Volume 6 Paper 100
Metal dusting of iron in CO-H2-H2O gas mixtures at 600Ã?Â°C
J. Zhang, A. Schneider and G. Inden
Keywords: Metal dusting; Carburisation; Cementite decomposition; Coke ; CO content ; Iron particles/layer; ilamentous carbon; Bulk columnar graphite; Graphite particle cluster
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JCSE Volume 6 Paper 100
Submitted 6th July 2003, fully published 17th July 2004
Metal dusting of iron in CO-H2-H2O
gas mixtures at 600ï¿½C
J. Zhang, A. Schneider, G. Inden
fï¿½r Eisenforschung GmbH, Max-Planck-Str. 1, D-40237 Dï¿½sseldorf, Germany, mailto2('j.zhang','mpie.de')
carburisation and coke formation were investigated at 600ï¿½C in (1-95)% CO-(98.8-4.8)%
H2-0.2% H2O gas mixtures. In all cases, cementite is
formed on the sample surfaces together with carbon deposits on the top. With 1%
CO, α-Fe particles or
an iron layer are formed at the cementite/graphite interface as a result of
cementite decomposition. The cementite under this condition vanishes after 21.5
h of reaction. However, for CO contents > 5%, no α-Fe is detected at the
interface. The observations of the cross-sections show a very thin cementite
layer directly contacted with a thick graphite layer.
morphologies of graphite can be identified in the coke of samples carburised by
the gas with 1% CO: filamentous carbon, columnar bulk graphite, and graphite
particle clusters with many fine iron-containing particles embedded inside.
With increasing CO contents, the first two morphologies become the main forms
of graphite in the coke. The examination of coke formation in different stages
of reaction in the gas with 75% CO shows that the graphite bulks are composed
of graphite columns. Filaments develop to the top of surface from the gaps
between these columns and distribute preferably along the grinding scratches.
Very fine and short filaments are observed in the gases with very low CO
content (1-5%) and very high CO content (95%). For CO contents between 5 and
95% long and coarse filaments are observed.
phase analysis by x-ray diffraction reveals that with 1% CO, both iron and
cementite are detected. However, above this CO content only cementite is
detected as an iron-containing phase in the coke. Thermo-gravimetric analysis
shows that the rate of carbon take-up in the early stage of reaction increases
with increasing CO content reaching a maximum at 75%.
§4 Keywords: Metal dusting;
Carburisation; Cementite decomposition; Coke ; CO content ; Iron
particles/layer; Filamentous carbon; Bulk columnar graphite; Graphite particle
dusting is a disastrous corrosion phenomenon, degrading iron, low and high
alloy steels and Ni- or Co-based alloys in a strongly carburising gas
atmosphere (carbon activity ac > 1) at a relatively high
temperature (400-800ï¿½C). The
detrimental consequence of this corrosion includes impairing the heat transfer
efficiency, reducing metal catalyst lifetime and damaging the material by
separating metal particles to the coke. So far many studies on metal dusting
have been published and several mechanisms have been proposed [crossref(1)-crossref(14)].
Grabke [crossref(1)-crossref(5)] proposed a mechanism, in which cementite is
formed first and then decomposed to fine particles into coke by the initiation
of graphite deposition on cementite surface. Recently an extensive study of
metal dusting on iron in CO-H2-H2O gas mixtures at 700ï¿½C was conducted in our
laboratory, including the effect of gas composition, cementite decomposition,
coke formation, and coke gasification.ï¿½
It was shown that with low CO content α-Fe particles or an iron
layer are formed between cementite and graphite [crossref(11)-crossref(12)].
Increasing the CO content in the gas results in the disappearance of this
phenomenon, i.e. cementite contacting directly with the coke layer.
Corresponding coke analyses [crossref(13)] indicate the presence of cementite
and the absence of iron in the coke if high CO contents are used. Three
morphologies of graphite on the surface have been identified as porous graphite
clusters, filamentous carbon and bulk dense graphite with a columnar structure
[crossref(12)]. The observation of reaction products in different stages of
the reaction reveals clearly the process of coke formation. The gasification of
carburised samples provides new information about cementite decomposition in
bulk material and in the coke [crossref(14)].
this work, a comprehensive research work has been conducted on the metal
dusting of iron at 600ï¿½C which is
considered as the worst temperature for metal dusting in CO-H2-H2O
gas mixtures [crossref(6)]. At this temperature cementite was reported to be
decomposed with a fastest rate [crossref(15)]. The aim of this work is to find
out whether all phenomena observed at 700ï¿½C will also occur at 600ï¿½C, e.g. α-Fe layer formation between cementite and graphite etc. The comparison of results at two different
temperatures is expected to be helpful to understand the mechanism of metal
§7 Pure iron samples (discs of 20 mm in diameter and 1.1
mm in thickness) were used for all metal dusting experiments. The samples were
first annealed at 850ï¿½C for 1 hour in a pure H2 gas atmosphere and then ground on
SiC paper to grade 1000. Afterwards, the sample was hung on a microbalance
(Sartorius 7287 with an accuracy of 1ï¿½g) by silica hooks. Helium gas was introduced into the chamber of the
microbalance in order to protect the balance. Reaction gases were composed of H2,
CO and H2O. The content of H2O was established by passing
H2 through a mixture of oxalic acid and its dihydrate at a certain
temperature [crossref(16)]. At first the sample was heated in a pure hydrogen
gas atmosphere. After reaching a stable temperature, the reaction gases were
introduced into the reactor. The inner diameter of this reactor is 27 mm. The
gas compositions were controlled by capillary flow meters. The gas flow rate
was fixed at 2 ml/sec. The weight change of the sample during reaction was
§8 After each carburisation experiment, the quartz
filament holding the sample was broken to let the sample drop down into the
cold zone for quenching. The surfaces of the samples were analysed by scanning
electron microscopy (SEM). After the SEM surface analysis, the specimens were
cut to obtain the cross sections for optical microscopy. In order not to alter
the surface by the preparation the samples were coated with nickel and then
mounted in epoxy resin. Mounted samples were polished and etched by a hot
solution of alkaline sodium picrate for optical microscope observation. In
order to detect the phases present in the dust, the coke layer was carefully
removed from the surface and analysed by X-ray diffraction (XRD). XRD analysis
was performed using monochromatic Co radiation.
3.1 Thermogravimetric analysis and cross-section
§10 Fig. 1. TGA results of iron samples carburised
at 600ï¿½C in CO-H2-H2O
gas mixtures with fixed H2O content (0.2%) and varied CO contents of
(1) 1%, ac=22.2; (2) 5%, ac=106.7; (3) 30%,
ac=470; (4) 50%, ac=559; (5) 75%, ac=418;
and (6) 95%, ac=102.5.
effect of gas composition on carburisation of iron was investigated by changing
the ratio of H2 and CO, H2/CO: (4.8-98.8)/(95-1), but
fixing the H2O content at 0.2%. The corresponding carbon activities
[crossref(12)] cover a large area from 22.2 to 559. The weight gains during
carburisation are recorded in Fig. 1. All TGA curves can be roughly divided
into two parts, corresponding to gradual increase in the rate of the weight
gain at the early stage and to a rapid raise of the rate after a certain time
of the reaction. Fig. 1 shows that with increasing CO content the time period
of the first stage becomes shorter. The rate of carburisation in the first
stage increases with CO contents to the maximum at 75% and then decreases with
further increasing CO content to 95%. The rate of carburisation in the second
stage keeps its increase with CO contents. Fig. 1 also shows that for CO content
higher than 50%, the tendency to increase the rate of carburisation slows down.
cross-sections of specimens after different stages of carburisation in 1%
CO-98.8% H2-0.2% H2O are shown in Fig. 2. After 4.5 h
reaction, cementite was formed together with graphite on the top of its
surface. Some iron particles are also found between cementite and graphite
(Fig. 2a). By increasing the reaction time to 9 h the thickness of graphite and
also the amount of iron, which forms even an iron layer between cementite and
graphite, are increased (Fig. 2b). After 21.5 h reaction almost all cementite
disappears (Fig. 2c).
§14 Fig. 2. Metallographic cross-sections of
specimens after (a) 4.5 h, (b) 9 h and (c) 21.5 h carburisation in the 1%
CO-98.8% H2-0.2% H2O gas mixture at 600ï¿½C.
phenomenon, however, does not occur for CO concentrations higher than 5%. The
metallographic cross-sections of samples after carburisation by gases with
5-95% CO show a thin cementite and a thick graphite layer on the top but no
iron particles or iron layer between them (Fig. 3b-3f). Fig. 3 also shows that
the cementite/graphite interface becomes more serrated by increasing CO contents
in the gas. Fig. 3f shows clearly the graphite penetration into cementite
§17 Fig. 3. Metallographic cross-sections of
iron samples carburised at 600ï¿½C in CO-H2-H2O
gases (H2O content fixed at 0.2%) with different CO contents for
different reaction times: (a) 1% CO, 9 h; (b) 5% CO, 5 h; (c) 30% CO, 4 h; (d)
50% CO, 2 h; (e) 75% CO, 2 h; and (f) 95% CO, 2 h.
§19 Fig. 4. SEM surface observation of the
graphite morphologies of iron samples after carburisation (a) for 21.5 h with
1% CO; (b) for 5 h with 5% CO; (c) for 4 h with 30% CO; and for 2 h with (d)
50% CO; (e) 75% CO and (f) 95% CO in CO-H2-H2O gas
mixtures (H2O fixed at 0.2%).
3.2 Surface micro-analysis
4 shows the morphologies of graphite on top of the sample surfaces after
carburisation with different compositions of the CO-H2-H2O
gas mixtures. Filamentous carbon is clearly seen in all cases. With low CO
contents, e.g. 1 - 5% CO, fine filaments are produced (Fig. 4a-4b). By
increasing the CO content to 30% the long and coarse filaments are formed (Fig.
4c). This tendency reaches its maximum when CO content is 75% (Fig. 4e). With
further increasing CO content to 95%, the filaments become fine and short again
the case of low CO contents, especially at 1% CO, very fine filament structures
are observed. Therefore, high resolution SEM was used for further analysis.
Fig. 5a shows a structure of filaments densely located on the top of a columnar
bulk graphite. Fig. 5b gives a high magnification of these filaments. The
diameter of these filaments is about 30 nm. In addition to this filamentous
carbon, another kind of graphite in the form of particle clusters was also
found as shown in Fig. 6. The corresponding back-scatter-electron image of the
graphite particle cluster in Fig. 6a shows small metallic particles embedded
inside of these graphite particles (Fig. 6b).
§23 Fig. 5. Morphologies of graphite on the
surface of the iron sample carburised at 600ï¿½C in the 1% CO-98.8% H2-0.2% H2O gas mixture
for 21.5 h: (a) low magnification showing very fine filaments and columnar bulk
graphite, and (b) high magnification of fine filaments as indicated by the
white square in (a).
§25 Fig. 6. Graphite with particle cluster
structures formed on the surface of the iron sample carburised at 600ï¿½C in the 1% CO-98.8% H2-0.2% H2O gas mixture
for 21.5 h: (a) secondary electron image; (b) corresponding back-scatter
structural evolution of carbon deposits on the surface of sample was
investigated under the condition of 75% CO-24.8% H2-0.2% H2O
for 10 min, 0.5 h and 2 h. After 10 min reaction, bulk dense graphite islands
are formed on the surface. Some areas of surface are still graphite free as
indicated as bright areas in the back-scatter electron image of Fig. 7b. Short
filaments are mainly accumulated along grinding scratches (Fig. 7a) and are
also bright in back-scatter electron image but with many white points inside
(Fig. 7b). After 0.5 h reaction, the whole surface was covered by bulk
graphite. Filaments are distributed non-uniformly on the top of bulk graphite
and preferably along grinding scratches (Fig. 8a).ï¿½ These graphite bulks seem to be separated from each other, as
shown in Fig. 8b. The filaments mainly grow outward from these gaps. By
increasing the reaction time up to 2 h, these filaments grow further forming
long filamentous carbon clusters (Fig. 8c and 8d).
§28 Fig. 7. Carbon deposits on the surface of
iron sample after 10 min carburisation in the 75% CO-24.8% H2-0.2% H2O
gas mixture: (a) a secondary electron image of bulk graphite and filaments
along scratches; and (b) the corresponding back-scatter electron image, where
bright areas represent filaments and also the surface without carbon coverage.
different reaction stages in this gas atmosphere (75%
CO-24.8% H2-0.2% H2O) are characterised by performing
optical microscopy of cross-sections,
as shown in Fig. 9. Very thin cementite layers are observed in all stages. The
thickness of these layers increases slightly with increasing reaction time. The
thickness of the graphite on the top of the cementite layer increases
remarkably with increasing reaction time. The cross-section of the sample after
2 h carburisation was also analysed by SEM, as presented in Fig. 10. The sample
was not polished after cutting. Figure 10 shows that the bulk graphite layer is
composed of many graphite columns. The filaments grow to the top of the surface
via the gaps between these columns. ï¿½ï¿½
§30 Fig. 8. Carbon deposits on the
surface of iron samples carburised in the 75% CO-24.8% H2-0.2% H2O
gas mixture for 0.5 h in (a) and (b) ((a) low magnification showing bulk
graphite and non-uniform filaments; (b) high magnification of this structure);
and 2 h in (c) and (d) ((c) secondary electron image and (d) corresponding
back-scatter electron image).
3.3 Coke phase analysis
coke of samples carburised in CO-H2-H2O gas mixtures with
different compositions was analysed by XRD. The results are shown in Fig. 11.
In the case of 1% CO, mainly α-Fe and some Fe3C
are detected, but α-Fe is a main
part of iron-containing phase in the coke. However, when the CO contents are
above 30%, cementite is the only detectable iron-containing phase in the coke.
§32 Fig. 9. Metallographic cross-sections of iron samples after
(a) 10 min, (b) 0.5 h and (c) 2 h carburisation in the 75% CO-24.8% H2-0.2%
H2O gas mixture at 600ï¿½C.
§33 Fig. 10. SEM cross-section observation of
the coke layer obtained at 600ï¿½C for 2 h
in the 75% CO-24.8% H2-0.2% H2O gas mixture showing the
columnar structure of bulk graphite layer.
4.1 Iron particles/layer formation between cementite and
phenomenon of iron particles/layer formation between cementite and graphite was
first reported by our past research of metal dusting on iron at 700ï¿½C [crossref(11)-crossref(12)].
At this temperature this phenomenon appears
when CO content is 5% (ac: 15.8) or below. It was proved that
α-Fe at the cementite/graphite
interface is a reaction product of cementite decomposition. With a low carbon
activity the iron from cementite decomposition accumulates forming iron
particles and even a layer between graphite and cementite. It is assumed that
the same phenomenon also occurs at other temperatures with a suitable low
carbon activity gas. The only expected difference is that the critical content
of CO for the formation of this phenomenon differs with temperature. The
present results show that this phenomenon occurs when CO content is as low as
1% (ac: 22.2). It implies that the occurrence of α-iron layer is determined by
the carbon activity of the gas mixture.
Fig. 11. XRD results of the coke obtained from iron samples carburised in
CO-H2-H2O (H2O fixed at 0.2%) gas mixtures for
21.5 h for 1% CO, 4 h for 30% CO and 2 h for 50-95% CO.
4.2 Graphite morphologies
§36 For a low carbon
activity gas, e.g. ac = 22.2, three kinds of graphite are
observed, very fine filaments with a diameter of about 30 nm, graphite particle
clusters with iron-containing particles embedded, and columnar bulk graphite.
The formation of fine filaments involves dissolution and diffusion of carbon
through the particle [crossref(17)]. Low carbon activity limits this kind of
diffusion especially for large particles. The so called graphite particle
clusters are very similar to the graphite structure called porous graphite
clusters obtained at 700ï¿½C with 5% CO
[crossref(12)]. Metallic particles are embedded inside of graphite particles,
which form a shell structure. These metallic particles are mostly α-Fe as indicated by XRD coke
analysis (Fig. 11). It is observed that these iron particles are mechanically
detached by inside growth of graphite from the decomposed iron directly
contacted with graphite.
§37 By increasing the
CO content to above 30% (ac: 470) only bulk columnar graphite
and filaments are observed. The morphology of bulk graphite is different from
that at 700ï¿½C. In that case
bulk graphite is rather dense and cracks are formed by underneath filament
growth with subsequent outward growth of filaments [crossref(12)]. However, at
600ï¿½C the graphite
columns are not closely compact. Carbon filaments can grow outward through the
gaps between these columns without breaking the bulk graphite layer.
formation of filaments was found to have a priority along the grinding
scratches. A similar observation was also reported by Schmid et al. [crossref(18)].
It was generally accepted that the deformed area benefits carbon diffusion and
graphite nucleation which in turn accelerates the formation of filaments and
4.3 Kinetics of iron carburisation and cementite
results in this work indicate that the rate of carburisation in the early stage
of reaction is increasing with CO content reaching its maximum value at 75% and
then decreasing with further increasing CO content. This observation is very
similar to the results found at 700ï¿½C [crossref(12)].
In both cases the kinetics of iron carburisation does not reach its maximum at
CO and H2 contents of about 50%. It once again suggests that the
rate of carburisation does not fully depend on the reaction of CO + H2
= C + H2O. The contribution of the reaction 2CO = C + CO2
should also be considered especially at high contents of CO [crossref(12)].
kinetics of cementite decomposition was investigated by Zhang and Ostrovski
[crossref(15)] in their study of iron carbide production. They found that
cementite produced from iron ore reduction and carburisation by CH4-H2
gas decomposes into iron and graphite in both carburisation gas atmosphere and
non-carburising atmosphere (Ar gas). They also found that cementite decomposes
with a fastest rate at 600ï¿½C. By comparing
the results in the present work with those at 700ï¿½C [crossref(12)], it is
found that the rate of cementite decomposition at 600ï¿½C is faster than that at 700ï¿½C. In the case of iron
particles/layer formation between cementite and graphite, after 21.5 h
reaction, cementite layer is disappeared completely at 600ï¿½C (Fig. 2c), while at 700ï¿½C after 20 h reaction, the
cementite is still existing but with a decreased amount [crossref(11)]. For
the cases without iron particles/iron layer formation, the cross-sections show
a very thin cementite layer together with very thick graphite on the top (Fig.
3b-3f). It implies that the decomposition of cementite is very fast at 600ï¿½C. The decomposed cementite
then catalyses the massive graphite formation. In fact the metal dusting of
iron was reported to be the worst at about 575ï¿½C by Chun et al. [crossref(6)].
carburisation and coke formation were investigated at 600ï¿½C in (1-95)% CO-(4.8-98.8)%
H2-0.2% H2O gas mixtures which cover a large area of
carbon activity from 22.2 to 559. In all cases cementite is formed underneath
the samples surface together with carbon deposits on its top. Cross-section
observations show that with 1% CO, α-Fe particles or
a α-Fe layer are formed at cementite/graphite interface. The cementite obtained under this condition
evanesces after 21.5 h reaction. The iron particles or iron layers are formed
as a reaction product of cementite decomposition. For CO contents larger than
5%, however, no α-Fe is detected
at the interface. The cross-sections in these cases show a very thin cementite
layer directly contacted with a thick graphite layer.
gas composition also affects the morphology of graphite on the surface. In the
case of 1% CO, three forms of graphite can be identified in the coke as very
fine filamentous carbon, bulk dense graphite with columnar structure and
graphite particle clusters with fine iron-containing particles embedded inside.
With increasing CO contents, the main forms of graphite are bulk dense graphite
and filaments. The examination of coke formation at different reaction stages
in a gas mixture with 75% CO reveals the coexistence of bulk graphite columns
and carbon filaments. Filaments are preferably located along the grinding
scratches and grow outward from the gaps between the graphite columns. Very
fine and short filaments are observed in the gases with very low CO content
(1-5%) and very high CO content (95%). For CO contents between 5 and 95% long
and coarse filaments are observed.
phase analysis by x-ray diffraction reveals that with 1% CO, both iron and
cementite are detected. However, only cementite is detected as an
iron-containing phase in other gas atmospheres with > 5% CO. Thermo-gravimetric
analysis shows that the rate of iron carburisation in the early stage of
reaction increases with increasing CO concentration reaching a maximum at 75%.
authors would like to thank Prof. Grabke for many valuable discussions. The
authors would also like to thank Mrs. H. Falkenberg, Mrs. M. Nellessen and Mrs.
Angenendt for preparing the metallographic cross-sections and the SEM analysis.
Support of this study by the Deutsche Forschungsgemeinschaft is greatly
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