Corrosion Behavior of Hastelloy-XR Alloy in O 2 and SO 2 Atmosphere

Transcription

1 Materials Transactions, Vol. 46, No. 8 (2005) pp. 882 to 889 #2005 The Japan Institute of Metals Corrosion Behavior of Hastelloy-XR Alloy in O 2 and SO 2 Atmosphere Rong Tu and Takashi Goto Institute for Materials Research, Tohoku University, Sendai , Japan As Hastelloy-XR alloy is a candidate structural material for the IS (Iodine-Sulfur) process in hydrogen production, oxidation and sulfidation of Hastelloy-XR alloy in Ar O 2 and Ar SO 2 atmospheres were studied by thermogravimetry at temperatures from 000 to 300 K. In Ar O 2 atmosphere, the mass change obeyed a linear-parabolic law at oxygen partial pressures (P O2 ) from 0.0 to 0 kpa. The oxidation scales consisted of inner Cr 2 O 3 layer and outer Mn :5 Cr :5 O 4 spinel layer. The surface morphology of the oxide scales changed from island-like to buckled and to porous texture with decreasing P O2. In Ar SO 2 atmosphere, the mass change obeyed a linear-parabolic law at SO 2 partial pressures (P SO2 ) from 0.05 to 5 kpa. The morphology of corrosion scales changed mainly with corrosion temperature. While oxidation was dominant at 073 and 73 K forming double-layer scales of inner Cr 2 O 3 and outer Mn :5 Cr :5 O 4 spinel, sulfidation was accompanied with oxidation at 273 K and P SO2 < 0:5 kpa with scales consisting of Fe 3 O 4, FeCr 2 O 4 and Cr 2 O 3 layers and Ni 3 S 2 dispersed particles together with CrS particles segregating at the grain boundary of Hastelloy-XR alloy. (Received May 0, 2005; Accepted June 9, 2005; Published August 5, 2005) Keywords: Hastelloy-XR alloy, SO 2 gas, oxidation, sulfidation, corrosion, Iodine-Sulfur process. Introduction Since hydrogen can be a new clean energy source to reduce CO 2 emission and the green house warming, it is a key issue to develop a mass production process of hydrogen. Hydrogen can be produced by a thermochemical decomposition of water, and recently IS (Iodine-Sulfur) process has succeeded to produce hydrogen from water in a continuosly operated closed cycle. IS process is fundamentally composed of three processes, i.e., sulfuric acid (H 2 SO 4 ) decomposition around 23 K, Bunsen reaction at 373 K and hydrogen iodide decomposition at 473 to 573 K.,2) In the process of H 2 SO 4 decomposition, SO 3 and H 2 O are produced, and then SO 3 decomposed into SO 2 and O 2. Such mixing atmosphere of SO 2 and O 2 is significantly corrosive to structural metallic materials. 3) Thus, in bench-scaled IS process, the tubes and vessels have been made of quartz glass and polymers (mainly Teflon). 4) However, in order to develop large-scaled industrial chemical plants, commercially available heat-resistant alloys should be employed to construct the facility. The hightemperature mechanical properties and corrosion resistance of many candidate alloys have been investigated in such harsh environment, 5,6) and then Hastelloy-X alloy (49Ni 2Cr 8Fe 9Mo 0.6Mn 0.8Si, in mass %) has been expected as the structural material because of its high-temperature strength and creep resistance. 7) However, this alloy had not enough corrosion resistance in IS process. On the other hand, to improve the oxidation resistance of Hastelloy-X alloy in a helium cooled very high temperature nuclear reactor (VHTR), the contents of Mn and Si were optimized and Hastelloy-XR alloy (49.47Ni 2.99Cr 7.80Fe 8.73Mo 0.88Mn 0.33Si, in mass %) has been developed. The oxidation behavior of Hastelloy-X alloy has been reported by Wlodek et al., 8) Charlot 9) and Shindo et al. 7,0) However, the oxidation behavior of Hastelloy-XR alloy has not been investigated in wide-ranged temperatures and oxygen partial pressures. Moreover, no corrosion resistance of Hastelloy- XR alloy in SO 2 atmospheres at high temperatures has been studied. In the present study, the corrosion behavior of Hastelloy-XR alloy as a candidate structural material for IS process was investigated in O 2 and SO 2 atmospheres at high temperatures. Since SO 2 would cause oxidation or sulfidation depending on temperature and P SO2, the oxidation in O 2 atmosphere was separately examined to understand the corrosion (oxidation and/or sulfidation) in SO 2 atmosphere. 2. Experimental Hastelloy-XR alloy disks (0 mm in diameter by mm thickness) were polished with an alumina paste ( mm) and supersonically cleaned in acetone. The specimens were exposed to Ar O 2 and Ar SO 2 atmospheres at oxygen partial pressure (P O2 ) and SO 2 partial pressure (P SO2 ) from 0.0 to 0 kpa at temperatures from 000 to 300 K for 43 ks. Ar O 2 and Ar SO 2 mixture gases were introduced from the bottom of a reaction tube at a flow rate of 6:7 0 6 m 3 s. The total pressure in the reaction tube was fixed at 0. MPa. The mass changes were continuously measured by thermogravimetry (A&T: HA202M, sensitivity: 0 mg). Crystal structure, microstructure and composition of corrosion scales were analyzed by X-ray diffraction (XRD, Rigaku: RAD-C), scanning electron microscopy (SEM, Hitachi: S-300H), transmission electron microscopy (TEM, JEOL: EX-II) and electron probe microanalysis (EPMA, JEOL: JXA62MX). The potential diagrams of Fe S O, Ni S O and Cr S O system were calculated by using the thermodynamic database to estimate the corrosion behavior in Ar SO 2 atmosphere at high temperatures.,2) 3. Results and Discussion 3. Oxidation behavior in Ar O 2 atmosphere Figure presents the XRD pattern of Hastelloy-XR alloy after the oxidation for 43 ks at 273 K and P O2 ¼ kpa. The oxide scales consisted of Cr 2 O 3 and Mn :5 Cr :5 O 4 spinel phases, and such two-phase oxide scales were observed independent of oxidation conditions. Figure 2 depicts the cross-sectional back-scattering SEM image and EPMA analyses for the scales formed at 273 K and P O2 ¼ 0 kpa. The oxide scale of 4 mm in thickness consisted of double layers with inner Cr 2 O 3 and outer Mn :5 Cr :5 O 4 spinel.

2 Corrosion Behavior of Hastelloy-XR Alloy in O 2 and SO 2 Atmosphere 883 Intensity (a. u.) Mn.5 Cr.5 O 4 Cr 2 O 3 Hastelloy-XR θ / (Cu kα) Fig. X-ray diffraction pattern of the scale on the Hastelloy-XR alloy after the oxidation for 43 ks at P O2 ¼ kpa and at 273 K. 00 µm Hastelloy-XR 0 µm Mn.5 Cr.5 O 4 Cr 2 O 3 (c) 20 µm Fe Ni Mn Fig. 2 Cross-sectional back-scattering SEM image and EPMA analyses of the scale on the Hastelloy-XR alloy after the oxidation for 43 ks at 273 K and P O2 ¼ 0 kpa. Figures 3 to (c) demonstrate the surface morphology of oxide scales on the Hastelloy-XR alloy after the oxidation at 273 K and P O2 from 0.0 to 0 kpa. Although the scales always consisted of Cr 2 O 3 and Mn :5 Cr :5 O 4 spinel layers, the surface morphology significantly changed with P O2. Island-like scales were observed at P O2 ¼ 0 kpa [Fig. 3]. It was confirmed by XRD that the islands in Fig. 3 was Mn :5 Cr :5 O 4 spinel and the flat area was Cr 2 O 3 by mechanically removing the islands. During the cooling process, sudden mass drops were often observed, implying the island-like scale was resulted from the partial delamination of spinel outer layer due to thermal expansion mismatch between Cr 2 O 3 and spinel layers. Buckled oxide scales were observed at P O2 ¼ 0: kpa [Fig. 3], which could be also Cr O 0 µm Fig. 3 Surface SEM images of scales on the Hastelloy-XR alloy after the oxidation for 43 ks at 273 K, P O2 ¼ 0, 0. and 0.0 kpa (c). caused of the thermal expansion mismatch between two layers with the spinel layer remaining on the Cr 2 O 3 layer. Well-adhered uniform scales with a large amount of micropores were formed at P O2 ¼ 0:0 kpa [Fig. 3(c)]. The micropores could relax the thermal stress between two layers. Ecer et al. conducted a marker test for the oxidation of Ni Cr alloy at 073 to 373 K. 3) The platinum marker stayed at the interface between alloy and oxide scales. This implies that the outward diffusion of cations could dominate the oxidation of Ni Cr alloys. Since Ni and Cr are major components in Hastelloy-XR alloy, the similar oxidation mechanism (i.e., outward diffusion of cations) can be dominant in the present study. Figure 4 presents the cross-sectional back-scattering SEM image after the oxidation for 432 ks at 273 K and P O2 ¼ 0 kpa. The thickness of Cr 2 O 3 inner layer increased

3 884 R. Tu and T. Goto µm Hastelloy-XR Cr 2 O 3 Mn.5 Cr.5 O 4 Fig. 4 Cross-sectional back-scattering SEM image of the scale on Hastelloy-XR alloy after the oxidation for 432 ks at 273 K and P O2 ¼ 0 kpa. Mass change, M / kg m Time, t / s 0 4 P O2 / kpa Fig. 6 Relationship between mass change and time for the oxidation of Hastelloy-XR at 273 K in Ar O 2 atmosphere. 0 5 Mn.5 Cr.5 O 4 Cr 2 O 3 Hastelloy-XR 200 nm Fig. 5 Cross-sectional TEM image of the scale on the Hastelloy-XR alloy after the oxidation for 43 ks at 273 K and P O2 ¼ 0:0 kpa. significantly with time, and the slight increase in the spinel outer layer was observed. Douglass et al. 4) and Shindo et al. 7) have obtained the similar results for the oxidation of Ni Cr Mn alloys. Douglass et al. reported that the double-layer scales of inner Cr 2 O 3 and outer MnCr 2 O 4 spinel were formed on a Ni 20Cr Mn alloy after the oxidation at 373 K in air. 4) Shindo et al. reported that the increase in Cr 2 O 3 inner layer thickness was much faster than MnCr 2 O 4 outer layer in the oxidation of Hastelloy-X alloys and suggested that the oxidation reaction should proceed mainly at the interface between inner and outer layers. 7) Present study clearly indicated that the growth rate of Cr 2 O 3 inner layer was significantly greater than that of Mn :5 Cr :5 O 4 outer layer, implying that the oxidation of Hastelloy-XR alloy in the later stage was mainly dominated by outward diffusion of Cr 3þ ion. Figure 5 presents the TEM micrograph for the crosssection of the oxide scale formed at 273 K and P O2 ¼ 0:0 kpa after 43 ks. The double-layer structure with inner Cr 2 O 3 and outer Mn :5 Cr :5 O 4 spinel was observed. The diffusion rates of metallic ions in oxides including Cr 2 O 3 were reported as Fe 3þ, Mn 2þ > Fe 2þ > Ti 3þ > Co 2þ > Ni 2þ > Mn 3þ > Cr 3þ. 5) It is assumed that the spinel layer formed at the outside of Cr 2 O 3 due to faster diffusion of Mn 2þ ion than that of Cr 3þ ion particularly in an early stage Log(Linear rate constant, k l / kg m -2 s - ) Temperature, T / K P O2 / kpa (air) 8) Reciprocal of temperature, T - / 0-4 K Fig. 7 Arrhenius plots of linear rate constants for the oxidation of Hastelloy-XR alloys. of oxidation. 5,6) The thickness of outer layer was thicker than inner layer. Figure 6 shows the mass change of Hastelloy-XR alloy as a function of oxidation time at 273 K. The mass change increased with increasing time and P O2. The time dependence of mass change obeyed a linear-parabolic law at P O2 from 0.0 to 0 kpa. The linear (k l ) and parabolic rate constants (k p ) for the oxidation of Hastelloy-XR alloy were calculated separately from the time dependence of mass change. Figure 7 depicts the Arrhenius plots of linear rate constants (k l ) for the oxidation of Hastelloy-XR alloy. The k l increased with increasing the oxidation temperature and P O2. Wlodek studied the oxidation of Hastelloy-X alloy in air from 40 to 470 K and reported the linear-parabolic mass change behavior at less than 255 K. 8) The transition from linear to parabolic behavior occurred between 2 and 36 ks, which

4 Corrosion Behavior of Hastelloy-XR Alloy in O 2 and SO 2 Atmosphere 885 Log(Parabolic rate constant, k p / kg 2 m -4 s - ) kpa(air) 8) P O2 / kpa K 9) Temperature, T / K P O2 =0-7 ~0-20 Pa 7) 00 P O2 / kpa µm Reciprocal of temperature, T - / 0-4 K - Fig. 8 Arrhenius plots of parabolic rate constants for the oxidation of Hastelloy-XR alloys. was longer than that in the present study (2 to 0 ks), and the k l obtained by Wlodek (P O2 ¼ 20 kpa) were close to that of P O2 ¼ 0: kpa in the present study. In the present study, the time for starting the parabolic behavior increased with decreasing P O2, which coincides the general trend of linear to parabolic transition. 7) These suggest that Hastelloy-XR alloy can form protective oxide scale more easily than Hastelloy-X alloy. Figure 8 depicts the Arrhenius plots of parabolic rate constants (k p ) for the oxidation of Hastelloy-XR alloy. The k p values increased with increasing temperature and P O2. Wlodek obtained the k p values of Hastelloy-X alloy in air 8) which were almost the same as those of Hastelloy-XR alloy in P O2 ¼ 0 kpa. Charlot et al. reported that the k p values of Hastelloy-X alloy increased with increasing P O2 from 5 Pa to 3.3 kpa at 393 K. 9) Their values are almost in agreement with the present values extrapolated to higher temperatures. Shindo et al. obtained slightly lower k p values of Hastelloy-X alloy than the present results. 7) They studied in a helium atmosphere containing a small amount of impurity (water vapor and carbon dioxide), in which P O2 was estimated as 0 7 to 0 20 Pa. Charlot et al. 9) and Shindo et al. 7) reported that the activation energy (E a ) for the parabolic oxidation of Hastelloy-X alloy was 234 kj mol, which is almost in agreement to that for the diffusion of Cr 3þ in Cr 2 O 3 reported by Giggins et al. (255 kj mol ). 8) Charlot et al. and Shindo et al. implied that the rate-controlling step for the parabolic oxidation of Hastelloy-X alloy could be the diffusion of Cr 3þ in Cr 2 O 3 layer. In the present study, the E a for the parabolic oxidation of Hastelloy-XR alloy was 220 kj mol, which almost agreed with those of Hastelloy-X alloy reported by Shindo et al. 7) and Wlodek (238 kj mol ). 8) Therefore, the diffusion of Cr 3þ in Cr 2 O 3 could be the rate-controlling step for the parabolic oxidation of Hastelloy-XR alloy. 3.2 Corrosion behavior in Ar SO 2 atmosphere Oxide scales consisting of Cr 2 O 3 and Mn :5 Cr :5 O 4 spinel 2 µm were formed on the Hastelloy-XR alloy after the corrosion in Ar SO 2 atmosphere between 073 and 273 K, as observed in Ar O 2 atmosphere. Figure 9 depicts the surface, crosssectional SEM images and EPMA analyses of the scale on the Hastelloy-XR alloy after the corrosion for 43 ks at 073 K and P SO2 ¼ kpa. Buckled scales were observed on the surface, being almost the same as that formed at 273 K and P O2 ¼ 0: kpa in Ar O 2 atmosphere as shown in Fig. 3. The EPMA analysis showed that the buckled scale consisted of inner Cr 2 O 3 layer and outer Mn :5 Cr :5 O 4 layer, and a Fe Ni Mn Fig. 9 Surface and cross-sectional SEM images of the scale on the Hastelloy-XR alloy after the corrosion for 43 ks at 073 K and P SO2 ¼ kpa. Cr O S

5 886 R. Tu and T. Goto Mass change, M / kg m K, P SO2 =0. kpa 073 K, P SO2 = kpa 273 K, P SO2 =0. kpa 273 K, P SO2 = kpa 2 00 µm Time, t / s 0 5 Fig. Relationship between mass change and time for the corrosion of Hastelloy-XR in Ar SO 2 atmosphere. (c) Hastelloy -XR CrS E D CrS Ni 3 S 2 C. B A 20 µm Ni 3 S 2 Fig. 0 Surface SEM, cross-sectional back-scattering SEM image and a schematic of cross-section of the scale on the Hastelloy-XR alloy (c) after the corrosion for 43 ks at 273 K and P SO2 ¼ 0: kpa. (A: Fe 3 O 4 (Ni 3 S 2 ), B: FeCr 2 O 4 (Ni 3 S 2 ), C: Cr 2 O 3, D: Ni Fe Mo(CrS), E: Hastelloy-XR) small amount of sulfur was identified near the surface of Hastelloy-XR alloy. No CrS or Ni 3 S 2 was detected by XRD. At 273 K, the corrosion scales showed two kinds of microstructures depending on P SO2.AtP SO2 > 0:5 kpa, the scale was almost the same as that shown at Fig. 9. At P SO2 < 0:5 kpa, on the other hand, the scale had a multi-layer microstructures with dispersions of sulfides as shown in Fig. 0, where the surface SEM, cross-sectional backscattering SEM image and a schematic of cross section (c) of the scale on the Hastelloy-XR alloy after the corrosion for 43 ks at 273 K and P SO2 ¼ 0: kpa were demonstrated. Partially delaminated scale with many bumps was observed [Fig. 0]. The corrosion scale consisted of Fe 3 O 4 with Ni 3 S 2 particles (layer A), FeCr 2 O 4 with Ni 3 S 2 particles (layer B), Cr 2 O 3 (layer C), Ni Fe Mo metal with CrS particles (layer D) and Hastelloy-XR substrate (layer E) as depicted in Fig. 0(c). It was reported that continuous Cr 2 O 3 layers were formed in SO 2 atmosphere at high temperatures for Ni Cr alloys containing high-content Cr. 9,20) Hancock et al. studied the corrosion of Ni 20Cr alloy in SO 2 atmosphere at 73 K, and reported that the Cr 2 O 3 scale contained Ni 3 S 2 particles, and the internal sulfidation caused the segregation of CrS at the grain boundary in the alloy. 9) Zurek et al. reported that the mass change of Ni 22Cr 0Al Y alloy almost obeyed a parabolic law, and scales consisted of Cr 2 O 3,Al 2 O 3, NiO, NiCr 2 O 4 and a small amount of AlS at 73 to 273 K in SO 2 atmosphere. 20) In the present study, continuous Cr 2 O 3 layers were formed on Hastelloy-XR alloy because Hastelloy-XR alloy contained enough amount of Cr. The dispersion of Ni 3 S 2 particles in the oxide scales and the segregation of CrS at the grain boundary in the present study were similar to the results by Hancock 9) and Zurek. 20) The corrosion of Hastelloy-XR alloy became more significant at high temperature and low sulfur potential (at 273 K and P SO2 < 0:5 kpa), and CrS was observed at the grain boundary of Hastelloy-XR alloy due to the internal sulfidation of Cr. This behavior was similar to that of Ni Cr, Fe Cr and Co Cr alloys in H 2 S H 2 atmosphere studied by Narita et al. 2) They reported that Cr-containing alloys were hardly sulfidized at high sulfur potentials, however sulfides were easily formed at low sulfur potentials. At low sulfur potentials, copious internal sulfidation (CrS x layer) was formed for Ni Cr alloys, whereas sulfidation was confined to grain boundary for Fe Cr alloys. As Hastelloy-XR alloy is a Ni Cr Fe based alloy, the internal sulfidation mainly occurred at the grain boundary and no CrS x layer was formed. Figure demonstrates the relationship between the mass change and time for the corrosion of Hastelloy-XR alloy at

6 Corrosion Behavior of Hastelloy-XR Alloy in O 2 and SO 2 Atmosphere 887 Log(Linear rate constant, k l / kg m -2 s - ) Log(Linear rate constant, k l / kg m -2 s - ) T / K Log(Partial pressure of SO 2, P SO2 / Pa) T / K Log(Partial pressure of O 2, P O2 / Pa) Fig. 2 Relationship between linear rate constant (k l ) of Hastelloy-XR alloy and P SO2 from 073 to 273 K. Relationship between linear rate constant (k l ) of Hastelloy-XR alloy and P O2 decomposed by SO 2 from 073 to 73 K. Log(Parabolic rate constant, k p / kg 2 m -4 s - ) Log(Parabolic rate constant, k p / kg 2 m -4 s - ) -9-0 T / K Log(Partial pressure of SO 2, P SO2 / Pa) T / K Log(Partial pressure of O 2, P O2 / Pa) Fig. 3 Relationship between parabolic rate constant (k p ) of Hastelloy- XR alloy and P SO2 from 073 to 273 K. Relationship between parabolic rate constant (k p ) of Hastelloy-XR alloy and P O2 decomposed by SO 2 from 073 to 73 K. 073 to 273 K and P SO2 ¼ 0: to kpa. The mass change obeyed a linear-parabolic law at the whole conditions, and increased with increasing temperature. The linear to parabolic transition occurred at about 20 ks, which was longer than that of oxidation (2 to 0 ks in Fig. 6) implying that it is more difficult to form protective scales in Ar SO 2 rather than in Ar O 2 atmosphere. At 073 K, the mass change at P SO2 ¼ kpa was higher than that at P SO2 ¼ 0: kpa, but the trend was opposite at 273 K. Since the oxidation was dominant at 073 K as above-mentioned, the mass change increased with increasing P O2 calculated from the decomposition of SO 2 as given by eqs. () to (4). When a temperature increases from 073 to 273 K at P SO2 ¼ 0: kpa, the equilibrium P S2 increases from 4:3 0 2 to 2:8 0 0 kpa and P O2 increases from 8:6 0 2 to 5:6 0 0 kpa. SO 2 ¼ 2 S 2 þ O 2 ðþ G T ¼ RT lnðp 2 S2 P O2 =P SO2 Þ ð2þ P O2 ¼ 2P S2 ð3þ therefore, 2 P O2 ¼ :44P SO2 exp G 3 T RT ð4þ Figure 2 shows the relationship between linear rate constant (k l ) and P SO2 for the corrosion of Hastelloy-XR alloy in Ar SO 2 atmosphere. At 073 and 73 K, the k l increased with increasing P SO2, but decreased with increasing the P SO2 at 273 K [Fig. 2]. As the corrosion of Hastelloy-XR alloy at 073 and 73 K was almost the same as the behavior of oxidation as described in 3., the relationship between k l and P O2 calculated from eq. (4) was demonstrated in Fig. 2. The k l slightly increased with increasing P O2. Figure 3 shows the relationship between parabolic rate constant (k p ) and P SO2 for the corrosion of Hastelloy-XR alloy. The trend of k p was almost the same as that of k l. The relationship between k p and P O2 calculated from eq. (4) was

7 888 R. Tu and T. Goto depicted in Fig. 3. The linear relation was observed at 073 and 73 K where k p / P =n O 2 (n ¼ 5 to 6). It is generally understood that Cr 2 O 3 was a p-type semiconductor, and the defect formation reaction could be expressed as eq. (5). 3 2 O 2 ¼ 3O O þ 2VCr 000 þ 6h ð5þ The equlibrium constant for reaction (5) is given by eq. (6) because the concentration of vacancyes formed is proportional to the concentration of electron holes. K eq ¼ ðv000 Cr Þ2 ðh Þ 6 ðp O2 Þ 3=2 ¼ ðv000 Cr Þ2 ð3v 000 Cr Þ6 ðp O2 Þ 3=2 Therefore, VCr 000 / const. P3=6 O 2 ð7þ The 3/6-power relationship can be predicted in the oxidation where the Cr 2 O 3 formation is the rate controlling step. Charlot et al. reported the relationship of k p / P =n O 2 (n ¼ 5). 9) Present results of n ¼ 5 to 6 can be close to that of Charlot et al. 9) and above-mentioned calculated results. The potential diagram can be useful to understand the corrosion behavior and the formation of oxide or sulfide scales in SO 2 atmosphere. The corrosion in SO 2 atmosphere may occur by dissociated O 2 or S 2, thus the oxygen and sulfur potentials may determine the corrosion process. We have calculated the potential diagrams by using thermodynamic database to compare with the experimental results. Figure 4 represents the potential diagrams for Ni S O, Fe S O and Cr S O systems at 273 K, where the three diagrams were superposed in one diagram. The P S2 and P O2 at P SO2 ¼ 0: kpa are :3 0 0 and 2:6 0 0 kpa, respectively. These values correspond to point P in Fig. 4 and located in the stable region of Cr 2 O 3,Fe 3 O 4 and Ni. The outermost layer of the corrosion scale consisted of Fe 3 O 4 with Ni 3 S 2 particles and no NiO [layer A in Fig. 0(c)]. In the layer A, the P O2 can be in the stable area of Fe 3 O 4 and Ni, and the P S2 can be in the stable area of Ni 3 S 2. Therefore, the P O2 and P S2 in the outermost layer (layer A) can be in the area A in Fig. 4. The second layer of the corrosion scale consisted of FeCr 2 O 4 and Ni 3 S 2 particles [layer B in Fig. 0(c)]. In the log(p S2 / kpa) Cr Cr 3 S 4 (s) CrS(s) E Cr(s) Fe FeS(s) Ni 3 S 2 (l) D NiS(s) C Cr 2 O 3 (s) Fe(s) FeO(s) B FeSO 4 (s) A P Cr 2 (SO 4 ) 3 (s) Fe 3 O 4 (s) Ni(s) NiO(s) Ni log(p O2 / kpa) Fe 2 (SO 4 ) 3 (s) NiSO 4 (s) Fe 2 O 3 (s) -3 2 Fig. 4 Potential diagrams for Ni S O, Fe S O and Cr S O systems at 273 K. ð6þ layer B, the P O2 can be in the stable area of FeO and Cr 2 O 3 because FeCr 2 O 4 may be formed by the reaction of FeO and Cr 2 O 3. The P S2 can be in the stable area of Ni 3 S 2. Therefore, the P O2 and P S2 in layer B can be in the stable area of FeO, Cr 2 O 3 and Ni 3 S 2 (area B in Fig. 4). The P O2 and P S2 in the third layer [layer C in Fig. 0(c)] can be in the stable area of Ni, Fe and Cr 2 O 3 (area C in Fig. 4) because the layer consisted of Ni, Fe and Cr 2 O 3. Since the layer D in Fig. 0(c) consisted of Ni and Fe with CrS particles, the P O2 and P S2 can be in the stable area of Ni, Fe and CrS (area D in Fig. 4). The P O2 and P S2 in the layer E of Fig. 0(c) can be in the stable area of Cr, Ni and Fe (area E in Fig. 4) because the layer was non-corroded Hastelloy-XR alloy. Since the P O2 and P S2 in the corrosion scale may continuously decrease from the outermost to inside, the P O2 and P S2 can be possibly changed along the broken line from point A to E in Fig. 4. The P S2 at the outermost of corrosion scale can be higher than the initial S 2 partial pressure. Since O 2 may be consumed significantly at the outermost layer, P S2 may be relatively increased more than the initial P S2. Therefore, the calculated potential diagrams could be applicable to understand the corrosion behavior in Ar SO 2 for Hastelloy-XR alloy. 4. Conclusions The corrosion behavior of Hastelloy-XR alloy in O 2 and SO 2 atmospheres were investigated in the temperature range between 073 and 273 K. In Ar O 2 atmosphere, the mass changes mainly obeyed a linear-parabolic law. The corrosion scales had a double-layer structure of inner Cr 2 O 3 and outer Mn :5 Cr :5 O 4 spinel layers. The surface morphology of scales changed from island-like to buckled and to smooth porous layers with decreasing P O2 from 0 to 0.0 kpa. In Ar SO 2 atmosphere, the mass changes obeyed a linear-parabolic law. The corrosion scales formed at 073 and 73 K were similar to the scales formed in Ar O 2 atmosphere at 073 K, because the oxidation was dominant at these low temperatures. At 273 K and P SO2 < 0:5 kpa, the corrosion scales were consisted of multi-layers changing from Fe 3 O 4 with Ni 3 S 2 particles (outmost), FeCr 2 O 4, Cr 2 O 3, and CrS particles (innermost). The sulfidation became more significant in less P SO2 at 273 K. The thermodynamic potential diagrams may be useful to understand the corrosion behavior of Hastelloy- XR alloy in Ar SO 2 atmosphere. Acknowledgments A part of this study was supported by Tokyu Foundation for Inbound Student. The authors thank Mr. Y. Murakami of Laboratory for Advanced Materials, Institute for Materials Research for EPMA analyses. REFERENCES ) M. Sakurai, H. Nakajima, K. Onuki, K. Ikenoya and S. Shimizu: Int. J. Hydrogen Energy 24 (999) ) A. Hammache and E. Bilgen: J. Energ. Resources Tech. 4 (992) ) Y. Kurata, K. Tachibana and T. Suzuki: J. Jpn. Inst. Met. 65 (200) ) T. Goto: Mater. Jpn. 40 (200)

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