UDK 669.1:669.784:620.179.11 Pregledni znanstveni članek ISSN 1580-2949 MTAEC 9, 36(6)297(2002) H. J. GRABKE: CARBURIZATION, CARBIDE FORMATION, METAL DUSTING, COKING CARBURIZATION, CARBIDE FORMATION, METAL DUSTING, COKING NAOGLJIČENJE, TVORBA KARBIDOV, KOVINSKO UPRAŠENJE, KOKSANJE Hans Jürgen Grabke Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Str. 1, D-40237 Düsseldorf, Germany grabkeŽmpie.de Prejem rokopisa - received: 2002-11-28; sprejem za objavo - accepted for publication: 2002-12-20 The kinetics of carbon transfer in the carburization of iron and steels is described, considering the surface reactions, their rate equations and their retardation by surface active, adsorbed or segregated elements. Furthermore, the mechanisms and morphologies of corrosion processes are presented which can be caused by carbon; the internal carbide formation in high alloy steels at aC < 1 and "metal dusting", a disintegration of metals and alloys to a dust of graphite and metal particles due to carburizationat aC > 1. Fine metal particles cause carbon deposition, thus metal dusting induces the annoying phenomenon "coking". Keywords: carburization, carbon transfer, steels, surface reactions, high temperature corrosion, internal carbide formation, metal dusting, sulfur effect. Opisana je kinetika prenosa ogljika pri naogljičenju železa in jekel, ob upoštevanju reakcij na površini, njihove enačbe kot tudi zakasnitev, ki jo povzročajo površinsko aktivni adsorbirani oziroma segregirani elementi. V nadaljevanju je prikazan mehanizem in morfologija korozijskih procesov, ki jih lahko povzroči ogljik, notranja tvorba karbidov pri visokolegiranih jeklih pri aC < 1 in"kovinsko uprašenje", degradacija kovininzlitinv prah ingrafit ter kovinske delce zaradi naogljičenja pri aC > 1. Drobni kovinski delci povzročajo nanos ogljika, tako kovinsko prašenje inducira neželen pojav "koksanja". Ključne besede: naogljičenje, prenos ogljika, jekla, reakcije na površini, visokotemperaturna korozija, tvorba karbidov, kovinsko uprašenje, učinek žvepla 1 INTRODUCTION The system Fe-C is of high technical importance and also of great scientific interest, not only because of the existence of the stable system Fe-graphite and the unstable system Fe-cementite, and in addition all the various microstructures which canbe attained by different C-contents and heat treatments. Also the reactions and processes inthe carburizationof ironand steels in gases are very complex and most interesting as will be demonstrated in this paper. Furthermore there are different corrosion processes caused by carburization, i.e. internal carbide formation in high alloy steels in carbonaceous environments and ’metal dusting’, a disintegration of metallic materials into a dust of graphite and metal particles in strongly carburizing atmospheres. In carburizing atmospheres carbon is transferred into solid solutionina- or ?-iron, this process is named carburizationand leads at aC < 1 (aC = 1, inequilibrium with graphite) to very well-known equilibria 1. But also a technical process is called ’gas carburization’, in this process low alloy steels are heat treated incarburizing atmospheres for carbon transfer into a surface layer, followed by controlled quenching, i.e. the ’case hardening’ of tooth wheels, shafts etc. The kinetics of this carburization is controlled by combined surface reactionand diffusioninthe work piece 1,2. The corrosionprocess ’carburization’ occurs by ingress of carbon into high alloy steels and subsequent internal carbide formation, which embrittles the steels and causes crack formation and loss of oxidation resistance 3,4. Inthe high alloy steels, e.g. Alloy 800 (rolled 20Cr-32Ni-steel) or HK40 (cast 25Cr-20Ni-steel) the chromium is precipitated inthe carbides M23C6 and M7C3 (M = Cr, Fe, Ni). This materials degradationis a problem especially for ’cracking tubes’ in the ethylene productionby pyrolysis of hydrocarbons at 900-1150 °C and aC < 1 but also for heating tubes and other metal components in industrial furnaces for case hardening of steels. The corrosion phenomenon ’metal dusting’ also occurs in these industrial furnaces, but mainly in the chemical and petrochemical industry. Especially the H2-CO-H2O-CO2 atmospheres, produced by methane conversion for synthesis of methanol, ammonia etc. and for the direct reductionof ironores, caused many failure cases. Metal dusting follows after carbon transfer into metals and alloys, and oversaturation at aC > 1 an d is due to the tendency for graphite formation. The mechanisms and kinetics have beenstudied indepth inthe recent years 5-10 and will be presented shortly in this paper. MATERIALI IN TEHNOLOGIJE 36 (2002) 6 297 H. J. GRABKE: CARBURIZATION, CARBIDE FORMATION, METAL DUSTING, COKING 2 GAS CARBURIZATION 2.1 Kinetics of the Carburizing Reactions Carbon reactions: is transferred into iron by the following CO + H2 = H2O + C (dissolved) 2 CO = CO2 + C (dissolved) CH4 = 2 H2 + C (dissolved) (1) (2) (3) The thermodynamics of these reactions is well known 1, their kinetics was investigated in depth since 1965 11-15, using the resistance-relaxation method or gravimetric measurements of the carburization and decarburizationonthinironfoils inflowing gas mixtures (Figure 1). Onthe thinfoils the surface reactions were rate controlling and the diffusion equilibrium was virtually established. Here only the main kinetic equations and dependencies will be presented and explained shortly. Reaction (1) is the fastest and most important carburization reaction, and takes place by the reactionsteps: CO = CO(ad) = O(ad) + C(dissolved) O(ad) + H2 = H2O (1a) (1b) where (ad) means adsorbed. Step (1a) is rate controlling and (1b) is virtually in equilibrium, since its forward and backward reactions are very fast, more rapid than those of step (1a). This follows from the rate equation which was determined experimentally 13-15: k1 pCO- 1 1 + KO pH2O/pH2 k'1-aC KO pH2O/pH2 1 + KO pH2O/pH2 (4) where v1 Šmol/cm2 sec] is the rate of C-transfer, k1 and k1’ are rate constants, pi the partial pressures and KO the equilibrium constant of the adsorption equilibrium H2O = H2 + O(ad). The forward reactionrate is proportional to pCO and the part of surface (1 - 9O) which is free of O(ad). The backward reactionis proportional to aC and GO, i.e. the degree of coverage with O(ad) which is described by a Langmuir-isotherm: eO = KO pH2O/pH2 1 + KO pH2O/pH2 Equation(4) canbe written: = k1 pCO- 1 1 + KO pH2O/pH2 -(1-a/aeq) (5) (6) where aCe q is the carbonactivity inequilibrium with the givengas atmosphere. Inthe case KOˇpH2O/pH2 >>1 a dependence results on the partial pressures, which is well known from practice: pCOpH2 pH2 O (7) The reaction(2) takes place inthe two steps: CO = CO(ad) = O(ad) + C(dissolved) (2a = 1a) O(ad) + CO = CO2 (2b) Figure 1: CarburizationinCH4-H2 and decarburizationinH2 of anironfoil (10 ľm) at 1000 °C, resistance-relaxationmethod 1112: a) recorder plot showing change of CH4 partial pressure and electrical resistance of the foil AR č ŠC] with time, b) evaluationof (a), plot of carburizationrate v3 decarburization v3, and forward reaction rate v = v3 + v Slika 1: Naogljičenje železne folije (10 pm) v CH4-H2 inrazogljičenje v H2 pri 1000 °C, metoda upornosti - relaksacije 1112: a) časovni zapis spremembe parcialnega tlaka CH4 in električna upornost folije A R č ŠC], b) ocena (a), odvisnost od hitrosti naogljičenja v3, razogljičenje v3 in napredovanje hitrosti reakcije v = v3 + v v v b a 298 MATERIALI IN TEHNOLOGIJE 36 (2002) 6 H. J. GRABKE: CARBURIZATION, CARBIDE FORMATION, METAL DUSTING, COKING Figure 2: Schematics of the partial reactions ingas carburizationof ironina CO-CO2-H2-H2O-CH4-H2S atmosphere, the thickness of the arrows characterizes the reactionvelocities, adsorbed or segregated atoms occupy and block reactions sites on the surface Slika 2: Shema hitrosti parcialnih reakcij. Adsorbirani ali segregirani atomi zasedejo in blokirajo reakcijska mesta pri plinskem naogljičenju železa v atmosferi CO-CO2-H2-H2O-CH4-H2S, debelina puščic je merilo na površini. Thus the first step, the CO-adsorptionand dissociation is the same for reactions (1) and (2). The second step, the removal of O(ad) by reactionwith CO, however, is much slower thanits removal by reaction with H2, so that the overall reaction(2) is slower than reaction(1). Additionof H2 clearly accelerates the carbontransfer from CO to iron 1315. The carbontransfer from CH4, reaction(3) is even very much slower than both reactions (1) and (2), it takes place by stepwise dehydrogenation of the relatively stable molecule CH4 11,12. So the carbontransfer from CH4 plays only a negligible role in the carburization of ironina CO-H2O-CO2-CH4 mixture. The rate equation of reaction(3) determined inthe early studies 1112 v3 = k3ˇpCH4/(pH2)1/2 - k'3ˇaCˇ(pH2)3/2 (8) indicates that after adsorption of CH4 one H-atom is lost, and the decomposition of adsorbed CH3 becomes rate determining for carburization, for the back reaction CH3 formation is rate controlling. That in fact, the backward reactionrate is proportional to aC and not to the solute carbon concentration is demonstrated in Figure 1, the back reaction rate increases stronger than proportional to the solute concentration, because of the well-knowndeviationfrom ideality inthe system y-Fe-C at high carbon concentrations. The rate constants for the forward reactions were determined onironat 920 °C 13: k1 =7.6ˇ10-4mol/cm2sbar k2= 1.5ˇ10-4mol/cm2sbar k3 = 1.9ˇ10-6mol/cm2sbar With this knowledge on the kinetics the atomistic model for carbontransfer results, which is shownin Figure 2. Methanconversionby H2O or CO2 needs decompositionof the CH4-molecule 16, either onhot furnace walls as inthe case of gas carburization, or by catalytic action e.g. of nickel catalysts. It may be noted that MATERIALI IN TEHNOLOGIJE 36 (2002) 6 CH4-decompositionis extremely slow only oniron. Studies onFe-Ni, Fe-Cr and Fe-Mnfoils have shown 17 that the alloying elements cause a drastic increase of the carburizationrate. From pure ironto pure nickel k3 is increasing by two orders of magnitude, therefore Ni is a good catalyst for methane conversion at high temperatures and methane formation at low temperatures. Case hardening of steels is also conducted by carburization, introducing organic compounds such as methanol, aceton, propanol, acetaldehyde and ethyl acetate 1819. These compounds are decomposed at the carburizationtemperature after short residence time by homogeneous gas reactions, delivering the simple gases CO, H2, CO2, CH4 and H2O so that the carburizationwill take place, as described mainly by reaction (1). For example methanol decomposes at high temperatures rapidly according to CH3OH -Č CO + 2H2, and carburizationexperiments showed equal rates, for a certainCH3OH partial pressure in the experiment, and for CO and H2 pressures as resulting from the decompositionof that CH3OH content1819. 2.2 Retardation of carburization by adsorbed or segregated elements The reactions (1) and (3) also have been studied in the presence of some H2S inthe flowing gas mixtures. Sulfur is adsorbed onironvery strongly according to H2S=H2 + S(ad) (9) Already at relatively low sulfur activities as č pH2S/pH2 the coverage with S(ad) approaches a monolayer, e.g. at 850 °C at pH2S/pH2 - 106 20,21. The adsorbed sulfur atoms are blocking the reaction sites for carbon transfer, similarly as the adsorbed oxygen. Thus with increasing pH2S/pH2 a decrease of carburization rate is observed, for the forward reaction(1) was found: vČk1pCO-------------------------------------------- (10) 1 + KSpH2S/pH2+KOpH2O/pH2 The carbon transfer is only possible on sites which are free of S(ad) and O(ad), see, Figure 2. The dependence of v1 onsulfur activity was measured inflowing CO-H2-H2S mixtures at 1000 °C, see Figure 3. Already at pH2S/pH2 = 105 the rate is clearly diminished. At higher sulfur activities v1 becomes inversely proportional to the sulfur activity, which means the sulfur coverage approaches a monolayer. However the adsorptionequilibrium (9) leaves always some vacancies for carbon transfer, up to the sulfur activity, where FeS canbe formed (pH2S/pH2 = 5.5ˇ103 at 1000 °C). Not only by adsorptionbut also by segregationfrom the metal phase a blocking of reaction sites for carbon transfer is possible. This was shown especially for case hardening steels containing antimony 22,23, already relatively small concentrations of about 100 ppm Sb cause a drastic decrease of the carbontransfer 299 H. J. GRABKE: CARBURIZATION, CARBIDE FORMATION, METAL DUSTING, COKING Figure 3: Initial rates of carburizationof anironfoil, from resistance-relaxation measurements, conducted in flowing H2-CO-H2S mixtures at 1000 °C, as a function of the ratio H2S/H2 (sulfur activity) Slika 3: Začetna hitrost naogljičenja železne folije iz meritev upornosti - relaksacije v toku zmesi H2-CO-H2S pri 1000 °C kot funkcija razmerja H2S/H2 (aktivnost žvepla) coefficient. The surface segregation of Sb was demonstrated by surface analytical studies (AES) on Sb-containing steels after heat treatment at 930 °C. Cu, Snand As are much less effective. 3 HIGH TEMPERATURE CORROSION BY CARBURIZATION 3.1 Internal carbide formation Carburization of high alloy steels, leading to internal carbide formation, see Figure 4a, is a problem inthe ’cracking’ of hydrocarbons for ethylene production 3,24-27. At 900 - 1100 °C hydrocarbons and water vapor are passed through the cracking tubes, typical materials for such tubes are steels based onFe-20Cr-32Ni (Alloy 800) and the cast steels Fe-25Cr-20Ni (HK40) and Fe-25Cr-35Ni (HP40). Inthe pyrolysis process, carbon is deposited onthe tube walls and this ’coke’ must be removed repeatedly by decoking with water vapor and air. The tube materials should form protective Cr2O3-scales which hinder the ingress of carbon into the steels. Carbonis virtually insoluble inchromia and can be transferred into the steel only by diffusion of molecules through pores and cracks of the scale 28,29. So cracking tubes can be operated for 5-10 years if critical conditions are avoided. Overheating is critical since Cr2O3 is converted to carbide at >1050 °C and at aC = 1 24-27, as givenwhenthe inner wall is covered with coke and heated too high, e.g. upon decoking. There are also tendencies to operate the cracking units at temperatures >1050 °C and it must be emphasized that chromia forming steels are not capable for such operation, - not only the conversion of Cr2O3 to carbides at the process gas side but also evaporationof CrO3 and CrO2(OH)2 at the fireside will rapidly destroy the tubes 30. Present alloy development for process operation at >1100 °C aims at Ni-base alloys forming alumina or silica-scales. 300 Figure 4: Internal carbide formation in high alloy materials by carburizationat aC < 1, metallographic cross sections: a) of a thoroughly carburized Fe-32Ni-20Cr sample (Alloy 800), after carburizationinCH4-H2 at 1000 °C, crack formationby internal stresses 25,26, b) of the protectiontube of anoxygenprobe, made of Ni-16Cr-8Fe (Alloy 600), after service ina carburizationfurnace, internal carbide formation followed by internal oxidation 32. Slika 4: Nastanek notranjih karbidov v visoko legiranem materialu pri naogljičenju z ac < 1, metalografski prerez: a) popolnoma naogljičen vzorec Fe-32Ni-20Cr (zlitina 800), po naogljičenju v CH4-H2 pri 1000 °C. Nastanek razpok zaradi notranjih napetosti 25,26, b) varovalna cev kisikove sonde iz Ni-16Cr-8Fe (zlitina 600) po uporabi v naogljičevalni peči; notranji nastanek karbidov in notranje oksidacije za njim 32. Innormal operationthe oxide scale will have defects, by cracking and spalling due to creep of the tubes and due to the thermal cycling, connected with the decoking procedure. To avoid ingress of carbon at these defects, additionof sulfur to the process gases is favourable. Organic sulfur compounds, e.g. dimethyl-disulfide decompose under formation of H2S, and on free metallic spots the adsorptionequilibrium is established. Studies onthe carburizationof Alloy 800 inCH4-H2-H2S 31 have established the optimum ratios H2S/H2 for protection, for 1000 °C the value is H2S/H2 ˜ 10-4, for higher temperatures higher values are necessary, for lower temperatures lower values, see Figure 5. Internal carbide formation also means loss of oxidation resistance since the Cr is tied up in the carbides M7C3 and M23C6. Internal oxidation may follow if the material is exposed to oxidizing conditions. The internal carbides are oxidized to Cr2O3 and the material MATERIALI IN TEHNOLOGIJE 36 (2002) 6 H. J. GRABKE: CARBURIZATION, CARBIDE FORMATION, METAL DUSTING, COKING Figure 5: Effect of sulfur on internal carbide formation, mass gain of 20 % Cr - 32 % Ni steel samples after 100 h carburizationat 900, 1000 and 1100 °C in CH4-H2-H2S at aC = 1, plotted vs. the varied H2S/H2-ratios 31 Slika 5: Vpliv žvepla na notranji nastanek karbidov, pridobitev na masi vzorcev jekla 20 % Cr - 32 % Ni po 100 urah naogljičenja pri (900, 1000 in1100) °C v CH4-H2-H2S zmesi inaC = 1 v odvisnosti od različnega H2S/H2 razmerja 31 disintegrates. This phenomenon was called ’green rot’, since often nicely green Cr2O3 is formed. Cases of internal carbide formation and green rot have occurred also in furnaces for heat treatment, i.e. in components made of high alloy steels: heating tubes, furnace walls, ventilation wings, protection tubes for thermoelements and oxygen probes, see Figure 4b. 3.2 Metal Dusting Metal dusting is a disintegration of metals and alloys to a dust of metal particles and dust, occurring in strongly carburizing atmospheres at aC>1. The final reason for metal dusting is the tendency for graphite formation ?G = -RT lnaC. Graphite grows after over-saturationof the metal phase with dissolved carbonand destroys the material. The mechanism may involve different steps depending on the material, as described below. Metal dusting is observed in the colder parts of industrial furnaces for the case hardening of steels. At the carburizationtemperatures > 900 °C the carbon activity of the CO-H2-H2O-CO2 mixture is aC < 1 but for reactions (1) and (2) aC increases with decreasing temperature and in colder parts, near the furnace wall or inholes of the wall for thermoelements or oxygenprobes the critical condition aC>1 is given, accordingly metal dusting attack was observed, see Figure 6. Problems caused by ’syngas’ or reduction gas from the conversion of natural gas are widespread, metal dusting attack has been observed in plants for methanol, ammonia, and hydrocarbon production, and in plants for direct reductionof ironores. But also inhydrocarbons metal dusting is possible and recently problems in refineries have beenreported 33. MATERIALI IN TEHNOLOGIJE 36 (2002) 6 Figure 6 Failure cases by metal dusting in an industrial furnace for gas carburization of case hardening steels, attack of heating tubes in less hot regions of the furnace, where aC > 1: a) austenitic 15Cr-35Ni-steel, b) Ni-16Cr-8Fe (Alloy 600) 7 Slika 6: Poškodbe zaradi uprašenja kovine v industrijski peči za plinsko naogljičenje jekel za cementacijo, uprašenje ogrevnih cevi v manj vročih delih peči, kjer je aC < 1 a) avstenitno jeklo 15Cr-35Ni, b) Ni-16Cr-8Fe (zlitina 600) 7 Onironand low alloy steels the attack may start with wide pits but becomes more or less general, so that uniform wastage is observed. The mechanism, see Figure 7, involves intermediate formation of the unstable cementite 5-9, the following steps are taking place: (i) carbon transfer and oversaturation of the metal matrix, (ii) formationof cementite M3C (M=Fe,Ni) at the surface and at grain boundaries. The cementite layer is a diffusionbarrier for further carboningress, causing increase of aC, an d (iii) nucleationof graphite onthe surface, causing decrease of aC ? 1. The cementite becomes unstable and starts to decompose (iv) by inward growth of graphite. In the decomposition M3C ? 3M + C (11) the C-atoms attach to graphite planes, which grow more or less vertical into the cementite. The metal atoms migrate through the graphite and agglomerate under formationof fine metal particles (č 20 mm), which (v) catalyze carbondepositionfrom the gas phase, oftenunder growth of carbonfilaments from the metal particles. 301 H. J. GRABKE: CARBURIZATION, CARBIDE FORMATION, METAL DUSTING, COKING Figure 7: Schematics of the mechanism of metal dusting on iron and low alloy steels 5-9: a) oversaturationof the metal phase with dissolved carbon, b) growth of cementite at the surface, c) deposition of graphitic carbononthe cementite, aC ? 1, d) decompositionof the cementite, and carbondepositionby catalytic actionof the metal particles, arising from cementite decomposition Slika 7: Shema mehanizma uprašenja železa in malo legiranih jekel 5-9: a) prenasičenje kovinske faze z raztopljenim ogljikom, b) rast cemen-tita na površini, c) nanos grafita na cementitu, aC ? 1, d) razpad cementita in nanos ogljika s katalitsko reakcijo zrn kovine, ki so nastale z razpadom cementita The steady state of this mechanism is demonstrated in Figure 8. Onhigh alloy steels and Ni-base alloys with sufficient Cr-content generally an oxide layer is formed and metal dusting starts locally, at defects where the layer has a crack or pore or has spalled. Inthese materials an internal carbide formation takes place at first, the chromium carbides M23C6 and M7C3 precipitate and the carbides of other stable carbide formers such as Ti, Nb, W, Mo etc. This process causes some delay of the oversaturation and the start of metal dusting (which is however not significant, considering the requested life times). A fringe with precipitates is formed around the point of attack. In the temperature range about 600 °C 302 Figure 8: Metallographic cross sectionof anironsample after 4 hours metal dusting in 30 % CO - 70 % H2 - 0,2 % H2O at 600 °C, optical micrograph demonstrating the steady state of inward cementite growth and Fe3C decomposition by growth of graphite into the cementite, outward coke formation Slika 8: Metalografski prerez vzorca železa po 4 urah kovinskega uprašenja v 30 % CO - 70 % H2 - 0,2 % H2O pri 600 °C. Optični posnetek dokazuje stabilno rast cementita v notranjost in razpad Fe3C z rastjo grafita v cementit, navzven nastanek koksa. Figure 9: Metallographic cross sections of alloys attacked by metal dusting: a) Fe-20Cr-32Ni (Alloy 800) with carburized zone and "coke" inthe pits, b) Ni-16Cr-8Fe (Alloy 600), same protectiontube as in Figure 2b, but ina regionof lower temperature aC >1 Slika 9: Metalografski obrus zlitin napadenih s prašenjem kovine: a) Fe-20Cr-32Ni (zlitina 800) z naogljičeno zono in koksom v zajedah, b) Ni-16Cr-8Fe (zlitina 600), ista varovalna cev kot na sliki 2b, vendar področje z nižjo temperaturo ac > 1 MATERIALI IN TEHNOLOGIJE 36 (2002) 6 H. J. GRABKE: CARBURIZATION, CARBIDE FORMATION, METAL DUSTING, COKING Figure 10: Arrhenius-plot of wastage rates by metal dusting of several steels and Ni-base alloy 600 versus reciprocal temperature (K-1), the data were determined in H2 - 24 % CO - 2 % H2O whenthe attack on the surface was uniform 9 Slika 10: Arrheniusov diagram odvisnosti hitrosti prašenja več jekel in nikljeve zlitine 600 od recipročne temperature (K-1), podatki iz H2 -24 % CO - 2 % H2O v primeru enakomernega prašenja površine 9 which is most critical concerning appearance of metal dusting, these precipitates are very fine and the etched metallographic cross section shows only a dark zone, see Figure 9. The local attack spreads and leads to formation of pits, which are more or less hemispherical. In laboratory experiments worms of coke can be seen, growing from each pit. In industrial units the coke is mostly carried away by the fast flowing process gases, and can be found deposited in bends or dead ends of the system. It should be noted that the metal dusting mechanism for Ni and Ni-base alloys is different from that for iron and steels. No unstable carbide is formed as intermediate but the graphite directly grows into the oversaturated metal and destroys the materials 34,35. The difference in the mechanism for steels and Ni-base Alloy 600 canbe seenalso inthe Arrhenius-diagram Figure 10, which collects rates of metal consumptionmeasured onsamples which were corroding on their whole surface. The Arrhenius-line for the steels yields anactivationenergy of about 167 kJ/mol for the temperature range up to about 540 °C, and that value most probably canbe attributed to reaction (11), i.e. the cementite decomposition. At higher temperatures a lower activation energy was found, about 55 kJ/mol which probably is valid for the carbontransfer reaction, here reaction(1). This assumptionwould be MATERIALI IN TEHNOLOGIJE 36 (2002) 6 well in agreement with the partial pressure dependence of the metal consumption, which is v č pCOˇpH2 10 , corresponding to the well-known dependence of the carbontransfer coefficient as already giveninequation (7). The carbon deposition, according to step (v) of the above reaction mechanism was found to be proportional to pCO and to increase quadratically with time 6, which follows if it is assumed that the depositionis proportional to the amount of metal particles formed in step (iv). Carbondepositionis generally higher for the austenitic materials compared to the ferritic steels, and its activation energy contains the activation energy for the formationof the catalytic particles reaction(11). The complex processes inthe metal dusting of ironand steels, however, are not fully understood. Changes of the mechanism result from a change of morphology of the reactionproducts with temperature. At > 700 °C the cementite decompositionreaction(11) canresult in formationof a dense ironlayer onthe cementite, see Figure 11, through which the carbonmust diffuse for graphite growth 36,37. Thencarbondiffusioninthe iron layer becomes rate controlling and the velocity and severity of metal dusting is drastically decreased. 3.3 Effects of Sulfur on Corrosion by Carbon While the retarding effect of sulfur is unwanted in the gas carburizationof steels, this effect is welcome on carburizationas a high temperature corrosionprocess. Additions of sulfur bearing compounds is used in the ethylene production by cracking of hydrocarbons. A quantitative study using CH4-H2-H2S mixtures for carburizationof Alloy 800 inthe temperature range 900 - 1100 °C, see Figure 5, showed that at 1000 °C the carburization and internal carbide formation is Figure 11: Metallographic cross sectionof anironsample after 4 hours inH2 - 5 % CO - 0,2 % H2O at 700 °C, optical micrograph showing inward growth of cementite and its outward decomposition under iron layer formation, the carbon diffuses through this layer so that graphite grows into the iron layer Slika 11: Metalografski obrus vzorca železa po 4 urah v H2 - 5 % CO - 0,2 % H2O pri 700 °C. Optični posnetek prikazuje rast cementita v notranjost in njegov razpad proti zunanjosti pod nastajajočim slojem železa. Ogljik difundira skozi ta sloj, zato grafit raste v sloj železa. 303 H. J. GRABKE: CARBURIZATION, CARBIDE FORMATION, METAL DUSTING, COKING Figure 12: Thermodynamic plot onthe effect of sulfur onthe occurrence of metal dusting, log(pH2S/pH2) versus 1/T, presenting the regions with low sulfur coverage and fast occurrence of metal dusting (black dots), and regions where monolayer coverage with S(ad) is approached (hatched), and region with monolayer coverage - in both the latter regions metal dusting is effectively suppressed, uppermost field - FeS stable 40,41 Slika 12: Termodinamična odvisnost vpliva žvepla na pojav uprašenja kovine, log(pH2S/pH2) v odvisnosti od 1/T, ki prikazuje področje z majhnim žveplovim prekritjem, hiter pojav uprašenja kovine (črne točke) in področja, kjer je doseženo približno enoatomsko pokritje z S(ad) (šrafirano) in področja z doseženim enoatomskim prekritjem. Na obeh področjih je učinkovito preprečeno uprašenje, zgornje polje -stabilenFeS 40,41. suppressed optimally at pH2S/pH2 Ä 10-4, at higher ratios the material is endangered by sulfidation of the alloying elements Cr and Mn. From the kinetic studies of reactions (1) and (3), it was known already 20,21 that for this ratio the rate of carbontransfer is decreased by two orders of magnitude, compared to the rate in absence of sulfur. As knownfrom studies about adsorptionand segregationof sulfur oniron38,39 for such H2S/H2 ratio at 1000 °C the coverage with S(ad) approaches a monolayer, see Figure 12. Presence of sulfur is also an effective remedy against metal dusting. Adsorbed sulfur hems the carbon transfer, step (i) of the metal dusting mechanism, but in addition it suppresses step (iii), the nucleation of graphite effectively. To nucleate graphite, an ensemble of free sites is necessary and even if the coverage with sulfur is much less than a monolayer, graphite nucleation cannot take place. Thus, the carbonactivity onthe cementite layer stays high and its decomposition cannot start. Continued cementite growth has beenobserved inCO-H2-H2O-H2S or CH4-H2-H2S mixtures. Generally the stability of cementite achieved depends on the carbon activity and time of exposure, but a diagram was derived from studies on iron and low alloy steels, describing, the ranges of immediate metal dusting and suppressed metal dusting, i.e. continued cementite growth, in dependence on pH2/pH2 and 1/T, see Figure 12 40,41. Inthe range of low pH2S/pH2 the coverage with S(ad) is very low and not sufficient to suppress metal dusting for long time. In the hatched range of conditions the monolayer coverage is approached (? = 0.90-0.99) and the start of metal dusting is effectively retarded. Inthe range of high pH2S/pH2 no metal dusting occurs and cementite is largely stabilized. It may be noted that the hatched area at high temperatures 900-1100 °C corresponds to the pH2S/pH2-values determined to be optimal for retardationinthe carburiza-tionof high temperature alloys. Furthermore the ’stabilizationof cementite’ is of great scientific interest to study properties of the otherwise unstable compound and for example to determine diffusivities of C in Fe3C from its growth rate. And the growing of cementite also has important practical interest, since cementite would be a valuable product inthe direct reductionof ironores. This ’iron carbide production’ has beenstudied indetail 42-45 and also tried inlarge scale but with no great success. 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