Effect of strengthening additions on the oxidation and sulphidation resistance of cast Ni-base superalloys

摘要

In order to increase the efficiency and decrease the costs of power generation, gas turbines have to be able to operate using a wide range of alternative fuels, such as crude oil, biogas or unclean syngas. Many of the alternative fuels contain substantial amounts of contaminants, especially sulphur. This means that gas turbine components will be exposed to environments in which not only oxidation but also sulphidation will be an issue. Such components are typically coated with protective coating systems, including MCrAlY-coatings and thermal barrier coatings. However, during service, after the long term exposure to cyclic temperature coatings may crack or spall, thereby allowing direct access of the hot gases to the superalloys surface. Therefore the oxidation resistance of the Ni-base superalloy to oxidizing/sulphidizing atmospheres becomes an important factor for the lifetime of the turbine components. In the present work the oxidation behaviour of Ni-base superalloys in high pO2, SO2-containing environments have been studied. For this purpose, screening tests of commercial alloys commonly used in stationary gas turbines, were performed to establish the sensitivity of alloys to the presence of SO2 in the test gas (synthetic air). Based on the screening test results, commercial Ni-base superalloys with similar Cr and Al contents were chosen to determine which parameters affect the material resistance against the enhanced attack by SO2. In the first part of the thesis, the behaviour of alloys with 12-14% Cr and 3-4% Al, namely PWA 1483 and Rene 80, in synthetic air is compared with that in synthetic air + 2% SO2 at 1050°C. The second part of the present investigation concerns the behaviour of Ni-base superalloys with 5-6% Al and 6-8% Cr. For the latter comparison two alloys were chosen: CMSX 4 and CM 247. The results of the present work clearly show that Ni-base alloys such as PWA 1483 and Rene 80, as well as CMSX 4 and CM 247 exhibit tremendous differences in resistance to SO2 attack in high pO2 gas, in spite of possessing similar Cr and Al contents. The overall corrosion behaviour in the SO2 containing gas critically depends on the other alloy constituents. The far better resistance of PWA 1483 and CMSX 4 in the SO2-containing environment is shown to be related to the rapid development of a dense alumina scale, which prevents access of molecular SO2 to the metal surface and effectively supresses sulphidation. In contrast, a porous chromia based scale formed on Rene 80. The latter type of surface scale allows easy molecular access of SO2, which eventually results in breakaway oxidation triggered by formation of internal chromium sulphide. The formation of fast growing porous chromia scale on Rene 80 was attributed to the Ti addition of 5 wt. %, which increases the growth rate of the Cr2O3-scale by p-type doping thereby suppressing the formation of a protective alumina scale. Furthermore, Ta addition to Ti containing Ni-base alloys, such as in PWA 1483, was found to promote external alumina scale formation by forming a mixed Ti/Ta oxide compound, hence preventing the enhancement of chromia growth by Ti incorporation. The effect of Ti and Ta on the scale formation was verified by using model alloys of the same base composition Ni-9Co-14Cr-3Al. Owing to a high Al-content of 5.6 wt. % CMSX 4 formed rather pure alumina scale after relatively short period of transient oxidation. In contrast, CM 247 with the same Al-content formed an Al-rich oxide scale with high amounts of Hf and B-containing oxides, which compromised its resistance to sulphidation and resulted in rapid failure in synthetic air + 2% SO2. Using model alloys it was shown that the addition of B is detrimental for the oxidation resistance of Ni-base alloys with 8 wt. % Cr and 5-6 wt. % Al. The formation of Al/B mixed oxides can explain why after short time exposure the Al2O3 formation was locally hindered, thereby allowing transport of SO2 and enhanced formation of internal Cr-sulphide. The reason for the rapid enrichment of B within the scale is that it forms a thermodynamically very stable oxide combined with much faster boron diffusivity in the Ni-rich matrix as compared to Al and Cr. The rapid incorporation of boron into the oxide scale apparently resulted in boron depletion from the alloys. This was indicated by specimens exposed for 500 h to synthetic air which showed continuous Al2O3 scale formation for all three studied model materials.