sumitomo ogawa metal dusting [formatted] · 2019-01-12 · - 129 - ⇌pp improved metal dusting...

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Improved Metal Dusting Resistance of New Sumitomo 696

Ni-Base Alloy for Synthesis Gas Environments

Y. NISHIYAMA, H. OKADA, T. OSUKI, S. KURIHARA Sumitomo Metal Industries, Ltd

Amagasaki, Japan

H. OGAWA Sumitomo Metal Industries, Ltd, European Office

London, UK

Long-term tests were conducted for several Fe- and Ni-base alloys to evaluate their metal dusting behaviour. Test specimens with ground surfaces were reacted with a CO-H2-CO2-H2O gas mixture which simulates synthesis gases produced in reforming plants. Several pits developed on the ground specimens of conventional Ni-base alloys after exceeding 10,000h when the heating at 650ºC for 50h was periodically followed by cooling. A Ni-base alloy containing a proper content of Cu still had no pits on its surface even after 22,000h exposure. The cyclic heating test was also conducted on test specimens treated by electrolytic-polishing to accelerate the pit initiation. It was demonstrated that the developed alloy had excellent resistance against metal dusting while other Ni-base alloys had several pits in a relatively short time. A solid solution of Cu greatly restrains interaction between the CO molecule and the metal where oxide scales are damaged, leading to a complete healing of the protective oxide scale. In addition, Ni-base alloy has excellent phase stability which contributes long-term creep strength and toughness. A matching filler metal for the new alloy is under development and shows good resistance to metal dusting corrosion.

INTRODUCTION Metal dusting, a type of corrosion resulting from catastrophic carburization or graphitization of steels and alloys occurring in a carbonaceous atmosphere, is a prominent cause of corrosion damage for high-temperature materials used in ammonia, methanol, dimethyl ether and gas-to-liquids production plants [1-4]. It is often encountered when steels and alloys are exposed in carbon-bearing gas mixtures including CO, H2, CO2, and H2O. Once the metal dusting occurs on the metal surface, a pit like wastage continues to grow during exposure to the carbon-bearing gases at intermediate temperatures. This catastrophic wastage due to metal dusting is a more severe problem than carburisation because it seriously decreases the service life of materials used as a high-temperature component.

Y. Nishiyama, H. Okada, T. Osuki, S. Kurihara, H. Ogawa

130 Nitrogen+Syngas 2011 International Conference (Düsseldorf, 21-24 February 2011)

One of the most common techniques to prevent metal dusting is to form a protective oxide scale on alloy surfaces. Sheller [5] has proposed that theoretical explanation for effective metal dusting resistance can be calculated by the equivalent equation:

Creq = Cr% + 2 × Si% > 22. Both chromium and silicon can form protective oxide scales as Cr2O3 and SiO2, respectively. Schillmoller [6] has modified the equivalent equation in a recent paper. He considered the effect of Al as an Al2O3 scale and proposed it as follows:

Creq = Cr% + 3 × (Si% + Al%) > 24. These scales can act as a barrier against the carbonaceous gas. Growth of a pit initiated through a defect of the oxide scale has also been reported for alloys containing various alloying elements. Nishiyama [7] has clarified the effect of chromium and nickel both on pit initiation and its growth in several Fe-Ni-Cr alloys with differing contents of Cr and Ni. Baker [8] investigated the metal dusting behaviour of Ni-base alloys and has shown the regression analyses for wastage rate on alloying elements from a laboratory corrosion test result. Moreover, laboratory corrosion tests for development of eminent alloys required for severe metal dusting attack have been conducted [9-13]. The formation of the protective oxide scale can be effective for some time. Once any defects occur in the oxide scale, however, carbon dissociated from CO molecules diffuses into the metal matrix through the defects. Unless the oxide scale is repaired, the carbon ingress progresses, followed by pit initiation. Even the enhanced alloys that contain high concentrations of Cr, Si, and Al did not resist metal dusting completely. Recently, it has been established that copper is greatly beneficial in protecting against metal dusting [14, 15]. Nishiyama [16] examined the metal dusting behaviour of Ni-Cr alloys with various Si and Cu contents and has concluded that the effect of Cu can be applied in a practical material design of Ni-Cr alloy for use as a high-temperature component in reforming units. In the chemical reaction between synthesis gas and metal, CO molecules are first adsorbed on the surface of the metal, and dissociated carbon atoms then penetrate the metal. The basic experiments revealed that the added Cu reduces the atomic interaction of CO with the metal and restrains carbon penetration, which has been successfully proved by electronic structure analysis [14]. For a Cu-containing Ni-base alloy, excellent metal dusting resistance may be expected by means of a hybrid-suppression technique; i.e. formation of protective oxide scales and reduction of reactivity with CO gas in

protective oxide scale

Cr and Si

metal matrix

Cu

(b) Suppression of CO-dissociation by a Cu solid solution at defect sites of the oxide scale

Carbonaceous atmosphere (ac >1)

CO molecule

metal matrix

(a) Oxide scale formation: barrier to reaction between CO and the metal

(c) Healing of the protective oxide scale at the defect sites

Fig. 1: Concept of a new metal dusting prevention technique

Improved Metal Dusting Resistance of New Sumitomo 696 Ni-Base Alloy for Synthesis Gas Environments

Nitrogen+Syngas 2011 International Conference (Düsseldorf, 21-24 February 2011) 131

synthesis gas environments. As illustrated in Fig. 1, protective oxide scales such as Cr2O3 and SiO2 act as a barrier against CO attack during a certain period. A solid solution of Cu in the metal matrix plays a role of the surfactant-mediated resistance on the metal surface where any defects of the oxide scale occurred. In the meantime, the healing of the protective oxide scale can be accomplished successfully, keeping the metal free of a pitting nucleation. In this study, we conducted a long-term metal dusting test for a new Ni-base alloy containing Cu. We intend to clarify whether our novel “hybrid-suppression technique” approach can be preserved efficiently into the long time. We also researched the mechanical and thermal stability. In addition, a matching filler metal of the new alloy is under development. We investigated the resistance to metal dusting corrosion of welding of new alloy. The new alloy has already been designated as ASTM UNS (1) N06696 and ASME code case 2652 (Table 1).

Table 1 Standards of Sumitomo 696

Standard No.

ASTM UNS N06696 B-163,166,167 and B-168

ASME* Code Case 2652 Section I and Section VIII, Division1

* The maximum allowable stress values are certified and the maximum temperature is 1000ºC(Section VIII)

Table 2

Chemical Composition of the Test Alloys Mark UNS no. C Si Cr Ni+Co Al Others A800 N08810 0.09 0.3 19.9 30.7 0.5 0.5Ti A601 N06601 0.02 0.3 23.0 60.0 1.4 - A690 N06690 0.02 0.3 29.9 59.6 - - A160 N12160 0.05 2.7 28.2 65.5 - Ti A602 N06025 0.16 0.1 25.2 62.7 2.3 Y A693 N06693 0.02 0.3 30.1 59.6 3.0 Nb A696 N06696 0.08 1.5 30.2 61.0 - 2Cu, Mo

EXPERIMENTAL One steel and seven nickel-base alloys were tested. Their chemical compositions are listed in Table 2. The newly developed alloy is named as A696. Alloys of UNS nos. N 08810, N 06601, and N 06690, were prepared from commercial production tubes. For other alloys, ingots were made in a small-scale vacuum furnace. These were hot forged and hot rolled to plates of thickness 12 mm. After annealing at 1150ºC in air, these plates were cold rolled to a thickness 8.4 mm and then subjected to solution heat treatment at 1180ºC. Coupon specimens were cut from the tubes or sheets to 3 × 15 × 20 mm, and a small hole of 2 mm diameter was drilled in them for support. Their surface was mechanically ground to a 600-grit emery paper, followed by ultrasonic cleaning in acetone. For some alloys, the surface were electrolytic-polished in 10% H2SO4-20% methanol -70% ethanol (in vol-%) solution at ambient temperature on 10 V, which can accelerate metal dusting initiation. A metal dusting test was conducted in a horizontal reaction chamber. The reaction gas composition of 60% CO, 26% H2, 11.5% CO2, and 2.5% H2O (in vol.%), which gives the carbon activity of 10 (without graphite precipitation) and oxygen partial pressure of 4.6 × 10-26 MPa at 650ºC obtained from Equations (1) and (2), respectively, was chosen to simulate the actual reforming plants.

CO + H2 ⇌ C + H2O (1) 2H2O ⇌ 2H2 + O2 (2)

The cycling heating test consisted of heating at 650ºC for 50h and cooling down to 100ºC in 5h, followed by holding the temperature at 100ºC for 0.5h. After every 20 cycles (corresponds to 1,000h heating), the test



specimens were cooled down to room temperature. Each specimen was weighed after removing the coke by ultrasonic cleaning in acetone, and then the pits generated on its surface were counted using an optical microscope. The growth of pit which was generally selected from the one which appeared at the very first stage of the test was evaluated by a measurement of its depth using a stylus depth indicator. The metallographic cross-section of the specimen surface was investigated for some selected test specimens using an optical microscope. Oxide scales formed on the specimens were analysed by a transmission electron microscope with EDS (JEM-2100). To research the mechanical properties, creep rupture tests were conducted at elevated temperature. The temperature was from 500 to 1,100 ºC, respectively. In addition, Charpy impact tests were carried out on A696 and Al-bearing Ni- base alloys after aging at 500, 650 and 800ºC for 1,000, 3,000 and 10,000 hours. A metal dusting test was also conducted using matching filler metal. The chemical composition of the matching filler and the welding conditions are listed in Tables 3 and 4, respectively. The surface and test conditions were the same as in the base metal test. The test was carried out at 650ºC for 60 heating cycles (corresponding to 3,000h heating).

Table 3 Chemical Composition of Matching Filler

Mark C Si Cr Ni+Co Others MDSP28 0.051 1.35 30.7 63.0 2Cu MDSP30 0.16 1.39 30.6 63.7 2Cu

Table 4

Welding Conditions Shape of groove U groove (Root face = 2.5 mm) Interpass temperature below 150 C Pre-heat None PWHT None

Arc current (A)

Arc voltage (V) Welding speed (cm/min) Welding heat input

(kJ/cm) 1st pass 100 12 10 7.20 2nd pass 120 13 10 9.36

Welding condition

More than 3rd pass 130 12 10 9.36

RESULTS AND DISCUSSION Resistance to metal dusting corrosion As listed in Table 5, total exposure time for each test specimen depends on the degree of metal dusting due to pitting. The strong corroded specimens had to be removed from the test. Though the specimens of A696 had no pit after an exposure of 22,000h corresponding to 440 cycles of the cyclic heating test, they were taken out to examine the micro structures. Except alloy A696, all the test alloys had pits on their ground surface. On the A800 steel, many pits have developed after a short period of 500h, indicating that it was highly susceptible to metal dusting. Nickel-base alloy A601 yielded to the metal dusting attack at shorter time than other Ni-base alloys. Alloys A690 and A160 with about 30% chromium showed a better resistance to metal dusting, suggesting that Cr more than 30% is needed to improve metal dusting resistance. But, pits were eventually initiated on their surface after approximately thirteen thousand hours. Copper content alloy A696 remained unattacked even after the 22,000h exposure, resulting that it has an excellent resistance to metal dusting. Corrosion kinetics in terms of the mass loss of the test specimens were obtained for the final period of each exposure and are also summarized in Table 5. Alloys A800 and A601 underwent significant mass loss. Both of the final mass loss rates reached at the order of 10-3 mg.cm-2.h-1, though they differed in total exposure



time, apparently showing that the electrolytically-polished A601 instantly suffered pitting attack on its surface. The electrolytic-polishing treatment shortened the ‘incubation time’ for the initiation of metal dusting pits. In this test condition, pits have developed on the electrolytically-polished surface fifteen times faster than those on the ground one. It is considered that the electrolytically-polished surface forms less oxide scale to protect against CO attack than the ground surface because Cr comprising the protective oxide diffuses outward slowly in its microstructure. This propensity has been reported in a past paper in which metal dusting behaviour was investigated for 18-8 stainless steel [18]. Alloy A602 had pits after the 500h of cyclic heating, which is somewhat longer than for alloy A601. The high content of Al may enhance the integrity of the oxide scale, though A602 and A601 have an almost similar Cr content (see Table 2). An electrolytically-polished specimen of A693 with 3% Al content exhibited better resistance than alloys A602 and A601: it endured metal dusting attack for up to 2,000h under cyclic heating conditions. Only alloy A696 among the electrolytically-polished specimens survived after an exposure of 22,000h, when the other ground-finished alloys had already suffered from metal dusting. The vertical growth of pits indicated by the maximum pit depth on the specimens is presented in Fig. 2 for the ground specimens and Fig. 3 for the electrolytically-polished ones, respectively. For the ground specimens,

Table 5 Time to Initiate Pits and Mass Loss Rate Upon Cyclic Heating in

60 vol-% CO-26% H2-11.5%CO2-2.5% H2O at 650ºC

Mark Surface condition*

Total exposure time (h) **

Time to initiate pits (h) **

Final mass loss rate (mg.cm-2.h-1)

A800 GR 4,000 (80) 500 (10) 2.710-3 A601 GR 13,000 (260) 3,000 (60) 5.310-3 A690 GR 27,000 (540) 13,000 (260) 7.510-5 A160 GR 27,000 (540) 14,000 (280) 5.310-4 A696 GR 22,000 (440) No Pitting Trace A601 EP 4,000 (80) 200 (4) 3.410-2 A602 EP 4,000 (80) 500 (10) 3.810-3 A693 EP 16,000 (320) 2,000 (40) 5.010-5 A696 EP 22,000 (440) No Pitting Trace

* Abbreviations GR and EP mean ground and electrolytic-polished, respectively. ** Number in ( ) denotes the number of 50h-heating and cooling cycles.

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Fig. 2: Vertical growth of pits on ground-surface specimens



A800 steel suffered from the formation of deep pits on its surfaces, and the maximum pit depth was approximately 350 µm after the 4,000h exposure. For alloy A601, the maximum pit depth increased sharply with increasing exposure time, suggesting that the pit grows without the healing of the oxide at the boundary of pit and metal. Pits initiated on A160 and A690, which have a resistance to pitting up to ten-thousand hours, developed slower than those on A800 and A601. For the electrolytically-polished specimens, the pit depth

growths of A601 and A602 increased with increasing time, whereas that of A693 was relatively small, i.e., below 200 µm for 14,000h from the pit initiation. Metallographic cross sections of the pit on the ground specimens reacted for each exposure time are shown in Fig. 4. For alloys A601 and A690, the bottom surface of the pits was relatively smooth, while that for alloy A160 was irregular. It suggests an imperfect healing of the protective oxide scale. In case of Fe-Cr-

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Fig. 3: Vertical growth of pits on electrolytically-polished specimens

300m

(a)

300m

(b)

Fig. 4: Metallographic cross-sections of the ground specimens after cyclic heating test: (a) A601 after 13,000h; (b) A690 after 27,000h; (c) A160 after 27,000h.

300m

(c)



Ni and Ni-Cr alloys, a protective Cr2O3 layer is hard to repair because the ingressive carbon continuously reacts with Cr in the metal matrix and blocks its supply outward. On the other hand, in alloys containing Si as well as Al, it is possible for the oxide scale to form at pit surfaces, even though the healing is not entirely completed. This resultant yields nucleation of several secondary pits at the bottom of the primary pit. Figure 5 shows the metallographic of the ground A696 exposed for 17,000h in the cyclic heating test. Only the thin oxide scale of 1.0 to 1.5 µm was uniformly formed on the specimen surface, where there were no signs of pitting attack. Alloying elemental analyses in the oxide scales were conducted using STEM/EDS for both the ground and electrolytically-polished specimens of A696 and are shown in Figs 6. Chromium was mainly distributed in the oxide scale including a small amount of Mn. Silicon as SiO2 was found to be enriched beneath the Cr-oxide. In the Cr-oxide near interface between Cr-oxide and SiO2, islands containing Ni, Fe and Cu were dispersed which the metal matrix was left during the oxidation. Note that Cu was not enriched at any region of the metal beneath the SiO2. This result proves that Cu of approximately 2 mass-% does not form any compounds such as oxides and carbides but being a solid solution in the Ni-base matrix at 650ºC. In addition, our research group investigated Cu segregation in the metal of the oxide-metal interface using hard X-ray photoelectron

300m

(a) (b)

5m

metal

oxide scale

SEI O Si Cr

Mn Fe Ni Cu

Fig. 5: Metallographic cross sections of the ground A696 reacted with simulated synthesis gas at 650ºC for 17,000h. (b): Higher magnification of (a) using a CCD microscope

Fig. 6: STEM/EDS analyses of cross sections of the oxide scale formed on the ground specimen of A696 alloy after an exposure of 17,000h at 650ºC in a simulated CO-H2-CO2H2O gas mixture



spectroscopy (HX-PES), which is of higher-resolution than EPMA [19]. According to the study, a considerable amount of Cu has segregated as a solid-state solution within only a couple of atomic layers at the interface between the Cr-oxide and the metal for Ni-20%Cr-(1 and 2)%Cu model alloys. This segregation may enhance the Cu effect; restraint of the dissociative adsorption of CO molecule on the metal surface where the oxide scales suffer any damages, i.e. spalling and/or flaw.

Mechanical properties The stress versus time-to-rupture curves of A696 tubes and plates are shown in Fig. 7.

The long-term test over 10,000h proves stable creep rupture strength of A696 at elevated temperature. Figure 8 shows the Charpy impact value of A696 aged at 500, 650 and 800ºC for up to 10,000h comparing with that of a 3% Al-bearing Ni-base alloy such as A693 in Table 2, which had relatively excellent metal dusting in this study. Toughness after aging of A696 is superior to that of Al-bearing Ni-base alloy. Figure 9 shows TEM images of A696 and Al-bearing Ni-base alloy after aging at 650ºC for 3,000h. The precipitates observed in

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Fig. 8: Toughness after aging at 500, 650 and 800ºC of (a) A696 and (b) Al-bearing Ni-base alloy



aged A696 are mainly M23C6. On the other hand, those in Al-bearing Ni-base alloy are fine γ’(Ni3Al) phase dispersed in γ matrix with high density, which degrade the toughness. The formation of the protective oxide scale such as Cr2O3, SiO2 and Al2O3 can be effective in preventing metal dusting corrosion. However, Al-bearing Ni-base alloy has an adverse effect on the toughness. So A696 has excellent phase stability which contri-butes long-term creep strength and toughness at elevated temperature. According to hybrid-suppres-sion technique containing Si, Cr and Cu, A696 balances metal dusting resistance with phase stability.

Resistance of welds to metal dusting corrosion The chemical composition of the filler metal was listed earlier in Table 3. The appearance for both the ground and electrolytically-polished specimens of A696 weld after exposure to the simulated syngas at 650ºC for the 60-cycle heating test (corresponding with 3,000h heating) are shown in Fig 10(a) and (b), respectively. There are no signs of pitting attack at the weldment. The matching filler metal for A696 shows good resistance to metal dusting corrosion.

CONCLUSIONS Long-term tests were conducted for several Fe- and Ni-base alloys to evaluate their metal dusting behaviour upon cyclic heating and cooling. In addition, the mechanical properties, thermal stability and resistance to metal dusting corrosion of weldment have been researched. The results obtained are as follows:

1. Many pits associated with metal dusting developed within 3,000h on the ground specimens of alloys with Cr less than 25 mass%. High-Cr content alloys with and without Si showed good metal dusting resistance, but eventually generating pits after exceeding 10,000h. In contrast, the developed alloy of Ni-30%Cr-1.5%Si-2%Cu remained unattacked, even after the 17,000h exposure, showing that it has an excellent resistance to metal dusting.

(a)

(b)

M23C6

Fig. 9: TEM images after aging of (a) A696 and (b) Al-bearing Ni-base alloy

(a)

(b) 10mm

Fig. 10: Appearance of ground specimens of A696 weld after exposure to simulated synthesis gas at 650ºC during 60-cycle heating test. (a) MDSP28. (b) MDSP30.



2. The developed alloy treated with electrolytic polishing also had no pits after 22,000h exposure, while other electrolytically-polished Ni-base alloys suffered severe metal dusting in a distinctly short time.

3. The hybrid-suppression technique, a new concept for prevention of metal dusting, is based on the formation of protective oxide scales and the reduction of reactivity with CO gas in syngas environments. A solid solution of Cu in the metal matrix plays the role of a surfactant-mediated resistance on the metal surface where any defects of the oxide scale have occurred, leading to a complete healing of the protective oxide scale. The test results have proved that the novel technique holds great promise in severe carbonaceous environments.

4. The new alloy has excellent phase stability, which contributes long-term creep strength and toughness. A matching filler metal for the new alloy is under development and shows good resistance to metal dusting corrosion.

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NACE, 1999) (Paper No.429). 5. Schueler, R.C.: Hyd. Proc. (1972), p. 73. 6. Schillmoller, C.M.: Chem. Eng. 93 (1), 83 (1986). 7. Nishiyama, Y.; Otsuka, N.; Kudo, T.: Corros. Sci. 48, 2064 (2006). 8. Baker, B.A. ; Smith, G.D.; Hartmann, V.W.; Shoemaker, L.E.: “Nickel-base material solutions to metal dusting

problems”. CORROSION /2002 (Houston, TX, NACE, 2002) (Paper No.2394). 9. Fabiszewski, A.S.; Watkins, W.R.,et al.: “The effect of temperature and gas composition on the metal dusting

susceptibility of various alloys”. CORROSION/2000 (Houston, TX: NACE, 2000) (Paper No.532). 10. Baker, B.A.; Smith, G.D.: “Alloy selection for environments which promote metal dusting”. CORROSION/2000

(Houston, TX: NACE, 2000) (Paper no.257). 11. Klöwer, J.; Grabke, H.J., et al.: “Metal dusting and carburization resistance of nickel-base alloys”. CORROSION/97

(Houston, TX: NACE, 1997) (Paper no.139). 12. Klarstrom, D.L.; Grabke, H.J.; Paul, L.D.: “The metal dusting behaviour of several high-temperature nickel-based

alloys”. CORROSION/2001 (Houston, TX: NACE, 2001) (Paper No.1379). 13. Szakalos, P.; Lundberg, M.; Pettersson, R.: Corros. Sci. 48, 1679 (2006). 14. Nishiyama, Y.; Moriguchi, Y.; Otsuka, N.; Kudo, T. : Mater. Corros. 56 , 806 (2005). 15. Zhang, J.; Cole, D.M.I.; Young, D.J.: Mater. Corros. 56, 756 (2005). 16. Nishiyama, Y.; Otsuka, N.: Mater. Sci. Forum, 522/523, 581 (2006). 17. Grabke, H.J., Müller-Lorenz, E.M.: “Occurrence and prevention of metal dusting on stainless steels”. CORROSION

/2001 (Houston, TX: NACE, 2001) (Paper No.1373). 18. Doi, T.; Kitamura, K.; Nishiyama, Y., et al.: Surf. Interface Anal. 40, 1374 (2008).

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