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 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.     

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.     

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.     

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.     

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.     

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

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.     

Coke formation during metal dusting of iron in CO‐H2‐H2O gas with high CO content

Iron carburisation and coke formation during metal dusting of iron have been investigated in the gas mixture of 75%CO‐24.8%H₂‐0.2%H₂O at 600°C and 700°C. In all cases, cementite is formed at the surface, together with a coke layer on the top. In the coke layer, two morphologies of graphite are identified: compact bulk graphite with a uniform thickness and a columnar structure, and filamentous carbon with iron‐containing phases at the tip or along its length. The examination of coke formation in different stages of reaction at 700°C reveals that the coke contains two layers. The inner layer is composed of filaments, while the outer layer consists of the compact columnar graphite. After 2 h reaction the top compact graphite layer has suffered a serious deformation and has formed fractures because of the growth of catalytic filamentous carbon underneath. These filaments grow outside from these fractures and finally cover the whole surface after 4 h reaction. At 600°C, however, the coke contains a thick bulk graphite layer and non‐uniformly distributed filaments on the top. The bulk graphite layer is composed of many graphite columns which are loosely piled and are vertical to the surface. Each graphite column consists of many fine graphite fibres in parallel with the columnar axis. Filaments grow outside preferably from the gaps among these graphite columns and along the grinding scratches. TEM analysis of the coke detects very convoluted filaments with iron‐containing particles at the tip or along their length. XRD and TEM analyses show that these particles are Fe₃C rather than metallic iron.     

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.     

Effect of copper on metal dusting of austenitic stainless steels

Three steels, 304SS, 310SS and 800H, were alloyed with 5%, 10%, and 20% (by weight) copper, and then exposed to 68%CO-31%H₂-1%H₂O gas at 680 °C (a_C = 19 and p_O2=5.4×10^-25 atm) under thermal cycling conditions. Kinetic measurements showed that copper-free alloys all dusted, with 304SS experiencing the greatest metal wastage. Copper additions did not have any effect on metal wastage of 304SS, but reduced the attack on 310SS and 800H markedly at levels of 5% and 10%. However, increasing the copper content to 20% produced large copper-rich precipitates which accelerated dusting by promoting internal graphitisation.Dusting was associated with surface coking. When pitting occurred, on copper-free alloys and on copper containing 304SS, large coke structures grew above the pits. Internal grain boundary carburisation always took place, and intragranular carbides also precipitated when dusting occurred. A lamellar surface layer of internally precipitated spinel and austenite also developed in association with dusting. The copper effect is discussed in terms of its alloy solubility and its known beneficial effect in Ni-Cu binaries.     

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 coke formation and metal dusting of electroplated Ni3Al–CeO2‐based coatings in CO–H2–H2O

Pre‐oxidation was introduced to improve the resistance of electroplated pure, 5 µm CeO₂‐dispersed, and 9–15 nm CeO₂‐dispersed Ni₃Al coatings to coke formation and metal dusting in 24.4%CO–73.3%H₂–2.3%H₂O at 650 °C. Coke formation and metal dusting of pre‐oxidized Ni₃Al‐based coatings were retarded up to 200 h owing to a thin Al₂O₃ scale induced during pre‐oxidation. The long‐term effectiveness of pre‐oxidation nonetheless depended on the integrity of Al₂O₃ scale. The pure Ni₃Al coating suffered severe spallation after pre‐oxidation and thereby showed the worst long‐term resistance. Two pre‐treated 9–15 nm CeO₂‐dispersed Ni₃Al coatings exhibited the best long‐term resistance to carbon attack because nano‐CeO₂ particles maintained a full coverage of Al₂O₃ scale on the coatings. Two 5 µm CeO₂‐dispersed Ni₃Al coatings showed significant spallation after pre‐oxidation because of an overdoping effect and experienced coke formation and metal dusting during long‐term exposure.     

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 and coking of alloy 803

Investigation was made by SEM examination on metal dusting and coking behaviours of alloy 803 in a flowing gas mixture of H₂-CO-H₂O. It was found that an oxide scale arisen on the sample surface at the beginning of exposure. Metal dusting started when graphite deposition occurred earlier at the local defects in the oxide scale than the defects were repaired by enough supply of chromium from the interior of alloy matrix. Coke consisted of graphite filaments and metallic particles produced by disintegrating of alloy matrix, and grew up from the defects in the oxide scale with pit left in the sample surface. Increasing chromium content, doping a small amount of silicon and reducing grain size to create fast diffusion paths for chromium and silicon to alloy surface, all promote the formation of a dense oxide scale and favor early self-repairing of the defects in the oxide scale before occurrence of graphite deposition. The resistance of an alloy to metal dusting can be improved generally by means of these methods.     

Metal Dusting of Nickel–Aluminium Alloys

Alloys of γ-Ni(Al), γ–γ′ Ni(Al)–Ni₃Al, γ′–Ni₃Al and β-NiAl were exposed in 1 h cycles to a carbon-supersaturated CO–H₂–H₂O gas mixture (a _C = 36.7, ${{p_{{\rm O}_2}}} $  = 2.83 × 10^?26 atm) at 650 °C and an overall pressure of 1 atm. It was found that all alloys except β-NiAl had been attacked by metal dusting, leaving a layered structure of nickel particles, graphite and catalytically grown nano-sized carbon filaments as the corrosion product. Carbon uptake and metal wastage rates were slowed with increasing aluminium content for the single-phase alloys. However, the γ–γ′ two phase alloy had the overall highest metal loss rate. Surface morphologies reflected uniform attack for the γ and γ–γ′ alloys, whereas on γ′ a pitting type of attack was observed. Amorphous alumina formation was identified on the surface of the γ′ and β alloys, and is thought to be the major factor providing protection against dusting attack.     

Metal Dusting of Alumina-Forming Creep-Resistant Austenitic Stainless Steels

Three developmental alumina-forming austenitic stainless steels were exposed to metal dusting conditions at 650?°C in a gas of 50%CO–49%H₂–1%H₂O (a _C: 36.7 and $ p_{{{\text{O}}_{2} }} $ : 2.83?×?10^?26?atm) under thermal cycling conditions. Metal wastage measurement showed initially slow kinetics followed by a fast weight loss. This observation is attributed to the formation of protective chromia/alumina oxide scales in the early stage of the reaction, followed by local oxide failure/spallation during cyclic reaction. Metal dusting initiated from these local defects, and pitting-type attack was observed after 131 cycles of reaction. After 352 cycles, severe dusting had developed, forming heavy and distinctive “tentacles” of superficial coke. This carbon deposit was composed of fine carbon filaments. Examination by TEM of the coke-metal reaction front showed direct surface metal disintegration, indicating that the dusting follows the classical mechanism for austenitic materials. Etching with aqua regia revealed a carburised zone formed in the alloy underneath the coke layer. Analysis by TEM of this zone revealed the formation of ultra-fine, needle-shaped chromium carbide precipitates within a chromium depleted austenite matrix.