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.     

Hochtemperaturkorrosion von Schutzgasglühhauben

High temperature corrosion of covers for batch annealing Covers made of austenitic FeNiCr-steels are used for annealing of cold rolled steel coils in inert atmospheres at about 700°C. Thick corrosion layers of oxides, carbides, coke and Fe-Ni are formed at the inside walls, the formation occurs upon changing gas compositions and temperatures, by local reduction of the oxide layer, internal carbide formation, external coke deposition and repeated oxidation, similar as in “metal dusting”. The corrosion phenomenon was simulated with the steel 1.4828 and with Ce- and Nb-doped steels upon cycling the temperature between room temperature und 800°C and changing the N₂-H₂-CH₄-H₂O atmosphere from oxidizing to reducing and carburizing compositions. Furthermore experiments were performed under compressive stress and at elevated temperature up to 950°C. Strong corrosive attack occurred on the surface with the oxide layer which stems from the hot rolling of the material, much less attack after surface finishing by grinding, polishing, machining or sand blasting. The following technological measures are favorable for avoiding the high temperature corrosion of the covers:

• Surface finishing of the inner walls before use.
• Sufficiently high H₂O-content of the atmosphere.
• Alloying addition of Ce to the material.
• Avoidance of too high temperatures

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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

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.     

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.     

Effect of Pressure on Metal Dusting Initiation on Alloy 800H and Alloy 600 in CO-rich Syngas

The effect of pressure on metal dusting initiation was studied by exposing conventional alloys 600 and 800H in CO-rich syngas atmosphere (H₂, CO, CO₂, CH₄, H₂O) at ambient and 18 bar total system pressure and 620 °C for 250 h. It was verified that, at constant temperature, increasing the total system pressure increases both oxygen partial pressure (pO₂) and carbon activity (a _C), simultaneously. Both samples exposed at ambient pressure showed very thin oxide scale formation and no sign of metal dusting. By contrast, samples exposed in the high-pressure experiment showed severe mass loss by metal dusting attack. Iron- and chromium-rich oxides and carbides were found as corrosion products. The distinct pressure-dependent behavior was discussed by considering both thermodynamic and kinetic aspects with respect to the protective oxide formation and pit initiation.     

Damage of a heat-treatment retort by metal dusting attack

The damage of a heat-treatment retort which was made of cast Fe36Ni21Cr steel and used under a carburizing atmosphere at about 900 °C has been investigated. Strong corrosion attack had occurred, leading locally to pitting on the surface or even to hole formation in the retort walls. An oxide layer rich in chromium had formed on the outside of the metal followed by a precipitation zone of carbides. These carbides decomposed near the surface into graphite and the corresponding metal, similar to “metal dusting”. The strong corrosion attack was often correlated with preexisting imperfections in the cast material like shrinkage cavites or microporosity.     

Influence of Silicon and Aluminum Additions on the Oxidation Resistance of a Lean-Chromium Stainless Steel

The effect of Si and Al additions on the oxidation of austenitic stainless steels with a baseline composition of Fe–16Cr–16Ni–2Mn–1Mo (wt.%) has been studied. The combined Si and Al content of the alloys did not exceed 5 wt.%. Cyclic-oxidation tests were carried out in air at 700 and 800°C for a duration of 1000 hr. For comparison, conventional 18Cr–8Ni type-304 stainless steel specimens were also tested. The results showed that at 700°C, alloys containing Al and Si, and alloys with only Si additions showed weight gains about one half that of the conventional type-304 alloy. At 800°C, alloys that contained both Al and Si additions showed weight gains approximately two times greater than the type-304 alloy. However, alloys containing only Si additions showed weight gains four times less than the 304 stainless. Further, alloys with only Si additions preoxidized at 800°C, showed zero weight gain in subsequent testing for 1000 hr at 700°C. Clearly, the oxide-scale formation and rate-controlling mechanisms in the alloys with combined Si and Al additions at 800°C were different than the alloys with Si only. ESCA, SEM, and a bromide-etching technique were used to analyze the chemistry of the oxide films and the oxide–base-metal interface, in order to study the different oxide film-formation mechanisms in these alloys.     

Increasing the Upper Temperature Oxidation Limit of Alumina Forming Austenitic Stainless Steels in Air with Water Vapor

A family of alumina-forming austenitic (AFA) stainless steels is under development for use in aggressive oxidizing conditions from ~600?C900 °C. These alloys exhibit promising mechanical properties but oxidation resistance in air with water vapor environments is currently limited to ~800 °C due to a transition from external protective alumina scale formation to internal oxidation of aluminum with increasing temperature. The oxidation behavior of a series of AFA alloys was systematically studied as a function of Cr, Si, Al, C, and B additions in an effort to provide a basis to increase the upper-temperature oxidation limit. Oxidation exposures were conducted in air with 10% water vapor environments from 800?C1000 °C, with post oxidation characterization of the 900 °C exposed samples by electron probe microanalysis (EPMA), scanning and transmission electron microscopy, and photo-stimulated luminescence spectroscopy (PSLS). Increased levels of Al, C, and B additions were found to increase the upper-temperature oxidation limit in air with water vapor to between 950 and 1000 °C. These findings are discussed in terms of alloy microstructure and possible gettering of hydrogen from water vapor at second phase carbide and boride precipitates.     

Protective behaviour of newly developed coatings against metal dusting

Metal dusting is a corrosion phenomenon whose mechanisms and effects depend on different parameters such as temperature, pressure, time, material, etc. and which still leads to unexpected failure cases in several high temperature industries. The present work deals with the development and testing of coating systems against metal dusting attack and the evaluation of their protective behaviour at different temperatures. The recently developed coatings are based on high amounts of strong oxide formers, like Si, Ti, Cr and Al, which are able to form protective oxide layers even under the highly reducing metal dusting atmospheres. The coatings were applied on ferritic and austenitic steel substrates by HVOF and by (co) diffusion of one or up to three elements via a pack cementation process. The process parameters of the diffusion coatings were optimized with respect to the different substrate materials and diffusing species. Isothermal tests at temperatures of 400°C, 620°C and 700°C under metal dusting atmospheres were carried out for up to 2022 h for coated and uncoated specimens. Discontinuous mass change measurements were performed in order to determine the kinetics of attack. After the corrosion tests metallographic cross sections of the specimens were investigated by optical and electron microscopy (SEM, EPMA). Especially the interdiffusion of substrate and coating and the formation of potentially protective oxide layers on top of the coatings were studied using elemental mapping and concentration profiles. The results obtained so far indicate that coatings have a high potential for significantly increasing the life‐time of components under metal dusting conditions. The different systems investigated are classified, evaluated, and discussed with respect to their protection potential and the responsible protection mechanisms.     

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 behaviour of welded Ni‐base alloys with different surface finish

Welded Ni‐base alloys and Alloy 800 were exposed under metal dusting conditions at 600°C and 650°C for up to 6000 hours. Alloy 800 was attacked very strongly already in the first days and Alloy 600 also rather soon and widespread, on both materials attack started mainly in the heat affected zone. Several surface states of Alloy 600: brushed, ground, sand‐blasted and pickled were tried only grinding caused a modest delay and decrease of metal dusting attack. Generally the attack was less widespread but deeper at 650°C than at 600°C, also for Alloy 601 and 602. The latter alloys show minor, mainly local attack, but especially the welds are affected. TIG welding led to better resistance than hand‐welding.     

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.     

Microprocesses of coke formation in metal dusting

The microprocess of coke formation during metal dusting on iron in a carburizing atmosphere with medium and extremely high carbon activities as well as the influence of sulphur have been studied down to the nanometer scale using high resolution electron microscopy (HREM) and analytical electron microscopic techniques (AEM). While for medium carbon activities the metal dusting proceeds via a formation, disintegration and further decomposition of a metastable carbide Fe₃C into Fe and C, the additional formation of the carbide Fe₅C₂ and the stabilization of carbides in the coke region have been observed for extremely high carbon activities. If sulphur is present in the atmosphere metal dusting takes place solely in the S-free surface areas. Furthermore, sulphur deposited from the atmosphere will suppress the nucleation of graphite in the coke. In addition, the results reveal that, irrespective of the degree of the carbon activity, there is a fundamental initial reaction micromechanism of metal dusting characterized by a vertically oriented deposition of graphite lattice planes with respect to the original surface of the substrate and with free ends affecting the decomposition of the carbides and thus forming a coke of carbon and iron, or of carbide particles, depending on the carbon activity.     

High-Temperature Corrosion Behaviour of Aluminized-Coated and Uncoated Alloy 718 Under Cyclic Oxidation and Corrosion in NaCl Vapour at 750 °C

To evaluate the suitability of HR3C and 22Cr–25Ni–2.5Al AFA steels as the heat-resistant alloys, the oxidation behavior of them was investigated in air at 700, 800, 900 and 1000 °C. The evolution of oxide layer on the surface and subsurface was investigated using a combination of compositional/elemental (SEM, EDS) and structural (XRD, GDOES) techniques. A dense and continuous Cr₂O₃ healing layer on the HR3C was formed at the temperature of 700 or 800 °C, but the Cr₂O₃ oxide film on HR3C was unstable and partly converted into a less protective MnCr₂O₄ with the increase in temperature to 900 or 1000 °C. The composition and structure of oxide film of 22Cr–25Ni–2.5Al AFA steels are significantly different to the HR3C alloys. The outer layer oxides transformed from Cr₂O₃ to Al-containing oxides, leading to a better oxidation resistance at 700 or 800 °C compared to HR3C. Further, the oxide films consist of internal Al₂O₃ and AlN underneath the outer loose layer after 22Cr–25Ni–2.5Al AFA oxidized at 900 or 1000 °C. It can be proved that the internal oxidation and nitrogen would make 22Cr–25Ni–2.5Al AFA steels have worse oxidation resistance than HR3C alloys at 900 or 1000 °C.     

High temperature corrosion of alloy Haynes 556 in carburizing/oxidizing environments

The iron based alloy Haynes 556 has been recently used in hydrocarbon and carbonaceous environments due to its excellent resistance to carburization. This alloy outperforms stainless steels and some of the best commercial carburization‐resistant nickel‐based alloys. This paper is concerned with the behavior of alloy Haynes 556 in high temperature carburizing environments containing trace amounts of oxygen. Thermal cyclic exposures were conducted in 2% and 10% CH₄/H₂ gas mixtures at 800, 900, 1000, and 1100°C for 10 cycles, 50 h each. Carbon activities, oxygen partial pressures, and stabilities of oxides and carbides were used to identify the role played by the reaction products in providing protection. Thermodynamic analyses, weight changes, and microstructural characterization were correlated with environmental parameters and alloy composition to elucidate the causes of its marked resistance to carburization. The results indicate protective character in both gas mixtures under all exposure conditions except the most aggressive, namely 1100°C in 10% CH₄/H₂ gas mixture. Below 1000°C, the formation of Cr, Al, and Si oxides along with Cr carbides provides the primary means of protection. Catastrophic failure at 1100°C was manifested by extensive fracture and crack development within the outer substrate surface in the 10% CH₄/H₂ gas mixture resulting in a dramatic increase in weight gain. This has been attributed to the increased carbon pick‐up, coupled with the loss of the protective outer scales.     

The influence of alloying elements on the chlorine-induced high temperature corrosion of Fe-Cr alloys in oxidizing atmospheres

The effect of the alloying elements Al, Cr, Mn, Mo, Si and Ti on the corrosion behaviour of ferritic Fe-15Cr model alloys was studied in a N₂/He-5 vol.% O₂ gas mixture with and without additions of 500–1500 vppm HCl at 600°C. The main corrosion mechanism is “active oxidation”, characterized by the formation of volatile metal chlorides at the metal/oxide interface. Volatilization and subsequent conversion of the chlorides into oxides results in the formation of porous and poorly adherent oxide scales. Large mass gains were observed for Fe-15Cr, Fe-35Cr and Fe-15Cr with additions of 5 wt.% Ti, 10 wt.% Mn or 10 wt.% Mo. The specific morphology of the corrosion products depends strongly on the alloying elements. For the Fe-Cr alloys, a model for the formation of the scales, which are characterized by alternating dense and porous layers, is presented. The addition of 5 wt.% Si or Al to Fe-15Cr leads to much better corrosion resistance by the formation of protective Cr₂O₃/Al₂O₃-layers, however in the case of Al addition the behaviour depends strongly on the experimental conditions, as surface treatment and flow velocity. In Fe-15Cr-10Mo preferential removal of the more reactive metals Fe and Cr was observed resulting in a Mo-enriched porous metal zone underneath the metal-oxide interface. The effect of carbon on the corrosion behaviour was examined by addition of 0.3–0.8 wt.% C to the model alloys. Cr-rich M₂₃C₆-carbides were attacked preferentially while Mo-rich M₆C-carbides are very stable relative to the matrix and the attack occurs in regions surrounding the carbides.     

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).