Metal dusting

This introductory review paper summarizes shortly the research on metal dusting, conducted in the MPI for Iron Research during the last dozen years. Metal dusting is a disintegration of metals and alloys to a dust of graphite and metal particles, occurring in carburizing atmospheres at a_C > 1 and caused by the tendency to graphite formation. The cause of destruction is inward growth of graphite planes into the metal phase, or in the case of iron and low alloy steels into cementite formed as an intermediate. The kinetics of metal dusting on iron and steels was elucidated concerning dependencies on time, temperature and partial pressures. High alloy steels and Ni‐base alloys are attacked through defects in the oxide scale which leads to pitting and outgrowth of coke protrusions, after initial internal formation of stable carbides M₂₃C₆, M₇C₃ and MC. A dense oxide layer prevents metal dusting, but formation of a protective Cr‐rich scale must be favored by a fine‐grain microstructure and/or surface deformation, providing fast diffusion paths for Cr. Additional protection is possible by sulfur from the atmosphere, since sulfur adsorbs on metal surfaces and suppresses carburization. Sulfur also interrupts the metal dusting mechanism on iron and steels, causing slow cementite growth. Under conditions where no sulfur addition is possible, the use of high Cr Nickelbase‐alloys is recommended, they are largely protected by an oxide scale and if metal dusting takes place, its rate is much slower than on steels.     

Coking by metal dusting of nickel-base alloys

Metal dusting of nickel and Ni-base alloys occurs by inward and internal growth of graphite in the metal phase, leading to extrusion and ejection of metal particles, which catalyze carbon deposition, i.e. coking. Compared to metal dusting of iron and steels which occurs via intermediate carbide formation and leads to much finer metal particles, coking on the Ni-base alloys is much less. This is caused by the larger size and smaller amount of metal particles formed by metal dusting and also by the clearly smaller rates of metal wastage on Ni-base alloys.     

Metal dusting of nickel-base alloys

Metal dusting, the disintegration of metallic materials into fine metal particles and graphite was studied on nickel, Fe Ni alloys and commercial Ni-base alloys in CO H₂ H₂O mixtures at temperatures between 450–750°C. At carbon activities a_c > 1 all metals can be destroyed into which carbon ingress is possible, high nickel alloys directly by graphite growth into and in the material, steels via the intermediate formation of instable carbide M₃C. Protection is possible only by preventing carbon ingress. Chromium oxide formation is the best way of protection which is favoured by a high chromium concentration of the alloy and by a surface treatment which generates fast diffusion paths for the supply of chromium to the surface. The metal dusting behaviour of Alloy 600 is described in detail. A ranking of the metal dusting resistance of different commercial nickel-base alloys was obtained by exposures at 650°C and 750°C.     

Metal dusting of high temperature alloys

Metal dusting, i.e. disintegration into fine metal particles and carbon, was induced on a selection of chromia forming high temperature alloys in a flowing CO-H₂-H₂O atmosphere in exposures at 650°C, 600°C, 500°, and 450°C. The materials were pretreated by annealing in H₂ at 1000°C and electropolishing, this leads to large grain size and low surface deformation, both is disadvantageous for formation of a Cr₂O₃ scale. The resistance to metal dusting is only dependent on the ability to form a protective Cr₂O₃ scale, thus the high Cr ferritic steels proved to be very resistant, the ferritic steels with 12–13% Cr were less resistant. Due to the lower Cr diffusivity in the austenitic steels, these were very susceptible, especially two alloys with about 30% Ni (Alloy 800, AC 66). The appearance of metal dusting was somewhat different for Ni-base materials but they were also attacked under pitting. The metal dusting is preceded in all cases by internal carburization whereby the chromium is tied up, afterwards the remaining Fe or Fe-Ni matrix can react to the instable intermediate carbide M₃C which decomposes to metal particles and carbon, in case of Ni-base materials a supersaturated solid solution of carbon is the intermediate.     

Metal Dusting-Mechanisms and Preventions

Metal dusting attacks iron, low and high alloy steels and nickel-or cobalt-base alloys by disintegrating bulk metals and alloys into metal particles in a coke deposit. It occurs in strongly carburising gas atmospheres (carbon activity aC>1) at elevated temperatures (400℃～1000℃). This phenomenon has been studied for decades, but the detailed mechanism is still not well understood. Current methods of protection against metal dusting are either directed to the process conditions-temperature and gas composition-or to the development of a dense adherent oxide layer on the surface of the alloy by selective oxidation. However, metal dusting still occurs by carbon dissolving in the base metal via defects in the oxide scale. The research work at UNSW is aimed at determining the detailed mechanism of metal dusting of both ferritic and austenitic alloys, in particular the microprocesses of graphite deposition, nanoparticle formation and underlying metal destruction. This work was carried out using surface observation, cross-section analysis by focused ion beam and electron microscopic examination of coke deposits at different stages of the reaction. It was found that surface orientation affected carbon deposition and metal dusting at the initial stage of the reaction. Metal dusting occurred only when graphite grew into the metal interior where the volume expansion is responsible for metal disintegration and dusting. It was also found that the metal dusting process could be significantly changed by alterations in alloy chemistry. Germanium was found to affect the iron dusting process by destabilising Fe<,3>C but increasing the rate of carbon deposition and dusting, which questions the role of cementite in ferritic alloy dusting. Whilst adding copper to iron did not change the carburisation kinetics, cementite formation and coke morphology, copper alloying reduced nickel and nickel-base alloy dusting rates significantly. Application of these fundamental results to the dusting behaviour of engineering alloys is discussed.     

Coking by metal dusting of steels

In process industries coking is an annoying phenomenon, the carbon deposition causes decrease of heat transfer and hinders gas flow. Coking in a process may indicate metal dusting, i.e. the disintegration of metals and alloys in carbonaceous atmospheres under formation of graphite and fine metal particles. The metal particles act as catalysts for vast coke formation. The thermodynamics, mechanisms and kinetics of metal dusting have been studied on iron and steels in synthesis respectively reduction gas CO-H₂- H₂O, here the aspects are presented of coking due to metal dusting. From the interplay of the metal disintegration and carbon deposition rather complex coupled kinetics are resulting, even different in a low temperature range where the decomposition of the intermediate cementite is rate determining and in a higher temperature range where the carbon transfer from the atmosphere is rate controlling. Coking by metal dusting can be suppressed in the same way as metal dusting, by sulfur addition to the atmosphere and/or by a stable dense protective oxide layer.     

Metal dusting behaviour of Cr-Ni steels and Ni-base alloys in a simulated syngas mixture

Long-term laboratory exposure tests for various Cr and Ni content steels and Ni-base alloys were conducted at 650 °C in a 60vol.%CO-26%H₂-11.5%CO₂-2.5%H₂O gas mixture simulating syngas environments. Upon isothermal heating, alloys with 15% and 20% Cr had many pits on the surface after a brief exposure, while no pit was found on alloys containing of 60% Ni and more than 23% Cr exposed for up to 5000 h. The thermal cycling accelerated pit initiation drastically, resulting that all test specimens except 30%Cr-60%Ni alloy suffered from metal dusting. From a measurement of pit depths, Ni proved to be an effective alloying element to retard the pit growth: growth rate for 75% Ni alloy has achieved double-digit decrease compared to that for 20% Ni. Microscopic observations has revealed that platelet graphite aligned perpendicular at the boundary of gas/metal of pits. The length of the platelet graphite for high Ni alloys was appreciably longer than that for low Ni steels. This can be interpreted from the difference of super saturation of carbon.     

On the mechanism of catastrophic carburization: ‘metal dusting’

Metal dusting is a disintegration of metals and alloys into small metal particles and carbon (graphite) occurring at carbon activities of a_C > 1 in a range of intermediate temperatures 400–800°C. The phenomenon was simulated in CO---H₂---H₂O atmospheres at 650°C. For iron and low alloy steels a mechanism was confirmed in which the unstable carbide M₃C is an intermediate, which decomposes according to M₃C = 3M + C, the metal particles serving as catalysts for further coke deposition. According to thermodynamic considerations this mechanism might be suppressed by alloying with Ni or Mn. Exposures with Fe---Ni alloys, however, showed that at high Ni contents another mechanism applies, the disintegration of a supersaturated solid solution. Also Fe---Mn alloys were susceptible to metal dusting after Mn depletion of the surface-near region by selective Mn oxidation; similar behaviour is to be expected for Fe---Cr alloys after selective Cr-oxide or Cr-carbide formation. Thus, in principle no alloys are resistant against metal dusting if no protective oxide layer is formed.     

Recent advances in understanding metal dusting: A review

Recent experimental investigations have widened the understanding of metal dusting significantly. Microscopic observations have been used to dissect dusting mechanisms. Iron dusts by growing a cementite surface scale, which catalyses graphite nucleation and growth. The resulting volume expansion leads to cementite disintegration. Cementite formation on iron can be suppressed by alloying with germanium. Nonetheless, dusting occurs via the direct growth of graphite into the metal, producing nanoparticles of ferrite. This process is faster, because carbon diffusion is more rapid in α‐Fe than in Fe₃C. Austenitic materials cannot form cementite, and dust via formation of graphite at external surfaces and interior grain boundaries. The coke deposit consists of carbon nanotubes with austenite particles at their tips, or graphite particles encapsulating austenite. TEM studies demonstrate the inward growth of graphite within the metal interior. It is therefore concluded that the dusting mechanism of austenitic materials like high alloy Cr–Ni steels and Ni base materials is one of graphite nucleation and growth within the near surface metal. In all alloys examined, both ferritic and austenitic, the principal mass transfer process is inward diffusion of carbon. Alloying iron with nickel leads to a transformation from one mechanism with carbide formation to the other without. Copper alloying in nickel and high nickel content stainless steels strongly suppresses graphite nucleation, as does also an intermetallic Ni–Sn phase, thereby reducing greatly the overall dusting rate. A surface layer of intermetallic Ni–Sn Fe‐base materials facilitates the formation of a Fe₃SnC surface scale which also prevents coking and metal dusting. Current understanding of the roles of temperature, gas composition and surface oxides on dusting rates are summarised. Finally, protection against metal dusting by coatings is discussed in terms of their effects on catalysis of carbon deposition, and on protective oxide formation.     

Role of alloying elements in steels on metal dusting

Metal dusting of Fe-Ni-Cr alloys has been observed in industrial processes in strongly carburizing atmospheres at temperatures from 450°C to 800°C. At temperatures below 650°C the alloys are generally not able to form dense, well adherent oxide layers in spite of relatively high Cr-contents, therefore, metal dusting can take place. Already a lot of experimental work has been done to elucidate the mechanism and to compare the resistance against metal dusting for high alloy steels [1]. The intention of this study was to obtain additional information concerning the role of alloying elements and the effects of carbide precipitates in austenitic high alloy steels such as Alloy 800. The susceptibility to metal dusting was determined by measuring the metal loss under metal dusting conditions of Fe-20%Cr-32%Ni alloys modified with additions of different carbide formers (W, Mo, Nb) or oxide formers (Si, Al). The samples were exposed at 600°C in a CO-H₂-H₂O-gas mixture for repeated periods up to 500 – 1500 h. The attack by the oxidizing and carburizing atmosphere leads to the precipitation of internal carbides and metal dusting and more or less to formation of an oxide layer. In comparison to the undoped material, the addition of carbide formers retards the initiation of metal dusting attack. The additions of Si and Al seem to prevent metal dusting under the given laboratory conditions. When carbides are present at the metal surface, they affect the initial oxide growth and have a negative effect on the protectivity of scales. Very striking is the effect of Ce, this rare earth element is generally known to favour Cr-oxide formation and to improve the adherence of the oxide layer [2], but in the case of metal dusting it clearly enhances metal dusting and metal wastage.     

Metal dusting of Fe3Al and (Fe,Ni)3Al

Metal dusting is a deterioration of metallic materials in strongly carburizing atmospheres under disintegration into a dust of carbon and fine metal particles (coke). The intermetallic compound Fe₃Al is also very susceptible to metal dusting and disintegrates under formation of vast amounts of coke. The mechanism corresponds to the metal dusting of iron and steels, Fe₃C is formed as an intermediate and the Al is oxidized. With increasing Cr-addition and with increasing Ni-content in alloys (Fe,Ni)₃Al-Cr the materials become more resistant, Ni₃Al is not attacked by metal dusting.     

Alloying with copper to reduce metal dusting of nickel

Copper is thought to be noncatalytic to carbon deposition from gas atmospheres, and owing to its extremely low solubility for carbon, inert to the metal dusting reaction. Thus, the addition of copper to nickel, which forms a near perfect solid solution, may be able to suppress or greatly retard the metal dusting of the alloy, without the need for a protective oxide scale on the surface. The dusting behaviour of Ni‐Cu alloys containing up to 50 wt% Cu, along with pure Cu, was investigated in a 68%CO‐31%H₂‐1%H₂O gas mixture (a_C: 19) at 680°C for up to 150 h. Surface analysis showed that two types of carbon deposits, graphite particle clusters and filaments, were observed on pure Ni and Ni‐Cu alloys with Cu contents of up to 5 wt%. Alloys with more than 10 wt% Cu showed very little coking, forming filaments only. SEM and TEM analyses revealed metal particles encapsulated by graphite shells within the graphite particle clusters, and metal particles at filament tips or embedded along their lengths. A kinetic investigation showed that alloy dusting rates decreased significantly with increasing copper levels up to 10 wt%. At copper concentrations of more than 20 wt%, the rate of metal dusting was negligible. Although pure copper is not catalytic to carbon formation, scattered carbon nanotubes were observed on its surface. The effect of copper on alloy dusting rates is attributed to a dilution effect.     

Metal dusting behaviour of several nickel‐ and cobalt‐base alloys in CO?H2?H2O atmosphere

The metal dusting behaviour of total 11 nickel‐ and cobalt‐base alloys at 680 °C in a gas of 68%CO? 31%H₂? 1%H₂O (a_C = 19.0, = 5.4 × 10^?25 atm) was investigated. All samples were electropolished and reacted in a thermal cycling apparatus. On the basis of their reaction kinetics, these alloys can be classified into three groups: the first, with rapid carbon uptake and significant metal wastage, consists of alloys of relatively high iron content (AC 66, 800H and NS‐163); the second, with intermediate rates, consists of some Co‐base alloys (HAYNES 188, HAYNES 25 and ULTIMET) and the third, with very low reaction rates, consists of nickel‐base alloys with high chromium levels (601, HAYNES HR 160, 230, G‐35 and EN 105). An external chromia scale protected group 3 alloys from carburization and dusting. However, this protective scale was damaged and not rehealed for group 1 and group 2 alloys, allowing carbon attack. In all cases, coke deposited on the surface with two typical morphologies: filaments and graphite particle clusters. Subsurface spinel formation in high iron‐content alloys led to rapid dusting due to the significant volume expansion. Alloy carbon permeability was calculated from a simple law of mixtures, and shown to correlate reasonably well with initial dusting rate except for one cobalt‐base alloy in which iron spinel formation was significant.     

Micromechanisms of metal dusting on Fe-base and Ni-base alloys

Phenomena of metal dusting on iron and nickel (and their alloys) were studied by characterizing both microstructure and nanochemistry of the reaction zones using TEM and AEM techniques. While in case of iron a metastable carbide (cementite) is formed nickel directly disintegrates. In the chromium-rich steel HK40 and the chromium-rich Ni-base alloy Inconel 600 some microstructural chromium dependent features were observed with protective effects against metal dusting. Independent of these differences, in all groups of materials a fundamental common starting mechanism on the atomic scale could be deduced, which first of all comprises the arrangement of basal graphite lattice planes perpendicularly oriented to the carbide (or metal) surface acting as active sites in the disintegration process.     

Metal dusting of nickel-base alloys

Nickel-base alloys are generally less susceptible to metal dusting than steels and the attack is slower. Exposures in strongly carburizing CO-H₂-H₂O mixtures at 650°C and 750°C have shown, however, gradually increasing attack on the alloys with lower Cr-content. Alloy 600 and even 601 were gradually attacked by pitting, whereas for alloys with >25 % Cr the materials loss was negligible even after 10,000 h. For these alloys such as 602 CA and 690 the formation of a protective chromia scale is strongly favored compared to carbon ingress and metal dusting.     

Metal dusting

Metal dusting is a catastrophic carburization of steels which leads to disintegration of the material to a mixture of powdery carbon and metal particles leaving pits and grooves. The phenomenon was simulated by carburization of low-and high-alloy steels in CO-H₂-H₂O mixtures at carbon activities > 1 in the temperature range 600–700°C. The occurance of an unstable carbide M₃C (M=Fe, Ni), as an intermediate in metal dusting, was proven—the reaction sequence involves over saturation of the metal matrix with carbon, M₃C formation at the surface, subsequent decomposition of the M₃C layer M₃C3 M+ C, leading to carbon with interspersed metal particles which act as catalysts for additional carbon deposition from the gas atmosphere. With increasing Ni content in Fe-Ni alloys, typical metal dusting is suppressed, but another mode of deterioration was observed, involving graphite growth on the grain boundaries. The high-alloy, chromia-forming alloys showed metal dusting only when chromia formation was suppressed by electropolishing the materials.     

Study of the metal dusting behaviour of high‐temperature alloys

The corrosion phenomenon named metal dusting has been observed in many high‐temperature industrial plants. An experimental research programme is being carried out into the degradation resistance of wrought and cast commercial and development high‐temperature alloys in H₂/CO gas mixtures at temperatures of 550°C to 750°C. Emphasis is placed on very high carbon activities, consistent with the next generation of steam‐reforming and similar plants that are susceptible to metal dusting. The overall programme is concerned with the mechanisms of initiation and propagation of dusting and the sensitivity to damage of the more resistant alloys, as a function of environmental parameters. Initial tests have been carried out on a number of commercial alloys: Alloy 600, 693, 602CA, 601, 603 XL, 671, 617, 690 (wrought), and H46M (cast). The specimens were exposed to a gas mixture of high carbon activity at 650°C for a total of 1000 hours. Many of the alloys showed at least the initial stages of metal dusting. Preliminary analysis using electron microscopy revealed that initiation of metal dusting is influenced by microstructure, stress state and composition. In some cases, attack was enhanced at stress points, such as corners and edges. Sample holders were found to influence strongly the length of the initiation period for the onset of the corrosion phenomenon. The reaction layers in the alloy beneath areas of damage were analysed by EDX and EPMA. Mechanical characterisation of such areas has been carried out using nanoindentation methods. These early results are discussed in terms of the effectiveness of oxide scales in inhibiting the onset of damage and presence of impurities in the ceramic holder in initiating the onset of damage.     

Metal dusting of binary iron aluminium alloys at 600 °C

In this work experiments on metal dusting of binary iron aluminium alloys with 15, 26 and 40 at.% Al were performed in strongly carburising CO‐H₂‐H₂O gas mixtures at 600 °C. The mass gain kinetics was measured using thermogravimetric analysis (TGA). The carburised samples were characterised by means of light optical microscopy (LOM), scanning electron microscopy (SEM), X‐ray diffraction (XRD) and X‐ray photoelectron spectroscopy (XPS). It was found that the mass gain kinetics depends on the CO content of the gas mixtures and on the Al content of the alloys. With decreasing carbon activity the carburisation reaction kinetics decreases and the onset of metal dusting is retarded for increasing time periods. With increasing Al content of the alloys the carburisation reaction is slower and metal dusting sets on at later times. The samples were not pre‐treated for the formation of a protective oxide scale. By X‐ray Photoelectron Spectroscopy (XPS) analyses of the carburised iron aluminium samples it was found that the formation of Al₂O₃ layers has taken place in the CO‐H₂‐H₂O gas atmospheres. Needle‐ or plate‐like κ‐phase (Fe₃AlC_x) precipitates close to the surface of the carburised Fe‐15Al sample were detected by means of XRD and LOM. The coke on top of the carburised samples mainly consists of filamentous carbon with metal particles at their tips.