Surface Chemical Analysis of Corroded Alloys in Subcritical and

The surface chemical analysis of corroded alloys in the subcritical and supercritical water oxidation (SCWO) of 2-chlorophenol were studied by using a...
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Ind. Eng. Chem. Res. 2008, 47, 2265-2272

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Surface Chemical Analysis of Corroded Alloys in Subcritical and Supercritical Water Oxidation of 2-Chlorophenol in Continuous Anticorrosive Reactor System Sang-Ha Son,† Jae-Hyuk Lee,† Sang-Hoon Byeon,‡ and Chang-Ha Lee*,† Department of Chemical Engineering, Yonsei UniVersity, Seoul 120-749, South Korea, and Department of EnVironmental Health, Korea UniVersity, Seoul, 136-701, South Korea

The surface chemical analysis of corroded alloys in the subcritical and supercritical water oxidation (SCWO) of 2-chlorophenol were studied by using a continuous anticorrosive SCWO system. The corrosion phenomenon of metal in the supercritical condition (250 atm and 380 °C) was different from that in the subcritical condition (220 atm and 320 °C). However, the corrosion of metal alloys contributed to the elevation of destruction efficiencies in both subcritical and supercritical conditions. Furthermore, the degree of improvement of destruction efficiency at the subcritical condition was higher than that at the supercritical condition regardless of alloy types. Although all the tested alloys were severely corroded under both conditions, the catalytic effect of Monel K-500 with Cu on the destruction of 2-chlorophenol was higher than the other alloys in both subcritical and supercritical conditions. The results indicated that well-selected metal alloys could be applied to both subcritical and supercritical oxidation as catalysts by corroding themselves. Introduction In the past two decades, supercritical water (P > 221 bar, T > 374 °C) oxidation has been actively developed as a means of destroying hazardous organic waste.1,2 Most organic compounds, as well as oxidant and combustion gases, are miscible in all proportions with water in supercritical condition. Therefore, organic materials and wastes can be oxidized easily in supercritical water oxidation (SCWO) conditions with an oxidizer, and the destruction efficiency can reach over 99.9% in SCWO conditions.3,4 The major species produced during oxidation of the hydrocarbons were CO2 and H2O, even under short residence times. However, in the treatment of halogenated hydrocarbons, a corrosion problem produced by the formation of acidic conditions is one of the main obstacles to commercializing the SCWO process.5-7 Many researchers have focused on solving this problem using a variety of methods and reactor designs.8-11 In addition, various materials have been employed for the construction of SCWO systems.9,12,13 Corrosion phenomena of metal alloys have also been investigated at supercritical condition using a batch reactor.14 However, it has been difficult to control the corrosion of the reactor itself when halogenated hydrocarbon is oxidized at supercritical condition. Moreover, the corrosion of the metal reactor affects the destruction in the SCWO such that the destruction efficiency is hampered without the influence of corrosion on the reaction. In addition, the possibility is pointed out that the metals from the stainless steel reactor or Inconel preheating tubing can become solvated in the reaction mixture due to corrosion, and act as a homogeneous catalyst.15 To successfully apply the SCWO system to environmental treatment, it is important to understand corrosion phenomena of metals and their catalytic effects on the destruction efficiency of halogenated hydrocarbons. Recently, we reported on a continuous anticorrosive SCWO reactor system.11 In this study, the effects of metal corrosion on the SCWO of 2-chlorophenol were investigated at subcritical * To whom correspondence should be addressed. Tel.: 82-2-21232762. Fax: 82-2-312-6401. E-mail: [email protected]. † Yonsei University. ‡ Korea University.

and supercritical conditions by using the anticorrosive SCWO system. In addition, to elucidate the corrosion characteristics of the selected metals, such as stainless steel 316, Inconel 625, and Monel K-500, at both conditions, a surface chemical ana lysis of corroded metal alloys was conducted by various methods. Experimental Methods Chemicals. 2-Chlorophenol of 99.9% purity was mixed with primary distilled water as a feed solution of 1000 ppm. As oxidizing and neutralizing agents, a 100% stoichiometric amount of hydrogen peroxide (H2O2, 30 wt %) and a 200% stoichiometric amount of sodium hydroxide (NaOH) were used, respectively. The theoretical amounts of H2O2 and NaOH for complete neutralization were calculated under the assumption of the complete decomposition of 2-chlorophenol in the SCWO as follows:

C6H5ClO + 13H2O2 f 6CO2 + HCl + 15H2O

(1)

Experimental Apparatus and Procedure. The experimental reactor is presented in Figure 1. The corrosion test was conducted using a continuous anticorrosive SCWO reactor as presented in the previous study.11 The main modification of the system from the previous study was to an inlet part of the reactor. Since two influents were mixed inside the ceramic tube after passing at the edge of the ceramic partition in the reactor, the inlet lines and the bottom part of reactor could be prevented from corrosion. The reactor, all valves, and connecting parts were made of stainless steel 316. The nonporous ceramic tube (99% Al2O3, 7 mm i.d., 9 mm o.d.) was installed inside the stainless steel reactor as a shell and tube type. Four high-pressure pumps (maximum 6000 psi, Lab Alliance, Prep. 100) with 0.1100 mL/min (2% precision were used to convey wastewater, oxidant, pure water, and neutralizer, respectively. The flow rates of all influents were set to 2 mL/min. The wastewater and oxidant were heated to less than 100 °C in a preheating unit and added to the reactor via a high-pressure pump, separately. Simultaneously, at the bottom of the reactor, pure water was supplied through a preheater to the space between the wall of the vessel and the wall of the nonporous ceramic tube. This

10.1021/ie0709281 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/06/2008

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Figure 2. Destruction efficiencies of 2-chlorophenol without metal alloy at various conditions.

Figure 1. Schematic diagram of a continuous anticorrosive SCWO reactor.

was to prevent possible damage of the ceramic tube from the pressure difference. In addition, the neutralizing NaOH solution was fed into the top of the reactor. Since the NaOH solution was supplied at room temperature, it also acted as a cooling device and this helped to prevent fouling problems. Temperatures were monitored using five thermocouples (Ktype) located between the ceramic tube and the metal-shell reactor. Four pressure transducers were located at the inlet lines for the wastewater, H2O2, and NaOH solutions and at the outlet line after the cooling unit. A back-pressure regulator (BPR; Tescom, USA) with a line filter (0.45 µm) was installed in the effluent line. For the corrosion test of alloys under subcritical (320 °C, 220 atm, 0.6 g/cm3) and supercritical conditions (380 °C, 250 atm, 0.2 g/cm3), the metal coupons of stainless steel 316, Inconel 625, and Monel K-500 were located in the middle of the reactor. The total surface area of each metal coupon was about 250 mm2. Since the temperature profile in the reactor occurs in the floatingtype reactor used,11 the reaction temperature and reactive species are simultaneously changed at the position of the reactor. Therefore, all of the tested metal coupons were located at the same position in the reactor. To mount the metal coupons in the reactor, several pieces of ceramic, the same material as the inner ceramic reactor, were piled up from the bottom to the middle part (12 cm) of the reactor. Then, the test coupon was mounted on top of these pieces. The solution in the reactor was heated to the reaction temperature by an electric furnace with built-in heating wire. After reaching the desired temperature, the system was operated for another 2 h with water to confirm the steady state. The reaction temperature could be controlled within (5 °C. In this study, since the temperature of the inner ceramic reactor was 20 °C lower than that of the reactor shell, the temperature where the metal alloy was located in the ceramic reactor was specified as a reaction temperature. In the experiments, the feed was directly changed from the purified water to 2-chlorophenol solution after the desirable temperature and the pressure were reached. Therefore, it took a certain period of time to completely remove the purified water filling the reactor system at the initial stage. Therefore, each effluent sample was collected for 5 min after passing the first 30 min. Analytical Methods. Surface chemical analysis of corroded metal alloys was conducted by using Auger electron spectros-

copy-scanning Auger electron microscopy (AES-SAM; Perkin-Elmer, PHI Model 670). The conditions of the survey scan and sputter depth profile of the AES were recorded as the following: primary beam energy Ep ) 5 keV, primary beam current Ip ) 0.01 µA, and beam diameter ∼0.4 µm. The resolution of the cylindrical mirror analyzer was set to 0.6%. The argon ion beam, with an ion energy level of 3 keV and current density of 0.6 µAm2, was produced by a differentially pumped ion gun. The sputter profiles were analyzed using the PC PHI-MATLAB software package. Morphological analysis of the surfaces of the alloys was done by the SAM. The sputter rate was a depth of 199 Å/min, based on the SiO2 in the AES. The structure of powder gathered from the line filter (0.45 µm) was analyzed by X-ray diffraction (XRD; D5005, Bruker) scanned from 10 to 80 °C (2θ) with a scan rate of 5 °C (2θ) min-1. The amount of depleted metal components from the coupon was measured by inductively coupled plasma mass spectrometry (ICP-MS; Perkin-Elmer Sciex, ELAN 6100) and destruction efficiencies were analyzed with a total organic carbon (TOC) analyzer (Shimadzu). Gas chromatography (GC; 6890 Series, Agilent) with a DB5MS column and time-of-flight mass spectrometry (TOF-MS; Pegasus, Leco Corporation) were used to detect intermediates in the effluents formed by subcritical and supercritical water oxidation of 2-chlorophenol. The detector voltage of MS was 1800 V, and the electron energy was -70 eV. The interface temperature and ion source temperature were 300 and 230 °C, respectively. Results and Discussion Figure 2 shows the destruction efficiency of 2-chlorophenol without metal alloys in the continuous anticorrosive SCWO reactor. Destruction efficiencies were analyzed with a TOC (Shimadzu) analyzer. When the oxidation of 2-chlorophenol was conducted under various conditions (pressure from 220 to 250 atm and temperature from 300 to 380 °C), the destruction efficiency was 94-96% in the experimental range. At 100% stoichiometric amount of H2O2 condition, the incomplete decomposition of the halogenated compound11 and nonhalogenated compounds4,16 was also reported at similar experimental conditions. The reaction temperature of 300-380 °C was chosen as the subcritical and supercritical conditions to observe the change of destruction efficiency with the metal corrosion even though the application temperature of SCWO process is often about

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Figure 3. Surface composition of corroded stainless steel 316 after exposure to supercritical water oxidation of 2-chlorophenol (250 atm, 380 °C).

600 °C or even higher.8 The destruction efficiency increased with an increase in temperature; the change of destruction efficiency was trivial with the pressure change at the same temperature condition (340 °C). In this study, the conditions showing relatively lower destruction efficiency, instead of complete destruction, were selected as the experimental conditions to observe the contribution of metal corrosion to the reaction. Therefore, the subcritical condition (220 atm and 320 °C) with 94.43% destruction efficiency and the supercritical condition (250 atm and 380 °C) with 95.92% destruction efficiency were selected as the base conditions of catalytic effects of corroded metals on the destruction. Corrosion of Metal Alloys. AES-SAM analysis of stainless steel 316 surface reveals that iron oxides cover the surface of stainless steel 316 after exposure to the SCWO of 2-chlorophenol. Figure 3 shows the surface chemical analysis of stainless steel 316 exposed to supercritical water oxidation. Three parts of the stainless steel 316 surface, which were small granules, flat surface, and large granules, were analyzed, respectively. Strong oxygen and Fe peaks were detected in both granulated surface types. Also, small Ni peaks were simultaneously observed in the small granule surface type, while these peaks were almost completely absent in the large granule surface type. In addition, Cr peaks in the both granulated surfaces were minute because Cr was dissolved in the water as an ion instead of as an oxide.17-19 On the other hand, the flat surface shows weak Ni and O peaks with Fe and Cr peaks, implying that Ni is depleted from the surface even though the metal surface appears to be flat. The results of the AES-SAM survey scan and microscopy images for all the corroded metal alloys are shown in Figure 4. From the enlarged surface image magnified 300× using scanning Auger electron microscopy (SAM), the flat surface of the corroded metal alloy was selected and analyzed. Each metal alloy was exposed for 1 h to the oxidation reaction at subcritical (220 atm and 320 °C) and supercritical (250 atm and 380 °C) conditions, respectively. The term “Before sputtering” refers to the chemical compositions of the surfaces of the metal alloys exposed to the subcritical and supercritical conditions. “After 0.25 min sputtering” and “After D-profile” indicates the chemical compositions of the metal alloys at a depth of 50 Å and after the completion of Auger electron sputtering, respectively. As shown in Figure 4, the surface of each metal alloy exposed to the oxidation of 2-chlorophenol at the supercritical condition (right side of the figure) was severely changed in terms of

surface morphology and color, compared to the surface of each metal alloy exposed to the subcritical condition (left side of the figure). This implies that corrosion mechanisms at the subcritical and supercritical conditions are different, a phenomenon probably due to the variation of electrolyte dissociation and water density. The corrosion process at near-critical temperatures may involve acid attack, with the concentration of H+ being a function of both the dissociation constant of HCls which is a major product of the oxidation of chlorinated organic wastesand the density of the solution.18,19 In addition, at the subcritical condition, temperature up to 320 °C, shallow pitting corrosion is mainly observed. The microscopic image of metal alloys exposed to the subcritical condition reveals that the surfaces of all the metal alloys have small pits on them. However, at supercritical temperature, where only nondissociated HCl is present and the conductivity of the solution is very low, corrosion occurs via a nonionic mechanism and the main corrosion mechanism is slight intergranular attack. Therefore, formation of metal oxide or metal salt occurs favorably rather than corrosion. The microscopic images of the metal alloys exposed to the supercritical condition also support that the surfaces of all the metal alloys are wholly covered by small granules, expected as metal oxides or metal salts. The surface color of stainless steel 316 exposed to supercritical water oxidation (Figure 4b) was changed to black, and the surface was covered by a greater amount of small granules. This results from the exposure of iron oxide mentioned in Figure 3, because Fe is the most abundant component in stainless steel 316 (main constituents: Fe, 66.35%; Cr, 17%; Ni, 12%; and Mn, 2%). In addition, even though oxygen molecules were added by the corrosion, mass reduction percentages of stainless steel 316 exposed at the subcritical and supercritical conditions were 0.094% and 0.181%, respectively. However, this does not clearly suggest that stainless steel 316 is more corroded at the supercritical condition than at the subcritical condition because the Ni concentration in the effluent solution was much higher at the subcritical condition than at the supercritical condition in Table 1. The results of ICP-MS will be discussed in the next section in detail. As shown in Figure 4d, the change of surface color of Inconel 625 was different from that of stainless steel 316. Because Ni and Cr, the main constituents of Inconel 625 (main constituents: Ni, 57.4%; Cr, 22%; Mo, 9%; and Fe, 5%), were depleted from the surface during corrosion, the amount of granules on the Inconel surface exposed at both subcritical and supercritical conditions was much smaller than that on the stainless steel surface. However, the percentage of mass reduction of Inconel

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Figure 4. Depth profiles of each alloy using AES-SAM. (a, b) Stainless steel 316; (c, d) Inconel 625; (e, f) Monel K-500. Sputtering rate of 199 Å/min; D-profile means a final supporting depth profile.

625 was much smaller than that of stainless steel 316. The mass reduction at the supercritical condition (0.074%) was higher than that at the subcritical condition (0.026%), similar to the results for stainless steel. In Figure 4f, the surface color of Monel K-500 was changed to opaque green. This implies that Cu, one of the major components of Monel K-500 (main constituents: Ni, 66.5%; Cu, 30.0%; Fe, 2.0%; Mn, 1.50%; and Ti, 0.5%), was changed to copper oxide by corrosion. Therefore, the change of surface morphology of Monel K-500 was more severe than that of Inconel 625 at both subcritical and supercritical water oxidation. It was noted that the mass reduction of Monel was most severe in the tested alloys: 0.580% at the subcritical condition and 0.514% at the supercritical condition. Moreover, the mass reduction at the subcritical condition was larger than that at the supercritical condition, which was different from the other alloys. According to the AES-SAM analysis in Figure 4, strong oxygen peaks and weak carbon peaks were detected at the surfaces of all the metal alloys. Although Monel K-500 does not have a carbon constituent, a carbon peak was observed.

Then, the carbon peak in all the alloys was absent in the depth profile. Therefore, it was concluded that the carbon came from the oxidation of 2-chlorophenol. After passing through a certain period of sputtering time, the constituents of each metal alloy showed respective peaks, but the oxygen peak exhibited reduced intensity. In the cases of stainless steel 316 and Inconel 625, after the depth profile, the intensity of each constituent component of the metal exposed in the supercritical condition was slightly stronger than that exposed in the subcritical condition. This was because a small oxygen peak was still observed at the subcritical condition. On the other hand, after the depth profile of Monel K-500, the peak intensities at both conditions were similar. In the case of Monel K-500 exposed at both conditions, the Auger electrons in the AES-SAM could not penetrate to the surface. It was assumed that the adsorbed H2O on Cu, one of the major components of Monel K-500, hindered the penetration of the Auger electrons to the surface. Since the surface of Monel K-500 could be analyzed after hot nitrogen purge, it was concluded that the copper oxide layer is covered on the surface.

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Figure 5. AES depth profiles of each metal alloy at subcritical (solid symbols) and supercritical (blank symbols) conditions and AES montage displays at subcritical and supercritical conditions. (a, b) Stainless steel 316; (c, d) Inconel 625; (e, f) Monel K-500.

Figure 5 shows the AES depth profiles with sputtering time (a, c, and e) and AES montage display of main constituents (b, d, and f) for each metal alloy exposed to the subcritical and supercritical conditions. The results indicate that oxygen dominates at the surfaces of metal alloys. Although carbon contamination was not severe, it was relatively more significant at the supercritical condition than at the subcritical condition. This seems to have resulted from the higher destruction of 2-chlorophenol at the supercritical condition than at the subcritical condition. It was also reported that carbon contamination was significant when SCWO of 2,4dichlorophenol was conducted for 30 h in a batch metal reactor, with 700% stoichiometry of H2O2 at the supercritical condition.14 This implies that the destruction reaction occurs not only in

the fluid but also on the metal surface. In addition, the AES montages clearly show that the corrosion phenomena at the supercritical condition were different from those at the subcritical condition. Figure 5a indicates the depth profile of stainless steel 316. Even after 40 min of sputtering time, the atomic percent of Cr and that of Fe were smaller at the supercritical condition than at the subcritical condition, but that of Fe was almost the same in both conditions. In addition, the atomic percent of oxygen in the supercritical condition was still higher than that of Ni. In Figure 5c, after 10 min of sputtering time, the atomic percent of Cr and that of Ni were smaller at the supercritical condition than at the subcritical condition, but that of Mo was almost the same in both conditions. The atomic percent of

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oxygen at the subcritical condition approached zero after 15 min of sputtering time, while the penetration depth of oxygen at the supercritical condition was still maintained at the same sputtering time. As shown in Figure 5c, while the atomic percent of oxygen was maintained, the depletion of nickel was more significant than the depletion of the other metals. This also implies that Ni and Cr are depleted from the surface of Inconel 625 even though the mechanism of metal depletion is slightly different in each condition. Furthermore, although a greater amount of oxygen atom was detected on stainless steel 316 and Inconel 625 exposed at the supercritical condition, the mass reduction of the metals exposed at the supercritical condition was higher, as mentioned before. In Figure 5e, it was found that the atomic percent of oxygen at the surface of Monel K-500 was lower than that of the other metal alloys. However, this does not imply that Monel K-500 is more anticorrosive than the other alloys. According to Figures 4f and 5e, it seems that, after oxygen penetrated and formed metal oxides such as copper oxide and nickel oxide, the metal oxides around the surface were depleted at both the subcritical and supercritical conditions because the mass reduction of Monel K-500 was most significant in the tested alloys. In addition, the oxygen penetration depth was deeper at the supercritical condition than at the subcritical condition. It is interesting that, in the results for 5 min of sputtering time, Ni and Cu at the subcritical condition were significantly affected by the penetrated oxygen and the atomic percent of Cu was smaller than that of Ti. After that sputtering time, the atomic percent of them at the subcritical condition became larger than that at the supercritical condition. On the contrary, such phenomena were not observed in the supercritical condition. Catalytic Effect of Corroded Metal. Figure 6 shows the comparison of destruction efficiencies of 2-chlorophenol at both subcritical and supercritical conditions with the corrosion of metal alloy. It was evident that the corrosion of stainless steel 316 contributed to the improvement of destruction of 2-chlorophenol at both the subcritical and supercritical conditions, compared to the destruction efficiencies without metal corrosion. Moreover, it was noted that the catalytic effect of corroded metal on the destruction efficiency was more significant at the subcritical condition than at the supercritical condition as shown in the improved level of destruction efficiency at the subcritical condition in Figure 6a. The improved level of destruction efficiency stemming from metal corrosion at the subcritical condition was greater than that at the supercritical condition. Moreover, the destruction efficiency of 2-chlorophenol became slightly higher at subcritical condition than at supercritical condition. However, the destruction efficiency at the subcritical condition became similar to that at the supercritical condition after 1 h reaction. With corrosion of Inconel 625 and Monel K-500, the catalytic effect of corroded metal on the destruction efficiency was more significant at the subcritical condition than at the supercritical condition, while the final destruction efficiency at the supercritical condition was higher than that at the subcritical condition, which was different from the result of stainless steel 316. Furthermore, the catalytic effect of Monel K-500 on destruction efficiencies at both conditions was higher than those of the other two metal alloys. With the results of AES-SAM analysis, it is concluded that, at the subcritical condition, a hydrochloric solution facilitates the depletion of metal constituents from alloys and these metal constituents then mainly exist as a form of dissolved ions, which can act as homogeneous catalysts. However, at the supercritical condition, a relatively

Figure 6. Comparison of destruction efficiencies of 2-chlorophenol with corrosion of metal alloys at the subcritical (blank symbols) and supercritical (solid symbols) conditions. (a) Stainless steel 316; (b) Inconel 625; (c) Monel K-500.

large amount of oxygen is used to penetrate the alloys and make metal oxide. This can be one of the possible reasons that the elevation of destruction efficiency is more significant at the subcritical condition than at the supercritical condition. Because a 2-5 µm in-line filter was used, nanosized metal particles as well as metal ions with solution could pass through the filter. Table 1 shows the ICP-MS results of effluent samples with corrosion of stainless steel 316, Inconel 625, and Monel K-500, respectively. The target metal components for the analysis of ICP-MS were selected by the proportion of metal

Ind. Eng. Chem. Res., Vol. 47, No. 7, 2008 2271 Table 1. ICP-MS Results of Stainless 316, Inconel 625, and Monel K-500 at Subcritical and Supercritical Conditions

sample

condition

stainless steel subcritical 316

supercritical

sample

condition

Inconel 625 subcritical

supercritical

sample

condition

Monel K-500 subcritical

supercritical

mass time amount of component (ppb) reduction range (%) (min) Cr Ni Mn 0.094

0.181

35-40 40-45 45-50 50-55 55-60 35-40 40-45 45-50 50-55 55-60

mass time reduction range (%) (min) 0.026

0.074

35-40 40-45 45-50 50-55 55-60 35-40 40-45 45-50 50-55 55-60

0 12.12 12.80 10.88 11.56 28.72 41.34 55.73 53.05 44.22

204.31 214.48 153.76 107.76 99.76 1.08 1.94 1.38 1.61 1.19

142.08 151.28 131.06 99.04 90.96 18.33 25.73 28.88 21.60 21.35

amount of component (ppb) Cr

Ni

Mo

52.37 70.15 61.07 60.90 55.28 74.69 73.61 72.74 67.48 64.48

707.90 606.95 553.05 500.37 466.32 268.14 258.62 230.50 204.50 208.20

0.42 0.51 0.48 0.42 0.46 0.44 0.43 0.42 0.48 0.46

mass time amount of component (ppb) reduction range (%) (min) Ni Mn Cu 0.580

0.514

35-40 40-45 45-50 50-55 55-60 35-40 40-45 45-50 50-55 55-60

Figure 7. GC/MS results of effluent collected after exposure of Monel K-500 to the supercritical condition.

2558.25 2280.87 2254.63 2239.26 2359.67 72.98 71.30 71.59 74.36 75.31

654.58 599.00 602.98 598.57 611.61 32.88 32.54 34.75 37.39 38.49

1108.23 867.58 1111.59 1130.40 1010.53 797.94 810.68 847.15 1020.69 920.24

constituents in the alloys according to the chemical composition. However, Fe could not be detected by ICP-MS due to ArO+ interference with Fe. As shown in Table 1, more Cr depletion was measured in stainless steel 316 at the supercritical condition than at the subcritical condition, whereas the other metal components such as Ni or Mn exhibited the opposite results. In addition, even though the amount of Mn (2 wt %) is much smaller than that of Ni (12 wt %), the amount of Mn in the effluent was comparable to that of Ni in the subcritical condition and even higher in the supercritical condition. Moreover, the concentration of Ni in the supercritical condition was low compared to that of Cr and Mn. Because a larger amount of Cr was detected at the supercritical condition than at the subcritical condition, it was assumed that greater amounts of Ni and Mn were also depleted at the supercritical condition. However, small amounts of Ni and Mn were detected at the supercritical condition as shown in Table 1. According to the Cr concentration from ICP-MS, the depleted Ni and Mn might exist as forms of metal oxides or metal chlorides and they might be trapped in the line filter. In addition, it could be expected that stainless steel 316 is more corroded at the supercritical condition, which agreed with the results of mass reduction. Since the depleted Ni and Mn might be trapped in the line filter as a form of metal oxide or metal chloride, XRD analysis was performed for the powder trapped by the line filter. The

main peak line indicates that the main species was aluminum oxide, stemming from the reaction of ceramic inner reactor. However, it was difficult to identify the existence of metal oxide or metal chloride because the amount of metal was too small. The presence of several other small peaks indirectly revealed that there were other species than the various metal constituents of alloys present in the powder. In the case of Inconel 625 (Table 1), more Cr depletion was measured at the supercritical condition than at the subcritical condition, whereas the concentration of Ni exhibited the opposite results. However, the difference in percentage between the two conditions was much smaller than that of stainless steel 316. Moreover, the concentrations of Mo in the two conditions were almost the same. It is noteworthy that greater amounts of Cr and Ni were detected at the effluent of Inconel 625, even though the mass reduction of Inconel 625 was smaller than that of stainless steel 316. The ICP-MS results for Monel K-500 were different from those for the other alloys. The amounts of Ni and Mn at the subcritical condition were much higher than those at the supercritical condition, a trend similar to the results for stainless steel 316. On the contrary, the amounts of Cu were similar at the subcritical and supercritical conditions, which was similar to the result for Cr in Inconel 625. In Figure 6 and Table 1, it can be seen that the contributions of metal corrosion to the elevation of destruction efficiency were similar to each other at 1 h even though metal corrosion of stainless steel 316 was more severe than that of Inconel 625. This indicates that Ni, the major constituent of Inconel 625, can contribute to the oxidation reaction more than Fe, the major constituent of stainless steel 316. It was also noted that Monel K-500 showed the best contribution to the elevation of destruction efficiency, as evidenced not only by the abundant amount of metal depletion but also by the Cu and Mn components. These components are known to be effective catalysts in the supercritical water oxidation of refractory hydrocarbons.15,21,22 Therefore, it is concluded that the metal components depleted from each metal alloy by corrosion are either solvated in the reactant as metal ions or form metal oxide, and that they act as catalysts to elevate the destruction efficiency of 2-chlorophenol at both the subcritical and supercritical conditions. It is reported that metal cations play a certain role in the elimination of chlorine from 2-chlorophenol, and also reduce the formation of heavy polycyclic aromatic hydrocarbon (PAH) byproduct.20 Figure 7 illustrates the GC/MS analysis of effluent obtained from the supercritical water oxidation of 2-chlorophenol with the corrosion of Monel K-500. It is apparent that the major intermediates found in the effluent are phthalates and amide groups and they might be caused by cations from metal corrosion. However, from the results of ICP-MS and surface

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chemical analysis, the reaction seems to be homogeneous as well as heterogeneous in both conditions because the alloys contain various constituents. In sum, metal corrosion during the oxidation of 2-chlorophenol at the subcritcal and supercritical conditions can contribute to the improvement of destruction efficiency due to the formation of metal oxide and depletion of the metal component. It was found that the metal corrosion elevated the destruction efficiency of 2-chlorophenol by 1-3%. However, metal corrosion with a large surface area will lead to the deterioration of destruction efficiency due to excess consumption of oxidant.23 Considering that the improvement of destruction efficiency from 340 to 380 °C at 250 atm is less than 1% in Figure 2, the change of the destruction efficiency by metal corrosion is not small. However, since the concentration of oxidant contributes to the decomposition efficiency, severe corrosion of metal can result in a decrease of reaction rate.4,14 Conclusion Corrosion phenomena of various metal alloys and the effect of corrosion on the destruction efficiency of 2-chlorophenol at both subcritical condition and supercritical condition were examined by using a continuous-type anticorrosive SCWO reactor. After the exposure to both subcritical and supercritical conditions, all the tested alloys changed their surface colors and morphologies even though the degree of change was different according to the reaction condition and metal constituents. The metals exposed to supercritical condition showed more significant surface changes than those exposed to subcritical condition. In AES-SAM analysis, oxygen penetration was severer at supercritical condition than at subcritical condition. The surface of stainless steel 316 shows that the granules covering the surface are various metal oxides, mainly iron oxides. On the other hand, the surface change of Monel K-500 stemmed from the change of copper, which is one of the major constituents of Monel K-500. After passing through a certain period of sputtering time, the constituents of each metal alloy showed respective peaks, but the oxygen peak exhibited reduced intensity. Destruction efficiencies without corrosion of metal alloys at the subcritical condition (220 atm, 340 °C) and supercritical condition (250 atm, 380 °C) were 94.43% and 95.92%, respectively. It was found that higher destruction efficiency was obtained as the condition goes severe, and the effect of pressure on destruction efficiency was not significant compared with the effect of temperature. Destruction efficiencies of 2-chlorophenol with corrosion of stainless steel 316, Inconel 625, and Monel K-500 were investigated, respectively. The elevation of destruction efficiency was proportional to the amount of metal depletion, and the contribution of metal corrosion to the elevation of destruction efficiency was greater at subcritical condition than at supercritical condition. This implies that the metal oxides formed by the penetration of oxygen and the depleted metal oxide and metal ions act as catalysts and contribute to the elevation of the destruction efficiency of 2-chlorophenol at both subcritical and supercritical conditions. Moreover, Monel K-500 showed the most significant contribution to the elevation of the destruction efficiency of 2-chlorophenol at both subcritical and supercritical conditions because Cu and Mn are promising constituents for effective catalysts in the SCWO of refractory hydrocarbons. The result indicates that a well-selected metal can supply catalysts for the reaction continuously and metal oxide particles

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ReceiVed for reView July 9, 2007 ReVised manuscript receiVed January 22, 2008 Accepted January 26, 2008 IE0709281