1.3280583

CORROSIONVol. 56, No. 8 801

CORROSION SCIENCE SECTION

0010-9312/00/000151/$5.00+$0.50/0 2000, NACE International

Submitted for publication November 1999; in revised form, May2000.

* Max-Planck-Institut fr Eisenforschung, Postfach 140 444, D-40074 Dsseldorf, Germany. E-mail: [email protected].

Nickel-Based Alloys in Carbonaceous Gases

H.J. Grabke*

ABSTRACT

High-temperature corrosion in carbonaceous gases can occurby carburization (i.e., internal carbide formation at C activi-ties [aC] < 1 and at temperatures > 800C) or by metaldusting, a disintegration of alloys into graphite and metalparticles observed at aC > 1 and in a lower temperaturerange. In both cases, C entry and diffusion in the metal ma-trix is a decisive step, and in comparison to high-alloy steels,the C ingress in Ni-based alloys is retarded by the decreaseof C solubility and diffusivity with increasing Ni content.Therefore, Ni-based alloys are recommended for applicationsin carbonaceous gases (e.g., for the steam-cracking of hydro-carbons). Since operating temperatures > 1,100C aredesired in ethylene (C2H4) production, where chromium oxide(Cr2O3) is no more stable, the alloys must contain Si and/orAl to form a protective scale. At temperatures 650C, highNi-based alloys with 25% Cr and some Al proved to be veryresistant to metal-dusting attack by the carbonaceoussyngas (CO-H2) environment. Such alloys showed no attackin laboratory and pilot plant exposures for up to 10,000 h,whereas all high-alloy steels were susceptible.

KEY WORDS: carbonaceous gas, carburization, high-alloysteels, metal dusting, nickel-based alloys

INTRODUCTION

Strongly carburizing gases occur in many chemicaland petrochemical processes, in the direct reduction

of Fe ores, and in the heat treatment of steels andalloys. In such atmospheres, C can exert corrosiveeffects after entering the metal matrix. Since mostlychromium oxide (Cr2O3)-forming alloys are used inthe process industries with temperatures rangingfrom 500C to 1,000C, generally C dissolutionshould be prevented by the protective oxide layer,which is impermeable for C.1-2 But when the oxidescale fails or is not formed properly, C ingress is pos-sible and leads to two types of failure: carburizationand metal dusting.

Carburization is the most common cause of fail-ure of cracking tubes for ethylene (C2H4) productionand drastically can reduce the life of metal compo-nents used in heat-treating operations. The ingressof C causes precipitation of Cr-rich carbides and,therefore, a loss of ductility and oxidation resistance(Figure 1).3-4

Metal dusting results from oversaturation ofmetals and alloys with C activities (aC) > 1 and leadsto disintegration of the materials in a dust of metalparticles, graphite, and sometimes oxides and car-bides (Figure 2).5-10 The latter are formed in apreceding carburization from the alloying elementsforming stable carbides. The remaining Fe-Ni matrixis destroyed by the graphite formation, either viacementite (Fe3C) as intermediate or in the caseof Ni-based alloys by direct inward growth ofgraphite.11-12

For carburization and metal dusting, the dissolu-tion and diffusion of C into the metal phase areimportant steps. After consumption of Cr by oxida-

802 CORROSIONAUGUST 2000


(1) UNS numbers are listed in Metals and Alloys in the UnifiedNumbering System, published by the Society of AutomotiveEngineers (SAE) and cosponsored by ASTM.

tion and/or carbide formation, the C solubility anddiffusivity depend mainly on the Ni/Fe ratio in theremaining metal matrix. C solubility13 anddiffusivity14 have been investigated for Fe-Ni alloys,and both show a distinct decrease with increasingNi content to a minimum at ~ 70 wt% to 80 wt% Ni.For higher contents up to 100% Ni, there is a smallincrease of solubility and diffusivity (Figure 3). Thispeculiar behavior of C in the system Fe-Ni13-14 playsan important role in the carburization and metal-dusting resistance of alloys, as will be demonstrated.

CARBURIZATION

Corrosive damage by carburization is most wide-spread in the steam-cracking of hydrocarbons forproduction of C2H4 or other olefins. Centrifugally casttubes of 25% Cr-20% Ni or 25% Cr-35% Ni steel arefired on the outside, and a mixture of steam andhydrocarbons process gas is passed through thesetubes at high velocity. Coking occurs on the inside ofthe tubes, and they must be decoked repeatedly bysteam and air. The process gas is a weak oxidizer,and the steels used should form a protective scaleconsisting of an outer layer of spinel ([Fe,Mn]Cr2O4)

(a)

(b)FIGURE 1. Failures by carburization: (a) Alloy 800 (UNS N08810)(1)thoroughly carburized in a laboratory experiment,3-4 in CH4-H2 at1,000C, internal carbide formation and cracking and (b) Alloy 600(UNS N06600) after use as tube of an oxygen probe in an industrialfurnace for carburization of case-hardening steels, thoroughlycarburized and internally oxidized starting from the outer surface.

(a)

(b)FIGURE 2. Appearance of metal dusting on Alloy 600 after laboratoryexposure in H2-24%CO-2%H2O at 650C for 3 days: (a) scanningelectron microscopy (SEM) top view of the surface with pits filled withcoke and (b) metallographic cross section of a pit on Alloy 600, cokewith relatively large metal particles and internally carburized zonewith very fine carbides.



and an inner layer of Cr2O3. The carburization ofthe tube walls is slow at operating temperatures< 1,050C, proceeding only by gaseous transport of Cthrough pores and cracks,1-2 which may occur in thescale because of high-temperature creep of the tubematerial.3-4 Thus, the cracking tubes can be in opera-tion for up to 10 years at modest temperatures.15-16

However, in some plants, the operation tempera-tures are 1,100C to 1,200C. Such high tempera-tures also may occur during the (exothermic)decoking if it is not controlled carefully. Tempera-tures > 1,050C, however, are disastrous for thelifetime of most cracking tubes, since the protectiveoxide layer will be converted into unprotective Crcarbides if coke is present (aC = 1). Thermodynamiccalculations clearly show that Cr2O3 becomes un-stable with increasing temperature and should reactto form chromium carbides (Cr3C2 or Cr7C3) (Figure4).17-18 This reaction is slow at 1,050C, but its rateincreases with temperature.18

The increasing susceptibility to carburization attemperatures > 1,050C was demonstrated in thepack carburization test, which is used by alloy pro-ducers for testing.17,19 Samples of alloys were packedwith C in a box of heat-resistant steel and heated to

the testing temperature, the change of thermody-namic conditions in this test is shown in Figure 4.Enhanced carburization started at > 1,050C, wherethe protective scale was converted to carbides. Underthese conditions without oxide scale, the carburiza-tion was governed by C diffusion, and the progress ofinternal carbide formation can be described by theequation:

xD c

ctC C

M

2 2= (1)

where DC is the diffusivity, cC is the solubility (molefraction) of C in the metal matrix, is a labyrinth fac-tor, is the stoichiometric factor for the carbide MC,and cM is the concentration (mole fraction) of themetals involved in the carbide formation. Thus, thedepth (x) of internal carbide formation for a givenalloy mainly is determined by the values of DC and cC,which refer to the alloy after carbide precipitation(i.e., in the Fe-Ni matrix). The value (DC cC)1/2

shows a minimum at the ratio Ni/Fe = 4/1;17 corre-spondingly, the pack carburization kinetics at1,100C of several steels and Ni-based alloys showed

(a) (b)FIGURE 3. C in Fe-Ni alloys: (a) C solubility determined at 1,000C at different C activities by Smith3 and (b) C diffusivitymeasured at 1,000C at different C activities by Bose and Grabke.4



a minimum at the same ratio (Figure 5). One mayconclude that alloys with such compositions shouldbe relatively resistant to carburization, and this con-clusion is in agreement with practical experience.However, C ingress and internal carbide formation istoo rapid for long-term applications, if no protectivescale is present. As pointed out, Cr cannot providesuch a scale at temperatures > 1,050C and onlysufficient concentrations of Si or Al can help. Silicondioxide (SiO2) and aluminum oxide (Al2O3) also arestable at very high temperatures in strongly carburiz-ing atmospheres, even at very low oxygen pressures.Contents of ~ 2% Si have been used in the Cr-Nisteels to attain protection by an inner SiO2 layer,20

which forms below the outer scale and stops C in-gress if the outer scale also is converted to carbides.

For Ni-based alloys, the positive effect of Al addi-tions recently has been demonstrated.21 For variousalloys (Table 1), the carburization in methane-hydrogen (CH4-H2) mixtures was investigated ataC = 0.8 and very low water vapor contents (i.e., oxy-gen activities lower than necessary to obtain Cr2O3).The carburization at 1,000C and 1,100C was notdecreased significantly for the Si-containing alloysbut clearly diminished for the Al-containing alloys.21

For Alloy 602 (UNS N06025), no increase in C con-

tent was detected (Figure 6), and the Charpy V-notchimpact energy remained unchanged.21 Al2O3 formedat the surface of this alloy and completely sup-pressed the carburization. All other alloys showedsome carburization. Ni-based alloys showed less thanFe-based alloys because of the reduced solubility anddiffusivity of C, as discussed previously. But for Alloy602, the Ni/Fe ratio is near optimum for reducingthe ingress of C, and the Al concentration (2.3 wt%Al) combined with additions of Y and Zr appears tobe sufficient for formation of a protective Al2O3 scaleat high temperatures. Positive effects of the reactiveelements Y and Zr on the nucleation, growth mecha-nism, and adherence of Al2O3 scales are well known,especially from the development of the Fe-Cr-Al foils forcatalyst carriers in automotive exhaust systems.22-23

FIGURE 4. Thermodynamics of the reactions at high temperaturesand aC = 1 (equilibrium with graphite, respectively, coke) in thesystem Cr-O-C. At temperatures > 1,050C, protective Cr2O3 scalesare converted to unprotective carbides. (pCO corresponds to the valuein the pack carburization test).17

FIGURE 5. Data for (DC cC)1/2, which is decisive for internal carbideformation in absence of a protective scale for the system Fe-Ni-C17at 1,000C, compared to results of the pack carburization test onvarious alloys (denoted by their Cr/Ni ratio) at 1,100C where theCr2O3-scale is converted to carbides and C diffusion determines therate of internal carbide formation.



METAL DUSTING

In recent years, metal dusting of Fe and Ni steelsand Ni-based alloys has been studied in carbonmonoxide-hydrogen-water vapor (CO-H2-H2O) mix-tures,5-10 simulating the synthesis gas or gas fordirect reduction, which is obtained from natural gas(methane [CH4]) conversion. In such atmospheres,high C activities (aC >> 1) are possible, especially atlow temperatures, which means that there is a strongtendency for graphite formation (in equilibrium withgraphite [aC = 1]). If the gas has access to a metallicsurface, C will be transferred into solid solution, andthe metal phase becomes oversaturated:

CO H H O C in solid solution+ = +2 2 ( ) (2)

In the case of Fe and steels, Fe3C is formed as anintermediate,5-7,11 but in the case of Ni and Ni-basedalloys, graphite directly grows into the metalphase:5,8-9,12

C (from oversaturated solid solution) graphite (3)

This graphite growth starts at the surface and atgrain boundaries. Here, the graphite nucleates, andC atoms from the oversaturated solution are attachedto the graphite planes. If these graphite planes aredirected more or less vertical to the surface or thegrain boundary face, this inward growth of graphitedestroys the metal matrix by the volume increase.Transmission electron microscopic studies haveshown such inward growth of graphite roots ortongues into the metal during metal dusting (Figure7), which leads to compressive stresses and metalparticles being pressed outward.12 This ejection ofmetal particles into an outer layer of coke growing onthe metal surface also can be seen in metallographiccross sections of Fe-Ni alloys after metal-dusting at-tack (Figure 8). In comparison to the metal particlesformed by the metal-dusting mechanism on Fe andsteels (i.e., by decomposition of Fe3C), the metal par-ticles are relatively large in the case of alloys with

50% Ni, and accordingly, their catalytic effect onC deposition is less:

CO H H O C+ +2 2 (in and on the metal particles) (4)

The mass of coke formed decreases, and its metalcontent increases with increasing Ni content in Fe-Nialloys.5,8

In the case of Ni-Cr-Fe alloys, such as Alloy 600,internal carbide formation precedes the metal-dust-

TABLE 1Compositions of Alloys Investigated

Alloy Material No. Ni Fe Cr Si Al C Others

DS 1.4862 36 42 18 2.2 < 0.06 0.15 Ti800H 1.4858 31 47 21 0.5 0.25 0.07 0.3 TiAC66 1.6877 32 39 28 0.06 0.8 Nb, 0.7 Ce45-TM 2.4889 47 23 27 2.7 0.06 0.05 Re600H 2.4816 74 9 16 0.07 0.2 Ti601H 2.4851 60 14 23 1.4 0.04 0.5 Ti617 2.4663 54 1 22 1.0 0.06 0.5 Ti, 6 Mo, 12 Co602 CA 2.4633 62 9 25 2.3 0.2 0.1 Y, 0.15 Zr

Nominal Composition (mass%)

(a)

(b)FIGURE 6. Results of carburization tests21 in CH4-H2 at aC = 0.8 andvery low oxygen activity (Cr2O3 not stable) on various commercialalloys (Table 1): (a) thermogravimetric curves obtained at 1,100Cand (b) C concentration profiles after carburization at 1,000C.



ing attack. When the C enters and diffuses in such amaterial, the elements forming stable carbides arefirst tied up by carbide precipitation before theoversaturation, which causes metal dusting.

The fine precipitates formed in Alloy 600 at650C are M23C6 (Figure 2); their growth causes for-mation of many dislocations. For a lower reactiontemperature, the finer carbides are formed. Thegraphite growth on and into the surface afteroversaturation leads to the ejection of metal particlesbeing pressed outward, as described. These metalparticles and a zone below the coke appear whitebecause of Cr oxidation and the dissolution of Crcarbides in this near-surface white layer. The oxida-tion of Cr was possible because of the atmosphere ofthe experiments and the H2O generated by the car-burization and coking reactions. However, the oxidedoes not form a protective scale but is included inthe coke. The white layer is the carbide-denudedzone, as often observed in the oxidation of Cr2O3-forming alloys, and it is not a mysterious cause ofthe dusting24 but rather the oversaturated Fe-Ni thatdecomposes into metal phase and graphite.

As seen in Figure 2, metal dusting started locallyand gradually spread laterally over the surface of thealloys. Long-term laboratory tests have been con-ducted, which repeatedly were interrupted to scratchoff the coke. This corrosion product was weighed andanalyzed for its metal content; from these data, ametal wastage rate was calculated. This rate gradu-

ally rose and approached a limiting maximum valuewhen the total surface was attacked (Figure 9). At650C, this state was reached for Alloy 600 afterabout 3,000 h. But after 10,000 h, Alloy 601 (UNSN06601) still showed local attacksome hemispheri-cal pits growing into the material (Figure 9).

Some materials hardly were attacked at all: 45-TM, 617, 690, and 602 CA (UNS N06045, N06617,N06690, and N06025, respectively).9 Very smallflakes of coke and some local carburization wereobserved on these materials. No attack of the alloywas detected in the case of 602 CA; only a protectiveCr2O3 layer had formed (Figure 10). By surface analy-sis with Auger electron spectroscopy (AES) it waslearned that the Al in the alloy helped in the forma-tion of the protective Cr-rich scale by immediatepassivation of the surface in the carburizing atmo-sphere through a thin Al2O3 film.

CONCLUSIONS

Two corrosion phenomena are caused by C:Carburization: an internal carbide formation in

alloys at high temperatures and aC 1.Metal dusting: a disintegration of alloys into

graphite and metal particles at intermediate tempera-tures and aC > 1.

Since solubility and diffusivity of C in alloysdecrease with increasing Ni content up to NNi 0.8,Ni-based alloys generally show a better resistance

(a) (b)FIGURE 7. Transmission electron micrographs of Alloy 600 after 23 h exposure under metal-dusting conditions in H2-25%CO-2%H2O (aC = 38): (a) showing inward growth of graphite in tongues or roots (from the left) and (b) coke deposit,graphite with metal particles.



against these C corrosion phenomena than high-alloysteels. Carburization largely is suppressed by the forma-tion of a protective Cr2O3 scale, and high alloy steelswith 25% Cr can be used (e.g., as cracking tubes inC2H4 production for many years, if temperatures> 1,050C are not exceeded). At higher temperatures,Cr2O3 scale is converted to unprotective carbides. Forhigh Ni materials under these conditions, carburiza-tion is retarded by the relatively limited and slowingress of C. However, for long-term operation attemperatures about 1,100C, as desired currently in

the process industry, new alloys are needed. Becauseof the necessary creep resistance, Ni-based alloyssuggest themselves, and alloying additions of Si and/or Al could provide the formation of protective oxidelayers. In a recent publication, superior carburizationresistance was demonstrated for Ni-based alloys with1 wt% to 2 wt% Al.21

Austenitic steels such as Alloy 800 with 20% Crand 32% Ni are most susceptible to metal dusting. Inthe temperature range of 450C to 600C, they imme-diately are attacked all over the surface, the metalwastage rate reaches high values (~ 5 mm/year at600C), and vast amounts of coke are produced.With increasing Ni-content in the alloys, the mecha-nism of attack changes, the rate of attack lessens(~ 0.15 mm/year for Alloy 600 at 600C), and theamount of coke produced is much less. As a result of its low Cr-content (< 16%), Alloy 600has insufficient high-temperature corrosion resis-tance, and many failure cases by metal dusting havebeen reported for this alloy. Better resistance againstmetal dusting can be expected for Alloy 601 (22% to23% Cr) and Alloy 602 (25% Cr). Both alloys containsome Al, which helps in the initial stages of protec-

(a)

(b)

(c)FIGURE 8. Metallographic cross sections of Fe-Ni alloys after 1 dayof exposure under metal-dusting conditions at 650C, coke layer withmetal inclusions: (a) Fe-40%Ni (the coke was coated with Ni beforeembedding), (b) Fe-50%Ni, and (c) Fe-80%Ni.

FIGURE 9. Metal wastage rates determined for Alloys 800, 600, 601,and 602 in discontinuous exposures at 650C in flowing H2-CO-H2Omixtures (aC = 10), included: photo of the Alloy 602 sample after6,700 h.



tive scale formation. It is most important that theformation of a Cr-rich scale occur rapidly on steelsor Ni-based alloys in the carbonaceous environmentbefore local or general C ingress initiates the metal-dusting attack. The preconditions for suppressing Cingress are favorable for Ni-based alloys such as Al-loy 602 since C solubility and diffusivity are near theminimum, and for the formation of a protective Cr2O3scale, the Cr-concentration should be 25%. In fact,Alloy 602 proved to be resistant in discontinuouslaboratory and pilot plant tests for thousands ofhours. The 50% Cr-50% Ni alloy, which also is usedas a special heat- and corrosion-resistant materialand some Cr-based materials also have proved to beresistant to metal dusting,25 but these materials gener-ally are expensive and difficult to produce and handle.

REFERENCES

1. H.J. Grabke, I. Wolf, Mater. Sci. Eng. 87 (1987): p. 22.2. I. Wolf, H.J. Grabke, H.P. Schmidt, Oxid. Metals 29 (1988): p.

289.3. A. Schnaas, H.J. Grabke, Oxid. Met. 12 (1978): p. 387.4. A. Schnaas, H.J. Grabke, Werkst. Korros. 29 (1978): p. 635.5. H.J. Grabke, R. Krajak, J.C. Nava Paz, Corros. Sci. 35 (1993):

p. 1,141.

(a) (b)

(c)FIGURE 10. Metallographic cross sections of the specimens from the discontinuous exposure at 650C (Figure 9): (a) Alloy800, attacked all over the surface, after 95 h; (b) Alloy 600, attacked all over the surface, after 5,000 h; (c) Alloy 601, attackedlocally (Figure 9), after 6,700 h; and (d) Alloy 602, no metal-dusting attack detected, Cr2O3 scale and some internal Al2O3formation.

(d)

6. H.J. Grabke, R. Krajak, E.M. Mller-Lorenz, Werkst. Korros. 44(1993): p. 89.

7. H.J. Grabke, Corrosion 51 (1995): p. 711.8. H.J. Grabke, R. Krajak, E.M. Mller-Lorenz, S. Strauss,

Werkst. Korros. 47 (1996): p. 495.9. H.J. Grabke, E.M. Mller-Lorenz, J. Klwer, D.C. Agarwal, MP

37 (1998): p. 58.10. H.J. Grabke, Mater. Corr. 49 (1998): p. 303.11. E. Pippel, H.J. Grabke, S. Strauss, J. Woltersdorf, Steel Res. 66

(1995): p. 217.12. R. Schneider, E. Pippel, J. Woltersdorf, S. Strauss, H.J.

Grabke, Steel Res. 68 (1997): p. 326.13. R.P. Smith, Trans. Met. Soc. AIME 224 (1962): p. 10.14. S.K. Bose, H.J. Grabke, Z. Metallkd. 69 (1978): p. 8.15. T.A. Ramarayanan, R. Petkovic-Luton, Proc. 6th Conf. High-

Temperature Corrosion, 1981, ed. R.A. Rapp (Houston, TX:NACE International, 1983), p. 430.

16. S. Ling, T.A. Ramarayanan, Oxid. Metals 40 (1993): p. 179.17. H.J. Grabke, U. Gravenhorst, W. Steinkusch, Werkst. Korros.

27 (1976): p. 291.18. W.F. Chu, A. Rahmel, Oxid. Metals 15 (1981): p. 331.19. H.J. Grabke, CarburizationA High-Temperature Corrosion

Phenomenon, MTI publication no. 52 (St. Louis, MO: MaterialsTechnology Institute of the Chemical Processing Industries, 1997).

20. W. Steinkusch, Werkst. Korros. 30 (1979): p. 837.21. J. Klwer, U. Heubner, Mater. Corr. 49 (1998): p. 237.22. H.J. Grabke, M. Siegers, V.K. Tolpygo, Z. Nat.forsch. A, Phys.

Phys. Chem. Kosmophys. 50 (1995): p. 217.23. J. Klwer, A. Kolb-Telieps, M. Brede, Metal-Supported Automo-

tive Catalytic Converters, ed. H. Bode (Frankfurt, Germany:Werkstoff-Informationsgesellschaft, 1997), p. 33-46.

24. J.A. Richardson, Nitrogen 205 (1993): p. 49.25. H.J. Grabke, E.M. Mller-Lorenz, H.P. Martinz, to be published.

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