Role of alloying elements in steels on metal dusting

Metal dusting of Fe-Ni-Cr alloys has been observed in industrial processes in strongly carburizing atmospheres at temperatures from 450°C to 800°C. At temperatures below 650°C the alloys are generally not able to form dense, well adherent oxide layers in spite of relatively high Cr-contents, therefore, metal dusting can take place. Already a lot of experimental work has been done to elucidate the mechanism and to compare the resistance against metal dusting for high alloy steels [1]. The intention of this study was to obtain additional information concerning the role of alloying elements and the effects of carbide precipitates in austenitic high alloy steels such as Alloy 800. The susceptibility to metal dusting was determined by measuring the metal loss under metal dusting conditions of Fe-20%Cr-32%Ni alloys modified with additions of different carbide formers (W, Mo, Nb) or oxide formers (Si, Al). The samples were exposed at 600°C in a CO-H₂-H₂O-gas mixture for repeated periods up to 500 – 1500 h. The attack by the oxidizing and carburizing atmosphere leads to the precipitation of internal carbides and metal dusting and more or less to formation of an oxide layer. In comparison to the undoped material, the addition of carbide formers retards the initiation of metal dusting attack. The additions of Si and Al seem to prevent metal dusting under the given laboratory conditions. When carbides are present at the metal surface, they affect the initial oxide growth and have a negative effect on the protectivity of scales. Very striking is the effect of Ce, this rare earth element is generally known to favour Cr-oxide formation and to improve the adherence of the oxide layer [2], but in the case of metal dusting it clearly enhances metal dusting and metal wastage.     

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.     

金属材料在高温碳气氛中的结焦与渗碳行为

叙述了碳气氛温度、金属材料种类及表面状态、构成碳源的气体种类、比例及流速等因素对金属材料结焦行为的影响，介绍了催化结焦的引发机理及其应用．然后以FeCrNi合金为例，分别介绍了内部渗碳和金属粉化这两种渗碳行为，并叙述了温度、碳气氛、合金材料晶体结构及成分构成的影响．最后分析了结焦和渗碳的关系，并介绍了添加结焦抑制剂、调整金属材料结构及成分构成、表面涂层化3种防护对策。     

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.     

Coking by metal dusting of steels

In process industries coking is an annoying phenomenon, the carbon deposition causes decrease of heat transfer and hinders gas flow. Coking in a process may indicate metal dusting, i.e. the disintegration of metals and alloys in carbonaceous atmospheres under formation of graphite and fine metal particles. The metal particles act as catalysts for vast coke formation. The thermodynamics, mechanisms and kinetics of metal dusting have been studied on iron and steels in synthesis respectively reduction gas CO-H₂- H₂O, here the aspects are presented of coking due to metal dusting. From the interplay of the metal disintegration and carbon deposition rather complex coupled kinetics are resulting, even different in a low temperature range where the decomposition of the intermediate cementite is rate determining and in a higher temperature range where the carbon transfer from the atmosphere is rate controlling. Coking by metal dusting can be suppressed in the same way as metal dusting, by sulfur addition to the atmosphere and/or by a stable dense protective oxide layer.     

焊接残余应力对接头尘化腐蚀影响的有限元模拟

金属尘化腐蚀是发生在含碳气氛中的一种高温腐蚀现象,碳在金属基体中的扩散是其过程的第一步.文中开发了碳扩散的耦合计算程序,进行了加热炉炉管焊态和工况温度状态下焊接残余应力对碳扩散影响的数值模拟,并与无应力状态下的碳扩散情况进行了比较.结果表明,在热影响区附近焊接残余应力梯度最大,对碳扩散的影响也最大,并且形成一个碳浓度峰值,在峰值两侧则会形成相应的碳浓度低谷,这是碳向高应力梯度区扩散所致.经过升温至工况温度后,残余应力得到松弛,对碳扩散的影响有所降低,可以延缓炉管的损坏时间.但由于残余应力的影响仍然存在,因此在炉管检修期间要注意焊接接头附近的尘化腐蚀情况.     

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.     

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.     

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.     

Orientation dependence of metal dusting nanoprocesses on nickel single crystals

Nanoprocesses of metal dusting have been studied on nickel single crystal surfaces by TEM and AEM techniques. The samples had been exposed to strongly carburizing conditions (a_C > 1) for 4 h at 650°C. On Ni(111) and Ni(110) epitaxial growth of graphite was observed, graphite layers with their basal planes had grown parallel to the surface and there was no indication of metal dusting attack. In contrast, on Ni(100) metal dusting had started by inward growth of graphite lattice planes oriented more or less vertically to the surface.     

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.     

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.     

Thermodynamics,mechanisms and kinetics of metal dusting

A survey is given on recent research on “metal dusting” i.e. a catastrophic carburization or rather graphitization of metals and alloys occuring in carbonaceous atmospheres at carbon activities a_C>1. The thermodynamics are explained, the mechanisms for iron, low and high alloy steels, nickel and Ni-base alloys are described and the kinetics derived for iron and low alloy steels. Protection against metal dusting is possible by the presence of sulfur in the atmosphere, since adsorbed sulfur retards carbon transfer and hems graphite nucleation. Also dense oxide layers are protective, the preconditions for the formation of Cr-rich protectivee layers on steels and Ni-base alloys are shortly presenteed.     

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-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.     

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 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.     

Coke formation in metal dusting experiments

Selected samples of carbonaceous deposit from metal dusting corrosion have been investigated. The morphology and the internal structure were examined by light microscopy, environmental scanning electron microscopy, and energy-dispersive analysis of X-rays. Alternating structures within the carbonaceous deposits have been observed, which suggest that metal dusting is an oscillatory corrosion process. The results are discussed in the light of reported examples of oscillatory high-temperature reactions. Nonequilibrium phases have been observed, and it is speculated on their origin and influence on metal dusting. The localized character of this corrosion type is emphasized. Conclusions about the influence of oxygen in the mechanism of metal dusting are given, leading to the idea of alternating carburizing and oxidizing conditions occurring in the metal dusting process.     

Influence of H2S on metal dusting

Presence of H₂S in a carburizing atmosphere causes S-adsorption which retards carbon transfer and deposition and can suppress metal dusting of iron and steels. In the latter process cementite Fe₃C is an intermediate, graphite deposition would initiate its decomposition but graphite nucleation is prevented by adsorbed sulfur. Thus continued Fe₃C growth can be observed in the presence of H₂S. Thermogravimetric studies in CO-H₂-H₂O-H₂S mixtures have been conducted at 500°C at various carbon activities a_C and H₂S/H₂-ratios. With increasing a_C higher H₂S/H₂-ratios are needed to suppress metal dusting, with increasing H₂S/H₂-ratio the kinetics of Fe₃C growth change from diffusion controlled parabolic kinetics to linear carbon transfer controlled kinetics. At very high a_C≥1000 besides Fe₃C also the Hägg carbide Fe₅C2 was observed as an outer layer on the cementite.     

外力作用对尘化腐蚀过程中碳扩散的影响

通过讨论当前常用的力学-化学耦合模型,用力学-化学耦合的激活体积模型研究外部载荷产生的应力对尘化腐蚀中碳扩散的影响。结果表明,拉应力促使碳元素的扩散,而压应力抑制其扩散。