Effects of Trace SO2 and Na2SO4 Deposit on the Reaction Behavior of Al2O3-scale Forming Alloys

This study focused on the effects of minor alloying elements (Si, Pt), trace SO2, and Na2SO4 deposit on oxidation and 900°C sulfate-deposit-induced hot corrosion behavior of Al2O3-scale forming alloys.\ud In the first part of this study, the effects of Si, Pt and Si+Pt on the oxidation and hot corrosion behavior of γ΄-Ni3Al-based alloys were studied. This was done by conducting exposure tests at 900°C in air, and in air with an Na2SO4 deposit (hot corrosion). All alloys showed similar behavior during oxidation in air, while an improved hot corrosion resistance was achieved with the addition of Si and Pt. The tracking of alloy performance during different stages of hot corrosion showed that: 1) Formation of a continuous and tenacious oxide scale of Al2O3 or (Al,Cr)2O3 is necessary for hot-corrosion protection; 2) Maintenance of a protective alumina scale or sub-scale depends on the scale adhesion and the re-healing capability of the alloy subsurface; and 3) Onset of breakaway corresponds to the alloy’s inability to maintain protective alumina scale formation. It was further found that the presence of Na2SO4 deposit favored α-Al2O3 establishment. The inferred reasons for this are presented.\ud The next focus of this study was on the effects of SO2 and Na2SO4 deposit on the oxidation and corrosion behavior of alumina-scale forming alloys. In one set of experiments, low Cr model and commercial alloys were oxidized in atmospheres with and without 0.1% SO2 or 1%SO2 at 900 or 1000˚C. It was found that a small amount of SO2 can be beneficial by promoting the transition from internal to external Al2O3-scale formation and favoring the early kinetic establishment of the thermodynamically stable α-Al2O3. In another set of experiments, Na2SO4-deposited PWA1484 were exposed at 900˚C in air and O2-SO2 gases for containing up to 0.1% SO2. With an increase in SO2 content, the extent of attack increased, particularly the extent of internal sulfidation. Thus, with a deposit, SO2 can be detrimental. This was due to SO2 introducing internal sulfidation as a mode of degradation beyond that found with 900°C hot corrosion. \u