO) were selected for the further experiments to compare the corrosion characteristics of all of the alloys and coatings. At 950oC with deposits of CaO, which are the selected experimental conditions obtained from the preliminary tests, accelerated cyclic oxidation experiments were performed with all uncoated and coated superalloys in extra dry air and wet (PH2O=0.1atm) air to compare corrosion characteristics of each with one another. Experimental details will be described followed by the presentation of experimental results and discussion. Additionally, uncoated GTD 111 specimens were exposed to different contaminants and moisture level environments to study the effect of contaminant level and water vapor pressure on CaO-induced degradation. Then, CaO deposits were coated on thermal barrier coatings (TBCs) and specimens with TBCs were exposed to the cyclic oxidation environments. The effects of deposits other than CaO, such as Fe2O3 and SiO2, on the oxidation characteristics of the specimens were also investigated. Finally, a mechanism for high temperature oxidation induced by CaO deposits was developed. It turns out that CaO directly reacts with protective oxides, such as Al2O3 and/or Cr2O3, to form non-protective ternary Ca compounds. Cracks are initiated and propagate along the weak interface between Ca compounds and underlying oxide layers resulting in spallation of Ca compound layers