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.     

Metal Dusting of Fe–Cr and Fe–Ni–Cr Alloys Under Cyclic Conditions

Metal dusting is the disintegration of alloys into carbon and metal particles during high-temperature exposure to carbon-bearing gases. Model Fe–Cr and Fe–Ni–Cr alloys were studied to test the hypothesis that M₃C formation is necessary for metal dusting to occur. The alloys were exposed to a 68% CO–26% H₂–6% H₂O gas mixture at 680°C (a_c=2.9) under thermal cycling conditions. Equilibrium calculations predicted the formation of M₃C at the surface of Fe–25Cr, but not Fe–60Cr. All compositions were expressed in w/o, weight percent. Alloys of Fe–25Cr with 2.5, 5, 10, and 25 w/o nickel additions were also exposed to the same conditions to study the role of nickel in destabilizing the precipitation of M₃C and, hence, altering the resistance to metal dusting. Metal dusting was observed on all the alloys except Fe–60Cr. For Fe–25Cr, Fe–25Cr–2.5Ni, and Fe–25Cr–5Ni, the carbonization and dusting process was localized, and its incidence decreased in Fe–25Cr–2.5Ni, consistent with the increased destabilization of M₃C precipitation. However, Fe–25Cr–10Ni and Fe–25Cr–25Ni both underwent extensive dusting in the absence of protective Cr₂O₃ formation. The carbon deposits formed consisted of carbon filaments, which contained particles at their tips. These were shown by electron diffraction to be exclusively Fe₃C in Fe–25Cr, Fe–25Cr–2.5Ni, and Fe–25Cr–5Ni, and a mixture of austenite and (Fe,Ni)₃C in Fe–25Cr–10Ni and Fe–25Cr–25Ni.     

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 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.     

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

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.     

Microprocesses of metal dusting on iron‐nickel alloys and their dependence on the alloy composition

Metal dusting of iron proceeds via the formation and disintegration of the metastable carbide Fe₃C, and the resulting fine Fe particles in the coke further catalyse carbon deposition. By contrast, nickel disintegrates directly, and larger grains are released. As revealed by TEM and AEM techniques, in both cases the disintegration proceeds by inward growth of thin graphite filaments, the atomic basal planes of which being oriented perpendicular to the surface thus effecting a high reactivity at the growth front. Consequently, successive alloying of iron with nickel should lead to a change over from one disintegration mechanism to the other, and, in fact, we could evidence that the carbide formation takes place only up to a nickel content of about 5 wt.%. Already at a Ni concentration of 10 wt.% a direct disintegration of the metal proceeds, as it is typical for pure nickel. Furthermore, in all investigated Ni‐Fe alloys a surface‐near enrichment of Ni was observed which indicates a selective corrosion of Fe, decreasing with increasing Ni content of the basic alloy.     

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.     

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.     

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.     

Relationship between coking and metal dusting

Metal dusting is a severe form of corrosive degradation that Fe, Co and Ni base high temperature alloys undergo when subjected to environments supersaturated with carbon (a_c > 1). This corrosion process leads to the break-up of bulk metal into metal powder. The present study focuses on the fundamental understanding of the corrosion of Fe and Ni in carbon-supersaturated environments over the temperature range, 350–1050 °C. Building on earlier research, the role of deposited carbon in triggering corrosion is further clarified. For Fe, the corrosion rate peaked at ∼ 575 °C with a sharp decrease in rate on either side of the maximum. High-resolution electron microscopy revealed, in addition to metal particles, a mixture of graphitic carbon, amorphous carbon and filamentous carbon in the corrosion product. While the presence of a surface layer of Fe₃C was characteristic of corrosion up to 850 °C, such a layer was absent at the higher temperatures. The corrosion rate maximum that typified the metal dusting of Fe was absent in the case of Ni where no surface carbide occurs until temperatures well below 350 °C. The mechanistic differences between iron corrosion and nickel corrosion are compared and contrasted.     

Improving metal dusting resistance of. transition‐metals and Ni‐Cu alloys

The present study focuses on a new technique for the prevention of metal dusting in carbonaceous gas environments at intermediate temperature. Preliminary laboratory metal dusting test was conducted for transition‐metals and Ni‐x%Cu binary alloys in a simulated 60%CO‐26%H₂‐11.5%CO₂‐2.5%H₂O (in vol.%) gas mixture at 650°C for 100 h. The metal dusting caused no coke deposition on transition‐metals of Cu, Ag, and Pt, while those of Fe, Co, and Ni have a large amount of coke and lost mass. Whether or not coking behavior of Ni‐Cu binary alloys formed any oxide scales in the simulated gas environment depended on the Cu content. Specimens containing low Cu were entirely covered with coke and showed rough metal surfaces due to the degradation of metal. Alloys of 20% and more Cu, on the contrary, had no coke deposition and smooth metal surfaces, suggesting alloys with an adequate Cu do not react with CO in the gas mixture without an oxide scale barrier. Based on these results, we conclude that Cu does not protect by formation of the oxide scale but has a “Surfactant‐Mediated Suppression” against metal dusting. This effect can be explained in terms of atomistic interaction of CO with transition‐metal surfaces by electronic structure analyses. The concept can be also useful for the practical material design of Ni‐Cr base alloy with excellent metal dusting resistance.     

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.     

Effect of Pre-oxidation on Carburisation and Metal Dusting of Nano-crystalline Ni–Cr Alloys

Nanocrystalline (NC) Ni–Cr coatings, containing 5, 10 and 20 wt% Cr were prepared using magnetron sputtering deposition, on the substrates of the same composition materials. These alloys were tested in 47 %CO–47 %H₂–6 %H₂O for 50 h at 650 °C. Weight gain kinetics showed that increasing Cr content decreased the carburisation kinetics. After reaction, the NC coatings containing high Cr (10 and 20 wt%) remained, with the formation of surface and inner Cr₂O₃ and internal precipitates of fine carbon deposits for Ni–10Cr and Cr₇C₃ for Ni–20Cr. In contrast, the Ni–5Cr coated sample suffered a severe metal dusting with whole coating scale was destroyed completely. Preoxidation of these alloys and their miro-grained counterparts was conducted before metal dusting. It was found that preoxidation significantly reduced weight gain kinetics. This reduction effect is more significant for NC Ni–Cr alloys than the micro-grained alloys. The critical chromium concentration for protective chromia scale formation and for internal chromium carbide formation were discussed using Wagner’s analysis and product solubility calculation, respectively. The effects of preoxidation and grain size on oxide formation and carburisation/metal dusting were also discussed.     

Carbon Permeability of Nickel and Ni–Cu Alloys

A Ni–20Cr alloy and variants containing 5, 10 and 20Cu (all in wt.%) were carburised in H₂-5% CH₄ at 1,000 °C. All alloys formed internal carburisation zones containing Cr₃C₂ and Cr₇C₃. The Ni–20Cr alloy also developed a surface deposit of graphite, but the copper-bearing alloys did not. Measured parabolic rate constants for intragranular carburisation were used to calculate carbon permeabilities from Wagner’s diffusion analysis. The value obtained for Ni–20Cr was in good agreement with the product of independently measured carbon solubility and diffusion coefficient values for nickel. Permeabilities found for copper-bearing alloys were similar, showing that the presence of copper had little effect on carbon diffusion in nickel. This finding is used in analysing the mechanism by which nickel undergoes metal dusting.     

Study of Mn activities in Mn–Ni–C alloys by EMF measurements

The activities of manganese in Mn–Ni–C alloys have been studied by solid-state galvanic cell technique with CaF₂ as the solid electrolyte. The measurements of electromotive force (EMF) have been carried out in the temperature range 920–1240 K. The main phase compositions of the alloys have been analyzed by X-ray diffraction (XRD). It was established that the substitution of Mn by Ni in the (MnNi)₂₃C₆ carbide was limited, that the lattice parameter decreased slightly with increase in the Ni content and that a solid solution is formed between Mn and Ni. It was also found that the activity of manganese decreases with increase in the nickel content when the ratio of C/(Mn+C) is less than 8.3 wt.%, and that the negative effect of Ni on the activity of Mn in Mn–Ni–C ternary system decreases as the carbon content increases. However, when the ratio of C/(Mn+C) is equal to 8.3 wt.% or more, the activity of manganese is independent of the nickel content.     

In situ XRD studies on Al–Ni and Al–Ni–Sr alloys prepared by rapid solidification

In this paper the structure and stability of Al–17 wt.%Ni(Al–17Ni) and Al–17 wt.%Ni–2 wt.%Sr alloys prepared by rapid solidification was investigated by means of XRD techniques. Our work demonstrates that both alloys are crystalline and composed of fcc (Al–Ni) solid solution and orthorhombic Al₃Ni phases. The ternary alloy shows in addition the presence of small amount of tetragonal Al₄Sr phase. In situ XRD experiment demonstrates the stability of the solute solution up to 650 °C, Al₃Ni above 750 °C while Al₄Sr overcomes melting of the major phases at 800 °C. High-temperature structure analysis proved strong bindings between Al and Ni atoms in Al₃Ni phase, corroborating its covalent nature, linear and faster increase of the fcc volume with annealing temperature. The linear correlation between constituting atoms decreases with increase of the temperature.The work also documents the applicability of pair distribution function (PDF) analysis to the study of multiphase crystalline systems.     

Metal dusting resistance of high chromium alloys

Samples of 5 high Cr‐alloys were discontinuously exposed for 10,000 hours under severe metal dusting conditions, i. e. in flowing 49%CO‐49%H₂‐2%H₂O at 650°C. After each of the 11 exposure periods the mass change was determined and any coke removed and weighed. Metallographic cross sections were prepared after about 4,000 h and 10,000 h. The high Cr‐alloys: 1. PM 2000 (Fe‐19%Cr‐5.5%Al‐0.5%Ti‐0.5%Y₂O₃), 2. Cr‐44%Fe‐5%Al‐0.4%Ti‐0.5%Y₂O₃, 3. Cr‐50%Ni, 4. Cr‐5%Fe‐1%Y₂O₃ and 5. porous chromium showed no or only minute metal dusting attack. Compared to the attack on reference samples of Alloy 601 (Ni‐23%Cr‐14%Fe‐1.4%Al), the metal dusting symptoms were negligible on the 5 high Cr‐alloys, minor coking and pitting and no internal carburization was observed. Because of the high Cr‐content, carbon solution and ingress should be minute, and in addition are inhibited by the formation of a chromia scale, as confirmed for four of the Cr‐rich alloys, and formation of an alumina scale on PM 2000. These alloys could be used for parts exposed to severe metal dusting conditions, and in fact, 50Cr‐50Ni has been applied successfully under such conditions.     

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.     

Metal dusting on an alumina forming Ni-base alloy

A high-performance commercial alumina-forming Ni-base alloy was studied after a 2 years field exposure at 540 °C in a methanol plant with a gas composition of 10-20%CO and 20-40%H₂O, with some CO₂ and the remainder H₂. The same material was also used in laboratory studies performed at 650 °C using a gas mixture with higher CO and lower H₂O content; 50%CO + 3%H₂O + 47H₂ (carbon activity a_c = 39). Post-exposure metallographic examinations together with thermodynamic calculations were used to identify and describe the metal dusting processes.A growth mechanism for metal dusting in nickel base alloys, which is independent of metal bulk diffusion, is identified. The process involves a separation of the carbon-saturated metal into a network of discontinuous precipitated carbides and a depleted Ni-austenite matrix followed by selective oxidation of the carbide network. The corrosion product consists of Cr-depleted Ni-particles, Cr-rich oxides and free carbon. The estimated metal dusting corrosion rate in the field exposure was 20-25 μm/year, based on metallography and it was correlated to a theoretical model based on boundary diffusion processes.