3412
Ind. Eng. Chem. Res. 2006, 45, 3412-3419
Surface Chemical Analysis on the Corrosion of Alloys in the Supercritical Water Oxidation of Halogenated Hydrocarbon Hyeon-Cheol Lee,† Sang-Ha Son,‡ Kyung-Yub Hwang,§ and Chang-Ha Lee*,‡ Manufacturing Engineering R&D Institute, Samsung Electro Mechanics Company, Ltd., Suwon, 443-743, Korea, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea, and Department of Chemical Engineering, Yonsei UniVersity, Seoul 120-749, Korea
A surface chemical analysis on the corrosion of various alloys under supercritical water oxidation (SCWO) conditions with relatively high concentrations of halogenated compounds and hydrogen peroxide was conducted using an Auger electron spectroscopy/scanning Auger-electron spectroscopy. The tested alloys (Inconel 600, Hastelloy C-276, Monel 400, stainless steel (SUS 316), Titanium G2, and Zirconium 702) were exposed to the same conditions: 4000 mg/L of 2,4-DCP at 400 °C and 250 bar, with 700% stoichiometric H2O2 in a Hastelloy C-276 batch reactor. Even under the supercritical water (SCW) condition of 4.8 pH without H2O2, a certain degree of corrosion was observed on the surface of all the alloys, especially SUS 316. Under the severe SCWO condition with excess H2O2, the surface corrosion of all the alloys was significant, but the types of corrosion on the alloy surfaces differed. Chromium in the SCWO process that contained H2O2 for halogenated compounds could potentially lead to the corrosion type and degree of alloys, but a certain amount of nickel depletion was also observed. Among the alloys tested, Titanium G2 was the most resistant to corrosion, under the conditions of the experiment. Considering that the surfaces of the alloys were covered by a carboncontaminated layer, it may be concluded that the metal oxides or metal ions on the surface of the alloy have a role in forming the carbon-contaminated layer in the decomposition of halogenated compounds under SCWO. 1. Introduction In the last two decades, supercritical water (at a pressure of P > 221 bar and temperature of T > 374 °C) oxidation (SCWO) has been actively developed as a means of destroying hazardous organic waste.1-6 Most organic compounds, as well as oxidant and combustion gases, are miscible in all proportions with water under supercritical conditions. Therefore, organic materials and wastes can be oxidized easily under SCWO conditions with an oxidizer. It has been demonstrated that hydrogen peroxide (H2O2) is a far more efficient oxidizing agent for SCWO than oxygen.7 The higher oxidizing power of H2O2 is largely attributed to the rapid decomposition of H2O2 into hydroxyl radicals at high temperatures.8,9 Therefore, using H2O2 as an oxidant, the decomposition efficiency of the hydrocarbons was able to reach a value of >99.9% under SCWO conditions.8-11 The major species produced during oxidation of the hydrocarbons were CO2 and H2O, even under short residence times. In addition, as a result of the relatively low operating temperature, NOx and SOx compounds were not produced.12 Because organic waste that contain halogenated hydrocarbon is toxic and hazardous to the environment, such waste must be treated before discharge into the environment. However, the heteroatoms Cl, S, or P that exist in the organic wastes were transformed to the mineral acids HCl, H2SO4, or H3PO4, respectively, under SCWO.13 Most notably, the presence of HCl at a very low pH and excess oxygen led to active corrosion and metal loss at high temperatures and pressure, because of the formation of a metal chloride and/or oxychloride.14 * To whom correspondence should be addressed. Tel.: (822)21232762. Fax: (822)312-6401. E-mail:
[email protected]. † Manufacturing Engineering R&D Institute, Samsung Electro Mechanics. Co., Ltd. ‡ Department of Chemical Engineering, Yonsei University. § Korea Institute of Science and Technology.
This corrosion problem is one of the main obstacles to commercializing the SCWO process.7,13 Mitton et al.15 determined, through experiments under high subcritical conditions, that there is a strong correlation between feed pH and the relative dissolution of nickel and chromium for various nickel-based alloys. In addition, acidic conditions have a tendency to favor nickel dissolution in these materials and the most pronounced degradation is observed at high subcritical temperatures,16 whereas the most severe corrosion was stress corrosion cracking in the SCWO of wastewater that contained chloride.17 Foy et al.18 reported that the corrosion of titanium was observed to be slight in the hydrothermal processing of chlorinated hydrocarbons in relatively high concentration. Recently, many researchers have focused on solving this problem using a variety of methods and reactor designs.19-22 In addition, various materials have been used for the construction of SCWO systems, including exotic and expensive alloys and ceramics.13,15-18,23 Therefore, the database of potential materials for the fabrication of SCWO systems remains important, because several new or modified reactor designs have emerged. In addition, most of the previous work was performed at dilute concentrations to avoid attack by the reaction product HCl on the high-pressure reactor and focused on analysis of the effluent solution. Although SCWO is a very efficient technology for the treatment of hazardous wastes and wastewater, the process must be conducted in a reactor that is capable of accommodating elevated temperatures, pressures, and, potentially, a very aggressive environment. As a result, the selection of appropriate material and an understanding of corrosion phenomenon are very important in designing a safe SCWO system. In this study, surface chemical analysis on the corrosion of various alloys in SCWO was conducted using an Auger electron spectroscopy/scanning Auger-electron microscopy (AES/SAM), instead of resorting to an analysis of the effluent solution. The materials chosen for testing included three nickel-based alloys
10.1021/ie050663v CCC: $33.50 © 2006 American Chemical Society Published on Web 08/31/2005
Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006 3413 Table 1. Chemical Compositions of Alloys Composition (wt %)
Inconel 625 Hastelloy C-276 Monel 400 SUS 316 Titanium Gr2 Zirconium 702
Al
C
Co
Cr
0.4
0.1 0.02 0.3 0.08 0.1 0.05
1 2.5
22 15.5 17 0.015
Cu
Fe
31
5 5.54 2.5 66.35 0.2
H
0.015 0.005
Hf
Mn
Mo
0.5 1 2 2
9 16
4.5
(Inconel 600, Hastelloy C-276, Monel 400, stainless steel (SUS 316), Titanium G2, and Zirconium 702). The corrosion test was performed under severe SCWO conditions, such as near-critical temperatures, a relatively high concentration of chlorine compound, and H2O2 conditions. The coupons of all the alloys were exposed to the same conditions: 4000 mg/L of 2,4-dichlorophenol (2,4-DCP) at 400 °C and 250 bar, with 700% stoichiometric H2O2 in a Hastelloy C-276 batch reactor. This study will contribute to the database of potential materials for the fabrication of SCWO systems. 2. Experimental Section 2.1. Experimental Apparatus and Procedure. In this study, 2,4-dichlorophenol (2,4-DCP, C6H4Cl2O, 99% purity; Acros Organics Co., Ltd.), as a representative halogenated compound, was used for the corrosion test of various alloys under SCWO conditions. After 2,4-DCP was dissolved in secondary purified water, the solution was used as a feed for the SCWO experiments. In this study, hydrogen peroxide (30 wt % H2O2, Junsei Chemical Co., Ltd.) was used as an oxidant. For the corrosion test of alloys under SCWO conditions, SUS 316, Inconel 625, Hastelloy C-276, Monel 400, Titanium G2, and Zirconium 702 were selected. The constituent elements of each alloy are listed in Table 1. The alloys were prepared as coupons with a length of 10 mm, a width of 10 mm, and a thickness of 1-10 mm. The experimental apparatus for the corrosion of alloys in SCWO is shown in Figure 1. The corrosion experiments were performed in a batch-type SCWO reactor that was made of Hastelloy C-276. The reactor volume was 588 cm3 (with an inner diameter of 6.5 cm and an outer diameter of 9.0 cm). The apparatus consisted of two high-pressure syringe pumps (Lab Alliance Prep. 100) to feed 2,4-DCP and H2O2, a reactor, and a pressure relief valve, all of which were connected by high-
Figure 1. Schematic diagram of the experimental apparatus.
N
1.5 0.03 0.025
Nb
Ni
3.7
57.4 55.2 63.68 12
O
P 0.03 0.045
0.25
S
Si
Ti 0.4
0.03 0.024 0.03
0.5 0.08 0.5 1
Tu
V
3.75
0.35
Zr
99.41 95.42
pressure stainless-steel tubing and fittings. An electrical heater was installed on the reactor to maintain reaction temperature. To reach the desired supercritical water (SCW) conditions in the batch reactor (250 bar and 400 °C), the amount of pure water was first determined. At that time, reactor temperature was measured using two type-K thermocouples, which were inserted into the reactor through the upper part and located at the outside of the reactor. Pressure was measured using a pressure gauge and pressure transducer. The feed amount at 250 bar and 400 °C then was tested again, using a mixture of pure water and 700% stoichiometric H2O2, corresponding to a 2,4DCP concentration of 4000 mg/L. Using the determined feed amount, the pH variation and the conversion of 2,4-DCP of 1000 mg/L were measured without oxidant and with 300% stoichiometric H2O2, respectively. From the experiments, the reactor temperature and pressure were confirmed (400 °C and 250 bar, respectively). At that time, a sample size of ∼2 mL was collected using a two-stage needle valve, as shown in Figure 1. To study the corrosion of alloys, the same size Hastelloy C-276 reactor was prepared again. However, in this case, unnecessary partsssuch as the inserted thermocouple, the inserted sampling line, and one inserted feed lineswere not included in the system (see Figure 1). Initially, the alloy coupons were installed inside the reactor. Then, the determined amount of water with a 2,4-DCP concentration of 4000 mg/L, as determined from the pretest, was fed into the reactor. After reaching the desired temperature and pressure as monitored by the pressure gauge and thermocouple installed on the outside of the reactor, 700% stoichiometric H2O2 was injected into the reactor. 2.2. Analytical Methods. A gas chromatograph/flame ionization detector (Hewlett-Packard model HP-5980 Series 2) with a capillary column (Hewlett-Packard, model HP-1) was used to analyze the decomposition efficiency. The oven operating temperature was set at 150 °C, and the injection temperature was set at 300 °C. Helium was used as a carrier gas, at a flow rate of 1 mL/min. The pH of the sample was measured by a pH meter (Orion, model 520A). Before and after the corrosion test, surface analyses of the alloys were conducted, using scanning electron microscopy (SEM) (Hitachi, model S-1400) and AES/SAM (Perkin-Elmer, PHI model 670). In this study, the AES was used quantitatively to determine the chemical state of the elements in the alloys. The conditions of the survey scan and sputter depth profile of the AES were recorded as follows: primary beam energy, Ep ) 10 keV; primary beam current, Ip ) 0.0099 µA; and beam diameter, ∼0.4 µm. The resolution of the cylindrical mirror analyzer was set to 0.6%. The Ar-ion beam, with an ion energy level of 1.5 keV and current density of 0.6 µA/m2, was produced by a differentially pumped ion gun. The sputter profiles were analyzed using the PC PHI-MATLAB software package. Morphological analysis of the surface of the alloys was performed using SAM.
3414
Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006
Figure 2. Plot of pH versus conversion in the supercritical water oxidation (SCWO) of 1000 mg/L of 2,4-dichlorophenol (2,4-DCP).
3. Results and Discussion 3.1. pH Variation in Supercritical Water Oxidation (SCWO) of 2,4-DCP. The decomposition and pH variation of 2,4-DCP in SCWO were studied via experimentation at a temperature of 100-600 °C, a pressure of 250 bar, and a 2,4DCP concentration of 1000 mg/L. The thermal decomposition of 2,4-DCP without oxidant was
