c2014-4330 (1)

Nickel base alloys in high temperature applications in the Petrochemical Industry

Stephen A.McCoy PCC Energy Group

Holmer Road Hereford, HR4 9SL

United Kingdom

Gaylord D. Smith PCC Special Metals Corp.

Riverside Drive Huntington, WV

U.S.A.

Paul Hazledine PCC Special Metals Pacific Pte Ltd

24 Raffles Place Singapore

Brian. A. Baker PCC Special Metals Corp.


U.S.A

C. S. Tassen PCC Energy Group

Telge Road Houston, TX

U.S.A.

L. S. Shoemaker PCC Special Metals Corp.


U.S.A

ABSTRACT Nickel-base alloys designed for elevated temperature service can offer a high degree of mechanical integrity and corrosion resistance for extended service in the manufacture of petrochemicals. Proper selection of a material for a particular application involves consideration of several important factors with corrosion resistance, mechanical strength, toughness and stability being primary design parameters. Mechanical properties are an important consideration and would include elevated temperature tensile, rupture and creep strength as well as fatigue strength and toughness. Fabrication issues associated with the application of these materials in a range of intermediate and high temperature service environments typically used in processing syngas are also considered. This paper provides a summary of high temperature corrosion properties for a range of high temperature nickel base alloys, including recent alloy developments, with high temperature mechanical and corrosion information. Key words: Nickel base alloys, petrochemical, metal dusting, oxidation, carburization, fabrication

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Paper No.

4330

©2014 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACE International, Publications Division, 1440 South Creek Drive, Houston, Texas 77084.The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.

INTRODUCTION Gases containing high proportions of carbon monoxide and hydrogen, commonly referred to as syngas are used in a variety of petrochemical processes for the production of other molecules including ammonia, hydrogen, methanol and styrene. The typical plant equipment handling these gases are steam methane reformers, partial oxidation units, goal gasifiers and autothermal reformers. As the industrial trend is to increase the efficiency of these units it can lead to more aggressive high temperature environments within the 500 to 1000°C temperature range. The hot corrosive gases used in these manufacturing processes can lead to oxidation, nitridation, sulphidation, carburization. Increasingly the trend is to improve the energy efficiency of these processes by increasing the carbon activity of the gas to values greater than 1, which can also lead to severe metal dusting attack of standard commercial alloys. The wrought Nickel base alloys are typically used in applications that cannot be practically met by other available commercial materials due to their combination of corrosion resistance, strength and fabricability. The Ni-Cr-Fe alloys develop dense and tenacious oxide scales that limit the ingress of corrosive anions, such as halides and sulphur and the nickel matrix has a low solubility for the interstitial elements that tend to diffuse inwards, such as carbon and nitrogen. This paper provides information on the high temperature corrosion resistance of the nickel base alloys, mechanical strength and welding properties. The effect of alloying in these wrought and welded materials is also discussed. Table 1. Nominal contents of Nickel-base alloys Alloy UNS No. Ni Cr Fe Mn Si Al Ti C Mo Other

600 N06600 72 15.5 8 0.3 0.3 0.3 0.3 0.08 -

693 N06693 62 29 4 - - 3.1 - - - 0.6Nb, 0.02Zr

FM53MD N06693 62 29 4 - - 3.1 - - - 0.6Nb, 0.02Zr

625 N06625 61 21.5 2.5 - 0.1 0.2 0.2 0.02 9 3.6Nb

601 N06601 60.5 23 13 0.2 0.2 1.4 0.4 0.05 -

690 N06690 59 29 9 0.2 0.1 0.3 0.3 0.02 -

FM52 N06052 59 29 9 0.2 0.1 0.3 - - -

FM72 N06072 56 43 0.3 - - 0.15 0.5 0.02 -

617 N06617 55 22 1 0.1 1.2 0.4 0.08 9.7 12.5Co

671 - 53 46 - - - 0.3 0.3 0.03 -

740H N07740 50 25 - - .15 1.35 1.35

0.05 - 20 Co

890 N08890 43 24.5 27 0.8 1.8 0.15 0.4 0.1 1.5 0.4Nb, 0.25Ta

803 S35045 34 27 36 1 0.8 0.4 0.4 0.08 -

800H/HT N08810/N08811

32 21 45 0.9 0.1 0.4 0.4 0.07 -

188 R30188 24 22 1.5 0.8 0.3 0.2 - 0.1 0.5 37Co,14W

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RESULTS & DISCUSSION HOT CORROSION OXIDATION Oxidation resistance depends on development of a protective oxide scale which is compact and dense, through which diffusion of in-coming ions and out-going cations occurs at a slow rate. Figure 1 shows mass change data for several nickel- base alloys after oxidation in air +5% water vapor at 1000°C. Figure 2 shows depth of oxidation versus time for N06601, N06690, N06693, N08890, S35045, N08811. Depth of oxidation measurements were made at three points across the section of the samples, which were rectangular in cross section, yielding a total of six measurements (averaged to produce the results shown). The majority of nickel- base alloys used for their oxidation resistance are chromia formers (Cr2O3). In addition to their chromium content, each of these materials uses secondary alloying additions to augment the protectiveness of the chromium-rich oxide scale. The aluminum addition in alloys N06601, N06617 and N06693 forms a sub-layer which enhances the chromium rich oxide scale by blocking grain boundaries which normally act as fast diffusion paths for chromium, but also by mechanically supporting the chromium oxide layer and enhancing its resistance to buckling and spalling. Silicon in the Fe-Ni-Cr alloys of S35045 and N08890 is shown to be less effective in enhancing the chromia scale in this severely oxidizing atmosphere. It should be noted levels of silicon greater than 2% can cause spallation of the scale due to its brittle properties.1,2 Figure 3 shows alloy 617 samples exposed for 15,482 hours, Alloy 617 shows an extensive intergranular sub-scale comprised largely of aluminum oxide. Figure 4 shows a cross-section of an alloy 693 sample exposed under the same conditions for 24,015 hours. A continuous alumina scale is present, explaining the very stable mass change data shown in Figure 1.

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Figure 1. Mass change after exposure in air + 5% water vapor at 1000°C. Samples were cycled to room temperature once per week for weighing.

Figure 2. Depth of oxidation after exposure in air + 5% water vapor at 1000°C. Samples were

cycled to room temperature once per week for weighing.

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Figure 3. Light opticalmicrograph showing an extensive network of intergranular internal oxide in a cross section of alloy 617 sample exposed for 15,482 hours in air + 5% water vapor at 1000°C.

Figure 4. Photomicrograph showing cross section of alloy 693 sample exposed for 24,015 hours in air + 5% water vapor at 1000°C. A continuous aluminum oxide scale was identified using a scanning electron microscope

SULPHIDATION/ SULPHIDATION/ HOT CORROSION Sulphidation behavior of alloys is similar to oxidation but with some significant differences including the higher rates of reaction and more complex corrosion products.3 Unlike oxide scales which have higher melting points than those of the base alloy, sulphide scales can have

100 µm

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melting points lower than the base materials melting point. The rate of sulphidation can also become increasingly more severe at low oxygen potentials. Alloys with high iron and chromium content are often selected for resisting high temperature sulphidation conditions. Nickel is generally detrimental to the high temperature sulphidation resistance primarily because of the relatively low melting point of the nickel/nickel sulphide eutectic (645°C) with the iron/ iron sulphide eutectic melting at 988°C. Increasing the iron and chromium content can raise the melting point for any given nickel level. However, the sulphidation resistance is less dependent on nickel content than on the scale forming characteristics of the alloy.4 Figure 5 shows the effect of alloying in H2S reducing environments at 482°C for a range of nickel-base alloys. Figure 6 shows data for alloys N08810, S30400, N06617, N06601 and N06625 after exposure in H2-6% H2S from 482°C to 593°C.5 The general ranking of the alloys performance does not change with temperature in the range studied. The lower mass change values for the alloy N06625 and alloy N06617 suggest the most stable scale formation. The high negative mass changes for the alloy N06601, S30400 and alloy N08810 suggest scale spallation and eventual failure by breakaway corrosion.

Figure 5. Depth of Attack after Exposure in H2-2%H2S at 482°C.

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Figure 6. Mass change data after exposure in H2-6% H2S at 482°C, 538°C and 593°C for 500

hours.

Typically, most H2S containing atmospheres in petrochemical processing also contain varying amounts of H2, CO, CO2 and N2. Figure 7 shows mass change results after exposure to H2-45%CO2–1% H2S at 816°C to simulate a coal gasification environment for screening the resistance to sulfidising-oxidizing conditions over a long time period. This test demonstrates the effects of unpredictable breakaway corrosion in sulphidation environments. The Fe-Ni-Cr alloys perform well given the high iron and chromium contents. The Ni-Cr materials 671 and filler metal 72 also perform well due to their very high chromium contents.

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Figure 7. Mass change after exposure to H2-45%CO2-1%H2S at 816C.

.

CARBURISATION AND METAL DUSTING Nickel has a low solubility and diffusivity for carbon making nickel- base alloys ideal for use in carburizing conditions. Stable protective oxides developed using chromium in conjunction with other protective oxide forming elements of aluminum and silicon are very effective in resisting carburization attack. In conjunction with a high nickel content, however control of carbide-forming elements is important, especially for reducing-carburising conditions where protective oxides are not stable. However, high levels of strong carbide formers, such as chromium, niobium and titanium can be detrimental for carburisation resistance in low pO2 environments.6 Figure 8 shows mass change data for a wide range of nickel-base alloys after exposure for 672 hours in a reducing carburizing atmosphere with hydrogen-1% CH4 at 1000°C. Samples were cycled once per week to room temperature for mass change measurement. The alloy N06693, containing 3% aluminum forms a protective alumina scale and shows very low mass change compared with the alloy N06617 and N06601 containing lower aluminum contents of 1.1% and 1.4%, respectively. In contrast, the alloys N06690 and FM72 with very high Ni, Cr and low Al contents, exhibit high mass gain rates.

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Figure 8. Mass change data for samples exposed at 1000°C in H2-1%CH4. Samples were cycled to room temperature once per week.

Figure 9 shows mass change after exposure to a carburising/ partially oxidizing environment with H2-5.5% CH4–4.5%CO2 at 1000°C (pO2 – 2.7 x 10-21 atm). The Fe-Ni-Cr alloys N08803 and N08890 with high chromium contents are shown to have good performance in this environment and are used as ID finned tubes in ethylene furnaces using the formation of a chromia scale to protect against oxidation on the OD surface and carburization-oxidation on the ID surface.

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Figure 9. Mass change data for samples exposed at 1000°C in H2-5.5% CH4-4.5% CO2. Samples

were cycled to room temperature once per week. Metal dusting is a form of catastrophic carburization that can cause rapid and unpredictable failure in petrochemical plants producing methanol, ammonia, hydrogen and gas to liquids. Processing high temperature high pressure syngas in the temperature range 400 to 800°C using high ratios of carbon monoxide to carbon dioxide and low ratios of steam to hydrogen and carbon activities greater than 1, can lead to severe metal dusting attack of standard materials.7,8 New plant designs use heat exchangers to recover more process heat to improve efficiency and these environments can have very aggressive conditions which can be ideal for metal dusting of standard pressure containing materials. Figures 10 and 11 show mass loss rates and pitting depth data for samples exposed in CO-20% H2 at 621°C; testing procedures have been described previously.9 The carbon activity of this gas has been calculated as 57.9. The plateau’s in the maximum pit depth measurements for alloys N0690 and N0693 exhibit evidence of oxide healing behavior. While it is apparent that carbide forming elements such as Mo and Ti can enhance performance in nickel-base alloys with chromium levels significantly lower than 25% (e.g. alloys N06625 and N06617), reliance upon a sustainable and healable oxide is preferred for long term performance. Based upon observed pitting rates and mass loss rates in laboratory testing, and the results of in-situ field exposures, alloy N06693 shows excellent metal dusting resistance by forming a thin adherent oxide film. The surface condition also has an important effect on the metal dusting resistance of the material. Figures 12 shows alloy N06693 exposed in the laboratory environment at 621°C in a number of different supply conditions including following use of a post fabrication stress relief heat treatment. Pickling which acts to deplete the surface of chromia is shown to be the least protective surface condition, however the use of the stress relief at 950°C is shown to be effective in restoring the materials protective surface oxide.

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Figure 10. Maximum pit depth measurements for samples exposed to CO-20% H2 at 621°C.

Exposure Time, hrs0 2000 4000 6000 8000 10000 12000 14000 16000 18000

Mas

s Lo

ss R

ate,

mg/

cm2 h

r

10-6

10-5

10-4

10-3

10-2

10-1

100

N06693

N06617

N06690

671

S35045

N08800

N06601

N06600

N06625

Figure 11. Mass loss rate versus exposure time for samples exposed to CO-20% H2 at 621°C.

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Figure 12. Mass loss rate versus exposure time for N06693 samples exposed to CO-20% H2 at

621°C.

Figure 13. Mass loss rate versus exposure time for samples exposed to CO-20% H2 at 621°C.

The weld can also be the most vulnerable area to metal dusting attack. Figure 13 also compares the metal dusting resistance of the various weld metals that are candidates for the most severe environments. A combination of filler metal 52 used for its mechanical strength and toughness has been used with an overlay cap of filler metal 72 to enhance corrosion protection. The filler metal 53MD has the highest resistance to metal dusting attack in this environment. The surface condition of the weld is also shown to be influenced by grinding the surface to give the optimum condition for resisting the pitting attack.10 MECHANICAL PROPERTIES In addition to corrosion resistance the selection of a material is based on a number of considerations including strength at temperature, stability and fabricability. Nickel is useful for

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its high temperature strength and stabilizing the austenitic structure to maintain good stability and toughness. At temperatures above approximately 500°C the nickel-base alloys design strength is limited by time dependent behavior rather than mechanical strength. Figure 14 shows the stress to produce rupture in 10,000 hours for the candidate materials used in petrochemical processing. At temperatures below about 871°C (1600F), alloy 693 possesses greater stress rupture strength than alloy 601. However, the properties of alloy 693 at 982°C (1800F) are similar to those expected for alloy 690. The alloy 740H which is an age-hardenable material is demonstrated as having very high temperature creep strength over the 600 to 800°C temperature range. Thermal stability of the materials can be important in particular applications such reformer pigtails. Table 2 shows the effect of a 1000 hour exposure in air on the impact strength of the various alloys. The alloy 690 shows excellent retained impact strength after exposure with little effect on the metallurgical structure of the material while the compositions of the alloy 617, 740H and alloy 693 lead to gamma prime phase formation, which hardens the materials. Alloy 800H and 803 are good candidates for these applications due to their combination of corrosion resistance, strength and retained ductility.

Figure 14. The 10,000 hour rupture strength for several candidate alloys for petrochemical applications.

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Exposure Temperature alloy

N06690 alloy

NO6601 Alloy

N08810 alloy

S35045 alloy

N06617 alloy

N07740 alloy

N06693 no exposure 190 175 152 324 232 80 90

°C F 540 1000 121 85 565 1050 156 593 1100 195 126 302 55 649 1200 198 127 118 99.9 47 22 704 1300 65 31 21 760 1400 214 65 85 34 31 800 1470 48 871 1600 93

Table 2. Impact Strength CVN Joules following elevated exposure for various alloys.

FABRICABILITY The annealed wrought Nickel base alloys all have very good formability due to the high ductility of these materials. A number of factors have to be considered when selecting the appropriate welding consumable for a petrochemical application. For example, transfer lines connecting the steam methane reformer to the quench steam generator are often fabricated from alloy N08810 and welded with filler metal N06617 and/ or welding electrode 117. The N06617 type welding consumable produces an over-matching strength in the weldments by combining optimum amounts of Co and Mo for solid solution strengthening plus Cr and Al for corrosion resistance. In applications where corrosion resistance is as important as strength a matching welding consumable or a weld overlay should be considered. In a metal dusting type environment the FM 53 has been developed as a matching composition to the highly resistant alloy N06693. Even though the material contains 3% Aluminum, Table 2 shows the weld joints have good strength and have passed the ASME IX qualification side bends. In applications requiring higher strength and stability a weld consumable selected for mechanical properties can be capped with a more corrosion resistant weld consumable. Combined Weldments using FM 52 capped with FM 72 or FM 53 have been used to provide an optimum combination of strength, stability and corrosion resistance.

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Weld Joint Temp. 0.2% Proof Strength Tensile strength Elongatio

n RofA Mpa Ksi Mpa ksi % %

N0693/53/N0693 RT 545 79.1 804 116.7 32.2 50.7 N06693/53/N06693 704 596 86.5 669 97.1 5.0 17.2 N06693/52/N06693 RT 493 71.6 671 97.4 25.9 47.3 N06693/52/N06693 649 326 47.3 445 64.6 25.4 45 N06693/82/N08810 RT 531 77.1 669 97.1 24.4 64.4 N06693/82/N08810 639 365 53 524 76.1 22.8 46.4

Table 2. Mechanical properties of GTAW plate material.

STRESS RELAXATION CRACKING An important consideration when designing equipment to operate in the temperature range from 500 to 750°C is the potential failure mechanism of Stress Relaxation Cracking (SRC). This cracking phenomenon can occur when an austenitic material cannot withstand the inelastic strains imposed upon the material and cracks typically develop along the grain boundaries with cavitation typically preceding each crack. The crack zone is often associated with aging reactions and the potential for high residual stresses in weldments and/or cold deformed material.11,12 Materials susceptible to stress relaxation cracking in highly stressed regions associated with heavy cold deformation or constrained weldments include S30409 and S32109 stainless steels, alloy N08810, N06601, N06617 and N06693. Fabrication techniques to reduce the materials susceptibility include fine grain control and the use of a stress relief or annealing heat treatment.13 Generally, alloys with high chromium content have been found to be more resistant to relaxation cracking than those with less chromium. Studies conducted at the TNO Institute have shown that alloys with over 25% chromium are essentially immune to stress relaxation cracking. The Nickel base alloy S35045 with 27% chromium and alloy N06690 with 30% chromium were found to be fully resistant to relaxation cracking across the temperature range 550°C to 700°C.14

CONCLUSIONS Nickel-base materials have excellent properties of corrosion resistance and strength in petrochemical applications. Combinations of alloying elements have been shown to be effective in resisting high temperature corrosion in single and mixed gas conditions.

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Alloy N06693 with high levels of chromium and aluminum has a very good resistance to oxidation, carburization and metal dusting due to the formation of a chromia scale combined with an alumina subscale. The newly developed alloy N07740 combines excellent high temperature creep strength with good corrosion performance in coal gasification environments. Techniques have been developed to avoid relaxation cracking during fabrication with high chromium alloys S35045 and N06690 or by using a stress relieve or annealing prior to service to reduce the residual stresses in service.

REFERENCES 1. Douglass, D.L. and Armijo, J.S., Oxidation of Metals, 1970, Vol. 2, No. 2, pp 207-231. 2. Evans, H.E., et al., Oxidation of Metals, Vol 19, Nos. ½, 1983, pp.1-18 3. S.Mrowec, Oxidation of Metals 44, ½(1995) : p. 177. 4. J.C.Hosier and J.A. Harris, “Sulphidation Resistance of High Temperature Alloys Tested in Reducing and Oxidising Atmospheres,” CORROSION/80, paper no. 167 (Houston, TX: NACE International, 1980). 5. G.D.Smith, CORROSION/97, Paper No. 0524, 1997, NACE International, Houston, TX 6. B.A.Baker, CORROSION/2006, Paper No. 06229, 2006, NACE International, Houston, TX 7. H.J.Grabke, R. Krajak, E.M. Muller-Lorenz, Materials and Corrosion, 44, pp 89-97. 8. H.J.Grabke, et al., Materials and Corrosion, 47, 1996, pp 495-504. 9. Baker, B.A. and G.D.Smith, CORROSION/2000, Paper No. 0257, 2000, NACE International. Houston, TX. 10. Muller-Lorenz, E.M., Proc. EUROCORR 1999, Aachen, Germany, September, 1999 11. J.C. van Wortel, “Prevention of Relaxation Cracking in the Chemical Processing Industry”, Stainless Steel ’99: Science and Market, Sardinia, Italy 6-9 June, 1999, Pages 281-290. 12. J.C. van Wortel, “Relaxation Cracking in Austenitic Welded Joints – an Underestimated Problem”, TNO Institute of Industrial Technology, P.O. Box 541, 7300 AM Apeldoorn, The Netherlands 13. L.E. Shoemaker, G.D. Smith, B.A. Baker, J.M. Poole, CORROSION/2007, Paper No. 07421, 2007, NACE International, Nashville, TN. 14. Internal report, Special Metals Corporation and conducted by TNO Institute of Industrial Technology, Apeldoorn, The Netherlands

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