Materials Science Forum Vols. 522-523

Abstract: The long term oxidation behaviour of TP 347H FG at ultra supercritical steam conditions was assessed by exposing the steel in test superheater loops in a Danish coal-fired power plant. The steamside oxide layer was investigated with scanning electron microscopy and energy dispersive X-ray diffraction in order to reveal the effect of oxidation time and temperature on the microstructure. A double layered oxide formed during steam oxidation. The morphology of the inner Cr-containing layer was influenced by the oxidation temperature. At temperatures below approx. 585oC, it consisted of regions of Fe-Ni-Cr spinel surrounded by Fe-Cr oxide. At higher temperatures almost the entire inner oxide layer was composed of Fe-Cr oxide. Possible mechanisms for the oxide growth are discussed and it is suggested that faster Cr transport within the alloy at higher temperatures explains the change in morphology. This hypothesis is supported by thermodynamic calculations and kinetic data. The thickness of the inner oxide layer did not change significantly with oxidation time and temperature for exposures less than 30000 h; however after 57554 h the thickness had increased significantly at the lowest temperatures.

Abstract: Achieving higher plant efficiency in thermal power plants is one of the major global challenges from the viewpoint of reducing carbon dioxide emission levels, particularly in coal-fired boilers, irrespective of the type of coal being burned. In recent times, it has been possible to increase the steam temperature in coal fired ultra supercritical (USC) plants without too much of a cost impact. The temperature has already been increased to 600
for main steam and 610
for reheat steam. The main enabling technology is the development of stronger high temperature materials such as newly developed high Cr ferritic steels and austenitic steels, capable of operating under high stresses at increasing high temperatures. Other key demands for those materials are hot corrosion resistance such as coal ash corrosion in superheater and reheater tubes and sulfidation of waterwall tubes, and steam oxidation resistance. This paper will mainly present the hot corrosion and steam oxidation properties of newly developed high strength heat resistant steels for their application to USC boilers and long-term experience in an actual plant.

Abstract: The high-temperature oxidation in air and steam at 923 K was examined with pure iron, Fe-10Cr and Fe-10Cr-0.08C (0~0.03)S steels. The amount of hydrogen dissolved into samples during exposure to steam was measured with thermal desorption spectroscopy (TDS). It was found that the amount of dissolved hydrogen was related closely to the steam oxidation resistance of the steels. In case of pure iron, the thickness of the oxide scale formed in steam at 923 K for 360 ks was comparable to that of the scale formed in air. On the other hand, in case of the Fe-Cr binary alloy and the ternary ferritic steel, the oxide scale was much thicker in steam than in air. It was found that the amount of the dissolved hydrogen was much larger in both the binary alloy and the ternary ferritic steel than in pure iron, and then it leads to the more accelerated oxidation rate in the ternary steels in steam.

Abstract: To achieve higher power generation efficiency in steam turbines, operating temperatures are expected to rise from 550°C to 650°C. The use of oxidation resistant coatings on currently available materials, with high creep strength but inferior steam oxidation resistance, is being explored in order to accomplish this goal in the context of the European project “Coatings for Supercritical Steam Cycles” (SUPERCOAT). Coating techniques have been chosen on the basis of being potentially appropriate for coating steam turbine components: the application of metallic and ceramic slurries, pack cementation and the deposition of alloyed and cermet materials by thermal spray. The coatings were characterised by metallography, SEM-EDS and XRD and steam oxidation and thermal cycling laboratory testing was carried out at 650º C. In this presentation, the testing results of selected coatings will be shown including those which exhibit the most promising behaviour. For instance, slurry aluminides have been exposed to steam at 650°C for more than 38,000 h (test ongoing) without evidence of substrate attack. Some HVOF coatings such as FeAl, NiCr and FeCr also have shown excellent behaviour. The results have provided information regarding the mechanism of protection and degradation of these coatings as well as insight into new coating development.

Abstract: Corrosion properties were investigated on stainless steels and Ni base alloys in supercritical water containing 0.01mol/kg-H2SO4 at 400°C/30MPa as a function of oxygen concentration ranging from 3ppb to 800ppm. Alloys with high Fe content showed an interesting contrast in corrosion property in sulfuric acid-containing supercritical water between at the high oxygen condition (800ppm) and at the lower oxygen conditions (3ppb and 8ppm). At 8ppm of oxygen concentration or lower, corrosion rate was a unique function of Cr content of the alloys including both Ni base alloys and stainless steels. However, corrosion resistance of the iron-based alloys (316 stainless steels) was remarkably improved when oxygen concentration was increased up to 800ppm. Corrosion rate of alloy C-276, which contains 5.7% of iron, was also somewhat reduced under the high oxygen condition, while corrosion rate of the other “iron-less” Ni base alloys was accelerated as oxygen level was increased. Characteristics of oxide scales, in terms of chemical composition and compound structure, have been examined in connection with the corrosion properties of the alloys.

Abstract: The oxidation behavior of Pt+Hf-modified γ-Ni+γ′-Ni3Al alloys containing up to 20 at.% Pt and either 15 or 20 at.% Al was studied by oxidizing the alloys in air at 1150°C under both isothermal and thermal cycling conditions. It was found that the co-addition of Pt and Hf was extremely beneficial to oxidation resistance, to the extent that Ni-20Al-20Pt-Hf and Ni-20Al-10Pt-Hf alloys (all compositions are in at.%) oxidized at significantly slower rates than that of a Ni-50Al-15Pt β-NiAl alloy. A Ni-20Al-5Pt-Hf alloy also showed good oxidation resistance, with the steady-state oxidation rate being almost the same as that obtained for the β alloy. Over a period of up to 500 one-hour oxidation cycles, no oxide spallation from the modified γ+γ′ alloys was observed. From cross-sectional SEM examination coupled with X-ray diffraction analyses, it was found that a compact and planar exclusive scale layer of α-Al2O3 formed on the Ni-20Al-20Pt-Hf alloy. By contrast, the Ni-20Al-10Pt-Hf and Ni-20Al-5Pt-Hf alloys formed a very thin outer layer of NiAl2O4 and a planar inner layer of α-Al2O3. The thickness of the inner Al2O3 layer increased with increasing oxidation time relative to that of the NiAl2O4 layer, meaning that the latter primarily formed during the initial stages of scale formation. Both NiO and NiAl2O4 were found in the scales formed on the Ni-20Al-Hf and Ni-15Al-0~10Pt-Hf alloys, with the thickness of these oxide layers decreasing with increasing Pt content in the alloys. Further, it was found that the extent of internal HfO2 formation decreased significantly with increasing Pt content, to the extent that no HfO2 was found in the oxidized Ni-20Al-20Pt-Hf alloy. Inferences for the observed beneficial effects of Pt promoting protective Al2O3 formation and decreasing the tendency for Hf to oxidize in γ+γ′ alloys are discussed.

Abstract: The early-stage oxidation behavior of γ '-Ni3Al-based alloys of composition (in at.%) Ni-22Al and Ni-22Al with 10, 20, and 30Pt was investigated in terms of oxidation kinetics, scale evolution and resulting composition profiles during heating to 1150°C in air. Platinum addition did not appear to affect the nature of the native oxide layer present on the γ '-based alloys at room-temperature; however, it was found that the presence of increasing Pt content aided in promoting the establishment of a continuous Al2O3 scale during heating the γ '-based alloys through to about 700°C. This beneficial effect can be primarily ascribed to the fact that Pt is non-reactive and its addition decreases the chemical activity of aluminum in γ '. Related to the latter, Pt partitions almost solely to the Ni sites in the ordered L12 crystal structure of γ ', which has the effect of increasing the Al:Ni atom fraction on a given crystallographic plane containing both Al and Ni. Such an effective Al enrichment at the γ ' surface would kinetically favor the formation of Al2O3 relative to NiO. A further contributing factor is that the Pt-containing γ '-based alloys showed subsurface Pt enrichment during the very early stages of oxidation. This enrichment would reduce Ni availability and increase the Al supply to the evolving scale, thus kinetically favoring Al2O3 formation.

Abstract: The applicability of 2CaO·SiO2-CaO·ZrO2 ceramic coatings as thermal barrier coatings (TBCs) was investigated. Coatings consisting of various ratios of 2CaO·SiO2-CaO·ZrO2 bond-coated with NiCrAlY were prepared using the plasma spray process. The structure of the coatings was characterized by scanning electron microscopy and X-ray diffraction analysis. The resistance of the coatings to thermal shock was evaluated with acoustic emission techniques under a thermal cycle from 1273 K to room temperature, and the hot corrosion resistance of the coatings was investigated with V2O5 and Na2SO4 at 1273 K for 3 h. The 2CaO·SiO2-10~30mass%CaO·ZrO2 coatings had excellent thermal shock resistance, because the coatings contained a vertical micro-crack in a single flattened ceramic particle. These coatings possessed excellent corrosion protection preventing direct contact between the corrosive ashes and a NiCrAlY bond coating. The CaO in the coating reacted with vanadium compounds and inhibited the penetration of corrosive ashes to the bond coating. The developed 2CaO·SiO2-20mass%CaO·ZrO2 thermal barrier coating on stationary vanes was evaluated in an actual gas turbine. The ceramic coating did not separate from the bond coating and reacted with SOx in combustion gas to produce a stable sulfate (CaSO4), which fixed in the coating. The TBC effectively protected the metal substrate of the vanes in practical operating condition for 25,000 h.

Abstract: Thermal barrier coatings(TBCs) are used in high temperature gas turbines to reduce the surface temperature of cooled metal parts such as turbine blades[1]. TBC consist of a bondcoat (e.g. MCrAlY where M is Co, Ni, CoNi, etc.) and a partially stabilized zirconia ceramic topcoat. Usually, the MCrAlY bondcoat is applied by LPPS (low pressure plasma spray) or HVOF(high velocity oxi-fuel spray). The topcoat is applied by APS (atmospheric plasma splay) or EB-PVD (electron beam-physical vapor deposition). High temperature oxidation properties, thermal barrier properties and durability of TBC are very important to increase the reliability in high temperature service. In this study, new TBC has been investigated. The new TBC consists of a two-layered bondcoat (LPPS-MCrAlY plus dense PVD overlay MCrAlY) and the EB-PVD type YSZ columnar structure topcoat. As a result of evaluation tests, it was confirmed that the new TBC had better oxidation properties and durability than a conventional TBC system.

Abstract: In order to prepare a highly oxidation-resistant surface for TiAl and SUS 304 stainless steel, the molten salt electrodeposition of Al or Y on these metals was carried out. The electrodeposition was conduced using a potentiostatic polarization method at constant potentials in an equimolar NaCl-KCl melt containing AlF3 or YF3 at 1023 K. After the Al electrodeposition, homogenous deposit layers were formed on the TiAl and the stainless steel. The deposited layer formed on the TiAl consisted of TiAl3. The deposited layer formed on the stainless steel consisted of some Fe aluminides. The TiAl and the stainless steel covered by the electrodeposited layers were far more resistant than the bare TiAl and stainless steel to high temperature oxidation. The Y electrodeposition on the stainless steel induced the deposition of Y particles on the stainless steel. The cyclic-oxidation resistance of the electrodeposited stainless steel was remarkably improved as compared to the untreated stainless steel.