- Review-CarburlzatiOn and Metal Dusting

ISIJ International, Vol. 41 (2001 ), Supplement, pp.

on lron

SI-S8

HansJUrgenGRABKE,Else Marie MOLLER-LORENZ,Andr6 SCHNEIDER

Max-Planck-Institut fOr Eisenforschung GmbH,Max-Planck-Str. 1,40237DOsseldorf, Germany

Fundamentalstudies have been conducted on the kinetics and mechanismsof reactions of iron in CH4-H2andCO-H2-H20mixtures, also in the presence of H2S, to understand earburization, iron oarbide formation, metaldusting and carbon deposition. The rate equations and mechanismsof the surface reaction in oarburization of iron

are presented, the carburization in CH4-H2is muchslower than in CO-H2. Both reactions are retarded byadsorbed sulfVr, the carburization rate becomesinversely proportional to the sulfur activity as ~ PH2S/pH2withincreasing as. At carbon activities ac ) I cementite growth oan be started in both gas mixtures on iron, however,the decomposition of this unstable oarbide gives rise to a corrosion process 'metal dusting', i.e. the materialdisintegrates to a dust of metal particles and graphitic carbon. This disintegration ean be prevented by adsorbedsulfur which hinders the nucleation of graphite. The stabilizing effect of sulfur on cementite and higher carbidessuch as Haggcarbide, allows fundamental studies about their thermodynamics, non-stoichiometry and diffusionalgrowih mechanisms,on the other hand the presenoe of sulfur will allow iron oarbide production in the reduction ofiron ores in carburizing gas mixtures.

KEYWORDS:oarburization; metal dusting; iron earbides: CH4-H2mixture, CO-H2~H20mixture, eatbon transfer,surface reaction kinetics, adsorbed sulfur, sulfUr inhibition, stabilization, graphite nucleation.

1. IntroductionThe system Fe-C is of great technical importance and

high complexity, considering the existence of a stable

system Fe-graphite and an unstable system Fe-cementite,and the many different microstructures arising fromcementite formation in the iron matrix.

In carburizing atnospheres carbon is transferred into

solid solution in a- or Y-iron, this process is calledcarburization, it leads to well-established equilibria ofatmosphere and dissolved carbon. But also a technical

process is called carburization, in which case hardeningsteels are treated at temperatures around 920 'C withatmospheres such as endogas CO-H2-H20or natural gasCH4, for carbon transfer into a surface region and hardeningby quenching. The kinetics of this carburization for casehardening generally is described by a transition fromsurface reaction to carbon difftlsion control.

Onthe other hand, in the field of corrosion, carburizationis a corrosion process affecting steels and high temperaturealloys mostly at temperatures > 900 ~C. Ingress of carbonleads to precipitation of stable carbides, M23C6and M7C3,(M= Cr, Fe, Ni) with Cr as the mainmetal component. Thisprecipitation causes embrittlement, a volume increase of the

carburized zone, cracking in the tube wall and also loss ofoxidation resistance. This kind of carburization can bewidely suppressed by protective oxide scales, however,under certain conditions it is controlled also by surfacereaction and carbon diffttsion kinetics. So in all these casesof carburization the surface reaction kinetics is of interest,

and will be discussed in this review, in addition the veryimportant retarding effect of sulfilr on the carbon transfer.

Thecases of carburization listed above occur generally at

carbon activities ac I ,i,e. under conditions where neither

graphite nor cementite can be formed (ac = l, in

equilibrium with graphite, for formation of Fe3Ca higher acis necessary, therefore Fe3C is unstable in relation tographite). If iron is carburized in atmospheres with ac > I,after oversaturation carbon (graphite) can be deposited or,

moreofien, cementite starts to grow in the metal phase andthe iron maybe converted to Fe3C. This would be a processof great economic importance, since iron carbide would be

a valuable product in direct reduction of iron ores, easily

handable without great danger of reoxidation and ignition

and well suitable for steel production in electric arcfumaces. However, as noted already, cementite is anunstable compoundand tends to decomposeinto iron andgraphitic carbon. The latter reaction, Fe3C- 3Fe + c, is

also a main step of a corrosion process, metal dusting,

which attacks iron and steels in strongly carburizingatmospheres, at ac > I .

In metal dusting of iron and steels

Fe3Coccurs as an intermediate, decomposesand a dust ofmetal particles and carbon (coke) results. Webelieve that

the decomposition of cementite is started by graphitenucleation at favorable sites. This nucleation is stronglysuppressed by the presence of sulfur adsorbed from the

aimosphere (H2S) and thus sulfur prevents metal dustingwidely and allows Fe3Cgrowth. Sucheffect certainly wouldbe of great interest also in the direct reduction of iron oresfor iron carbide production. These processes ofcarburization and metal dusting will be described in this

review, the kinetics of the surface reactions, of graphite andcementite formation and the role of sulfur in these

processes.

2. Carburization kinetics

Thekinetics of the carburization reactions

CH4= 2H2+ C(dissolved) (1)

CO+ H2= H20+ C(dissolved) (2)

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ISIJ International, Vol. 41 (2001 ), supplement

had been investigated in earlier studies [1-6] using the

resistance-relaxation technique. This technique measuresthe electrical resistance of a thin iron foil against a standard

as a function of time, after the composition of the flowing

gas atnosphere is abruptly changed. The resistance changeis proportional to the carbon concentration, for thin foils the

concentration rises nearly uniformly in the cross section, i.e.

not difiilsion but surface reaction kinetics are rate

determining.Fig. I shows the carburization in CH4-H2 and the

decarburization in H2 Of an Fe-foil (lO um) at 1000 'C.

Suchmeasurementshave been conducted at various partial

pressures of H2andCH4,and the rate equation wasobtained3/2

vl = kl ' pCH4/ (pH2)1/2_

kl'' ac ' (PH2) (3)

where v is the rate of C-transfer into solid solution in

moucm2sec,kl and kl' are rate constants, and ac is the

carbon activity (ac = I in equilibrium with graphite). Thebackwardreaction rate is clearly proportional to the carbonactivity and not to the carbon concentration [C], as could beshownfor Y-iron [2] whereac is a nonlinear function of [C]

.

Therate equation (3) indicates that in a reaction sequence

= CH4(ad)CH4CH4(ad) = CH3(ad)+ H(ad)CH3(ad) = CH2(ad)+ H(ad)CH2(ad) = CH(ad)+ H(ad)CH(ad) = C(ad) + H(ad)

= C(dissolved)C(ad)

4H(ad) = 2H2the dissociation of CH3(ad) is the rate determining step for

the forward reaction and its formation for the backwardreaction. The forward reaction rate is proportional to the

surface concentration of CH3(ad)and the backwardreaction

rate to the product of the surface concentrations CH2(ad)and H(ad), these suface concentrations are established bythe preceding equilibria:

l12rcH3 ~ PCHd(pH2)andrcH2 ~ ac ' PH2, rH -

(PH2)1/2

so that the above experimentally deterrnined rate equation(3) results. The rate equation is valid, with different rate

constants for oL- and Y-iron, between 500 - 1000 ~C. Theactivation energies are high, on a-iron 213kcaymol and onY-iron 230 kca~mol, and the reaction rates are relatively

low, at 920 "C the rate constant for the carburization is kl =3,8 •

l0~13 moVcm2secbaru2

In contrast, the carburization of iron in CO-H2mixtures is

muchfaster, the corresponding rate constant at 920 'C is k2

= 6,6 •l0~6 moUcm2sec bar [6]. The reaction also had been

studied using the resistance-relaxation technique, and aswell at 600 "C [4] and at 920 'C [6] the rate has been foundto be proportional to the partial pressure of CO. Thus the

complete rate law for the reversible reaction (2) must bewritten:

v2 = k2 ' pCO-k2' ' ac ' PH20/pH2 (4)

since at v2 = Othe massaction law must result. This rate

equation corresponds to a mechanism:

CO= CO(ad)CO(ad)= C(dissolved) + O(ad)O(ad) + H2= H20

@2001 ISIJ

in which the dissociation and formation of CO(ad) is rate

detennining for the forward and backward reactions,

respectively, and the equilibrium of oxygen adsorption is

virtually established. Theabove rate equation is valid at lowpH20andhigh temperatures.

,~,c:_

~.E

4R ~,::l

C~JCJru:Z~11OV,~,c:_

E

pclt,

JaR • C,o

ocAR8uRlz,Irrav

pc,t

o 0.5 CARBONwt'l-

S2

Fig, I Resistance-relaxation measurement of thecarburization and decarburization of a 0.01 mmiron foil at 1000"C: changeof the CH+partial pressure, measuredusing a thermalconductivity cell, and change of the electrical resistance of thefoil which is proportional to the carbon concentration [2]

In a lower temperature range and at higher pH20 the

following equation is valid:

v2 = k2 ' pCO• I (5)l+KOPH20/pH2

-k;

• aKo ' PH20/pH2

c l+Ko'PH20/pH2

which contains the expressions for the area free of adsorbed

oxygen(I-O) and covered with adsorbed oxygenO:

l-0= ll+KOPH20/pH2

e= Ko'PH20/pH2l +Ko ' PH20/pH2

(6)

(7)

Adsorption and dissociation of COcan nke place only on

ISIJ International, Vol. 41 (2001 ), Supplement

sites free of oxygen, and for the oxygen coverage aLangmuir isotherm and near-equilibrium are assumed.Fromisotope exchangestudies it is knownthat the oxygentransfer from H20to iron surfaces is avery fast reaction [7].

3. Effects of sulfur on the carburization of iron

The carburization-decarburization reactions (I) and (2)

also have been studied in the presence of someH2Sin theflowing gas aimospheres [8-12].

imtial rates decrease with increasing sulfur activity andeventually becomeinversely proportional to pH2S/pH2seefig. 2. Suchbehaviour can be described by an expression forthe imtial forward reaction rate

lv~ = ki •P. ' ki •P. '(1 -Os) (9)l+KsPH2SlpH2~

a)

l~oco 100~E 80a,~6

~40co~ an~ tv:!

~~ 10o 8~ 6c9

75 10~8

~

e

,

(P

1ooo 'c

o

11oo 'c

o

e

b)

l~o'o 200

E,~e: IOOc 80o'- 60~caN= 40se,Do- 20o~S~9* 10

. I0~7~5

'E

1O~7

ratio pH2!S/ pH21o~

,

,

CO+ H2- [C] •H2p\. ,~\

1oOo'c

Fe -foil

,25prn

1cr6

ratio pH~/ pH21o~

Fig. 2 Initial rates of carburization of iron foils, fromresistance-relaxation measurements, as a function of the ratio

H,S/H, [9- 11Ja) in flowing CH+-H,-H,Smixtures at 1000"C and

l 100 ~C.

b) in flowing H2-CO-H,Smixtures at 1000"C

Controlled addition of H2Swas conducted by passing aslow partial stream of H2 through a fhrnace with a Fe-FeSmixture, here the equilibrium content of H2Sis establishedaccording to the reaction FeS+ H2= Fe + H2S[13]. TheH2-H2Smixture wasadded to the main strearn of H2so that

small well defmedH2S/H2ratios were obtained far belowthe value for FeS formation. Sulftir adsorption on iron is

very strong, and the reaction

H2S= S(ad) + H2 (8)

leads to considerable coverage with adsorbed Salready at

low sulfur activities

as = Ks ' PH2S/pH2The adsorbed sulfur blocks the reaction sites for the

carburization reactions (I) and (2), for both reactions the

The dependenceon the sulftir activity shows that the

carbon transfer can take place only on the part of the

suface 1-es Which is free of sulftir, describing theadsorption of sulftir approximately by aLangnuir isotherm

e Ks ' PH2S/pH2(lO)s ~ I+Ks ' PH2S/pH2

The carburization rate is proportional to p. = pCH4(seefig. 3) or pCOrespectively.

In fact this partial pressure dependenceis changed for

reaction (1), see rate equation (3), indicating that in the

presence of sulfur the adsorption and dissociation of CH4is

rate detennining, not the dissociation of CH3(ad).Obviously each CH4-moleculewhich finds a sulfur-free site

will dissociate andnot react back.

ua'u'

\E,~1

accooNl:o

o

oL_

O1,:'

=

,1'PH2s=0iile/'T'I

H '/'/~TrJee:1;1_2' ~'~p2

ele

PH2s= 1,4

•10~] '

PH2 e.'-

e1/1

e

0.1

hydrogen pressure in atm1

Flg 3 Carburization of Fe-foils in H2-1%CH+at 800 'C, inabsenceof H2Sinitiai carburization rate -

p*'2 and in presence ofH2S- p, total pressure p= pCH4+ pH2

S3 C 2001 ISIJ

ISIJ International, Vol. 41 (2001 ), Supplement

As knownfrom surface analyiical studies (LEED, ISS)sulhr is adsorbed in foufold coordinated sites, similar ascarbon [8] see fig. 4, and these sites mayact as an entry for

carbon into the metal lattice which is effectively blocked bythe big adsorbed Satoms. As one can see from the kinetic

measurements, the carburization rates are stronglydecreased at already rather low sulfur activities,

corresponding to very low bulk concentrations of Sin iron.

This is well in agreement with AESstudies on the sulfur

segregation on iron, where already at very small bulkconcentrations - I wi ppmSalways complete saturation ofthe surface wasobserved, evenup to high temperatures.

a)

b)

Fig. 4 Atomic model for the adsorption of sulfur on aFe( 100) surfacea) top view of the c(2 x 2) structureb) cross section in the [1 lO] direction.

The size of the adsorbed sulfur was assessed from LEEDintensity-energy profiles and corresponds nearly to the size of S'~

ions [101

The retarding effect of sulfur adsorption from the

aimosphere can be used in practice, e,g. in petrochemical

processes such as steam-cracking of hydrocarbons for theproduction of ethylene. It is well-known that dosing ofsulfur bearing compoundssuppresses the carburization ofcracking tubes. These tubes are madeof high Cr-Ni-steelsby centrifugal casting and their carburization leads tointernal carbide formation, embrittling the material andcausing loss of oxidation resistance. The retarding effect ofsulftir is based on blocking the surface against carbontransfer, this effect was studied quantitatively on a 200/0Cr-320/0Ni-steel (Alloy 800) in a CH4-H2-H2Smixture at 900-

l 100 'C [12]. By increasing the sulftir activity from H2S/H2= I0~6 the carburization wasprogressively reduced see fig.

5. At IOOO'C a minimumof attack wasobserved H2S/H2=l0~4 at higher H2Scontents simultaneous sulfidation andcarburization of the Cr-steel occurs. The optimumH2S/H2ratio is lower at lower temperatures, at 900 'C about I0~5

and higher at IIOO'C. It maybe noted already, that thesedata fit nicely in a diagram on the suppression of metal

S4

dusting by sulfur, see fig. 7.

20

~E

o~o)ES 10

~cuo,cofba,

E

11oo'c

1000'C

900'C

olo' Io~* 10+ Io*ratio H2S/H2

@2001 ISIJ

Fig. 5 Mass gain of 20 %Cr-32%Ni-steel samples after

IOOhcarburization at 900, 1000 and I100 ~C, a* = I in CH*-H,-H,S mixtures, plotted versus the varied H,S/H,-ratios [12]

4. Iron Carbide Formation andMetal DustingMetal dusting is a corrosion phenomenon, a

disintegration of metals into a dust of fme metal particles

and graphite, which occurs in strongly carburizingatmospheresat ac > I [14-20].

Metals and alloys which dissolve carbon and do not formstable carbides are oversaturated in such aimospheres, andthe resulting strong tendency to graphite formation leads todestruction of the materials. In the metal dusting of iron andsteels, cementite Fe3C(and possibly other carbides) occuras intermediates, whereas nickel disintegrates by directinward growth of graphite [2 1J.

The reaction mechanismof the metal dusting of iron andsteels had been elucidated by optical and electronmicroscopy, the following steps occur, see fig. 6[14, 18].(i) carbon is transferred into solid solution and the metalphase becomes oversaturated, conceming graphiteformation (at ac ~I)and cementite formation (at ac > I),

(ii) cementite starts to grow, mainly at the surface and atgrain boundaries,(iii) since the cementite is a kind of barrier against fiuthercarbon ingress, at somefavorable spots graphite nucleates,accordingly there the carbon activity is decreased to ac = I,(iv) at ac = I cementite is no longer stable but starts todecompose. This leads to formation of fme iron particles

which act as catalysts for

(v) catalyiic carbon deposition, in manycases growih ofcarbon filaments from these fine iron particles.

Thereaction in step (iv), the decomposition of cementiteFe3C-3Fe + C (ll)in fact occurs by growth of graphite into the carbide, thecarbon atoms are attached to graphite planes which growmore or less vertically into the cementite [18]. The iron

atoms move outward through or between the growinggraphite flakes and agglomerate to particles with an average

ISIJ International, Vol. 41 (2001 ), Supplement

diameter of about 20 nm. These particles are carburized

from the atmosphere and oversaturated, graphite nucleates

and starts to grow, this causes the 'filamentous growth' ofcarbon typical for metal dusting.

a) ac

CO * H2

- H20 + C(diss.)

oc = 1

b)

_'//////'/____//.

(/////~/~

CO + H2

- H20 + C(inFe3C)

c)

CO * H2

- H20+ C(grophite)

Fe3C~i~ - C(diss) + Fe

d)

CO*H2

- H20* C(grophite)

~///~;~:~/'/'

3Fe '

I//,__/.

~/,/////.-

~~~~

C- Fe3C

/___/;;

--'///;

"/////

,,

Fig. 6 Schematics of the mechanismof metal dusting oniron and low alloy steels [17-201a) oversaturation of the metal with dissolved carbonb) growth of cementite at the surface and at grain

boundariesc) deposition of graphitic carbon on the cementite surfaced) decomposition of the cementite and catalytic carbon

deposition on the metal particles, arising from Fe,Cdecomposition

5. Effects of sulfur on iron carbide formation and metaldusting

From industrial practice it was known that addition ofsulfur compounds (H2S, CS2, (CH3)2S2 etc.) to the

atmosphere inhibits metal dusting and this effect was usedto protect plants.

However, no quantitative data were available until 1995

on the amountof sulfur necessary for protection. First data

were obtained in a study [19] whereFe and low alloy steels

were exposed to flowing H2-240/0C0-20/0H20-XppmH2Smixtures at temperatures between 475 - 700 ~C for somehundred hours. Ratios H2S/H2 Were detennined,corresponding to the sulfur activity. These values will be

S5

decisive also in practice since any organic sulftiT

compoundswill decomposeand in the presence of H2formation of H2Swill occur. Occurrence of metal dusting(coke formation) wasnoted or no metal dusting, this meanscontinued growth of cementite. The data were plotted in athermodynamicdiagram (see fig. 7) Iog(H2S/H2) versus l/T,

showing also the line for the equilibrium FefFeS which is

far above the data points, and a hatched area whichcorresponds to the transition from no sulftir to a completesulfur coverage on iron, starting at Os = 0,9 to Os ~: I .

Thishatched area was derived from previous work on sulfur

adsorption on iron [8-lO, 22, 23] and corresponds rather

well to the conditions where metal dusting is gradually

suppressed.

T('C)

10~21000 900 800 700 600 500

1o~sulfur adsorbed FeS

10C~l

I,~ 10~5.\~lo~o.

1a7

"" ~et" '"**'"9~~:~//~//////////~////////~/~~

::

~'

.

~;~;~////////~;

no sulfur 'dso*bed

~;~;~~~/////~///~.

lc metal dusting e

la8 Io 12 139 11

1/T • I04 (11K)

Fig. 7 Thermodynamicsof the sulfur effect on metal dusting[19], plot of log (pH2S/pH2) vs l/T, showing the regions ofsulfur-free iron surface where metal dusting occurs (filled

circles), the hatched area for the transition to monolayercoverage and the region for sulfur saturated iron surface wheremetal dusting is suppressed (open circles), and the upper regionof FeS-formation

It should be noted, that the data at 950 'C stem from astudy on carbon deposition in CO-H2-H2Smixtures on iron[24] in which the authors observed growth of filamentouscarbon at low H2Sadditions, strongly indicating occurrenceof metal dusting. In the range 900 - I100 'C the range ofapproach of sulfilr adsorption to a monolayer wellcorresponds to the range of H2S/H2 - ratios wheresuppression of the internal carburization of a high alloysteel wasobserved, see fig 5[12].

This diagram is of great interest for practice and showsthat at low temperatures very low H2S contents aresufficient for protection. In fact, for CCRunits of refineries

one can conclude that they are protected at about 600 ~CbyI ppmH2S[25, 26]. For heaters of direct reduction plants,

operating up to 900 'C 25 ppmH2Shave beenproven to besufficient for protection.

Suppression of metal dusting is of interest but on theother hand also the growih of iron carbide, achieved in H2-CO-H20-H2Sand also in CH4-H2-H2S,mixtures is ofrelevance since iron carbide would be a desirable product in

C 2001 ISIJ

ISIJ International, Vol. 41 (2001 ), Supplement

the reduction of iron ores. Recently the preparation of ironcarbides has been studied in such gas mixtures [25-29].

More fimdamental interest in solid state properties,

thermodynamicsand difftision has induced further work oniron carbides [30-33], which will be summarized in thefollowing.

Thermogravimetric studies have been conducted at 500'C in varied H2-CO-H20-H2Satrnospheres at different

carbon activities and sulfur activities. The mass gainobserved results from carbon transfer into solid solution,

carbide and coke growth. The latter indicates start of metaldusting causing catalyiic carbon deposition on the fine ironparticles from the Fe3C-decomposition. Coke growth is

proportional to the square of the time t2 [6] and can berecognized by an inflection point in the thermogravimetric

curve.Fig. 8a showsthermogravimetric curves for the reactions

in absence of H2S at different carbon activities, fig. 8bshows curves measured at ac = IOOOand different H2Sadditions. It maybe noted that at ac > 100 besides Fe3Canother carbide wasformed, the 'Hagg-carbide' Fe5C2.

a)

1,o

0.8 ac=4580 ac=IOOO ac=1OO

GrE006'~ .o)E_

0.4*,E

0.2

o.8.o 2.5 5.0 7.5 10.0 12.5 15,0 17.5 20.0

t [h]

b)

no H2S1.o

OI ppmH,S

~' 0.80.3 ppmH2S 0.5 ppmH2S 0.75 ppmH2S

Eo~~0.6

,,1ppmH2S

E 0.4

0.2

o.oO 50 1oo 150 200 250 300t [h]

Fig. 8 Massgain of iron samples during carburization in H,-CO-H,Omixtures at 500 "C [30]

a) at different carbon activities without H,S additionsb) at a* = 1000with different H,S additions

The studies show that occurrence of metal dusting notonly dependson the amountof H2Saddition, but also on thevalue of ac and time. At ac = IOO, metal dusting is

suppressed for about 100 hby O.03 ppmH2S, however, at

ac = 1000morethan O.5 ppmH2Sare necessary for shifiing

the start of metal dusting to > IOOhours and even with lppmH2Smetal dusting starts after > 200 hours. During theperiod unto the inflection point, the carbides Fe3C andFe5C2are growing as can be seen from metallographic crosssections. With the onset of metal dusting graphitic layers aregrowing on and into these carbides.

It was shownby transmission electron microscopy that

both carbides disintegrate by inward growth of graphite[33]• Undercertain conditions the growth ofthe carbides is

controlled by solid state difrbsion of carbon in the carbidelattice, diffasivities for this process could be derived fromparabolic plots Am2/A2versus t, measuredat 500 'C andvarious carbon activities ac = 12 - 4600 and H2Sadditions[32]. Self diffusion coefficients D*c for carbon in bothcarbides have been derived, they are about 6 orders ofmagnitude lower than the carbon difftlsivity in oe-iron.

As reported in the chapter on the carburization kineticsthe carbon transfer from CH4is muchslower than from CO-H2 nuxtures, accordingly the growth of cementite in CH4-H2-H2Smixtures attains comparable rates to the growth in

CO-H2-H20-H2Smixtures only at a much highertemperature - 700 'C, see fig. 9. The addition of H2Sstrongly retards the cementite growth, the kinetics is neverparabolic and difiilsion-controlled but always linear andsurface reaction-controlled.

1.50no H2S 0.7 ppmH2S

1.25 OI ppmH2S 1ppmH2

Eo 1.00~)

o.s ppm

E_0.75H2S

~E0.50 5ppmHS

0.25 1SppmH2S

0.00050 1oo 150 200 250 300 350

t [h]

C 2001 ISIJ S6

Fig. 9Massgain of iron samples during carburization in CH4-H2-H2Smixtures at 700 'C and ac = 100

6. Discussion of the role of sulfur

As noted before, sulfur is chemisorbed very strongly onmetal surfaces, especially on iron. The enthalpy for theadsorption by dissociation of H2S, equ. (8), is -98 kl/mol,

so that a monolayer coverage is approached at already lowsulfur activities, i.e. H2S/H2ratios. The rates of the surfacereactions vj in carburization are proportional to the surface

area free of sulfur (I-Os), thus from the measurementsof viin dependenceon as the adsorption isotherm of sulfur oniron had been derived for sometemperatures: 850"C, 1000~C, I100 'C [8-1 l]. The kinetic results showthat one sulfur

atomblocks one reaction site for the dissociation of CH4orCO. Even though the surface coverage of Son Fe reachesonly one S-/two Fe- atoms on Fe(lOO), obviously the big Satoms, which in fact are negatively charged [8] andapproximate S2~ions in size, also block the surrounding

ISIJ International, Vol. 41 (2001 ), Supplement

sites. However, an adsorption equilibrium is dynamic andalso at high as there will be always somevacancies in the

sulfur monolayer allowing slow carburization. So sulfur canstrongly retard but not fully suppress carburization. Theretardation is a wantedeffect in the petrochemical processesmentioned, e.g, cracking of hydrocarbons, but certainly anunwanted effect in carburization for case hardening ofsteels. In the latter process sulfur is a problem only rarely,

more problems arise from other surface active impurities,

such as Sb, Sn, P[34].

In the metal dusting mechanism, see fig. 6, also the

surface reaction in the flfst step (i) will be retarded bysulfur, the oversaturation is reached later and metal dusting

maystart after a longer incubation time. However, moreimportant, the step (iii) is inhibited by adsorbed sulfur, i.e.

the nucleation of graphite which starts cementitedecomposition. Thenucleation of graphitic carbon needs anensemble of several sites and the occurrence of such anensemble can be disturbed already at a coverage below amonolayer. Experience from studies on nickel catalysts for

steam-reforming of methanesuggests that abovea coverageOs= O.7 - O.8 no deposition of graphitic carbon waspossible

[35]. Studies on the suface segregation of carbon and sulfur

on iron [36] have shownthat no graphite is forrned on asurf;ace covered with sulftir, but only on a surface free ofsulfur, if the suface is blocked by sulfur, cementite growsbelow the sulftir layer, see fig. 10.

a) Contaminated surface - b) Clean surface -

graphite tormation

g'~h ite

eeeeoeeeeeeeeeoeeeeee

dusting, one mayconclude, that sulfUr is adsorbed also oncementite, and that the adsorption behavior of cementite is

not too different from that of iron so that the diagram fig. 7is applicable for metal dusting. The sulfur adsorbed oncementite (and iron) suppresses the nucleation of graphite

and therefore interrupts the metal dusting mechanismandallows production of iron carbide.

1)

2)

3)

4)

5)

6)

7)

8)

9)

iron carbide tormation

eeooe~'*+,, oteoo oo

eLe_~t'-_'__ooo

OJJOOe sutphUr O'iron

ooeooooooooeeoeeocooo.r)Qooo

e carbon

Fig. 10 Schematic model 136] on the carbon segregation to aa) sulfur contaminated iron surface, yielding cementite

formationb) clean iron surface, yielding graphite formation

The studies cited before [35, 36] investigated the effects

of sulfur adsorbed on nickel or iron on graphite or cementiteformation. Also in fig. 7, the hatched area represents the

approach of sulfur coverage on iron to a monolayer. In fact

the sulfur adsorption on cementite is decisive for the

nucleation of graphite initiating cementite decomposition,i,e, metal dusting. The information about sulfur adsorption

on cementite, however, is poor. It has been shown that

sulfur can be adsorbed [37] or segregated to cementitesurfaces [38], but more quantitative data are missing.

Anywayfrom the evidence of the sulfur effect on metal

lO)

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22)

23)

24)

25)

26)

27)

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C 2001 ISIJ S8