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 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.     

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.     

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 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.     

The influence of surface condition on the metal dusting behavior of cast and wrought chromia forming alloys

The current work investigated the impact of surface condition on the metal dusting behavior of chromia forming alloys. Five commercial alloys were included in the study, wrought 800H, 353MA, and cast G4859, G4852 Micro, and ET45 Micro, these alloys have a chromium and nickel content in the range of 20–35 wt% and 32–45 wt%, respectively. The wrought alloys were tested in a pickled state and the cast alloys with a machined surface, all the alloys were tested using a laboratory ground surface condition for comparison. The exposures were performed using a gas with a composition of 44 vol% CO, 52 vol% H₂, 2 vol% CO₂, and 2 vol% H₂O at a temperature of 600 °C and a pressure of 5.5 bar. The samples were periodically characterized by measuring the mass loss, pit density, pit size, and pit depth. The results show that the pickled surfaces were sensitive toward metal dusting attack while the machined and the ground surfaces had better resistance. This shows that the surface pre‐treatment plays a crucial role.     

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 phenomenon during carburization of FeNiCr-alloys at 850–1000°C

Three austenitic FeNiCr steels, G 4848, G 4852 and 36 XM, have been corroded in CH₄ + H₂ mixtures at 850, 900 and 1000°C. Under these conditions a new metal dusting phenomenon has been observed where closed cavities near the exposed surfaces of the alloys are filled with a powdery mixture of graphite, iron metal, chromium carbide and also in cases SiO₂. It is tentatively concluded that the powder is formed through the decomposition of (Cr, Fe)₇C₃.     

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.     

On the “coke” growth in carburizing and sulfidizing atmospheres upon High temperature corrosion of iron and nickel base alloys

The graphite deposition from carbonaceous atmospheres can initiate a catastrophic deterioration of alloys in high temperature corrosion. The graphite nucleation and growth is catalyzed by metal surfaces and affected by the presence of sulfur. Gravimetric studies have been performed on the carbon transfer from CH₄? H₂ or CH₄? H₂? H₂S atmospheres to iron, nickel or ironnickel alloys at 1000°C. The carbon activities were a_c = 1 (equilibrium with graphite), a_c = 5 or a_c = 10; the sulfur pressure was in a range where the metal surfaces are nearly saturated with adsorbed sulfur. The carburization, i.e. the transfer of C into solid solution is retarded in the presence of sulfur since surface sites are blocked for the methane decomposition. In the sulfur-free environment graphite layers grow with their basal planes parallel to the metal surface – for nickel an epitaxial growth occurs which is extremely slow. In the presence of sulfur the graphite can only nucleate in small islets which grow to irregular nodules. This results in a retardation by sulfur of the graphitisation on iron, whereas the growth of graphite on nickel is accelerated by sulfur. The transition between these ways of graphitisation behaviour was studied for Fe? Ni.     

Effect of H2S on metal dusting of iron

A review is given on the effect of H₂S on metal dusting of iron which has been studied by gas carburisation in CO‐H₂‐H₂O‐H₂S and CH₄‐H₂‐H₂S mixtures at 500 and 700°C. The presence of H₂S in carburising gas atmospheres leads to sulphur adsorption on the iron surface, which retards carbon transfer. Segregation experiments and surface analyses have shown that sulphur segregates (and thus adsorbs) on cementite surfaces as well as on iron surfaces. The adsorbed sulphur also suppresses graphite nucleation and thus can stop the reaction sequence of metal dusting. Experiments by thermogravimetric analysis (TGA) have shown that the extent of retardation of metal dusting depends on temperature, carbon activity and H₂S content. The higher the carbon activity, the higher is the H₂S content required for suppression of metal dusting. At carbon activities a_C > a_C(Fe/Fe₃C) the metastable iron carbide, cementite (Fe₃C), occurs as an intermediate phase during metal dusting. Carburisation experiments in CO‐H₂‐H₂O‐H₂S mixtures at 500°C and X‐ray diffraction analysis (XRD) of carburised samples have revealed that at very high carbon activities a second iron carbide, Hägg carbide (Fe₅C₂), forms on the cementite surface. Microstructural investigations have shown that both metastable carbides decompose during metal dusting. Metal dusting experiments on iron at 700°C have been performed in CH₄‐H₂‐H₂S gas mixtures. By adding 15 ppm H₂S to the CH₄‐H₂ atmosphere the onset of metal dusting can be retarded for more than 350 hours. By means of Auger electron spectroscopy (AES), scanning electron microscopy (SEM) and energy dispersive X‐ray analysis (EDX) it was shown that coke contains graphite, cementite and iron particles with adsorbed sulphur.     

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 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.     

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.     

Metal dusting of ferritic Fe‐Al‐M‐C (M = Ti,V, Nb,Ta) alloys in CO‐H2‐H2O gas mixtures at 650 °C

Iron aluminides are known for their resistance to high temperature oxidation and sulphidation. Only little information is available about carburisation and metal dusting of Fe‐Al alloys. Metal dusting experiments with Fe‐15Al and Fe‐15Al‐2M‐1C alloys (in at.%) with M = Ti, V, Nb, or Ta were conducted at 650°C in CO‐H₂‐H₂O gas mixtures with the carbon activity a_c = 28. The kinetics of the carbon transfer was measured using thermogravimetric analysis (TGA). It is shown that the mass gain kinetics decreases by adding the alloying elements Nb, Ta, V, or Ti with C. Alloying with titanium and carbon leads to the most significant decreasing effect. The metallographic cross section observation showed a general metal wastage for Fe‐15Al, but local pitting for the Fe‐15Al‐2Nb‐1C and Fe‐15Al‐2Ta‐1C alloys. For the Fe‐15Al‐2V‐1C and Fe‐15Al‐2Ti‐1C alloys no significant attack was observed. Needle‐ or plate‐like Fe₃AlC_x precipitates were detected in the carburised samples. The existence of this ternary carbide with perovskite structure was predicted by thermodynamic calculations using the software Thermo‐Calc. The morphology of graphite on the surface was analysed by scanning electron microscopy (SEM). Mainly fine filaments with iron containing particles were detected. Cementite was detected in the coke layer by X‐ray diffraction analysis (XRD).     

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.     

Coking and Dusting of Fe–Ni Alloys in CO–H2–H2O Gas Mixtures

Metal dusting of Fe–Ni alloys was investigated in a CO–H₂–H₂O–Ar gas corresponding to a _C = 19.6 at 650 °C. Thermogravimetric analysis showed that increasing the nickel content in the alloy decreased the initial rate of carbon uptake. A uniform Fe₃C scale formed on pure iron, a layer with mixed structures of Fe₃C, γ and α-Fe developed on ferritic Fe–5Ni, and small amounts of Fe₃C developed at the surface of an austenite layer grown on two-phase (α + γ) Fe–10Ni. At nickel levels above 10%, no carbide appeared. These observations are shown to be broadly consistent with local equilibrium according to the Fe–Ni–C phase diagram. However, the failure of higher nickel austenitic alloys to form the (Fe,Ni)₃C expected at high carbon activities indicates a barrier to nucleation and growth of this phase. Graphite deposition was catalysed by (Fe,Ni)₃C on ferritics and by the metal itself on austenitics. The rates of carbon deposition on Fe–60Ni corresponded to the existence of three parallel and independent paths: the synthesis gas, the Boudouard and the carbon methanation reactions.     

α‐Fe layer formation during metal dusting of iron in CO‐H2‐H2O gas mixtures

The formation of an α‐Fe layer between cementite and graphite was observed and investigated during metal dusting of iron in CO‐H₂‐H₂O gas mixtures at both 600°C and 700°C. The condition to form this phenomenon is determined by the gas composition which depends on temperature. The iron layer formation was observed for CO content less than 1 % at 600°C and less than 5 % at 700°C. With increasing CO contents, no α‐Fe layer was detected at the cementite/graphite interface by optical microscopy. In this case cementite directly contacts with the coke layer. The morphologies of the coke formed in the gas mixtures with low CO contents were also analysed. Three morphologies of graphite have been identified with 1 % CO at 600°C: filamentous carbon, bulk dense graphite with columnar structure, and graphite particle clusters with many fine iron containing particles embedded inside. At 700°C with 5 % CO the coke mainly consists of graphite particle clusters with some filamentous carbon at the early stage of reaction. Coke analysis by X‐ray diffraction shows that both α‐Fe and Fe₃C are present in the coke. The mechanism of α‐Fe accumulation between cementite and graphite is discussed in this paper.     

Surface analytical studies of metal dusting of iron in CH4‐H2‐H2S mixtures

The metastable iron carbide, cementite (Fe₃C), occurs as an intermediate phase during the high temperature corrosion process called “metal dusting”. The kinetics and thermodynamics of metal dusting of iron have been studied by Grabke et al. [1–5] using CO‐H₂‐H₂O and CH₄‐H₂ gas mixtures. H₂S additions to carburising atmospheres impede the carbon transfer and retard the onset of metal dusting [6–14], thus allowing to study the early stages of the process. In this work the metal dusting process was studied in CH₄‐H₂‐H₂S atmospheres at 700 °C. Segregation experiments and surface analyses showed that S segregates on iron surfaces as well as on cementite surfaces. By means of Auger electron spectroscopy (AES), scanning Auger electron microscopy (SAM) and energy dispersive X‐ray analysis (EDX) it was shown that coke contains graphite, cementite and iron particles with adsorbed sulphur.     

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.