Corrosion Behavior of Ni-Based Alloys in Supercritical Water

Article pubs.acs.org/IECR

Corrosion Behavior of Ni-Based Alloys in Supercritical Water Containing High Concentrations of Salt and Oxygen Xingying Tang, Shuzhong Wang,* Donghai Xu, Yanmeng Gong, Jie Zhang, and Yuzhen Wang Key Laboratory of Thermo-Fluid Science and Engineering of MOE, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China ABSTRACT: The Ni-based alloys Incoloy 800, Incoloy 825, Inconel 625, and Hastelloy C-276 exposed to subcritical water (350 °C, 25 MPa) and supercritical water (450 °C, 25 MPa) with high concentrations of chloride and oxygen were analyzed by using scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. There is a strong synergistic effect between salt and oxygen, leading to severe corrosion. The selective dissolution of nickel is the severest of all alloying elements, and a stable oxide in oxidizing condition is formed by chromium. Molybdenum improves the resistance to pitting corrosion when chromium is present. Without molybdenum, Incoloy 800 exhibits the severest pitting corrosion of the test alloys under subcritical condition. Inconel 625 and Hastelloy C-276 exhibit good corrosion resistance under the condition of oxygen and salt existing. NiO, NiCr2O4, and Cr2O3 are the three main components of oxide films on Ni-based alloys. The possible corrosion mechanisms of Ni-based alloys are discussed.

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≫ Cr, and the dissolution of metals can promote as catalyst the destruction efficiencies of organic matter under both subcritical and supercritical conditions.10,25 The pitting corrosion and stress corrosion cracking occur as a result of the oxidation of halogenated compounds or the waste containing halide salt.11−13,20,26−33 In previous studies, although many corrosion tests of Ni-based alloys are conducted in supercritical water (SCW) with a single corrosion medium such as salt or oxidant, little work has been done on the corrosion conditions of SCW with high concentrations of salt and oxidant. Under the actual corrosion condition of the coexistence of salt and oxidant, a complicated corrosion will be generated from wastewater being treated by SCWO. In this study, Incoloy 800, Incoloy 825, Inconel 625, and Hastelloy C-276 were investigated by using a batch SCWO reactor system with a feed of pesticide wastewater. The four selected Ni-based alloys always exhibit good corrosion resistance at the high temperature conditions with a high concentration of salt. The objective of the present article is to obtain the quantitative database of potential materials and effectively improve the anticorrosion technology for fabricating the commercial SCWO system.

INTRODUCTION Supercritical water oxidation (SCWO) developed since the 1980s is an effective means for destroying aqueous organic waste above the critical point of pure water (374 °C and 22.1 MPa). In the supercritical state, the disappearance of the phase interface between organics and oxygen can accelerate reaction, resulting in a short reaction time (few minutes or seconds). Under appropriate conditions, the SCWO process can have conversion efficiencies higher than 99.99% while the hazardous waste is oxidized. CO2, H2O, N2 and inorganic salt are the harmless products produced in the process of SCWO as an ecofriendly technology for treating organic waste. However, the reaction medium in the SCWO process always contains corrosive components such as salt and oxygen which will cause much severer corrosion than that under the atmospheric condition, so the corrosion problem is a main obstacle to commercializing SCWO. Many researchers have made efforts to develop many methods by which to solve the corrosion problem, such as developing anticorrosion reactors, salt separators, corrosion-monitoring methods, new catalysts, and corrosion-resistant materials.1−6 To develop an effective anticorrosion method, it is important to reveal the corrosion mechanism which depends on corrosion tests. With high mechanical strength and good corrosion resistance at high temperature, Ni-based alloys are generally considered as the candidate materials for fabricating SCWO system. Much research has been devoted to the corrosion behavior of Ni-based alloys at subcritical and supercritical water oxidation conditions, and the most severe corrosion is found in the subcritical region at high temperatures.7−15 Ni-based alloy surfaces are covered by a duplex oxide layer that includes the loose outer layer consisting of a Ni-based oxide and the compact inner layer consisting of Cr-based oxide.7,12,16−21 Higher Cr and Mo contents ensure a higher corrosion resistance, but the depletions of Ni and Fe are observed obviously on the surface.16,18,20,22−24 In addition, the oxidation rates of alloying elements are in the order of Fe > Ni > Ti > Mo © 2013 American Chemical Society

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EXPERIMENTAL SECTION Materials. The characteristics of the wastewater from pesticide manufacturing as a feed solution are presented in Table 1. A 300% stoichiometric amount of hydrogen peroxide (H2O2, 30 wt %, Tianhao Chemical Co. Ltd.) was used as an oxidant for decomposing the pesticide wastewater. The theoretical amount of oxidant for removing organic matter completely was calculated according to the initial COD concentration of pesticide wastewater. As shown in Table 1, Received: Revised: Accepted: Published: 18241

April 20, 2013 November 4, 2013 December 3, 2013 December 3, 2013 dx.doi.org/10.1021/ie401258k | Ind. Eng. Chem. Res. 2013, 52, 18241−18250

Industrial & Engineering Chemistry Research

Article

after both pesticide wastewater and hydrogen peroxide were injected into the reactor, all valves were closed. The amounts of pesticide wastewater and hydrogen peroxide were calculated according to the parameters of temperature, pressure, and volume of vessel. The temperature and pressure of the reactor reached the set parameters including subcritical condition (350 °C and 25 MPa) or supercritical condition (450 °C and 25 MPa). The exposure time was 60 h including the heating time (about 1.5 h), and the effect on the coupons of the heating-up and cooling-down processes were factored into the results. At the end of each experiment, the power supply was cut, and the reactor was cooled rapidly to an ambient temperature by the use of a cooling loop. The tested alloys were taken from the vessel to clean and analyze. The corrosion test parameters are shown in Table 3. To investigate the oxidation corrosion, electrodialysis desalination technology was used to remove the salt from the tested wastewater. Analysis. A multiparameter water analyzer (Merck NOVA60) was applied to monitor the COD, ammonium (NH3−N), and Cl− concentrations. The pH and conductivity of the liquid were analyzed by a sartorius professional meter (PP-50 model). The conductivity and pH are measured offline. The total salt was determined by a gravimetric method (HJ/T 51-1999 of China). Surface morphologies of the exposed specimens were examined by a scanning electron microscope (SEM) with the model of JSM-6390A. The components of total salt and the variation of metals in the oxide films were identified by a JEOL JSM-6390A energy diffraction spectrum (EDS). The oxide crystal structures were analyzed using the D/max-Ultima IV X-ray diffraction (XRD) instrument. The chemical composition of the oxide films was analyzed by a PHI-5000 ESCA X-ray photoelectron spectrometer (XPS).

Table 1. Properties of the Pesticide Wastewater characteristics of the pesticide wastewater color brown

NH3−N (mg/L)

COD (mg/L)

Cl− (mg/L)

pH

total salt (g/L)

70000 76 10.5 5100 components of total salt (in wt %)

mass (wt %)

12.8

C

N

O

Na

Cl

K

6.12

1.22

14.45

12.92

41.51

18.92

in the tested pesticide wastewater, the anions contain Cl−, Br− and SO42−, and the cations contain Na+ and K+. Four Ni-based alloys as Incoloy 800, Incoloy 825, Inconel 625, and Hastelloy C-276 were selected as the tested alloys, and their chemical compositions are shown in Table 2. Table 2. Chemical Compositions of the Alloy Coupons (in wt %) Incoloy 800 Incoloy 825 Inconel 625 Hastelloy C-276

Ni

Cr

32 42.0 58.3 45.1

21 21.5 22.0 16.0

Mo

C

Fe

Ti

3.0 9.0 16.0

0.1 0.02 0.01 0.01

46.5 28.5 5.0 5.5

0.4 0.9 0.4

Apparatus and Procedures. The corrosion coupon dimensions are 20 mm × 25 mm × 30 mm, and each coupon has a small hole for securing it in the autoclave. The corrosion coupons were mechanically polished to a 1600 finish, degreased ultrasonically in acetone, and dried. Corrosion tests were conducted in a similar batch reactor system described in our previous literature.34 However, this reactor and lining were made of Hastelloy C-276, with valves and connecting parts made of stainless steel 316. The maximum operation temperature and pressure of the reactor were 550 °C and 32 MPa, respectively. The reactor had a volume of 300 mL, and two type-K thermocouples were applied for measuring the jacket-heating temperature and the reactor temperature. The temperature-controlled precision is ±2 °C by a temperature controller. The pressure was monitored by a pressure gauge and the jumping-up pressure of the safety valve was controlled at 32 MPa. In addition, the lining was examined or replaced once every two months in order to keep it valid. The alloy coupons were located inside the reactor at room temperature. After the reactor was flushed with nitrogen and

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RESULTS AND DISCUSSION In this work, the experiments for decomposing the pesticide wastewater were selected to investigate the corrosion behaviors of a Ni-based alloy under the subcritical condition (350 °C and 25 MPa) and supercritical condition (450 °C and 25 MPa). The subcritical test without oxygen and the supercritical test with oxygen were used to simulate actual operating conditions of the preheater and the reactor, respectively. The corrosion morphology, surface chemical property, corrosion rate, and corrosion mechanism were presented in this part. Surface Morphologies of Tested Alloys. Figure 1 presents the surface morphologies of the tested Ni-based alloys

Table 3. Conditions Used for Corrosion Tests expt run no.

alloy

dissolved oxygen (mg/L)

dissolved Cl− (mg/L)

temperature (°C)

pressure (MPa)

exposure time (h)

1

Incoloy 800 Incoloy 825 Inconel 625 Hastelloy C-276 Incoloy 825 Inconel 625 Hastelloy C-276 Incoloy 825 Inconel 625 Hastelloy C-276 Incoloy 825 Inconel 625 Hastelloy C-276

12 12 12 12 111200 111200 111200 15 15 15 111200 111200 111200

5100 5100 5100 5100 5100 5100 5100 5100 5100 5100 115 115 115

350 350 350 350 450 450 450 450 450 450 450 450 450

25 25 25 25 25 25 25 25 25 25 25 25 25

60 60 60 60 60 60 60 60 60 60 60 60 60

2

3

4

18242

dx.doi.org/10.1021/ie401258k | Ind. Eng. Chem. Res. 2013, 52, 18241−18250


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Figure 1. Surface morphologies of the tested alloys after exposure to (a) experiment 2, (b) experiment 3, and (c) experiment 4.

Figure 2. High magnification (×5000) SEM images of Incoloy 825 after exposure to (a) experiment 2, (b) experiment 3, and (c) experiment 4.

resistance in experiment 4, as shown in Figure 1c. It is wellknown that the Ni-based alloys are the candidate metals for constructing the SCWO system.29,30,36 Incoloy 825 took on the weakest corrosion resistance in all the three tested alloys under the three conditions, with the Ni-based alloys suffering a severer corrosion from the synergistic effect between salt and oxygen than that from the individual effect. The aggressive ions can break the protective oxide layer, exposing the matrix to the corrosion environment.7,18,20,28,30,37 The pressure-bearing capacity of the tube decreased as the corrosion accelerated. This decrease is a concern as safety accidents can occur once pressure reaches beyond the burst limit. Results showed that the coexistence of oxidant and salt should be avoided effectively so as to reduce corrosion damage on an SCWO system. Therefore, the oxidant and wastewater feed should be separated in the preheat process, and the inorganic salt should be removed from the wastewater before the reactor, when the SCWO system is planned for construction. The high magnification (×5000) SEM images of surfaces of Incoloy 825 after exposure to different conditions are shown in Figure 2. In oxidizing supercritical water, many oxide crystals developed into platelets fully covering the surface of Incoloy

after 60 h of exposure to supercritical water. The corrosion conditions of Ni-based alloys in experiments 2, 3, and 4 are presented in Figure 1 panels a, b, and c, respectively. Significant changes of the morphologies of the oxide films were observed at different exposure conditions. Under the severest corrosion condition, experiment 2 contains high concentrations of salt and oxygen which will cause the most severe corrosion. As shown in Figure 1a, Incoloy 825, Inconel 625, and Hastelloy C276 all exhibited general corrosion and pitting corrosion while the oxide scales of Incoloy 825 displayed the severest spallation of three alloys. All the alloys were covered with a homogeneous oxide film under experiment 3, indicating general corrosion (see Figure 1b). As Figure 2 825(b) illustrated, the oxide film of Incoloy 825 exhibited little spallation, and the alloy suffered further corrosion due to the inner layer being exposed to the corrosive medium. Zhang et al.17 found that the oxide of Hastelloy C-276 also had a strong tendency to spalling under the harsh condition. Significant oxide spallation of Incoloy 800H was also observed, and using the grain boundary engineering (GBE) technology could enhance the resistance to spallation.35 The clearly ground scratches indicated that these three Ni-based alloys had outstanding corrosion 18243



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825, as shown in Figure 2 825(b). Sun et al.16 found that rodlike and platelet-like oxide crystals grew on the surface of Inconel 625 in oxidizing supercritical water. Figure 2 825(c) illustrates that the alloy can hardly form a stable corrosion product in supercritical saline aqueous solution. This result could be explained by the fact that the attack anions, such as Cl−, with high potential would gain electrons from the alloying element, and then the element would convert into soluble high valence form resulting in pitting corrosion. The salt and corrosion products were almost insoluble in supercritical water, which would result in less corrosion at the supercritical condition than at the subcritical condition.14,33 Kim et al.20 proposed that stress corrosion cracking could occur in supercritical water by direct chemical attack of HCl. The oxidant and salt have a synergistic effect on corrosion, since the oxidant can deepen the depth of oxygen penetration and ions such as Cl− can lead to the decomposition of oxides.9,27 As shown in Figure 2, it is difficult to generate stable and protective oxide films when there are aggressive ions present in the SCW. Mitton et al.29 also observed that the Hastelloy C276 alloy exhibited dealloying and stress corrosion cracking in oxidizing supercritical water with chloride. Significant stress corrosion cracking and dealloying were found in stainless steel owing to the corrosion of chloride.23,28,38,39 Incoloy 800, Incoloy 825, Inconel 625, and Hastelloy C-276 were selected for the test at high subcritical temperature. The surface of the corroded metal was analyzed by the enlarged surface image magnified 1000× using SEM, as displayed in Figure 3. Under the subcritical condition, corroded alloys

critical condition. The alloys would suffer seriously dealloying corrosion at high subcritical temperatures due to free aggressive ions and high temperature.8−11,14,20 The selective dissolution of the alloying element is the most probable reason for pitting corrosion and stress crack corrosion. Somerday et al.23 observed that the severe dealloying of Ni and cracking resulted in the failure of alloy C276 burst disks in subcritical temperature water with acidic chlorination. Boukis et al.33 found that alloy 625 exhibited severe pitting and intercrystalline corrosion in the preheater and cooling line of the entire system at the approximate critical temperature. Under the supercritical condition, the nonpolar organic compounds and gases become completely miscible with water, and the conductivity of the solution is extremely poor. The nonionic mechanism can explain the phenomenon in which small granules comprising metal salts overspread the metal alloys. As shown in Figure 2, the SEM micrographs of the coupons exposed to experiment 1 can better support the corrosion mechanism mentioned above. Incoloy 800 had the weakest resistance among the tested alloys under experiment 1, as seen from the severest pitting corrosion of all. Chemical compositions of all the tested Nibased alloys are in Table 2, and the Incoloy 800 lacks molybdenum. Because of the lack of molybdenum, the aggressive ions would cause a severer pitting corrosion and stress cracking corrosion on stainless steel in the subcritical water at high temperature (T = 350 °C) than in the supercritical water at low temperature (T = 450 °C). The corrosion process of the insoluble oxide scale would change to the soluble metal chloride under the attack of chloride ions.12,30,31,40 Many researchers have found that the corrosion resistance greatly increases as molybdenum content increases with a large proportion of the chromium existing in the metals.12,18,25 Surface Chemical Analysis of Tested Alloys. Under the condition of high concentration of salt and oxygen, different corrosion types are observed in different specimens. Experiment 2, experiment 3, and experiment 4 have been described in Table 3. The mass percentages of the main alloying elements were detected through the energy diffraction spectrum (EDS), as shown in Figure 4. The results of 4a,b indicated that oxygen dominated at the surfaces of metal alloys because of the oxidizing condition. The intensities of nickel decreased more significantly than the other detected elements. Then Ni departed from the surface of the corroded alloy and flowed into the reaction fluid. The intensity of carbon increased on the surfaces of the corroded Ni-based alloys, which suggested that the organic matters in pesticide wastewater were decomposed not only in the fluid but also on the metal wall. A comparison of the carbon content in three conditions showed that the intensity of carbon increases significantly on the surface in (c), because the incomplete decomposition of organic matters without oxygen caused the severest carbon contamination. The intensities of chromium, iron, and molybdenum had a smaller diminution than that of nickel. These results indicate that unstable corrosion products of nickel can dissolve in supercritical water easily, causing the pitting corrosion. Oxygen dominated at the surfaces of the metal alloys in (a) and (b). As shown in Figure 4, the oxygen intensity in (a) was weaker than that in (b), which could be explained by the fact that chloride ion caused the dissolution of oxide in the supercritical saline aqueous solution. As shown in 825(a) and 825(b), the intensity of iron had a lesser degree of decrease in the oxide film than that of nickel or chromium when oxygen

Figure 3. Surface morphologies of Incoloy 800, Incoloy 825, Inconel 625, and Hastelloy C-276 after exposure to experiment 1.

exhibited the corrosion morphologies which were different from those of specimens exposed to the supercritical condition, and Incoloy 800 exhibited the severest pitting corrosion in all the coupons in experiment 1. This difference implies that corrosion mechanisms under the subcritical and supercritical conditions are different, as a result of the variations of electrolyte dissociation and water density. Kritzer et al.13 reported that Inconel 625 exhibited pitting corrosion at temperatures above 130 to 215 °C, and stress corrosion cracking occurred near the critical temperature water containing chloride. Because of the great decrease of ionic product constant, the aggressive ions will cause severer pitting corrosion under a subcritical condition than under a super18244



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Figure 4. Surface composition of corroded alloys exposed to three medium conditions: (a) experiment 2, (b) experiment 3, and (c) experiment 4. (I) Incoloy 825, (II) Inconel 625, (III) Hastelloy C-276, and (IV) side scan of Incoloy 825.

Figure 5. SEM image and X-ray mappings of the cross-section of Incoloy 825 after exposure to experiment 2.

to experiment 2. The oxidative depth of Incoloy 825 was clearly observed, and distribution densities of alloying elements were also detected, as presented in Figure 5. The density of the white points represent the concentration of elements in the X-ray mapping. The X-ray mappings revealed that Ni was seriously depleted at the surface and the distribution densities of Fe, Mo, and Cr were almost unchanged.20,21,26 Kim et al.40 also observed the selective dissolution of Ni and selective oxidation of Cr in Inconel 625. This indicates that Fe, Mo, and Cr can form stable products in order to avoid dissolving in supercritical water. The depletion of nickel promoted by acidic chlorination

was present. It indicated that iron could generate a stable oxide which isolated the alloy and corrosive medium. However, the intensity of the iron signal decreases more than that of nickel and chromium in 825(c), an indication that iron changes to ionic forms more easily under the influence of aggressive anions. As shown in Figure 4IV, oxide film was generated on the matrix of Incoloy 825 under experiment 2. The chemical composition of the oxide layer can reveal the corrosion mechanism. To investigate the corrosion behavior, the cross-section of Incoloy 825 was analyzed by SEM and EDS after being exposed 18245



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was also observed in Hastelloy C-276.22 The Ni-based alloys exhibit different degrees of dealloying of Ni, which would cause pitting corrosion and stress corrosion cracking. In addition, the high concentration of oxygen in the outer layer indicated the formation of stable oxide and the corrosion depth. Figure 6 depicts the XRD spectra of three Ni-based alloys after being exposed to experiment 2, experiment 3, and experiment 4. The different peaks of XRD represent the different configurations of oxides on the Ni-based alloys surface.

The intensities of matrix signals were much stronger than those of the oxide signals, indicating that the oxide surface film was thin. According to the XRD analysis (Figure 6), Fe3O4, which was generated on the surface at the characteristic peaks, could prevent the metal from further corrosion. The thickness of oxide film on the tested alloy is dependent on the exposed time if the alloy and corrosion environment are definite. The intensities of MoO2 and Cr2O3 were slight in XRD of three alloys in subcritical water. Under the single oxygen condition, the oxide Cr2O3, NiCr2O4, and NiO were found in oxide layers of all Ni-based alloys.21,37 The peaks of Fe3O4, Ni(OH)2, and MoO2 phases were found in Incoloy 825, Inconel 625, and Hastelloy C-276, respectively. All the intensities of oxide signals except for MoO2 were weaker under the condition of the coexistence of oxygen and salt than under the condition of single oxygen, which occurs because the aggressive salt could cause the decomposition of oxide. Little difference in MoO2 signals indicated that the MoO2 was stable under the attack of salt, and the resistance to pitting corrosion increased as the Mo content increased. Sun et al.16 found that the stable Cr2O3 formed a protective film, and Ni(OH)2, NiO, MoO2, and NiCr2O4 were also observed in the oxide layer of Inconel 625. Ren et al.41 also observed that the oxide layer of Inconel 625 was made up of NiCr2O4, Cr2O3, and NiO in the oxidizing supercritical water. Zhang et al.17 pointed out that Hastelloy C-276 formed the dual-layer structure oxide scale consisting of the outer NiO layer and inner Cr2O3/ NiCr2O4-mixed layer. These results indicate that Cr content can improve the corrosion resistance of the Ni-based alloy, and the chromizing treatment for alloy being applied to decrease the corrosion rate.23 These findings confirmed that metal oxide stability was the main factor for corrosion resistance. Adschiri et al.42 found that the solubility of chromium oxide was lower than that of iron and nickel oxide, which was an important factor affecting the corrosion resistance of the alloy. Sue et al.43 reported that oxide solubilities of Fe, Ni, and Cr were in this order: Fe > Ni > Cr, and the test was conducted in oxidizing supercritical aqueous solution of HCl. The oxide scales of nickel-based alloys were examined by XPS to determine the chemical states of Ni, Cr, Fe, and Mo under experiment 2, experiment 3, and experiment 4, respectively. Figure 7 shows the core level spectra of Ni 2p, Cr 2p, Fe 2p, and Mo 3d. The Ni 2p peaks were decomposed into three components: NiO located at a binding energy (BE) of 853.2 eV, Ni(OH)2 located at the BE of 862.1 eV, and NiCr2O4 located at the BE of 880.1 eV. The signal at the BE of 576.2 eV was assigned to Cr3+ in Cr2O3. The signal located at BE of 710.5 eV was assigned to Fe3O4, and 227.4 eV was assigned to MoO2. Figure 7 825(a), 825(b), and 825(c) show the Ni 2p, Cr 2p, and Fe 2p core level spectra for Incoloy 825, respectively. Using XPS techniques, NiO, NiCr2O4, Cr2O3, and Fe3O4 were observed in the oxide film of Incoloy 825. Figure 7 625(a), 625(b), and 625(c) also show the Ni 2p, Cr 2p, and Mo 3d core level spectra for Inconel 625, respectively. The oxide film of Inconel 625 was composed of NiO, Ni(OH)2, NiCr2O4, Cr2O3, and MoO2 as observed by XPS. Figure 7 C276(a), C276(b), and C276(c) also show the Ni 2p, Cr 2p, and Mo 3d core level spectra for Hastelloy C-276, respectively. The oxide film of Inconel 625 was composed of NiO, NiCr2O4, Cr2O3, and MoO2 as observed by XPS. The aggressive ions might cause the decomposition of oxides which were helpful for preventing further corrosion, and the severe corrosion would be

Figure 6. XRD spectra for Ni-based alloys after exposure to experiment 2, experiment 3, and experiment 4: (a) Incoloy 825, (b) Inconel 625, and (c) Hastelloy C-276. 18246



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Figure 7. XPS of oxide films grown on Incoloy 825, Inconel 625, and Haselloy C-276 after exposure to experiment 2, experiment 3, and experiment 4.

observed as a result of the synergism between salt and oxygen. On the whole, the results of XPS analysis are in accord with those of XRD and SEM/EDS analysis, confirming the composition of the oxide scales on nickel-based alloys in the three different conditions. Corrosion Rates of All Metals. As is summarized in Figure 8, Inconel 625 exhibited the highest corrosion resistance in experiment 2 with oxygen condition, and Hastelloy C-276 displayed the best corrosion resistance in experiment 3 with salt condition, but Incoloy 825 had the worst performance under all

the conditions. The corrosion rates of alloys are relative values because of the corrosion of the reactor, sleeve of thermocouple, and cooling line. All tested Ni-based alloys displayed the highest corrosion rates at experiment 2, and all of them had a good resistance in experiment 4. Incoloy 825 had the highest corrosion rate (14.6 mmpy) under experiment 2 with a high concentration of oxygen and salt, and Hastelloy C-276 had the lowest corrosion rate (0.9 mmpy) under experiment 3 with salt condition among the tested alloys. Incoloy 825 contains the least content of nickel and molybdenum, resulting in Incoloy 825 displaying the weakest corrosion resistance among the three alloys under each condition. In contrast with the three conditions, Incoloy 825, Inconel 625, and Hastelloy C-276 showed good performances and all presented an acceptability corrosion rate

Corrosion Behavior of Ni-Based Alloys in Supercritical Water

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