Innovations in Supercritical Fluids - American Chemical Society

Chapter 22

Corrosion Studies in Supercritical Water Oxidation Systems

Downloaded by UNIV OF CINCINNATI on February 18, 2015 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0608.ch022

D. B. Mitton, J. C. Orzalli, and R. M. Latanision H. H. Uhlig Corrosion Laboratory, Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139

Supercritical water oxidation (SCWO) is a technology which can effectively destroy various civilian and military wastes, including nerve gas, by oxidation in water at high temperature and pressure. Although, as a technology SCWO can destroy such wastes, the process must be carried out in a reactor which will withstand not only the high temperature and pressure conditions but also a very corrosive environment. As process conditions are severe, corrosion could, ultimately, be the limiting factor in the useful application of this technique; nevertheless, little information relevant to degradation within such systems is available in the current literature. Results from laboratory and pilot-scale SCWO systems presently in operation indicate that wastage (accelerated general corrosion), pitting and stress corrosion cracking (SCC) are corrosion phenomena likely to be encountered in the various sections of SCWO systems. In order to increase our understanding of corrosion phenomena within such systems, systematic testing employing both exposure and electrochemical experiments will be carried out. Exposure samples are monitored for weight loss and subsequently examined to provide comprehensive information on both the rate and form of degradation. Additionally, in order to provide a fundamental understanding of degradation phenomena, an elevated temperature electrochemical facility has been constructed. This incorporates an Inconel-625 vessel designed to permit standard d.c. techniques as well as electrochemical impedance spectroscopy (EIS). Finally, results from the analysis of failures which have occurred within other SCWO systems have been employed to provide a more comprehensive understanding of potential failure modes. The design and construction of the corrosion test facility at MIT is described in detail elsewhere (7). While the majority of the facility is constructed of Inconel 625, Hastelloy C-276 and stainless steel type 316 have been employed to a limited degree. The high nickel alloys are considered important for severe service applications (2); nevertheless, for conditions associated with SCWO, selective dissolution of Cr and Mo from Inconel-625 and severe selective dissolution of Cr, Mo and W from C-276

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1995 A m e r i c a n Chemical Society

In Innovations in Supercritical Fluids; Hutchenson, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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are reported (5). Solution analysis for Cr and Ni of the effluent from an Inconel-625 reactor carried out at Sandia National Laboratories suggested that non-chlorinated feeds produced little evidence of corrosion while chlorinated feeds resulted in relatively high amounts of metal in the effluent (4). When exposed at 580°C., Inconel 625 exhibits a general corrosion rate of about 80 mpy, SCC, and pitting for a feed stock of methylene chloride and isopropyl alcohol (neutralized with NaOH). For the same feed, but at the lower temperature of 380 C, Hastelloy C-276 exhibits similar localized phenomena; however, the rate of corrosion increases to approximately 480 mpy (5). As a result of the likelihood of significant corrosion damage to the system during experiments, the exposure vessel at MIT was designed with a thick wall to permit future machining. Additionally, to reduce the potential for severe corrosion within the preheater section, the water and corrosive delivery systems were separated until after the preheater. Thus, only that portion of the system after the preheater will be subjected to conditions which are both hot and corrosive. Preliminary exposure studies were carried out to provide a base-line assessment of the materials of construction of the corrosion test facility. Short term exposure studies (96 or 24 hours) have been conducted in deionized water at 300, 400 and 500 C. The results are presented in detail (6) and in summary (7) elsewhere. A l l alloys were found to exhibit a weight gain, which, apparently, resulted from the formation of a relatively thick film. At the lowest temperature tested (300 C), both of the high-nickel alloys produced films and discrete raised regions, assumed to be reaction product caps on pits. There was some suggestion of a minor amount of intergranular corrosion for the stainless steel. At the highest temperature tested (500 C), there was an indication of crevice corrosion at the periphery of the washer for 316 stainless steel, and 1-625 exhibited pitting. Even after only 24 hours at 400°C., pitting was observed for the 1-625; however, neither the stainless steel nor the C-276 exhibit localized attack during this test. The effect of exposure time in deionized water at 500 C has also been reported (7). Inconel-625 and Hastelloy C-276 exhibit a similar trend over the duration of the experiment (10 days), which may suggest the formation of a protective oxide. Conversely, the stainless steel initially revealed less weight gain (4 days); however, after 10 days exposure, this alloy exhibited a significantly larger gain in mass than either of the nickel alloys. This suggests the formation of a comparatively thick film on the stainless steel (at the longer exposure time), possibly indicative of nonprotective (breakaway) oxidation. Additionally, after 4 days there was some indication of selective oxidation or deposition of Mo for the Inconel-625 sample. After 10 days, both the Inconel-625 and the Hastelloy C-276 coupons revealed localized phenomena. The relatively thick film development for the stainless steel alloy at the longer exposure time obscured the metallic surface and although discrete regions were seen on the coupons, these were not specifically correlated to any form of localized attack.


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Results and Discussion. Electrochemical Test Facility Design. While details of the original design of the corrosion test facility have been presented previously (7), the modified version is represented schematically by Figure 1. This figure shows the electrochemical equipment (1), which comprises a Schlumberger 1286 Electrochemical Interface (potentiostat) connected to a Schlumberger 1260 Impedance/Gain-Phase Analyzer. Electrochemical Impedance Spectroscopy (EIS) as well as d.c. techniques will be accomplished by employing software produced by Scribner Associates Inc. The general electrochemical cell configuration (Figure 2(a)) has been presented elsewhere (7,7). One potential problem in conducting experiments at these temperatures and pressures involves the working electrode (WE) design. Details of the working electrode assembly which has been designed for tests at both sub- and


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Corrosion Studies in Supercritical Water Oxidation


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Figure 2. The general cell configuration, Figure 2 (a), (Adapted from réf. 1.) and details of the working electrode assembly, Figure 2 (b), which will be used for experiments at both sub and supercritical temperatures.


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supercritical conditions are presented in Figure 2 (b) for the first time. As the cell is constructed from Inconel-625, it is necessary to electrically isolate the working electrode from the cell. This has been accomplished by the use of a thermally oxidized zirconium alloy. Added isolation is provided by using both Zr rod and tube. The connection between the sample and the Zr tube employs a PTFE seal at low operating temperatures; however, a ceramic seal is needed for work at supercritical temperatures. Exposure Test Facility Design. After conducting a number of exposure tests, several design faults became obvious, and these have necessitated some modification to the facility (Figure 1). The preheater (2) was located too close to a portion of the Lexan shield, which melted. This region of Lexan was replaced and the preheater was insulated and relocated at a greater distance from the shield. The influent and effluent capacity has been increased and a thermocouple (3) has been added to the effluent lines. The latter will prevent a further escape of steam as a direct result of the lack of cooling water, which occurred during one experiment. If a temperature excursion occurs in the effluent line, the computer will now be able to shut the system down. Preliminary Failure Analysis of Hastelloy C-276 Preheater Tubes. After almost ten years of essentially corrosion free service during experiments employing nonchlorinated organics, an axial through-wall rupture occurred in a Hastelloy C-276 preheater tube of a tubular plug-flow reactor (PFR) system schematically represented in Figure 3. This failure occurred approximately 104 hours after initiating tests with methylene chloride ( C H 2 C I 2 ) (9). The rupture occurred near the top of the second sandbath, and although no thermocouple was located in this region, evidence indicates that the temperature was in the high subcritical range. The tube in the region of both the inlet (ambient temperature) and outlet (supercritical temperature) suffered near negligible corrosion (10). The penetration rate (based on 104 hours) as a function of position is presented in Figure 4. Penetration rates in this figure refer to the maximum cross section of wall which has been degraded both uniformly and at grain boundaries. As revealed in this figure, although the penetration rate is near negligible at the extremities of the tube, it is significant in the region of the throughwall failure. A second Hastelloy C-276 preheater was subsequently installed; however, this failed (within 20 cm of the location of the first failure) approximately 45 hours after initiating experiments with methylene chloride. For a wall thickness of 0.01 inches, the first and second failures represent penetration rates at the failure site of the order 850 - 2000 mils per year (mpy) (77). For comparison, approximately 20 mpy is considered to be acceptable from an engineering standpoint. Due to the appearance of the rupture site, SCC must be considered as a potential failure mode (10). For nickel-base alloys, SCC is promoted by the following: elevated temperature (T>205°C), high CI" (% range), acidity (pH< 4), oxidizing conditions, H 2 S , high stress and/or high strength materials (2). Virtually all of these factors were present during these experiments. A SEM micrograph of the crosssection of the failure site is presented in Figure 5. The sample was etched with a mixture of hydrochloric, acetic and nitric acids in the ratio 3:2:1 plus 2 drops of glycerol per 30 ml acid. The eventual through-wall failure is obviously intergranular in nature. Additionally, there is an indication in other regions in this micrograph that corrosion is enhanced at grain boundaries. Solution analysis for Cr and Ni from a subsequent failure within this system during tests employing methylene chloride revealed that the effluent was significantly enriched in nickel (Ni/Ni+Cr = 97.6 - 98.6 % ) with respect to a nominal value (Ni/Ni+Cr = 78.6 %) in the alloy. Therefore, in addition to the possibility of SCC, both intergranular corrosion and selective dissolution or oxidation must be considered as contributory factors in the failure.



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Figure 3. Schematic representation of the tubular plug-flow reactor (PFR) system employed at MIT to study kinetics in SCWO (Adapted from ref. 8).

Figure 4. The penetration rate as a function of position of the a Hastelloy C-276 PFR preheater tube, which failed during experiments employing methylene chloride.


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Figure 5. Cross-section of the a Hastelloy C-276 PFR preheater tube, which failed during experiments employing methylene chloride.


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Figure 6. Auger line scan of the corrosion productfroma Hastelloy C-276 PFR preheater tube, which failed during experiments employing methylene chloride. The corrosion product is apparently primarily a chromium oxide. An Auger line scan is presented in Figure 6. This shows the relative amounts of Ni, Cr and oxygen in the metal and in the corrosion product. The dark solid horizontal line across the center of the micrograph shows the position of the scan. Both Ni and Cr levels are represented by white lines, which have been labelled, and the level of oxygen is depicted by a dashed line. In the alloy, as revealed by this scan, the oxygen intensity is negligible and the amount of nickel exceeds the chromium level. However, the relative intensity of these elements changes as the interface with the bulk corrosion product is reached. In the corrosion product, the chromium level is greater than that of nickel, and the oxygen values echo those of chromium. Thus, apparently, the bulk corrosion product is primarily a chromium oxide. Although the Auger line scan indicates that the corrosion product is primarily a chromium oxide, the appearance of bands (Figures 7 (a) and 8 (a)) within this oxide suggests a more complex structure. The relative intensity of oxygen and the remaining major alloying constituents of Hastelloy C-276 (Ni, Mo, W, Fe) are presented as elemental maps in Figures 7 (b) and 8 (b). Each map corresponds to the SEM micrograph presented in the same figure, and the white dots reveal both the location and intensity of the element of interest. Chromium intensity is so high as to reveal essentially a solid white region, thus, it has been omitted from these figures. In Figure 7, the banded structure can only be seen in the oxygen element map. In Figure 8, at higher magnification, again the oxygen analysis reveals the only indication of varying intensities corresponding to a banded structure. At face value, this suggests a structure which is composed of a number of oxides of chromium. However, sample preparation requires mechanical polishing, which can result in smearing and, thus, obscure subtle differences in elemental composition of the corrosion product. During high temperature oxidation of similar alloys (72), both Cr and Ni oxides form and, therefore, it is anticipated that future work will indicate that the bands reflect oxides of various elements.



Figure 7. (a) A SEM micrograph and (b) elemental maps (O, Fe, W, Mo) of the corrosion product from a Hastelloy C-276 PFR preheater tube, which failed during experiments employing methylene chloride.



Figure 8. (a) A higher magnification SEM micrograph and (b) elemental maps of the corrosion product (O, Ni, W, Mo)froma Hastelloy C-276 PFR preheater tube, which failed during experiments employing methylene chloride.


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Penetration rates for Hastelloy C-276 preheater tubes exposed to methylene chloride are higher in the subcritical region than in the supercritical region. The corrosion product is primarily chromium oxide; however, there are also bands of different oxygen intensity within this oxide. Although SCC must be considered as a potential mode of failure for these tubes, there is enhanced corrosion at grain boundaries and some indication of selective dissolution. The extremely high penetration rates associated with these conditions confirm that corrosion will be a pivotal issue in the eventual commercialization of SCWO as a viable technique for the destruction of chlorinated organic waste. Literature Cited. rd

1.

Mitton, D.B.; Orzalli, J.C.; and Latanision, R.M., Proc. 3 Int. Symp. on Supercritical Fluids; I.N.P.L.A.R: Strasbourg, France, 1994, Vol. 3, pp 43-48. 2. Asphahani, Α.; Metals Handbook 9th Edition; ASM International: Metals Park, OH, 1987, Vol. 13., pp 641-655. 3. Bramlette, T.T.; Mills, B.E.; Hencken, K.R.; Brynildson, M.E.; Johnston, S.C.; Hruby, J.M.; Feemster, H.C.; Odegard, B.C.; and Modell, M., "Destruction of DOE/DP Surrogate Wastes with Supercritical Water Oxidation Technology"; Sandia National Laboratories, Livermore, CA, Sand 90-8229 1990. 4. Rice, S.F.; Steeper, R.R.; and LaJeunesse, C.A., "Destruction of Representative Navy Wastes Using Supercritical Water Oxidation"; Sandia National Laboratories, Livermore, CA, SAND94-8203 1993. 5. Latanision, R. M. and Shaw, R.W., Co-Chairs, "Corrosion in Supercritical Water Oxidation System - Workshop Summary"; Massachusetts Institute of Technology Energy Laboratory, MIT-EL 93-006 1993. 6. Orzalli, J.C., Master's Thesis, Massachusetts Institute of Technology 1994. 7. Mitton, D.B.; Orzalli, J.C.; and Latanision, R.M., Proc. 12 ICPWS, Orlando, Florida, 1994, in press. 8. Holgate, H.R.; and Tester, J.W., Combustion Sci. and Tech. 1993, Vol. 88 pp. 369. 9. Mitton, D.B.; Latanision, R.M.; Orzalli, J.C.; Marrone, P.A.; Phenix, B.D.; Meyer,J.C.; and Tester, J.W., presented at the "University Research Initiative Meeting"; University of Delaware Newark, DE 1993, unpublished data. 10. Latanision, R.M., Willis Rodney Whitney Lecture presented at CORROSION 94, Baltimore MD 1994, in press. 11. Mitton, D.B.; Latanision, R.M.; Orzalli, J.C.; Marrone, P.A.; Phenix, B.D.; Meyer, J.C.; Lachance, R.; and Tester J.W., presented at the "University Research Initiative Meeting"; University of Texas at Austin, Austin , Texas 1994, unpublished data. 12. Shreir, L.L.; Corrosion 2 Ed; Newnes-Butterworths: London, U.K., 1976, Vol. 1.,pp7:79-7:114. th

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Innovations in Supercritical Fluids - American Chemical Society

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