Effects of exposure temperature and time on corrosion behavior of a ferritic–martensitic steel P92 in aerated supercritical water

Accepted Manuscript Effects of exposure temperature and time on corrosion behavior of a ferriticmartensitic steel P92 in aerated supercritical water Xiangyu Zhong, Xinqiang Wu, En-Hou Han PII: DOI: Reference:

S0010-938X(14)00479-X http://dx.doi.org/10.1016/j.corsci.2014.10.022 CS 6052

To appear in:

Corrosion Science

Received Date: Accepted Date:

2 May 2014 20 October 2014

Please cite this article as: X. Zhong, X. Wu, E-H. Han, Effects of exposure temperature and time on corrosion behavior of a ferritic-martensitic steel P92 in aerated supercritical water, Corrosion Science (2014), doi: http:// dx.doi.org/10.1016/j.corsci.2014.10.022

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Effects of exposure temperature and time on corrosion behavior of a ferritic-martensitic steel P92 in aerated supercritical water Xiangyu Zhong, Xinqiang Wu*, En-Hou Han Key Laboratory of Nuclear Materials and Safety Assessment, Liaoning Key Laboratory for Safety and Assessment Technique of Nuclear Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P. R. China

*Corresponding author: Xinqiang Wu Key Laboratory of Nuclear Materials and Safety Assessment Liaoning Key Laboratory for Safety and Assessment Technique of Nuclear Materials Institute of Metal Research Chinese Academy of Sciences 62 Wencui Road, Shenyang 110016, P.R. China Tel: +86-24-2384-1883 Fax: +86-24-2389-4149 E-mail:

[email protected]

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Abstract Oxidation behavior of a ferritic-martensitic steel P92 exposed to aerated supercritical water at 400-550 oC and 25 MPa was investigated by gravimetry, X-ray diffraction, Raman spectroscopy and scanning electron microscopy. The weight gain approximately followed near-power oxidation kinetics. The oxidation rate was strongly dependent on temperature and followed Arrhenius behavior. The oxide film consisted of porous outer layer with hematite and magnetite and dense inner layer with Fe-Cr spinel. Exposure temperature and time showed significant influences on oxide surface morphologies, thickness and constituents. Higher temperature accelerated the formation of pores in oxide film. Related oxidation mechanism was also discussed.

Keywords: A: Steel; B: SEM, C: High Temperature Corrosion, C: Oxidation.

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1. Introduction The supercritical water (SCW, above the critical point of water, 374.15 oC, 22.1 MPa), can be used as an environmentally benign solvent, a reactant, and a catalyst for supercritical water oxidation (SCWO) [1]. On the other hand, supercritical boiler power plants have been constructed for a long time, and the ultra-supercritical units are also constructed in some countries [2]. The supercritical water reactor (SCWR) is one of the most promising advanced reactor concepts for Generation IV nuclear reactors because of its high thermal efficiency and plant simplification [3]. Under such an elevated temperature and pressure environment, the corrosion resistance of candidate materials for SCW is one of the key requirements for these materials safe application in industry. In order to understand the fundamentals of oxidation behavior of the candidate materials under such an extreme environment, some in situ investigations [4-9] and ex situ analysis [10-17] of the oxide films formed on austenitic stainless steels and nickel based alloys were carried out. And the E-pH diagrams of the metals and alloys in SCW and sub-critical water also have been proposed [7, 17-20]. As one of candidate structural materials for SCWR, ferritic-martensitic steels (F-M steel) have attracted increasing attention because of their good high temperature strength and creep resistance, high thermal conductivity, low swelling under irradiation, low thermal expansion coefficients,

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and low susceptibility to stress corrosion cracking up to 600 oC [21]. A number of investigations on the corrosion behavior of 9-12% Cr F-M steels exposed to SCW have been carried out, and various growth mechanisms of the oxide films were also proposed [22-33]. These data showed that the oxide films on F-M steels followed parabolic oxidation kinetics in SCW, and the oxide scales have a multilayer structure where the Fe is enriched in the oxide layers nearest the outer surface and the Cr is enriched in the layer nearest the oxide-metal interface. Tomlinson and Cory [33] conclude that the oxidation mechanism for 9Cr-1Mo steel involved outward diffusion of Fe ions and electrons across the oxide layers, inward diffusion of H2O molecules along fine pores, and diffusion of protons in both directions. Other authors [32] have suggested inward transport of hydroxyl ions, with oxidation occurring at the metal/oxide interface and the hydrogen released diffusing through the metal to an appropriate interface. The appropriate interface means the defects in the metal and oxide. The hydrogen can be trapped defects such as voids in metal or oxide films, and the superabundant vacancies (SAVs) were formed [67]. Although considerable efforts have been made during past several decades, the oxidation mechanism of the candidate steels or alloys exposed to SCW is still not well understood. In the present study, the corrosion behavior of a F-M steel P92 in aerated pure water in 400-550 oC, 25 MPa SCW was investigated. The

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weight gain of exposure coupons and thickness, morphologies, composition profile, structure of the oxide films are investigated. The related oxidation mechanism is also discussed.

2. Experimental 2.1 Apparatus Exposure experiments were performed with a continuous flowing SCW system, which has been described in details in the previous work [13, 14, 34]. The system is consisting of an HPLC pump (Eldex Inc., AA-100-S), a preheater, a nickel-based Alloy 625 autoclave with a volume of 850 ml, a heat exchanger and a back-pressure regulator (BPR). The test solution was aerated pure water with 8 ppm (by weight) dissolved oxygen, and the flow rate was maintained at 5 ml/min. 2.2 Specimens The P92 steel used in the present work was annealed at 1050 oC for 30min and tempered at 730 oC for 3 h. Coupons (10 mm × 12.5 mm × 2 mm) were mechanically ground progressively with fine grit SiC paper up to 2000 grit, and final mechanical polished with 2.5 μm diamond pastes. Prior to each experiment, the specimens were cleaned with ethanol and ultrasonically rinsed with deionized water for 30 min. The surface of the P92 steel was etched to reveal the microstructure using a solution of ferric

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chloride (50 ml HCl, 5 g FeCl3 and 100 ml H2O). Fig. 1 shows the optical image of the microstructure of P92 steel. It consists of tempered martensite structure. Carbides precipitated at the prior austenite and martensitic lath boundaries outline the grain structure. Table 1 is the chemical composition of the steel. About 1.75 wt.% W is included in the steel for solution strengthen. 2.3 Methods During the exposure tests, the specimens were mounted on a rack and put into the autoclave made by nickel based Alloy 625. The testing pressure was maintained at 25 MPa and the exposure time was up to 500 h. After exposure tests, the coupons were cleaned and dried, and were characterized by weight gain measurement, surface analysis and cross section analysis. The mass of all coupons before and after exposure was measured using Sartorius BP211D microbalance, working with a resolution of 10-5 g. X-ray diffraction analysis was performed with a D/Max 2400 X-ray diffractometer o

using copper radiation (λ=1.542 A ). The surface morphologies and chemical compositions of oxide scales were performed using INSPECT F scanning electron microscopy (SEM) equipped with energy-dispersive X-ray analysis (EDX) system. The cross section morphologies and chemical compositions of oxide scales were performed using another SEM & EDX system (Phillips XL30). The Raman spectrometer (BWS905

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custom Raman spectrometer) used in the present work contains a powerful laser at 532 nm-1 with a maximum power of 1.5W. The Raman shift range is 175-2875 cm and the spectral resolution is 12 cm-1. The integration time used was 15 or 20 s depending on the signal intensity of the specimens. The mapping distribution of O, Fe, Cr and Mo elements on the cross-section of the oxide films were detected by electron probe micro-analyzer (EPMA) under the operating condition of U = 15 kV and I = 100 nA.

3. Results 3.1 Mass change and oxidation kinetics Fig. 2a shows the mass gain of P92 steel exposed to SCW for different times at 400 oC, 450 oC, 500 oC, 550 oC and 25 MPa. The mass gain increased with increasing temperature and exposure time up to 500 h, the mass gains at 550 oC are 2 times those at 500 oC and 7 times those at 400 oC. Comparison of weight gain data is only indicative of the trend because the reported weight gains are the sum of the weight of the oxide layer formed on the specimens minus the weight of any oxide lost by spallation and/or dissolution. And the plots can be fitted by the following equation [27, 29].

ΔW = kt n

(1)

where ΔW is the weight gain of P92 steel per unit area in mg/cm2, k is oxidation

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rate constant in mg/cm2·h-n [29], t is the time in h, and n is the time exponent that describes time dependence of oxide growth. The time exponents are 0.1702, 0.40672, 0.17562 and 0.3459 at 400 oC, 450 oC, 500 oC and 550 oC, respectively. R2 denotes the fitting quality with 1 for a perfect confidence fit. The temperature dependence can be fitted by an Arrhenius equation as following [27, 29].

k = k0 exp(−

Q ) RT

(2)

where k and k0 are the rate constants (mg/cm2·h-n), Q is the activation energy of oxidation reaction (J/mol), R is the gas constant and T is the temperature (K). Substituting equation (2) into equation (1) gives the weight gain as a function of time and temperature [27]. ΔW = k0 exp( −

Q n )t RT

ln(ΔW ) = ln k0 −

Q + n ln t RT

(3) (4)

For a constant exposure time (in the present paper, the exposure time is 500 h) the activation energy for oxidation, Q, can be calculated from the slope of a of ln(ΔW ) vs.

1 T . As shown in Fig. 2b. The activation energy of P92 steel calculated using the Arrhenius relation is 174.3 kJ/mol in the present work. And table 2 shows the activation energy data in the present study compared to those obtained from the literatures under

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different conditions [21, 27, 35-38]. Note that the activation energy of F-M steel decreases with increasing Cr content. And the activation energy of F-M steel is similar in both SCW and steam environment in the same temperature range.

3.2 XRD analysis Fig. 3a is the XRD results of the oxide film formed on P92 steel after exposure tests in SCW from 400 to 550 oC. All the XRD analyses are performed on the surface of the oxide films, not on a cross section. Fig. 3b is the XRD results of the oxide film on P92 steel exposed to 550 oC SCW for different time. The peaks of hematite (α-Fe2O3), magnetite (Fe3O4) and/or spinel (FeCr2O4) were observed. The lattice size of Fe-Cr spinel (FeCr2O4) is about 0.8379 nm (PDF card No. 34-0140), and the lattice size of magnetite is about 0.8396 nm (PDF card No. 19-0629), and the peaks of spinel are very close to magnetite. The intensity of hematite increase with increasing exposure temperature and time, while that of magnetite and/or spinel decrease, indicating that the content of hematite in the oxide film increase with increasing exposure temperature and time.

3.3 Raman spectra

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Fig. 4a shows the Raman spectra of the oxide film on P92 steel after exposure tests in SCW at different temperatures. And Fig. 4b shows the Raman spectra of the oxide film on P92 steel after tests in 550 oC SCW with different exposure time. The Raman peaks at 229, 292, 409, 599, 653, 1055 and 1310 cm-1 correspond to hematite structure [39, 40]. Spinel and magnetite was not detected by Raman spectra. The positions of peaks on the spectra show no obvious change with increasing exposure temperature and time. The intensity of the peak at 653 cm-1 is different at different exposure temperature and time. It may be related to the Raman scatter effects of different test sites with different crystal orientation and surface roughness. In the work of Olga N. Shebanova et al. [69], they found that the local site of sample will be heated by excitation laser during Raman test process. And the local temperature increased with the increasing of the laser power of Raman system. The laser can induce the magnetite oxidation and transform to hematite. Faria et al. [70] also found that increasing laser power causes the characteristic bands of hematite to show up in the spectra of most of the magnetite whereas the hematite spectrum undergoes band broadening and band shifts. When the temperature higher than 400 oC, the magnetite undergo phase transition to hematite. In our present paper, the test environment is 400 to 550 oC aerated supercritical water environment, the hematite was formed easily in such high oxygen partial pressure

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environment at high temperature. This is in agreement with the thermodynamic of iron oxidation [42]. And according to the XRD results, the oxide films formed on P92 in SCW consist of hematite, magnetite and/or spinel. So in our case, the hematite peaks of the Raman test results may not be induced by the laser thermal effects. Because the detected depth of Raman spectroscopy is limited, only the outer surface of oxide film can be detected. The present results indicated that the outer layer of oxide film is hematite, and the inner layer consists of spinel and/or magnetite.

3.4 SEM observation Fig. 5 shows the surface morphologies of the oxide films grown on P92 steel exposed to SCW for different time at 400-550 oC. Significant changes of the morphologies of the oxide films were observed after different exposure time. The oxide films formed on P92 steel after expose to SCW environment after 40 or 50 h are dense, full of granular polyhedral crystals (Fig. 5a, 5c, 5e and 5g). While the oxide film after long-term exposure (500 h) has much interconnected pores in comparison with that after short-term exposure (50 h). The size and number of the pores on the surface grew with increasing temperature (Fig. 5b, 5d, 5f and 5h). These results are similar to the works of Ampornrat et al. [27]. They also found that the pores of oxide films formed on T91

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expose to SCW at 400 to 600 oC increase with exposure temperature. But these results are in contradiction with the results of Yin et al. [68]. They observed that the surface porosity of the oxide film on P92 steel exposed to SCW at 500 to 600 oC decrease with exposure time and temperature. Fig. 6 shows the SEM image of the oxide film formed on P92 steel exposed to 550 oC SCW for 100 h. There are some cracks in the oxide film. In the upside, the oxide film is composed of granular polyhedral crystals, and in the downside, the oxide film change to be porous and has much interconnected porosity. The evolution of such a porous morphology could be related to the dissolution or vaporization of the oxides in SCW [41].

3.5 Cross-sectional analysis The cross-section morphologies of the oxide films were investigated using SEM, and the corresponding composition versus depth profiles was determined by EDX line scans. Fig. 7a to Fig. 7h show the cross-section images and elements distribution of the oxide films formed on P92 steel exposed to SCW for 500 h at different temperatures. Number 1 corresponds to the metal matrix, number 2 corresponds to the internal oxidation zone, number 3 corresponds to the inner oxide layer, number 4 corresponds to the outer oxide layer, and number 5 corresponds to the nickel coating and resin. Based on the

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morphology and elemental concentration distribution, the oxide film is generally composed of two layers, namely, a porous outer layer (labeled as number 4 in SEM images in Fig. 7) and a dense inner layer (labeled as number 3 in SEM images in Fig. 7). The thickness of the oxide film increased from 10 μm to 43 μm with increasing temperature from 400 oC to 550 oC. For all samples, the interfaces between the outer and inner oxide layers was distinct and is thought to be the original one between metal and SCW (the vertical black line between number 3 and 4 in Fig. 7a, 7c, 7e and 7g). This is in agreement with an overall oxidation mechanism of an outer oxide layer formed by outward diffusion of the iron cations (Fe3+ and Fe2+), and an inner oxide layer formation by inward diffusion of oxygen anions (O2-) [42]. The thickness of both the inner and outer layers increased with increasing temperature (Fig. 8a). And the ratio of thickness of the inner layer to outer layer kept at around 0.56±0.02 in the temperature range of 400-500 oC, but it increased suddenly at the exposure temperature of 550 oC (Fig. 8b). This indicated that the inward diffusion of oxidants from outside layer is faster than outward transport of the metal ions at a higher temperature, resulting in a high oxidation rate at 550 oC. For the wustite phase (FeO) can form at temperature close to 550 °C (>560 oC), this can be a reasonable explanation for the thicker inner oxide layer. But the FeO signal was not detected in XRD test results. This maybe due to the

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content of FeO in the oxide film is too low to be detected. The elemental concentrations in the oxides films display similar distributions for the samples after exposure tests at different temperatures. It was found that Cr was enriched in the inner oxide layer and depleted in the outer oxide layer. In contrast, Fe was depleted in the inner layer and enriched in the outer layer. W is depleted in the outer oxide layer and enriched in the inner oxide layer. In addition, a distinct thin layer in the alloy matrix close to the oxide/metal interface was also observed. This layer represents a diffusion or internal oxidation layer, because the concentration of oxygen across this layer varied gradually from oxide to bulk alloy concentration. Cr was also found to be enriched close to the outer surface of the oxide films for the samples at temperature above 400 oC (Fig. 7d, 7f and 7h). The EPMA mapping (Fig. 9) also shows that a Cr-rich thin layer exists close to the outermost surface of the oxide films. The surface Cr-rich layer could be related to Cr oxidized first at the initial oxidation stage [43, 44]. Such Cr-oxides may form after the process during cool-down of the sample in the apparatus, If SCW get enriched in Cr and Fe during the experiment and the dissolution of Cr is high, it may well be that a thin layer of Cr-oxides or hydroxides forms at the surface during cooling. This could explain the outer Cr-oxides for the samples, corroded at higher temperatures ( >400 °C), and the absence of Cr at the surface of Fig 7b. Combining the results of XRD, Raman

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spectroscopy and EDX, it could be deduced that the outer layer was mainly composed of hematite (α-(CrxFe1-x)2O3) and magnetite, and the inner layer was composed of magnetite and Fe-Cr spinel. These coincide with the thermodynamic calculation results of the oxidation of HCM12A in SCW proposed by Tan et al. [30]. They confirmed that the sequence of oxide layer formed on HCM12A in 500 oC SCW with high-oxygen partial pressure is spinel/magnetite/hematite ((Fe,Cr)2O3). Kahveci et al. [45] also found that the (Fe,Cr)2O3 layer was formed on the Fe - 3 wt.% Cr alloy exposed to 800 oC dry air. A regular laminated structure in the inner layer close to the matrix was also observed. The SEM image shows that the regular laminations are parallel to the oxide/metal interface in the inner layer (Fig. 7c, 7e, 7g and Fig. 10a). The EDX line scan results (Fig. 7f and 7h) and EPMA mapping show that the Cr-rich oxide was distributed as striations within the oxide films parallel to the oxide/metal interface. This has also been observed by the previous laboratory tests and power plants exposure for 9% Cr and 12% Cr steels [46-49]. In the internal oxidation zone, some small oxide precipitates fromed along the lath boundaries and grain boundaries (the dark areas in Fig. 10b), indicating that the lath and grain boundaries were oxidized first. The oxygen penetrated into the metal matrix along

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the defects and grain boundaries [47, 50-53].

4. Discussion 4.1 Effects of exposure temperature and time Exposure temperature caused a significant effect on the mass-gain change of P92 steel exposed to SCW (Fig. 2). The mass gain increased with increasing exposure temperature up to 550 oC and time up to 500 h. The temperature dependent mass gain can be fitted by the Arrhenius relation. The activation energy of P92 steel obtained from the slope of ln(ΔW ) vs. 1 T is 174.3 kJ/mol in the present work. This is consistent with the results of many other authors (Table 2). The average activation energy is around 211-264 kJ/mol for oxygen diffusion in magnetite, and 230-238 kJ/mol for iron diffusion [27]. In the Cr-rich spinel, the activation energy is even higher [54]. The measured oxidation activation energy for the F-M steels in SCW in the present work is about 174.3 kJ/mol. It is less than the activation energy for oxygen and iron diffusion in bulk crystals, but similar to those for grain boundary diffusion of oxygen (167 kJ/mol) [27, 33]. This suggests that the oxidation controlling step may be the inward diffusion process of O through the oxide layer along the grain boundaries. The time exponents are 0.1702, 0.40672, 0.17562 and 0.3459 at 400 oC, 450 oC, 500 oC

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and 550 oC, respectively. This is deviated from the expected parabolic kinetics. It is probably because of the microstructure of the oxide film is neither homogeneous nor uniform [27]. The parabolic law is derived from the transport of either vacancies or interstitial atoms through a uniform oxide film. In practice, the oxide films contain many defects, such as pores, cracks and grain boundaries, the defects may behave as diffusion short circuit paths. As a result, the diffusion rate may be determined by the defect state in the oxide film and the time dependence may deviate from parabolic law. 4.2 Morphology and structure of oxide film Similar surface oxide morphologies and oxide layer structures were observed for P92 steel exposed to SCW in the temperature range of 400-550 oC. The evolution of detailed surface morphologies and oxide film microstructures with the exposure temperature are shown in Fig. 5. The oxide film formed at the initial stage is dense, full of granular polyhedral crystals, while that formed after 500 h exposure has much interconnected porosity. Such a conversion of surface morphology from polyhedral particles to porous network is not yet understood very well. Some previous work reported that this phenomenon is likely to be attributed to defects of oxide at the surface [43]. After the initial oxidation stage, Fe ions penetrated the initial film, resulting in the localized nucleation of the polyhedral particles along the defects. Due to continuing outward

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diffusion of Fe ions through short-circuits such as grain boundaries, hematite was formed at the magnetite grain boundaries and grew up, resulting in the network porous outer layer [50]. Some works believed that this evolution may be related to the dissolution of the oxide in SCW. Previous investigations showed that the metal oxides can dissolve and deposit when the alloys exposed in the high temperature water environments [55-57]. When the oxides dissolve, the metal ions diffuse into SCW with increasing exposure time, leaving some vacancies on the oxide surface. As a result, the cavities or micro-pores formed at oxide surface due to vacancies accumulation [22-24]. Some other authors [41, 42, 58, 59, 73, 74] point out that Fe(OH)2 and CrO2(OH)2 are the active volatile species and may be involved in the metal wastage process when iron based alloys exposed to high temperature and high pressure steam. It is believable that the pores in the outer layer maybe attributed to the vaporization of Fe(OH)2 or CrO2(OH)2 formed via the reaction (1) to reaction (3) [41, 42, 58, 59, 73, 74].

1 Fe3O 4 +3H 2 O → 3Fe(OH)(g) O2 2 + 2

(1)

1 Fe 2 O3 +2H 2 O → 2Fe(OH)(g) O2 2 + 2

(2)

3 Cr2 O3 +2H 2 O+ O 2 → 2CrO 2 (OH)(g) 2 2

(3)

In the present work, it was found that the outmost surface of the oxide film formed on P92 steel is α-(Fe1-xCrx)2O3. It is believed that the α-(Fe1-xCrx)2O3 is easy to vaporize via

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the reaction (4) [41, 42, 58, 59, 73, 74], resulting in the pores on the surface of the oxide film.

1 3x (1-x) (Crx Fe1-x ) 2 O3 + O 2 +xH 2O → xCrO 2 (OH) 2 (g) + Fe2 O3 2 4 2

(4)

4.3 Oxidation mechanism Based on the present experimental results and analyses, the oxide films formed on the P92 steel exposed to SCW consist of hematite, Fe-rich magnetite and Cr-rich Fe-Cr spinel. These results are similar to much previous works [22-26, 38, 59] which focused on the corrosion behaviors and mechanisms of F-M steels in SCW and steam environment. Laverde et al. [38] identified the oxide film formed on T91 in steam contained (Fe,Cr)3O4 in the innermost layer, and porous magnetite (Fe3O4) followed by a compact thinner layer of hematite (Fe2O3) in the outer layer. Martinelli et al. [22-24] found that the oxide film formed on T91 exposed to Pb-Bi alloy at 470 oC exhibited a multi-layer structure, with columnar grains oxide Fe3O4 in the outer layer and small equiaxed grains oxides with Fe-Cr spinel ((Fe,Cr)3O4) in the inner layer, and the diffusion layer closed to the matrix. Chen et al. [26] also found that the oxide formed on T91

exposed

to

500

o

C

SCW

with

2

ppm

O2

exhibited

as

spinel

FeCr2O4/magnetite/hematite from inner layer to the outer layer. Bishoff et al. [60]

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suggested that the Fe-rich outer layer was formed at the metal/solution interface in associated with O adsorbs and Fe ion outward diffusion to the surface of metal, the inner Cr-rich layer was formed beneath the Fe-rich layer when the oxygen penetrates to the metal matrix through pores and/or grain boundaries. The internal oxidation zone simultaneously formed as the oxygen penetrated to the matrix. Laverde et al. [38] believed the spinel (Fe,Cr)3O4 was formed initially on the surface of T91 with O penetrating in to the matrix, and then the magnetite layer was formed above the spinel layer with Fe2+ outward diffusion. At last, the hematite layer was formed if the oxygen partial pressure at a high level. However, Jutte et al. [61] found that the hematite oxide was formed on α-Fe initially, and a magnetite layer was formed in-between the hematite and α-Fe at later oxidation stage. Greeff et al. [62] also found that the oxide film formed on 9Cr-1Mo steel consist of a mixture of FeO, Fe2O3 and Cr2O3 at the temperature between 400oC and 600oC in ultra-high vacancy system. Based on the above literatures and according the experiments results observed in present work, the oxidation process of P92 steel in the SCW could be reasonably described by the following steps. Fig. 11 is the schematic of the proposed oxidation mechanism of P92 steel in SCW. Because the oxygen affinity for Cr is higher than that for iron, the Cr content in surface layer of P92 steel is high enough to form Cr oxide at the initial stage of oxidation. But the oxygen is

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not high enough to form continues Cr2O3, the discrete Cr2O3 islands form at the metal/H2O interface (Fig. 11a and reaction (5)), leading to Cr depletion in the metal substrate. Fe diffuse outwards along oxide grain boundaries and short-circuit paths, and then Fe3O4 forms and H2 releases through the reaction (6) (Fig. 11a) [62]. 2Cr+3H 2 O → Cr2 O3 +3H 2

(5)

3Fe+4H 2 O → Fe3O 4 +4H 2

(6)

Local reducing conditions may be produced due to the release of H2. The E-pH diagrams of Fe and Cr in SCW environment [18, 20] and the Ellingham-Richardson diagram [42] show that the oxygen partial for chromia formation is lower than that for iron oxide formation, and the electrochemical potential of chromia reduction is also lower than that of iron oxide. These all indicated that iron oxide is easy reduced by hydrogen than chromia in such a high electrochemical potential environment. So Fe3O4 is reduced locally by the reaction (7). And then Cr2O3 further reacts with Fe through the reaction (8) and forms Fe-Cr spinel (FeCr2O4) and more H2 was released according to the reaction (8). The released H2 facilitates the reaction (7) again, which is a self-catalyzed process. As a result, the Fe-Cr spinel (FeCr2O4) layer is formed. The formation of Cr-rich spinel (FeCr2O4) at metal/oxide interface leads to Cr depletion in the substrate. When the concentration of Cr decreases below the minima, the Cr rich

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spinel is absent, and Fe rich spinel is formed. The Cr diffuses outer ward through the grain boundaries, resulting the increasing of the Cr content in the oxide/metal interface, where Cr-rich spinel can develop again [46]. These cyclic reactions can lead to an alternate band of Cr-rich oxide and Fe-rich oxide in the inner layer (Fig. 10a and Fig. 11b). Fe transports through the spinel oxide layer and reacts with H2O according to the reactioan (6), the Fe3O4 layer become thick gradually as exposure time increasing (Fig. 11b). At the magnetite/H2O interface, Fe2O3 is formed through the reaction (9) at the grain boundary of magnetite [38, 42, 50]. With inward transport of oxidation species along the oxide grain boundaries and short-circuit paths, both the Fe3O4 layer and Fe-Cr spinel layer became thicker. Some pores nucleated in the Fe3O4 layer by trapping or ejection of hydrogen (Fig. 11c). As a result, the oxide layer tends to exfoliate [50]. Considering the diffusion coefficient of cations is large than that of anions in the oxide crystals, it is easy for cations diffuse outward than anions diffuse inward. And some voids were formed in the oxide film. The micro-pores form at inner/outer oxide interface due to these voids accumulation may also be a reason for the exfoliation of oxide films. Fe3O 4 +4H 2 → 3Fe+4H 2 O

(7)

Cr2 O3 +Fe+H 2 O → FeCr2 O 4 +H 2

(8)

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2Fe3O 4 +H 2 O → 3Fe 2 O3 + H 2

(9)

The evolution of hydrogen is an expected consequence of oxidation in steam and high temperature water. If the hydrogen releases at the oxide/H2O interface, it may enter the water circuit. While if the hydrogen releases at the metal/oxide interface, it would diffuse inward to the metal matrix and diffuse outward through the oxide film. The mechanism of hydrogen affecting the oxidation of alloys is not considered to be sufficiently well understood, the arguments focus on possible effects of any accumulation of hydrogen in the steam or water circuit on the effective oxygen partial pressure (hence effects on oxide stability) and overall scale morphology, are considered to be specious. One possible role of water in the oxidation of metals can be an incorporation of hydrogen in the oxide, which can change the defects dependent properties of the oxide, resulting in increased outward diffusion of metal ions, and accelerated oxidation rate of alloys in high temperature water [63, 64]. Tomlinson and Cory [33] investigated the oxidation of 2-9Cr steels in steam using tubular steel specimens (steam on the inside and air on the outside), they measured the hydrogen evolved and found that up to 70% of the hydrogen was emitted into the steam, with 30% diffusing through the tube wall. Fujii and Meussner [65, 66] suggested that outward hydrogen diffusing from the metal/oxide interface may form water molecular where it

23

encounters a sufficient oxygen potential, such as in voids in the oxide film, possibly triggering dissociation of the film into the enlarging voids. The effects of hydrogen on the oxidation of steels or alloys and the mechanism of hydrogen enhanced oxidation rate of steels or alloys in high temperature water environments are not understood comprehensively, and further detailed investigations are needed.

5. Conclusions The corrosion behavior of a F-M steel P92 was investigated in aerated SCW in the temperature range of 400-550 oC for exposure times up to 500 h. The weight gain, phase constitutes, morphologies and chemical compositions of the oxide films were characterized in details. It is found that the mass gain of P92 steel increased with increasing exposure temperature and showed approximately near-power oxidation kinetics. The activation energy of P92 steel in SCW calculated using the Arrhenius relation is 174.3 kJ/mol. The oxide film is composed of three layers, an outer porous hematite layer containing some Cr followed by a magnetite layer, and an inner Fe-Cr spinel oxide layer. The exposure temperature and time showed significant effects on the surface morphologies, thickness and constituents of the oxide films. A high exposure temperature accelerated the formation of pores in the outer magnetite and hematite layer.

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The exposure time and temperature dependent surface morphologies of the oxide films on P92 steel is believed to be due to the dissolution of oxide or the Cr and Fe oxides volatilization.

Acknowledgments This study was jointly supported by the Science and Technology Foundation of China (51371174), the National Basic Research Program of China (2011CB610501), the Science and Technology Project of Yunnan Province, the Technology Development (Cooperation) Fund from Yunnan Wenshan Dounan Manganese Industry Co., Ltd. and the Innovation Fund of Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS).

References [1] P. Kritzer, Corrosion in high-temperature and supercritical water and aqueous solutions: a review, J. Supercrit. Fluids, 29 (2004) 1-29. [2] R. Viswanathan, J. Henry, J. Tanzosh, G. Stanko, J. Shingledecker, B. Vitalis, R. Purgert, U.S. program on materials technology for ultra-supercritical coal power plants, J. Mater. Eng. Perform., 14 (2005) 281-292.

25

[3] A Technology Roadmap for Generation IV Nuclear Energy Systems, in, US DOE Nuclear Energy Research Advisory Committee and the Generation IV International Furm (GIF-002-00), 2002. [4] I. Betova, M. Bojinov, P. Kinnunen, V. Lehtovuori, S. Peltonen, S. Penttila, T. Saario, In situ characterisation of the oxidation of Ni in ultrasupercritical water, Electrochem. Commun., 8 (2006) 311-316. [5] Y. Takeda, T. Shoji, M. Bojinov, P. Kinnunen, T. Saario, In situ and ex situ characterisation of oxide films formed on strained stainless steel surfaces in high-temperature water, Appl. Surf. Sci., 252 (2006) 8580-8588. [6] H. Sun, X. Wu, E.-H. Han, Effects of temperature on the protective property, structure and composition of the oxide film on Alloy 625, Corros. Sci., 51 (2009) 2565-2572. [7] J. Huang, X. Wu, E.-H. Han, Influence of pH on electrochemical properties of passive films formed on Alloy 690 in high temperature aqueous environments, Corros. Sci., 51 (2009) 2976-2982. [8] J. Xu, X. Wu, E.-H. Han, The evolution of electrochemical behaviour and oxide film properties of 304 stainless steel in high temperature aqueous environment, Electrochim. Acta, 71 (2012) 219-226.

26

[9] X. Liu, E.-H. Han, X. Wu, Effects of pH value on characteristics of oxide films on 316L stainless steel in Zn-injected borated and lithiated high temperature water, Corros. Sci., 78 (2014) 200-207. [10] Q. Zhang, R. Tang, K. Yin, X. Luo, L. Zhang, Corrosion behavior of Hastelloy C-276 in supercritical water, Corros. Sci., 51 (2009) 2092-2097. [11] M. Sun, X. Wu, Z. Zhang, E.-H. Han, Oxidation of 316 stainless steel in supercritical water, Corros. Sci., 51 (2009) 1069-1072. [12] L. Tan, X. Ren, K. Sridharan, T.R. Allen, Effect of shot-peening on the oxidation of alloy 800H exposed to supercritical water and cyclic oxidation, Corros. Sci., 50 (2008) 2040-2046. [13] M. Sun, X. Wu, Z. Zhang, E.-H. Han, Analyses of oxide films grown on Alloy 625 in oxidizing supercritical water, J. Supercrit. Fluids, 47 (2008) 309-317. [14] X. Gao, X. Wu, Z. Zhang, H. Guan, E.-H. Han, Characterization of oxide films grown on 316L stainless steel exposed to H2O2-containing supercritical water, The J. Supercrit. Fluids, 42 (2007) 157-163. [15] W. Kuang, X. Wu, E.-H. Han, Influence of dissolved oxygen concentration on the oxide film formed on Alloy 690 in high temperature water, Corros. Sci., 69 (2013) 197-204.

27

[16] W. Kuang, X. Wu, E.-H. Han, Influence of dissolved oxygen concentration on the oxide film formed on 304 stainless steel in high temperature water, Corros. Sci., 63 (2012) 259-266. [17] X. Liu, X. Wu, E.-H. Han, Influence of Zn injection on characteristics of oxide film on 304 stainless steel in borated and lithiated high temperature water, Corros. Sci., 53 (2011) 3337-3345. [18] W.G. Cook, R.P. Olive, Pourbaix diagrams for the iron–water system extended to high-subcritical and low-supercritical conditions, Corros. Sci., 55 (2012) 326-331. [19] W.G. Cook, R.P. Olive, Pourbaix diagrams for the nickel-water system extended to high-subcritical and low-supercritical conditions, Corros. Sci., 58 (2012) 284-290. [20] W.G. Cook, R.P. Olive, Pourbaix diagrams for chromium, aluminum and titanium extended to high-subcritical and low-supercritical conditions, Corros. Sci., 58 (2012) 291-298. [21] R.L. Klueh, A.T. Nelson, Ferritic/martensitic steels for next-generation reactors, J. Nucl. Mater., 371 (2007) 37-52. [22] L. Martinelli, F. Balbaud-Célérier, A. Terlain, S. Delpech, G. Santarini, J. Favergeon, G. Moulin, M. Tabarant, G. Picard, Oxidation mechanism of a Fe-9Cr-1Mo steel by liquid Pb-Bi eutectic alloy (Part I), Corros. Sci., 50 (2008) 2523-2536.

28

[23] L. Martinelli, F. Balbaud-Célérier, A. Terlain, S. Bosonnet, G. Picard, G. Santarini, Oxidation mechanism of an Fe-9Cr-1Mo steel by liquid Pb-Bi eutectic alloy at 470°C (Part II), Corros. Sci., 50 (2008) 2537-2548. [24] L. Martinelli, F. Balbaud-Célérier, G. Picard, G. Santarini, Oxidation mechanism of a Fe-9Cr-1Mo steel by liquid Pb-Bi eutectic alloy (Part III), Corros. Sci., 50 (2008) 2549-2559. [25] J. Bischoff, A.T. Motta, Oxidation behavior of ferritic-martensitic and ODS steels in supercritical water, J. Nucl. Mater., 424 (2012) 261-276. [26] Y. Chen, K. Sridharan, T. Allen, Corrosion behavior of ferritic-martensitic steel T91 in supercritical water, Corros. Sci., 48 (2006) 2843-2854. [27] P. Ampornrat, G.S. Was, Oxidation of ferritic-martensitic alloys T91, HCM12A and HT-9 in supercritical water, J. Nucl. Mater., 371 (2007) 1-17. [28] X. Ren, K. Sridharan, T.R. Allen, Corrosion of ferritic-martensitic steel HT9 in supercritical water, J. Nucl. Mater., 358 (2006) 227-234. [29] L. Tan, X. Ren, T.R. Allen, Corrosion behavior of 9-12% Cr ferritic-martensitic steels in supercritical water, Corros. Sci., 52 (2010) 1520-1528. [30] L. Tan, Y. Yang, T.R. Allen, Oxidation behavior of iron-based alloy HCM12A exposed in supercritical water, Corros. Sci., 48 (2006) 3123-3138.

29

[31] N.-Q. Zhang, H. Xu, B.-R. Li, Y. Bai, D.-Y. Liu, Influence of the dissolved oxygen content on corrosion of the ferritic-martensitic steel P92 in supercritical water, Corros. Sci., 56 (2012) 123-128. [32] G. Bamba, Y. Wouters, A. Galerie, G. Borchardt, S. Shimada, O. Heintz, S. Chevalier, Inverse growth transport in thermal chromia scales on Fe-15Cr steels in oxygen and in water vapour and its effect on scale adhesion, Scripta Mater., 57 (2007) 671-674. [33] L. Tomlinson, N.J. Cory, Hydrogen emission during the steam oxidation of ferritic steels: Kinetics and mechanism, Corros. Sci., 29 (1989) 939-965. [34] X. Zhong, E.-H. Han, X. Wu, Corrosion behavior of Alloy 690 in aerated supercritical water, Corros. Sci., 66 (2013) 369-379. [35] L. Marino, L.O. Bueno, High temperature oxidation behavior of 2.25Cr-1Mo steel in air. Part 1:Gain of mass kinetics and haracteriazaiton of oxide scale, J. Press. Vess. Technol., 123 (2001) 88. [36] M. Mongomery, A. Karlsson, Survey of Oxidation in Steamside Conditions, VGB Kraftwerkstechnik, 75 (1995) 235-240. [37] R.L. Klueh, D. R. Harries, High Chromium Ferritic and Martensitic Steels for Nuclear Applications, ASTM, West Conshohocken, PA, 2001.

30

[38] D. Laverde, T. Gómez-Acebo, F. Castro, Continuous and cyclic oxidation of T91 ferritic steel under steam, Corros. Sci., 46 (2004) 613-631. [39] J.E. Maslar, W.S. Hurst, W.J. Bowers, J.H. Hendricks, M.I. Aquino, In situ Raman spectroscopic investigation of aqueous iron corrosion at elevated temperatures and Pressures, J. Electrochem. Soc., 147 (2000) 2532-2542. [40] T. Miyazawa, T. Terachi, S. Uchida, T. Satoh, T. Tsukada, Y. Satoh, Y. Wada, H. Hosokawa, Effects of hydrogen peroxide on corrosion of stainless steel, (V) characterization of oxide film with multilateral surface analyses, J. Nucl. Sci. Technol., 43 (2006) 884-895. [41] S.R. J. Saunders, M. Monteiro, F. Rizzo, The oxidation behaviour of metals and alloys at high temperatures in atmospheres containing water vapour: A review, Prog. Mater. Sci., 53 (2008) 775-837. [42] N. Birks, G.H. Meier, F.S. Pettit, Introduction to the High-Temperature Oxidation of Metals, 2nd ed., Cambridge University Press, New York, 2006. [43] G. Liu, C. Wang, F. Yu, J. Tian, Evolution of Oxide Film of T91 Steel in Water Vapor Atmosphere at 750 °C, Oxid. Met., (2013) 1-10. [44] J. Shen, L. Zhou, T. Li, High-Temperature Oxidation of Fe-Cr Alloys in Wet Oxygen, Oxid. Met., 48 (1997) 347-356.

31

[45] A.I. Kahveci, G.E. Welsch, Oxidation of Fe-3 wt.% Cr alloy, Oxid. Met., 26 (1986) 213-230. [46] M.H. Hurdus, L. Tomlinson, J.M. Titchmarsh, Observation of oscillating reaction rates during the isothermal oxidation of ferritic steels, Oxid. Met., 34 (1990) 429-464. [47] J. Żurek, E. Wessel, L. Niewolak, F. Schmitz, T.U. Kern, L. Singheiser, W.J. Quadakkers, Anomalous temperature dependence of oxidation kinetics during steam oxidation of ferritic steels in the temperature range 550-650 °C, Corros. Sci., 46 (2004) 2301-2317. [48] M. Montgomery, S.A. Jensen, F. Rasmussen, T. Vilhelmsen, Fireside corrosion and steamside oxidation of 9-12%Cr martensitic steels exposed for long term testing, Corros. Eng. Sci. Techn., 44 (2009) 196-210. [49] X. Zhong, X. Wu, E.-H. Han, The characteristic of oxide scales on T91 tube after long-term service in an ultra-supercritical coal power plant, J. Supercrit. Fluids, 72 (2012) 68-77. [50] J. Zurek, E. De Bruycker, S. Huysmans, W.J. Quadakkers, Steam Oxidation of 9% to 12%Cr Steels: Critical Evaluation and Implications for Practical Application, Corrosion, 70 (2014) 112-129. [51] S. Lozano-Perez, D.W. Saxey, T. Yamada, T. Terachi, Atom-probe tomography

32

characterization of the oxidation of stainless steel, Scripta Mater., 62 (2010) 855-858. [52] K. Kruska, S. Lozano-Perez, D.W. Saxey, T. Terachi, T. Yamada, G.D.W. Smith, Nanoscale characterisation of grain boundary oxidation in cold-worked stainless steels, Corros. Sci., 63 (2012) 225-233. [53] J. Robertson, The mechanism of high temperature aqueous corrosion of stainless steels, Corros. Sci., 32 (1991) 443-465. [54] W.D. Kingery, D.C. Hill, R.P. Nelson, Oxygen Mobility in Polycrystalline NiCr2O4 and α-Fe2O3, J. American Ceram. Soc., 43 (1960) 473-475. [55] K. Sue, T. Adschiri, K. Arai, Predictive model for equilibrium constants of aqueous inorganic species at subcritical and supercritical conditions, Ind. Eng. Chem. Res., 41 (2002) 3298-3306. [56] S.E. Ziemniak, Metal oxide solubility behavior in high temperature aqueous solutions J. Solution Chem., 21 (1992) 745-760. [57] S.E. Ziemniak, M.E. Jones, K.E.S. Combs, Magnetite solubility and phase stability in alkaline media at elevated temperatures, J. Solution Chem., 24 (1995) 837-877. [58] I.G. Wright, R. B. Dooley, A review of the oxidation behaviour of structural alloys in steam, Int. Mater. Rev., 55 (2010) 129-167. [59] J. Ehlers, D. J. Young, E.J. Smaardijk, A.K. Tyagi, H.J. Penkalla, L. Singheiser, W.J.

33

Quadakkers, Enhanced oxidation of the 9%Cr steel P91 in water vapour containing environments, Corros. Sci., 48 (2006) 3428-3454. [60] J. Bischoff, A.T. Motta, Oxidation behavior of ferritic-martensitic and ODS steels in supercritical water, J. Nucl. Mater., 424 (2012) 261-276. [61] R. Jutte, B. Kooi, M. Somers, E. Mittemeijer, On the oxidation of α-Fe and ε-FezN1-z: I. Oxidation kinetics and microstructural evolution of the oxide and nitride layers, Oxid. Met., 48 (1997) 87-109. [62] A.P. Greeff, C.W. Louw, H.C. Swart, The oxidation of industrial FeCrMo steel, Corros. Sci., 42 (2000) 1725-1740. [63] B. Tveten, G. Hultquist, T. Norby, Hydrogen in Chromium: Influence on the High-Temperature Oxidation Kinetics in O2, Oxide-Growth Mechanisms, and Scale Adherence, Oxid. Met., 51 (1999) 221-233. [64] G. Hultquist, B. Tveten, E. Hörnlund, Hydrogen in Chromium: Influence on the High-Temperature Oxidation Kinetics in H2O, Oxide-Growth Mechanisms, and Scale Adherence, Oxid. Met., 54 (2000) 1-10. [65] C.T. Fujii, R.A. Meussner, Oxide Structures Produced on Iron-Chromium Alloys by a Dissociative Mechanism, J. Electrochem. Soc., 110 (1963) 1195-1204. [66] C.T. Fujii, R.A. Meussner, The Mechanism of the High-Temperature Oxidation of

34

Iron-Chromium Alloys in Water Vapor, J. Electrochem. Soc., 111 (1964) 1215-1221. [67] Y. Fukai, Superabundant Vacancies Formed in Metal-Hydrogen Alloys, Phys. Scripta, T103 (2003) 11-14. [68] K. Yin, S. Qiu, R. Tang, Q. Zhang, L. Zhang, Corrosion behavior of ferritic/martensitic steel P92 in supercritical water, J. Supercrit. Fluids, 50 (2009) 235-239. [69] O.N. Shebanova, P. Lazor, Raman study of magnetite (Fe3O4): laser-induced thermal effects and oxidation, J. Raman Spectroscopy, 34 (2003) 845-852. [70] D. L. A. S. and M. T. de Faria, Venancio Silva and de Oliveira, Raman microspectroscopy of some iron oxides and oxyhydroxides. J. Raman Spectroscopy, 28 (1997) 873-878. [73] A. Milewska, M. P. Hierro, J. A. Trilleros, F. J. Bolivar, F. J. Perez. Coating design using computational codes for steam oxidation conditions. Mater. Sci. Forum 461-464 (2004) 321-326. [74] Davies Hugh, Dinsdale Alan. Theoretical study of steam grown oxides as a function of temperature, pressure and p(O2). Mater. High. Temp. 22 (2005) 15-25.

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Table captions: Table 1 Compositions of the F-M steel P92 used (wt.%). Table 2 Activation energy of F-M steels in high temperature water and steam environments.

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Table 1 Compositions of the F-M steel P92 used (wt.%). C

Si

Mn

Cr

Nb

Mo

Ni

P

S

Al

N

W

V

Fe

0.12 0.18 0.50 8.96 0.068 0.36 0.13 0.013 0.005 0.01 0.062 1.75 0.18 Bal

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Table 2 Activation energy of F-M steels in high temperature water and steam environments. Alloys

Experimental

Cr content

Activation energy

temperature

(wt. %)

(kJ/mol)

2.25CrMo

Air 600-800 oC

2.25

290

Marino[35]

T22

Steam 450-650 oC

2.2

236

Mongomery[36]

T91

Steam 300-600 oC

9

199

Klueh[21, 37]

T91

Steam 400-650 oC

8.51

207.9

Laverde[38]

T91

SCW 400-600 oC

8.37

189.3

Ampornat[27]

T91

Steam 450-650 oC

9

199

Mongomery[36]

HCM12A

Steam 300-600 oC

12

166

Klueh[21, 37]

HCM12A

SCW 400-600 oC

10.83

177.1

Ampornrat[27]

HT9

SCW 400-600 oC

11.63

172.6

Ampornrat[27]

NF616

Steam 650-700 oC

9

193

Mongomery[36]

P92

SCW 400-550 oC

8.96

174.3

Present work

38

Reference

Figure captions: Fig. 1 The metallographic image of as-received P92 steel. Fig. 2 (a) Weight gain of P92 steel as a function of exposure time in 400-550 oC SCW. (b) Plots of ln(ΔW) ~1/T for P92 steel oxidized in aerated SCW. Fig. 3 (a) XRD patterns of the oxide film on P92 steel in 400-550 oC SCW for 500 h. (b) XRD patterns of the oxide film on P92 steel in 550 oC SCW for different exposure time. Fig. 4 (a) Raman spectra of the oxide film formed on P92 steel in SCW at different temperatures. (b) Raman spectra of the oxide film formed on P92 steel in 550 oC/25 MPa SCW for different exposure time. Fig. 5 SEM images of the oxide films on P92 steel exposed to SCW for 500 h at 400-550 oC: (a) 400 oC-40 h, (b) 400 oC-500 h, (c) 450 oC-50 h, (d) 450 oC-500 h, (e) 500 oC-50 h, (f) 500 oC-500 h, (g) 550 oC-50 h, (h) 550 oC-500 h. Fig. 6 SEM images of the oxide film on P92 steel exposed to SCW for 100 h at 550 oC. Fig. 7 Cross-sectional SEM images and elemental distribution of the oxide films on P92 steel exposed to SCW for 500 h at 400-550 oC. (a, b) 400 oC, (c, d) 450 oC, (e, f) 500 oC, (g, h) 550 oC. Fig. 8 (a) Thickness of the oxide film after exposure tests in SCW for 500 h at different temperatures. (b) Ratio of thickness of the inner layer to outer layer at different

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temperatures. Fig. 9 The distribution of elements on cross-section of the oxide films for P92 steel exposed to 550 oC, 25 MPa SCW for 500 h. Fig. 10 Cross-sectional SEM images of the inner layer of the oxide films formed on P92 steel in 450 oC SCW for 500 h, (a) the inner layer, (b) the internal oxidation zone. Fig. 11 Schematic of the oxide film formed on P92 steel in SCW environment.

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Highlights:  

Weight gain of P92 steel showed near-power kinetics in aerated SCW. The porous outer layer is enriched in Fe and dense inner layer is enriched in Cr.



Exposure temperature and time had significant influences on the oxide film. Oxide surface morphology change is due to dissolution or Cr and Fe volatilization.



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