Study of the metal dusting behaviour of high‐temperature alloys

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

Metal dusting 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.     

Corrosion resistance of Ni‐50Cr HVOF coatings on 310S alloy substrates in a metal dusting atmosphere

Metal dusting attack has been examined after three 168 h cycles on two Ni‐50Cr coatings with different microstructures deposited on 310S alloy substrates by the high velocity oxy‐fuel (HVOF) thermal‐spray process. Metal dusting in uncoated 310S alloy specimens was found to be still in the initiation stage after 504 h of exposure in the 50H₂:50CO gas environment at 620 °C. Dense Ni‐50Cr coatings offered suitable resistance to metal dusting. Metal dusting was observed in the 310S substrates adjacent to pores at the interface between the substrate and a porous Ni‐50Cr coating. The porosity present in the as‐deposited coatings was shown to introduce a large variability into coating performance. Carbon formed by decomposition of the gaseous species accumulated in the surface pores and resulted in the dislodgement of surface splats due to stresses generated by the volume changes. When the corrosive gas atmosphere was able to penetrate through the interconnected pores and reach the coating–substrate interface, the 310S substrate was carburized, metal dusting attack occurred, and the resulting formation of coke in the pores led to local failure of the coating.     

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.     

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.     

Effect of experimental conditions on the metal dusting phenomenon in several commercial nickel‐base alloys

The corrosion phenomenon, metal dusting, has been observed in many high‐temperature industrial plants, such as ammonia, syngas and steam‐reforming industries. An experimental research programme has been carried out into the degradation resistance of wrought and cast commercial high‐temperature nickel‐base alloys in H₂/CO gas mixtures at 650 °C. The results in this paper are focused on the effect of the experimental set‐up on the response of the alloy during exposure to the gas mixture; in particular, the specimen holders had an important role on the onset of carbon transfer from the gas to the alloy surfaces. Specimens of most alloys exposed to the gas mixture while suspended from quartz hangers were shown to have good resistance to the environment for periods up to 5000 h. On the other hand, specimens of these alloys, exposed while located in porous refractory ceramic holders, gained large amounts of coke and underwent severe damage. A series of short tests were carried out in order to ascertain the main factors influencing the alloy reactivities in the experimental rig. The presence of specimens of a less‐resistant alloy and slight variations in gas composition (with the addition of CO₂) were observed to have a marked impact on the alloy reactivities. Factors such as catalysts, contaminants and gas composition were considered and are discussed.     

Occurrence of metal dusting – referring to failure cases

Twelve years of research on metal dusting at the Max‐Planck‐Institut for Iron Research have led to comprehensive knowledge on the mechanisms and kinetics of metal dusting and on ways and means for prevention of this corrosion phenomenon, – this knowledge is shortly summarized in this paper. Inspite of this state of understanding, the present opinion in industry is that metal dusting is mysterious and not predictable and cannot be controlled. This paper is intended to show that the occurrence of metal dusting and its reasons can be well understood, by describing and explaining five failure cases.     

Internal oxidation and metal dusting of Fe-Si alloys

Iron and Fe-Si alloys (1, 2, 3 and 5 wt.% Si) were reacted at 680 °C with a gas mixture of 68% CO, 26% H₂ and 6% H₂O (a_C = 2.9, p_O2 = 2 × 10⁻²³ atm). Alloy reaction products consisted of internally precipitated SiO₂, an external scale of Fe₃C + SiO₂ and surface coke deposits containing cementite and silica particles. Coking and metal dusting rates both increased with alloy silicon level. This is proposed to be related to an increase in graphite nucleation sites at Fe₃C/SiO₂ phase boundaries, and the volume expansion accompanying graphite precipitation.     

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.     

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.     

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.     

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.     

Kinetics and mechanisms of nickel metal dusting I. Kinetics and morphology

The kinetics of nickel metal dusting were investigated at different gas compositions at 680 °C. The carbon uptake rate on nickel exposed to supersaturated H₂/CO/H₂O gas mixtures (nominal a_C = 19) increased as p_CO increased from 0.31 to 0.68 atm, but decreased with further increase in p_CO. This behaviour was ascribed to the existence of parallel, independent reaction paths, and the rate was well described by

     

Thermochemistry of some binary alloys of noble metals (Cu, Ag, Au) and transition metals by high temperature direct synthesis calorimetry

The standard enthalpies of formation of some noble metal–transition metal compounds have been measured by high temperature direct synthesis calorimetry at 1372 K. The following results (in kJ/mol of atoms) are reported; Cu₅₁Zr₁₄, −24.3±2.2; Cu₃Pd, −7.6±1.2; Cu₅₁Hf₁₄, −22.9±2.3; Cu₃Pt, −10.8±1.1; AgTi, −1.6±2.4; AgTi₂, −2.3±1.1; Ag₂Y, −26.6±1.2; Au₂Sc, −83.4±1.8; Au₄Sc, −57.3±1.5; AuMn, −25.3±1.7; Au₂Y, −84.6±1.8; Au₃Y, −69.1±1.9; Au₃Pd, −7.9±1.9. The results are compared with some earlier experimental values obtained by EMF, by vapor pressure and by calorimetry. They are also compared with the predicted values of Miedema and coworkers. We will present a systematic picture of how the enthalpies of formation may be related to the atomic number of the transition metal.     

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).     

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.     

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.     

Gas atomization of Cu-modified AB5 metal hydride alloys

Gas atomization together with a hydrogen annealing process has been proposed as a method to achieve improved low-temperature performance of AB₅ alloy electrodes in Ni/MH batteries and restore the original cycle life which was sacrificed by the incorporation of copper in the alloy formula. While the gas atomization process reduces the lattice constant aspect ratio c/a of the Cu-containing alloys, the addition of a hydrogen annealing step recovers this property, although it is still inferior to the conventionally prepared annealed Cu-free alloy. This observation correlates very well with the cycle life performance. In addition to extending the cycle life of the Cu-containing metal hydride electrode, processing by gas atomization with additional hydrogen annealing improves high-rate, low-temperature, and charge retention performances for both Cu-free and Cu-containing AB₅ alloys. The degradation mechanisms of alloys made by different processes through cycling are also discussed.