Microprocesses of coke formation in metal dusting by dfgh4bnmu


									628       Wei, Pippel, Woltersdorf and Grabke                                                Materials and Corrosion 50, 628±633 (1999)

Microprocesses of coke formation in metal
Mikroprozesse der Coke-Bildung beim Metal Dusting
                                                                         Q. Wei, E. Pippel**, J. Woltersdorf and
                                                                         H.J. Grabke

   The microprocess of coke formation during metal dusting on iron                                            È
                                                                            Unter Verwendung von hochauflosender (HREM) und analyti-
in a carburizing atmosphere with medium and extremely high car-          scher (AEM) Elektronenmikroskopie wurden die Mikroprozesse
bon activities as well as the influence of sulphur have been studied     der Coke-Bildung untersucht, die beim Metal Dusting von Eisen
down to the nanometer scale using high resolution electron micro-                     È
                                                                         in Atmospharen mit mittlerer und extrem hoher Kohlenstoffaktivi-
scopy (HREM) and analytical electron microscopic techniques               È                              È                È
                                                                         tat sowie geringen Schwefelzusatzen ablaufen. Wahrend der Metal-
(AEM). While for medium carbon activities the metal dusting pro-                                                         È    È
                                                                         Dusting-Prozeû bei mittleren Kohlenstoffaktivitaten uber Bildung
ceeds via a formation, disintegration and further decomposition of a     und anschlieûenden Zerfall von Fe3C fuhrt, entsteht bei hohen Ak-
metastable carbide Fe3C into Fe and C, the additional formation of            È
                                                                         tivitaten auûerdem Fe5C2, und die Carbide werden im Coke in Par-
the carbide Fe5C2 and the stabilization of carbides in the coke re-      tikelform stabilisiert. Schwefel kann Metal Dusting bereichsweise
gion have been observed for extremely high carbon activities. If                 È
                                                                         unterdrucken und verhindert eine Graphitbildung im Coke.
sulphur is present in the atmosphere metal dusting takes place so-                                                  È
                                                                            Die Untersuchungen zeigten, daû in Abhangigkeit von der Koh-
lely in the S-free surface areas. Furthermore, sulphur deposited                          È                          È
                                                                         lenstoffaktivitat ein grundlegender Schadigungsmechanismus
from the atmosphere will suppress the nucleation of graphite in          wirkt: Basisebenen des Graphitgitters wachsen senkrecht auf die
the coke.                                                                               È
                                                                         Carbidoberflache, wobei ihre reaktiven, frei endenden Kanten
   In addition, the results reveal that, irrespective of the degree of   als Zentren beim Zerfall der Carbide fungieren, so daû ein Coke
the carbon activity, there is a fundamental initial reaction microme-    aus Kohlenstoff und Eisen- bzw. Carbidteilchen entsteht.
chanism of metal dusting characterized by a vertically oriented de-
position of graphite lattice planes with respect to the original sur-
face of the substrate and with free ends affecting the decomposition
of the carbides and thus forming a coke of carbon and iron, or of
carbide particles, depending on the carbon activity.

1 Introduction                                                           dusting on iron and low-alloy steels have been elucidated as
   As is well known, metal dusting is a dangerous high-tem-                 After a supersaturation of the metal with dissolved carbon
perature corrosion phenomenon, which takes place on iron,                at aC b 1 a carbide layer is formed on the metal surface. As
steels, Ni and Ni-based alloys in a strongly carburizing atmo-           graphite from the atmosphere deposits on the surface of the
sphere (aC b 1) at temperatures between 400 8C and 800 8C.               carbide, the carbon activity at the interface between the car-
The metals are attacked by carbon from the atmosphere and                bide and the graphite reduces to aC ˆ 1. Thus the carbide be-
disintegrate into a dust consisting of fine metal particles and          comes unstable and decomposes into a coke of iron and gra-
carbon. The phenomenon was simulated in flowing CO/H2/                   phite. The metal particles in the coke act as a catalyst and
H2O mixtures using iron and model alloys. Thermodynamics                 promote the deposition of carbon from the atmosphere and
and kinetics were studied intensively by means of thermogra-             hence accelerate the whole metal dusting process.
vimetry (TGA) and scanning electron microscopy (SEM) [1 ±                   Furthermore, the transmission electron microscopy studies
5] as well as by microstructural and analytical transmission             of metal dusting on iron and model alloys exposed to a car-
electron microscopy methods [6 ± 8]. The main steps of metal             burizing atmosphere with a medium carbon activity (aC b 1)
                                                                         using high resolution electron microscopy (HREM), energy-
                                                                         dispersive X-ray spectroscopy (EDXS) and electron energy
                                                                         loss spectroscopy (EELS) revealed that (i) on the atomic
 * Presented at the EFC-workshop on `Coking and Decoking',               scale, the metal dusting process has a general reaction prin-
   Porto, Portugal, 6/7 May 1999                                         ciple, namely the oriented deposition of graphite on the sub-
** E. Pippel, Q. Wei, J. Woltersdorf                                     strate followed by its directed inward growth, and that (ii) the
   Max-Planck-Institut fur Mikrostrukturphysik,
                         È                                               details of the carbide disintegration and decomposition are
   Weinberg 2, D-06120 Halle (Germany)                                   modified by the kind of metal phase used.
   H. J. Grabke                                                             Recently, the influence of sulphur on metal dusting has
   Max-Planck-Institut fur Eisenforschung GmbH,
                         È                                               been investigated in [9] for different carbon activities using
   Max-Planck-Str. 1, D-40237 Dusseldorf (Germany)
                                  È                                      TGA, X-ray diffraction (XRD) and Auger electron spectro-

0947-5117/99/1111-0628$17.50‡.50/0                                              Ó WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
Materials and Corrosion 50, 628±633 (1999)                                                 Microprocesses of coke formation         629

scopy (AES). It turned out that sulphur in the carburizing at-
mosphere retards the metal dusting process. Moreover, for an
extremely high carbon activity (aC ) 1), with or without sul-
phur, besides cementite, a second carbide with a lower iron
content, viz. the Hagg carbide Fe5C2, has formed.
   It is the aim of the present paper to complete the above
results by corresponding microstructural and microchemical
investigations and to find out the differences in the metal dust-
ing behaviour compared to that in atmospheres with a medium
carbon activity (aC b 1). The investigations are intended to
expand the above-mentioned micro-model of the metal dust-
ing process developed for a medium carbon activity (aC b 1)
to the case of an extremely high carbon activity (aC ) 1) and
thus to complete it. Furthermore, a better understanding will
be achieved of both the retardation of metal dusting caused by        Fig. 1. Steps of the metal dusting on iron as observed by TEM:
                                                                      a) interface between iron and Fe3C, b) decomposition of Fe3C,
sulphur and the corresponding suppressing of the catalytic            c) laminated graphite, d) coke region
effect of the metal particles in the coke.
                                                                      Abb. 1. TEM-Abbildung der Stufen des Metal Dusting auf Eisen:
                                                                      a) Grenzflache zwischen Eisen und Fe3C, b) Zerfall von Fe3C,
                                                                      c) Graphitschicht, d) Coke-Bereich
2 Experimental

   Pure iron foils (12.5 lm) were exposed to a flowing gas            bon is transferred from the gas atmosphere, which leads to a
mixture of CO/H2/H2O with carbon activities aC ˆ 3.3 and              supersaturation of the metal with carbon. Subsequently, a me-
aC ˆ 4580. In a further number of samples, the sulphur con-           tastable F3C (cementite) phase is formed on the surface of the
centration ± produced via the gas mixture CO/H2/H2O/H2S ±             metal. The interface between iron (Fig. 1a, left) and F3C
in the atmosphere changed from 0 to 1 ppm. In a furnace, the          (right) is generally even and flat, with no microcracks, flaws
samples were heated at temperatures between 400 8C and                or other irregularities detectable.
650 8C for some minutes to several hours. At the same                    Since the cementite layer is a diffusion barrier to the ingress
time, thermogravimetric analyses were performed, which re-            of carbon within the process going on a graphite layer is de-
vealed the different metal dusting states. This was done at the       posited on the cementite. Over a large area (cf. Fig. 1c) this
MPI fur Eisenforschung, Dusseldorf. Since the process of me-
        È                    È                                        graphite layer exhibits a distinctly fibrous structure perpendi-
tal dusting does not change qualitatively with time and tem-          cular to the surface. Along the numerous cracks and fissures in
perature, the microstructural investigations were based on            this layer the reaction gas may reach the cementite so that
samples that exhibited well-developed metal dusting phenom-           carbon can be further deposited. In some areas the graphite
ena.                                                                  layer is completely closed to the atmosphere and the carbon
   Cross section specimens were prepared for transmission             activity at the interface between cementite and graphite de-
electron microscopy using the following procedures: At                creases to aC ˆ 1. In this case cementite becomes unstable
first, the iron foils with metal dusting products on both sides       and a decomposition into iron and carbon will locally start
were glued between ceramic blocks. Then cross section slices          by the reaction Fe3C R 3 Fe ‡ C. This interface (cf.
were cut across the interfaces of iron and ceramics, followed         Fig. 1b) is irregularly shaped and has a serrated structure.
by grinding, dimpling and final Ar-ion thinning down to elec-         Along its concentration gradient iron will diffuse through
tron transparency.                                                    the graphite layer, eventually forming small particles or clus-
   As the Mo-La/b and S-Ka peaks overlap at 2.3 keV in EDX,           ters of iron in an outer coke region (cf. Fig. 1d). These metal
it was tried to avoid a Mo contamination during the sample            particles catalyse the carbon deposition from the reaction gas
preparation. Thus, either a sample holder made of aluminium           and become covered with graphite layers of up to 100 nm. A
was used for ion thinning or a segmental method in the PIPS           HREM image of an iron particle covered with graphite is
(precision ion polishing system, Gatan 691) was chosen.               shown in Fig. 2. With a prolongation of the metal dusting pro-
   High resolution electron microscope investigations were
performed in a Philips CM20 FEG (200 keV) equipped
with a parallel EELS ( PEELS model 666 of Gatan) and an
X-ray detector (Ge crystal, Voyager system of Tracor) with
an ultrathin window. A cooling unit (Gatan model 636)
was used to keep the specimen at liquid nitrogen temperature
during the small-probe analysis to reduce the carbon contam-

3 Results

3.1 Medium carbon activity (aC b 1)

  The microstructural peculiarities of the main steps of the
metal dusting attack on pure iron in a carburizing atmosphere         Fig. 2. HREM image of an iron particle covered with graphite
with medium carbon activity (aC b 1) are demonstrated in              Abb. 2. HREM-Aufnahme eines von Graphit umgebenen Eisen-
Fig. 1, from left to right: At the first stage of the process, car-   partikels
630      Wei, Pippel, Woltersdorf and Grabke                                           Materials and Corrosion 50, 628±633 (1999)

Fig. 3. HREM image of the initial stage of the cementite decay
during metal dusting (model inserted, cf. text)
Abb. 3. HREM-Aufnahme des Anfangsstadiums des Zementitzer-
falls beim Metal Dusting-Prozeû (Modell eingefugt, siehe Text)

cess the decomposition of the cementite layer continues, how-                                                       È
                                                                   Fig. 5. HREM image of the initial stage of the Hagg carbide decay
ever, its thickness will keep in a steady state of 1 ± 2 lm by     (electron diffraction pattern inserted, aC ˆ 4580)
growing from the interior by the carbon diffusion from super-      Abb. 5. HREM-Aufnahme des Anfangsstadiums vom Hagg-Car-  È
saturated regions. Contrary to that the thickness of the coke      bid-Zerfall (Elektronenbeugungsbild eingefugt, aC ˆ 4580)
layer appreciably increases to several tens of micrometers.
   Both the local cementite decay and the oriented growth of
the graphite basal lattice planes at the serrated interface (cf.      Besides cementite, a second iron-carbon metastable phase,
Fig. 1b) are shown in the high resolution TEM image of Fig. 3.     i.e. the Hagg carbide (Fe5C2), has been observed in the carbide
The penetration of the graphitic planes into the cementite         layer of all samples exposed to an atmosphere of aC ) 1 with
layer on the left of this figure suggests that particularly the                                                   È
                                                                   and without sulphur. Both cementite and Hagg carbide areas
free ends of these lattice planes favour the cementite decom-      have formed above the metal substrate as shown in Fig. 4.
position by fixing the generated carbon atoms. The scheme             Like in metal dusting at aC b 1, in the further course of the
incorporated in Fig. 3 demonstrates this conception in more        process there is a decay of the carbide layer in areas, where
detail, here for an [100] oriented cementite crystal as an ex-     graphite is deposited on the carbide layer, with the carbon
ample. In opposition to that, in regions where the graphite        activity locally reducing to 1 at the interface. Both cementite
planes are running parallel to the cementite surface the de-               È
                                                                   and Hagg carbide become unstable and begin to decompose.
composition may be retarded or becomes even impossible.                                            È
                                                                   The decomposition of the Hagg carbide proceeds in a way
                                                                   similar to that of cementite: Fig. 5 shows a HREM image
                                                                   of a Hagg carbide decomposing via the free ends of the gra-
3.2 Extremely high carbon activities (aC ) 1)                      phite lattice planes in front of a graphite tongue favouring the
                                                                   decomposition (Fe5C2 R 5 Fe ‡ 2 C) by trapping the gener-
   In samples exposed to a carburizing atmosphere with an          ated carbon atoms. At the same time, Fe atoms diffuse
extremely high carbon activity (aC ) 1) and in those exposed       through the graphite layer.
to an atmosphere with a medium carbon activity (aC b 1) as            This local decomposition of carbide (i.e. breaking the che-
described above, three reaction areas are observed in the metal    mical bonds between carbon and iron) results in a disintegra-
dusting products, i.e. the carbide layer region, the laminated     tion of the carbide layer. The HREM images of Figs. 6 and 7
graphite region and the coke region. However, there are some       further demonstrate the local decay of carbides at the initial
differences:                                                       and prolongated stages. Oriented graphite bands divide the

                                                                                              Fig. 4. Layer of iron carbides (Fe3C
                                                                                              and Fe5C2) and coke area after metal
                                                                                              dusting (aC ˆ 4580, 1 ppm H2S)
                                                                                              Abb. 4. Eisencarbidschicht (Fe3C und
                                                                                              Fe5C2) und Coke-Bereich nach Metal
                                                                                              Dusting (aC ˆ 4580, 1 ppm H2S)
Materials and Corrosion 50, 628±633 (1999)                                                  Microprocesses of coke formation          631

carbide layer (cf. Fig. 6) into parts of several 10 nm in size (cf.      È
                                                                      Hagg carbide. Furthermore, such fine nano-carbides (cemen-
Fig. 7).                                                                          È
                                                                      tite and Hagg carbide as well) distributed randomly in the
   In contrast to a medium carbon activity (aC b 1), for an           amorphous carbon matrix have also occurred (Fig. 10). Unfor-
extremely high carbon activity after disintegration both ce-          tunately, the sulphur content in the coke has to be extremely
mentite and Hagg carbide particles do not further decompose           low. It could not be determined by the available spectroscopic
into iron and carbon but stay stable in the coke, which is in-
dependent of the sulphur content of the atmosphere. Fig. 8
shows a coke area composed of carbide and carbon. The inset
is a diffraction pattern of cementite taken of one of these par-
   However, depending on the presence or absence of sulphur
in the atmosphere there is one important difference in the coke
microstructure, which is demonstrated in Figs. 9 a,b and 10. If
sulphur is present, some carbide particles are covered with a
shell of crystals of only several nanometers in size (cf.
Fig. 9a), which have been identified as either cementite or

                                                                      Fig. 8. Coke area after metal dusting (aC ˆ 4580, inset: diffraction
                                                                      pattern of cementite)
                                                                      Abb. 8. Coke-Bereich nach Metal Dusting (aC ˆ 4580, Elektro-
                                                                      nenbeugungsbild von Zementit eingefugt)È

Fig. 6. Disintegration of iron carbide by the penetration with gra-
phite bands (aC ˆ 4580)
Abb. 6. Zerfall von Eisencarbid durch Einwachsen von Graphit-
bandern (aC ˆ 4580)

                                                                      Fig. 9. HREM images of iron carbide particles in the coke, a) cov-
                                                                      ered with nano-carbides, aC ˆ 4580, 0.5 ppm H2S, b) covered with
Fig. 7. HREM image of the interface area between coke and car-        graphite, aC ˆ 4580, without H2S
bide layer (aC ˆ 4580, 1 ppm H2S)                                     Abb. 9. HREM-Aufnahmen von Eisencarbidteilchen im Coke
Abb. 7. HREM-Aufnahme des Grenzbereiches zwischen Coke                a) eingehullt von Nano-Carbidteilchen, bei aC ˆ 4580, 0,5 ppm
und Carbidschicht (aC ˆ 4580, 1 ppm H2S)                              H2S, b) eingehullt mit Graphit, bei aC ˆ 4580, ohne H2S
632      Wei, Pippel, Woltersdorf and Grabke                                           Materials and Corrosion 50, 628±633 (1999)

                                                                   shown in Fig. 9b (cf. also Fig. 2 for the case of iron parti-
                                                                   cles). Neither were there any carbide nano-particles in the
                                                                   amorphous carbon matrix in this case.

                                                                   4 Discussion

                                                                       The special microstructural phenomena of the metal dust-
                                                                   ing process on iron at different atmospheres as revealed by
                                                                   electron microscopic methods are summarized in Fig. 11.
                                                                   The morphological similarities of metal dusting under differ-
                                                                   ent atmospheric conditions are: i) a serrated appearance of the
                                                                   interface between graphite and carbide, and ii) a graphite in-
                                                                   trusion perpendicular to the carbide surface. These features
                                                                   indicate a fundamental common reaction micromechanism:
                                                                       On the atomic scale the metal dusting process has one and
                                                                   the same reaction principle, viz. the deposition of graphite
                                                                   lattice planes in more or less vertical orientation to the origi-
                                                                   nal surface of the substrate, with their free ends effecting the
                                                                   decomposition of the carbides (cf. Fig. 11, bottom). The local
                                                                   decomposition of the carbides results in a disintegration of the
                                                                   carbide layer.
                                                                       For aC b 1 (cf. Fig. 11, left), after having formed cementite
                                                                   will continuously decompose into iron and carbon. The sur-
                                                                   face of iron particles in the coke catalyzes the further deposi-
                                                                   tion of carbon. A graphite border is being formed around the
                                                                   iron particles. Increasing the carbon activity to an extremely
                                                                   high level, e.g., to aC ˆ 4580 (aC ) 1) will result in a mod-
                                                                   ified mechanism: Besides cementite, the Hagg carbide Fe5C2
Fig. 10. HREM image of the coke area after metal dusting on iron   with a higher carbon content will be formed. This is consistent
at aC ˆ 4580 and 1 ppm H2S (carbide particles are marked by ar-    with [10] reporting that more and more carbon-rich phases
                                                                   from Fe3C (25 at.% C) via Fe5C2 (28.6 at.% C) to Fe2C
Abb. 10. HREM-Aufnahme des Coke-Bereichs nach Metal Du-            (33 at.% C) are formed with increasing carbon content in
sting von Eisen bei aC ˆ 4580 und 1 ppm H2S (Carbidteilchen
durch Pfeile gekennzeichnet)                                       iron. The crystallographic structures of cementite and Hagg  È
                                                                   carbide are very similar [11]. As the carbon content increases
                                                                   the arrangement of the carbon atoms in cementite requires
techniques. In contrast to that, with no sulphur in the reaction   only a slight displacement on {0k0} planes to yield the
gas the carbide particles in the coke have not been covered           È
                                                                   Hagg carbide structure. Frequently observed streaks in
with any nano-carbides but with a thin graphite band as            [010] direction in the cementite diffraction patterns, asso-

                                                                                              Fig. 11. Scheme of the metal dusting
                                                                                              products on iron using different reac-
                                                                                              tion atmospheres (cf. text)
                                                                                              Abb. 11. Schema der Metal Dusting
                                                                                              Produkte auf Eisen in verschiedenen
                                                                                              Reaktionsatmospharen (siehe Text)
Materials and Corrosion 50, 628±633 (1999)                                              Microprocesses of coke formation         633

ciated with stacking faults on {0k0} planes, support this sug-     6 References
gestion. Locally, both carbides decompose into iron and gra-
phite. This leads to a disintegration of the carbide layer with-    [1] J. C. Nava Paz, H. J. Grabke: Oxid. Metals 39 (1993) 437.
out any further decomposition of the formed carbide patches                                              È
                                                                    [2] H. J. Grabke, R. Krajak, E. M. Muller-Lorenz: Werkst. Korros.
into iron and graphite because of the extremely high carbon             44 (1993) 89.
activity in the atmosphere (cf. Fig. 11, middle part). The re-                                                          È
                                                                    [3] H. J. Grabke, C. B. Bracho-Troconis, E. M. Muller-Lorenz:
                                                                        Werkst. Korros. 45 (1994) 215.
sulting carbide particles in the coke no further catalyze the
                                                                    [4] H. J. Grabke, R. Krajak, J. C. Nava Paz: Corrosion Sci. 35
carbon deposition, thus preventing the quadratic mass gain              (1993) 1141.
during metal dusting, which has been proven by gravimetric          [5] H. J. Grabke: Solid State Phenom. 41 (1995) 3.
measurements [9].                                                   [6] E. Pippel, J. Woltersdorf, H.-J. Grabke, S. Strauû: steel re-
   If a sulphur-containing atmosphere is applied, metal dust-           search 66 (1995) 217.
ing will be suppressed by the adsorption of a sulphur mono-         [7] E. Pippel, J. Woltersdorf, R. Schneider: Materials and Corro-
layer [12]. However, at cracks and fissures within this layer           sion 49 (1998) 309.
metal dusting can take place, with cementite and Hagg car-
                                                        È           [8] R. Schneider, E. Pippel, J. Woltersdorf, S. Strauû, H. J.
bide forming as shown in Fig. 11 (right). Again, because of             Grabke: steel research 68 (1997) 326.
                                                                    [9] A. Schneider, H. Viefhaus, G. Inden, H. J. Grabke, E. M. Mul-
the extremely high carbon activity in the atmosphere, after
                                                                        ler-Lorenz: Materials and Corrosion, 49 (1998) 336.
disintegration of the carbide layer the carbide patches are pre-              È
                                                                   [10] A. Koniger, C. Hammerl, M. Zeitler, B. Rauschenbach: Phy-
served in the coke. At the same time, in the coke new carbide           sical Review B 55 (1997) 8143.
particles nucleate with Fe atoms from the local decomposition      [11] H. L. Yakel: International Metals Reviews 30 (1985) 17.
of carbides along the graphite bands (cf. Fig. 11, bottom). The                                È
                                                                   [12] H. J. Grabke, E. M. Muller-Lorenz: steel research 66 (1995)
observation of many nanometre-sized carbide particles, which            254.
either envelop the larger carbide patches or are distributed
randomly in the coke proposes their formation via the hetero-      (Received: August 5, 1999)                               W 3384
geneous or homogenous nucleation.

5 Acknowledgements

  The authors are grateful to Dr. A. Schneider, MPI fur Eisen-
forschung GmbH Dusseldorf, for providing the metal dusting
samples. Support of this study by the Deutsche Forschungs-
gemeinschaft is also acknowledged.

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