Roles of aluminium and chromium in sulfidation and oxidation of sputter-deposited Al- and Cr-refractory metal alloys

Our recent results of the sulfidation and oxidation behavior of sputter-deposited Al- and Cr-refractory metal alloys at high temperatures are reviewed, and the roles of the aluminum and chromium in sulfidation and oxidation of these alloys are discussed in this paper. Niobium, molybdenum and tantalum are highly resistant to sulfide corrosion. Their sulfidation resistance is further enhanced by alloying with aluminum. Although Cr-refractory metal alloys also reveal high sulfidation resistance, their sulfidation rates do not become lower than those of the corresponding refractory metals. The sulfide scales formed on the Al-refractory metal and Cr-refractory metal alloys consist of two layers, comprising an outer Al₂S₃ or Cr₂S₃ layer and an inner refractory metal disulfide layer. The inner layer has a columnar structure, and the growth direction of the refractory metal disulfides is perpendicular to 0 0 1 direction. Intercalation of Al³⁺ ions into NbS₂ and a decrease in the sulfur activity at the outer layer/inner layer interface by the presence of the Al₂S₃ layer are probably responsible for the improvement of the sulfidation resistance by the addition of aluminum. The oxidation resistance of niobium and tantalum is improved more effectively by the addition of chromium rather than aluminum. Although preferential oxidation of chromium does not occur, an outer protective Cr₂O₃ layer in the oxide scales is formed on Cr-rich Cr-Nb and Cr-Ta alloys due to outward diffusion of Cr³⁺ ions. In contrast, continuous alumina layer cannot be formed on the Al-Nb and Al-Ta alloys, and the alloys reveal a pest phenomenon at 1073 K, and at higher temperatures rapid oxidation occurs. Concerning the oxidation of molybdenum, the addition of aluminum, which has higher activity for oxidation than chromium, is more effective in improving the oxidation resistance of molybdenum than chromium addition, since preferential oxidation of aluminum suppresses the formation of volatile molybdenum oxide.     

High-temperature sulfide corrosion of cobalt-chromium alloys

The kinetics and mechanism of high-temperature sulfidation of cobaltchromium alloys were investigated. It has been found that the sulfidation rate of low-chromium alloys, up to 2% Cr, is higher than that of pure cobalt. The one-layer homogeneous scale on these alloys is a solid solution of chromium sulfide in cobaltous sulfide. In the case of alloys containing more chromium, up to 43%, sulfidation rate decreases gradually with chromium content, the scale consisting of two layers. The growth mechanism of scales as well as the influence of chromium concentration on sulfidation rate is discussed.This work was carried out under contract No. 03.10 coordinated by the Institute of Physical Chemistry, Polish Academy of Sciences.     

Kinetics and mechanism of the sulfidation of Fe-Mo alloys

Iron-molybdenum alloys containing up to 40 wt.% molybdenum were exposed to sulfur vapor at a partial pressure of 0.01 atm at temperatures of 600–900°C. Sulfidation kinetics were measured over periods of up to 8 hr using a quartz-spring thermogravimetric method. The sulfidation kinetics of all alloys studied obeyed the parabolic rate law. The sulfidation rate of iron was found to be reduced by factors of 60 at 900°C and 120 at 600°C by the addition of 40 wt.% molybdenum. Duplex sulfide scales formed on all alloys at all temperatures, the scales consisting of an inner layer of mostly MoS₂ and an outer layer of FeS. Platinum markers were located at the interface between the outer and inner scales, showing that outward iron diffusion and inward sulfur diffusion through the inner layer occurred. The improved sulfidation resistance was attributed to the formation of the MoS₂, which acted as a partially protective barrier to the diffusion of the reacting species. Sulfidation activation energies were found to range from 24.3 to 28.5 kcal mole for the alloys compared to 20.6 kcal/mole, for pure iron. The rate-controlling step was outward iron diffusion through the outer iron sulfide layer.     

The effects of aluminum alloying additions on the sulfidation behavior of iron

As part of an overall research program to examine the possible development of sulfidation-resistant alloys, the corrosion behavior of binary iron-based alloys containing 5, 10, and 20 wt. % of aluminum has been studied in a hydrogenhydrogen sulfide atmosphere for the temperature range 800–1000°C. Details of the observed overall reaction kinetics, the scaling kinetics, and the scales' morphology and composition are given. By considering these results in relation to those reported in an earlier paper concerning the sulfidation of an iron- 5 wt. % aluminum alloy at a lower temperature range, 500–700°C, it has been possible to ascertain the usefulness of aluminum as an alloying addition whereby the sulfidation resistance of iron might be improved. It is concluded that although aluminum can be quite effective in this context at relatively low reaction temperatures at only the 5 % concentration level, at the elevated temperatures likely to be of more industrial importance its usefulness is not maintained, even at the 20% level. The possible reason(s) for the inferior high-temperature performance of these alloys are discussed.     

Sulfidation-oxidation of advanced metallic materials in simulated low-Btu coal-gasifier environments

The corrosion behavior of structural alloys in complex multicomponent gas environments is of considerable interest for their effective utilization in coal conversion schemes. Little understanding of the degradation mechanisms of advanced high-temperature alloys in conditions typical of low-Btu coal gasifiers presently exists. Analysis of scale and subscale characteristics of several alloy types after exposure to the aggressive simulated low-Btu gasifier environments yielded a reaction model for these sulfidation-oxidation conditions. Initial competition between reactive metal oxide and base metal sulfide nuclei is followed by base metal sulfide overgrowth, chromium sulfide formation at the scale-metal interface, dissociation near voids in the subscale, and internal chromium sulfide precipitation. Additions of aluminum, titanium, and an oxide dispersion improve the sulfidation resistance by increasing the number of oxide nucleation sites and their growth kinetics on the surface in the crucial competition stage. Thermogravimetric tests carried out in three mechanistic regimes agreed with these hypotheses.     

Corrosion of nickel-base and iron-base alloys in a simulated fluidized-bed coal-combustion environment

The modes of initiation and propagation of corrosion attack on a series of high-temperature alloys were studied in synthetic gas mixtures at 900°C. The gas mixtures were intended to simulate the oxygen and sulfur partial pressures experienced in reducing zones in a coal-fired fluidized-bed combustor and comprised mixtures of CO, CO₂, and SO₂. The alloys studied were candidates for in-bed heat exchanger tubing for an air-heater cycle operating at 843°C and 300–500 psig and so ranged from type 300-series stainless steels to nickel-base alloys. With the exception of two FeCrAlY alloys and types 304 and 347 stainless steels, it was found that sulfidation corrosion could be initiated on all the alloys within 0.25 hr; the rate of propagation of the corrosive attack depended on the flux of SO₂ in the environment and on the nickel content of the alloys. The presence of iron in the alloys appeared to slow the initiation of sulfidation, by forming a continuous iron oxide layer. The effects of various alloying additions are discussed, and a schematic model for the initiation of sulfidation is proposed.     

Evaluation of high temperature corrosion resistance of a Ni3Al (Mo) alloy

The sulfidation/oxidation and carburization resistances of a Ni₃Al(Mo) (IC-6) alloy at high temperatures were investigated in this work. The corrosion kinetics of the IC-6 alloy was found to follow parabolic rate law in an environment of high partial pressures of sulfur (10⁻⁵ atm) and low partial pressures of oxygen (<10⁻²⁰ atm) at 700 °C. Because the Ni sulfides are readily formed at the testing temperature, the sulfidation/oxidation resistance of the IC-6 alloy is similar to that of commercial Ni–Cr alloys in the current environments, although IC-6 is alloyed with Al. Compared with the HP heat resistant steel which is commonly used in the petrochemical industry, the IC-6 alloy possesses significantly improved resistance to carburization at 1100 °C. The mechanisms governing the corrosion attack in the environments used in this investigation were also discussed.     

Influence of solid-state CaS-CaO-CaSO4 deposits on corrosion of high-temperature alloys in simulated FBC environments

This study addresses questions concerning the likelihood of sulfidation attack of heat-exchanger alloys beneath deposits of sulfur-sorbent material in fluidized-bed combustors. Alloy specimens were exposed at 900°C in calcium sulfate-calcium oxide and calcium sulfide-calcium oxide mixtures, in environments in which the oxygen partial pressures were fixed at values corresponding to the equilibrium values for each solids mixture, using controlled ratios of CO and CO₂. The only source of sulfur in these systems was the calcium sulfate or sulfide. Sulfidation attack of nickel-base alloys occurred in both mixtures, the calcium sulfide-calcium oxide mixture being the more aggressive. Iron-base alloys were less susceptible to attack, although susceptibility increased with increasing nickel content. FeCrAlY-type alloys were resistant to attack. Comparison with corrosion behavior under conditions in which the oxygen and sulfur partial pressures were the same as those used here, but in which the sulfur source was in the gas phase, indicates that the form of the sulfidation attack is similar but that its progress is much slower under solid deposits.     

Effect of presulfidation on the oxidation behavior of Co-based alloys. Part I. Presulfidation at sulfur partial pressures above the dissociation pressure of cobalt sulfide

The effects of presulfidation in H₂-H₂S atmospheres of sulfur activity sufficient to form cobalt and chromium sulfides on the oxidation rates of Co-Cr binary alloys containing 0–25 wt.% Cr and Co-25 wt.% Cr alloys containing 0–2 wt.% C have been investigated. Presulfidation increases the oxidation rate, but the effect is not very dramatic. Carbon additions to the Co-25 wt.% Cr alloy progressively increase the oxidation rate, but not to as great an extent as a simple model based on the reduction of the chromium activity in the alloy. Sulfur released from the preformed sulfides by oxidation diffuses into the alloy precipitating fresh sulfides, there appears to be no outward diffusion of sulfur through the oxide scale. These internal sulfides have a liquid-like morphology in cobalt-base alloys when the oxidation is carried out at 1000°C, as compared to 800°C in corresponding nickel-base alloys. When the sulfide layer produced during the presulfidation is thin, so that oxidation destroys the continuous sulfide layer, the subsequent scale morphologies after oxidation exhibit many features similar to samples subjected to hot corrosion in environments containing sodium sulfate.     

Effect of Al on the high-temperature sulfidation of Fe-30Nb

Fe-30Nb-Al alloys containing up to 9.1 wt.% Al were sulfidized at 0.01 atm sulfur vapor over the temperature range of 600–900°C. The sulfidation kinetics followed the parabolic rate law for all the alloys at all temperatures. The parabolic rate constants decreased with increasing Al content. Extremely slow sulfidation rates, even slower than that of pure Nb at low temperatures, were observed for alloys containing high Al (>4.8 wt.%). Duplex sulfide scales formed on alloys containing small amounts of Al. The outer layers were compact FeS, while the inner layers were a double sulfide, Fe _xNb₂S₄,containing partially sulfidized intermetallic islands. Very thin scales formed on the alloys containing high Al, but the nature of the scales is unknown. The intercalation of Al into the Nb-sulfides and the associated charge transfer induced a blockage of the transport of iron through the sulfide as well as a greater incorporation of Nb into the scale.     

Isothermal and cyclic oxidation resistance of boron-modified and germanium-doped silicide coatings for titanium alloys

Since titanium alloys with an adequate balance of mechanical properties and high-temperature oxidation resistance have not been developed, protective coatings are required. In our previous paper, B-modified and Ge-doped silicide diffusion coatings grown on CP Ti, Ti–24Al–11Nb, Ti–22Al–27Nb, and Ti–20Al–22Nb by the halide-activated, pack-cementation method were described. In this study, isothermal and cyclic oxidation were used to evaluate the oxidation performance of these coatings in comparison to uncoated substrates. The rate-controlling mechanism for isothermal oxidation at high temperature was solid-state diffusion through a SiO₂ scale, while the mechanism for low-temperature oxidation involved grain-boundary diffusion through TiO₂. Both isothermal and cyclic oxidation rates for the B-modified and Ge-doped silicide coatings were much slower than for pure TiSi₂. Oxygen contamination was not detected by microhardness measurements in the coated substrates after 200 oxidation cycles at 500–1000°C for the Ti–Al–Nb alloys, or at 500–875°C for CP Ti. The excellent oxidation resistance for the optimum coating compositions is discussed.     

Sulfidation properties of austenitic Fe-Mn and Fe-Mn-Al alloys under low sulfur vapor pressure (8 Pa) in the temperature range 873–1173 K

The sulfidation properties of austenitic Fe-Mn and Fe-Mn-Al alloys containing small amounts of carbon have been characterized with respect to the sulfidation kinetics, scale morphological development, structures, and composition of the sulfide phases. The alloys contained 21–40 wt. % Mn and 2.5–8 wt.% Al. The sulfide phase was monosulfide of manganese and iron containing the other metallic elements in solid solution. Two regimes of sulfidation categorized by slow and fast reaction rates were exhibited by all alloys when sulfidized in sulfur vapor at = 8 Pa and over the temperature range 873–1173 K. In the slow regime, a compact duplex -Mn(Fe)S/Fe(Mn)S scale evolved by a classical parabolic law associated with metal diffusion in scale. A porous microcrystalline mixed scale of the above sulfides evolved in the regime of rapid sulfidation by quasilinear kinetics associated with sulfur ingress through the porous scale.     

Oxidation and sulfidation behavior of Fe-20Cr-16Ni-4Al-1Y2O3 oxide-dispersion-strengthened alloy

High-temperature oxidation and sulfidation studies were conducted on an oxide-dispersion-strengthened alloy of composition Fe-20Cr-16Ni-4Al-1Y₂O₃. The oxidation studies were conducted in air and low-PO₂ environments over a temperature range of 650–1200°C. Results are also reported on the sulfidation resistance of preformed oxide scales and the influence of reoxidation of preformed sulfide scales. Detailed microstructural results and x-ray diffraction analysis data are presented to substantiate the corrosion behavior of the alloy over the wide range of conditions anticipated in fossil-energy systems. Data are also presented on the corrosion behavior of the alloy in oxygen/sulfur mixed gas atmospheres, and the results are used to compare the corrosion behavior of the present alloy with other chromia- and alumina-forming alloys.     

Corrosion of some pure metals in basaltic lava and simulated magmatic gas at 1150°C

The extraction of thermal energy from subterranean magma can be achieved by the use of a suitable heat exchanger extending into the molten rock. Although the engineering feasibility of this scheme has not been proven, engineering data, including materials compatibility information, will be required ultimately. The work summarized in this paper was designed to provide an understanding of the reaction mechanisms and modes of degradation of various metals so that the best generic types of alloys can be selected for structural components and instrumentation. Fifteen pure metals were studied. These included base metals such as iron, nickel, and cobalt; some precious metals: platinum, rhodium, and palladium (possible thermocouple or lead-wire materials); refractory metals: tungsten, molybdenum, tantalum, niobium, vanadium, and rhenium; plus other high melting metals such as titanium and zirconium. Samples were exposed to basaltic lava at 1150°C for periods of 24 and 96 hr. A cover gas was used to produce oxygen and sulfur fugacities corresponding to those of the gases dissolved in basaltic melts. The corrosion behavior can be classified into five categories: (A) no attack (Pt and Re); (B) slight oxidation (Cr and Mo); (C) heavy oxidation (W, Ta, Nb); (D) sulfidation (Fe, Ni, Co, Pd, Rh); and (E) reaction with lava constituents (V, Ti, Zr). Group (A) metals were inert for all practical purposes. Group (B) metals formed thin adherent oxides initially, under which sulfides eventually formed in the substrate. Attack was minimal. Group (C) metals exhibited extensive oxide formation and virtually no sulfidation. Some reaction between the base-metal oxides and those in the lava took place. Group (D) metals all formed liquid sulfides which penetrated the substrate grain boundaries. All of these metals except cobalt were completely degraded. Cobalt was only partially penetrated by the liquid sulfide formed. Group (E) metals formed silicates, oxides, mixed oxides, and dissolved oxygen in the metal which completely embrittled the metal substrate. A small amount of sulfidation occurred, but sulfidation played virtually no role in the corrosion of these metals. Extensive analyses of the reaction products by scanning electron microscopy, X-ray energy dispersive analysis, electron microprobe analysis, and metallography are presented for each metal. The products formed are discussed with reference to thermodynamic stability diagrams, and the reaction path concept is used to explain some of the corrosion product morphologies.This work supported by the United States Department of Energy under Contract AT(29-1)-789.on leave at Sandia Laboratories, Albuquerque, New Mexico.     

Effect of 1 wt.% yttrium addition on high-temperature sulfidation behavior of Fe-15Cr-4Al alloy

High-temperature sulfidation studies have been carried out on Fe-15Cr-4Al with and without 1% Y in the temperature range 700–1000°C in an H ₂-H₂ S environment over the sulfur pressure range of 10 ^–9–10^–3 atm. Two-layered and three-layered sulfide scales were observed in both alloys at low and high sulfur pressures, respectively. The pegging phenomenon, similar to that occurring in high-temperature oxidation, across the innermost layer and substrate was observed in the case of the yttrium-containing alloy. Yttrium was found to be associated with aluminum and chromium sulfides. The role of yttrium was more evident at low than at high sulfur pressures and was found to reduce the parabolic rate constants by a factor of about one-half to one-seventh, respectively.     

High-temperature oxidation of Fe-Cr-Al-Si alloys extruded into honeycomb structures

The oxidation behavior of Fe–20Cr–5Al–(0.5–5)Si and Fe–(12–20)Cr–(5–7)Al–(1–2)Si alloys extruded into honeycomb structures has been investigated at 1150°C in air for up to 500 hr. The oxidation weight gains decrease with increasing Si and Cr contents in the 5-Al alloys. Si additions are more efficient than Cr additions to reduce the weight gain. Increasing Si content in the 5-Al alloys suppresses the formation of an iron-chromium complex oxide, forming mullite and vitreous silica in the scale, although the location is not clearly indicated. The 5-Si alloy shows anisotropy in elongation of the honeycomb specimen during oxidation in the Fe–20Cr–5Al–xSi alloys, whereas alloying with Si and Cr does not improve the oxidation resistance of the 7-Al alloys significantly. These results are explained by Wagner's theory of a secondary getter. However, we point out additionally that the difference between Si and Cr in the Pilling-Bedworth ratio and the solubility of their oxides in the Al₂O₃ scale may contribute to the significant effect of Si additions. Finally, this paper demonstrates that the selected Fe–Cr–Al–Si honeycombs having walls 200 m thick show excellent oxidation resistance over 500 hr at 1150°C in air. The time to catastrophic oxidation is roughly proportional to the wall thickness in extruded honeycombs.     

THE SULFIDATION/OXIDATION RESISTANCE OF TWO Ni-Cr-Al-Y ALLOYS AT 700℃

The high temperature corrosion resistance of Ni-25.gCr-13.5Al-1.2Y-0.6Si and Ni-10.2Co-12.4 Cr-16.0A l-0.5 Y-0.2Hf alloys was assessed in sulfidation/oxidation environments.In the environment with a sulfur partial pressure of 1Pa.and an oxygen partial pressure of 10-19Pa,both these alloys exhibited three distinct stages in the weight gain-time curve when tested at 700℃.In the initial stage,selective sulfidation of Cr suppressed the formation of the other metal sulfides,resulting in lower weight gains.In the transient stage,breakdown and cracking of Cr sulfides and insufficient concentration of Cr at the outer zone led to the rapid formation of Ni sulfides and a rapid increase in weight.In the steady-state stage,corrosion was controlled by the diffusion of anions and/or cations,which led to a parabolic rate law.     

Mixed Sulfidation/Carburization Attack on Several Heat-Resistant Alloys at 900°C

A sulfidation/carburization study of seven commercial heat-resistant alloyswas carried out at 900°C in a H₂–25 vol.%CH₄–14.8N₂–4CO–0.6CO₂–0.6H₂Satmosphere. The equilibrium partial pressures for oxygen (O₂) andsulfur (S₂) were 1.1×10^–22 and 4.1×10^–8 atm,respectively, and the carbon activity for this system was unity. The time ofexposure was 500 hr. Relatively thick, mixed sulfide scales were formed onall of the alloys tested. In addition, internal carburization occurred inall of the alloys. Using metal loss (i.e., the reduction in samplethickness) plus internal attack (internal sulfidation plus internalcarburization) as a performance criterion, an alloy with a nominalcomposition of Ni–29 wt.% Co–28Cr–2.75Si performed thebest, showing 0.71 mm of attack. An alloy with a nominal composition ofFe–20 wt.% Ni–25Cr performed the worst, being totally consumedby the test (>3.18 mm of attack). Alloys containing relatively highamounts of silicon (>2.5%) showed a dramatic increase in theirsulfidation resistance compared to the other alloys containing lowersilicon contents. The amount of iron present within a given material playeda dominant role in the carburization attack that occurred, with as expected,high-iron alloys showing significant internal carburization because of ahigh solubility and diffusivity of carbon in the matrix. The importance ofthe various alloying elements with respect to sulfidation and carburizationresistance is discussed.     

The Sulfidation/Oxidation Resistance of Two Ni-Cr-Al-Y Alloys at 700℃

The high temperature corrosion resistance of Ni-25.9Cr-13.5Al-1.2Y-0.6Si and Ni-10.2Co-12.4Cr-16.0Al-0.5Y-0.2Hf alloys was assessed in sulfidation/oxidation environments.In the environment with a sulfur partial pressure of 1Pa. and an oxygen partial pressure of 10^-19Pα,both these alloys exhibited three distinct stages in the weight gain-time curve when tested at 700℃.In the initial stage, selective sulfidation of Cr suppressed the formation of the other metal sulfides,resulting in lower weight gains.In the transient stage, breakdown and cracking of Cr sulfides and insufficient concentration of Cr at the outer zone led to the rapid formation of Ni sulfides and a rapid increase in weight.In the steady-state stage, corrosion was controlled by the diffusion of anions and/or cations, which led to a parabolic rate law.     

饰用银合金的开发及应用现状

简要分析了饰用银合金变色的原因,指出银的硫化和铜的氧化红斑是导致银合金变色的主要原因;介绍了贵金属类、稀土类、其它氧活性元素类合金化抗变色银合金的研究概况,探讨了当前首饰企业在使用杭变色银合金中存在的杭变色性能不足、硬度偏低、铸造缺陷、抛光缺陷等问题,并提出了饰用银合金的性能评价方法.