An investigation of the growth-mechanism of Cr2O3 on pure chromium in 1 atm oxygen at 950° C

The growth mechanism of Cr₂O₃ scale formed when pure chromium was oxidized at 950°C in oxygen at 1 atm pressure has been investigated. Isotope tracer techniques were used to determine the growth sites of the oxide. The scale was buckled extensively, with the convex side always toward the gas, never the reverse. The following growth-mechanism is proposed. Initially growth occurs entirely by cation diffusion, with new oxide being formed at the oxide-gas interface. Then, at a time that is not the same for all parts of the scale, the growth mechanism changes to one in which new oxide is formed within the outer part of the scale.     

Chromium transport through Cr2O3 scales I. On lattice diffusion of chromium

Cr specimens preoxidized at 1100–1300°C to give Cr₂O₃ scales with varying thicknesses and microstructures have been treated at temperature in high vacuum. During the high vacuum treatment the specimens lose weight due to outward Cr transport through the Cr₂O₃ scales. The initial rate of weight loss gradually diminishes, but eventually the weight loss reaches a linear rate. Concurrently the Cr₂O₃ scale exhibits grain growth and densifies. It is concluded that the mode of outward chromium transport gradually changes during the high vacuum treatment: from lattice and grain-boundary diffusion and possibly vapor transport along microcracks during the initial stage to lattice diffusion only for the densified scales. It is concluded that chromium diffuses by an interstitial type mechanism. Self-diffusion coefficients of Cr in Cr₂O₃ at the Cr-Cr₂O₃ phase boundary have been calculated from the linear rates of chromium transport for different defect structure situations.     

Chromium transport through Cr2O3 scales. II. Changes in scale morphology during high vacuum treatment of oxidized chromium specimens

Chromium specimens oxidized at 1200 and 1300° O to give Cr₂O₃ scales with varying thicknesses have been high vacuum annealed for extended periods at temperature. During the high vacuum anneal chromium is transported through the scale and evaporates from the scale surface. Initially the rate of chromium evaporation decreases with time as a result of recrystallization and densification of the scale. On extended high vacuum treatment the rate of chromium evaporation again increases and major changes in scale morphology takes place. The outer scale surfaces develop hollows in the oxide grains while the grains protrude from the scale at the inner surfaces. The morphological changes are interpreted in terms of differences in diffusion rates along grain boundaries and through the lattice and resultant variations in surface energy along the surfaces.     

Diffusion of18O in massive Cr2O3 and in Cr2O3 scales at 900°C and its relation to the oxidation kinetics of chromia forming alloys

The lattice and grain-boundary diffusion coefficients of¹⁸O atP _{O 2}=0.1 atm and at 900°C were determined in massive Cr₂O₃ and in Cr₂O₃ scales which were grown on a Ni–30Cr alloy. The diffusion profiles were established by SIMS and analyzed considering two domains in the case of polycrystalline Cr₂O₃ (massive or scales), the first one relative to apparent diffusion and the second to grain-boundary diffusion. A ridge model is proposed for Cr₂O₃ scales to modify thef value, fraction of sites associated with the grain boundary. With such a model,f is equal to 0.0006 and 0.0005 for the scales formed during 15 hr and 165 hr, respectively. The oxygen-lattice diffusion coefficients determined in Cr₂O₃ scales are in very good agreement with those in massive Cr₂O₃. With some assumptions, our diffusion data lead to a calculated parabolic oxidation constant equal to the experimental one. Scale growth occurs by countercurrent diffusion of oxygen and chromium, mainly by grain-boundary diffusion.     

Evaporation of Cr2O3 in Atmospheres Containing H2O

Stainless steels in atmospheres containing H₂O form a Cr₂O₃ scale in the early stage of oxidation. However, the Cr₂O₃ scale gradually degrades with time. In order to determine the effect of H₂O on the deterioration of a Cr₂O₃ scale, the evaporation behavior of Cr₂O₃ in N₂–O₂–H₂O atmospheres was investigated. The rate of mass loss in an N₂–O₂–H₂O atmosphere was found to be one order of magnitude higher than the rates in N₂–O₂ and N₂–H₂O atmospheres, indicating that deterioration of the Cr₂O₃ scale is likely to occur in mixed atmospheres of oxygen and water vapor. Volatilization of Cr₂O₃ is probably based on the following reactions: 1/2Cr₂O₃(s)+3/4O₂(g)+H₂O(g)=CrO₂(OH)₂(g). However, it is also speculated that the reaction, Cr₂O₃(s)+2/3O₂(g)=2CrO₃(g), affects the evaporation of Cr₂O₃ at temperatures higher than 1323 K. The evaporation rate of Cr₂O₃ is roughly comparable to the growth rate of the Cr₂O₃ scale. Therefore, a Cr₂O₃ scale can be degraded by the evaporation of Cr₂O₃.     

Effect of Y2O3 additions on the plasticity of sintered Cr2O3

We have carried our constant strain-rate compression tests on polycrystalline Cr₂O₃ and Cr₂O₃ doped with 0.09 wt. % Y₂O₃ to establish whether there exists an effect of Y₂O₃ on the plasticity of Cr₂O₃. This study is motivated by previous work on the oxidation of alloys containing reactive-element additions. In that work, it has been observed that the addition of oxygen-active elements, such as Y to alloys that form Cr₂O₃ or Al₂O₃ oxide layers upon exposure at high temperature, strongly enhances the adhesion of the oxide layer to the base alloy as compared with alloys without reactive-element additions. We have found that at 1200°C (1) chromia exhibits limited plasticity at high temperatures, and (2) the presence of Y in the oxides does not enhance plasticity compared with addition-free oxides.     

Mechanical and tribological properties of plasma-sprayed Cr3C2-NiCr, WC-Co, and Cr2O3 coatings

Mechanical properties such as Young’s moduli and fracture toughness of plasma-sprayed Cr₃C₂-NiCr, WC-Co and Cr₂O₃ coatings were measured. The tribological properties of the three kinds of coatings were investigated with a block-on-ring self-mated arrangement under water-lubricated sliding. Furthermore, the influences of the mechanical properties on the tribological properties of the coatings were also examined. It was found that the Young’s moduli, bend strengths and fracture toughness of the coatings were lower than the corresponding bulk materials, which may be attributed to the existence of pores and microcracks in the coatings. Among the three kinds of coatings, the magnitude of wear coefficients, in decreasing order, is Cr₃C₂-NiCr, WC-Co and Cr₂O₃, and the wear coefficient of Cr₂O₃ coating was less than 1 × 10⁻⁶mm³N⁻¹m⁻¹. The wear mechanisms of the coatings were explained in terms of microcracking and fracturing, and water deteriorated wear performance of the coatings. The higher the fracture toughness and the lower the porosity and length of microcracking of the coating, the more the wear-resistance of the coating.     

Influence of a mixed Al2O3+Cr2O3 superficial coating on the non-isothermal oxidation behavior of Fe and Fe-Cr alloys

The non-isothermal oxidation behavior of electrolytic-grade iron and Fe-Cr alloys in dry air has been studied using linear heating rates of 6 K/min, 10 K/ min, and 15 K/min up to a final temperature of 1273–1473 K. Some of the iron and iron-chromium alloy samples were given a surface treatment by dipping them in an aqueous solution containing both Cr and Al ions before their oxidation studies. This pretreatment has resulted in improved oxidation resistance and scale adherence as depicted by no scale rupture even after a second thermal cycle. Mass changes were recorded gravimetrically, and scales have been characterized by SEM, EPMA, and x-ray diffraction analyses.     

Plasma spray coating of spray-dried Cr2O3/wt.% TiO2 powder

An agglomerated Cr₂O₃/wt.%TiO₂ powder has been fabricated by the spray drying process under different parameters. The spray-dried powder has well-agglomerated particles of spherical shape. In the conditions of the high slurry feed rate and low binder concentration in the slurry, the powder has large cavities inside some particles and ruggedness over their surface. The optimum plasma spray feed rate has been found by examining the spraying behavior of the powder and melted state of particles. The plasma spray coating has been performed under different process variables such as spraying distance and plasma power. These parameters strongly affect the characteristics of the coated layer: microstructure, hardness, and bond strength.     

The effect of preoxidation on the sulfidation of Ni-20Cr (2–5)Al Alloys

Ni-20Cr alloys with 2, 3.5, and 5 wt.% Al have been preoxidized up to 100 hr at 1000°C in dry H₂, in H₂/23% H₂O and in air and subsequently exposed to an H₂/5% H₂S atmosphere at 750° C. During the preoxidation treatment different types of oxide scales were formed which affect the sulfidation protection in different ways. Optimum results were obtained for alloys with 3.5 and 5 wt.% Al after 20 hr exposure to dry H₂ at 1000°C. A thin Al₂O₃ scale is formed which decreases the sulfur attack by more than one order of magnitude. Preoxidation conditions for Ni-20Cr-2Al alloys in H₂ and for Ni-20Cr-2Al and Ni-20Cr-3.5Al in H₂/H₂O were observed to be less effective. No improvement was found for preoxidation in air or for Ni-20Cr-5Al alloys preoxidized in H₂/H₂O.     

High-temperature corrosion of Cr2O3-forming alloys in CO-CO2-N2 atmospheres

The corrosion of Fe–28Cr, Ni–28Cr, Co–28Cr, and pure chromium in a number of gas atmospheres made up of CO–CO₂(–N₂) was studied at 900°C. In addition, chromium was reacted with H₂–H₂O–N₂, and Fe–28Cr was reacted with pure oxygen at 1 atm. Exposure of pure chromium to H₂–H₂O–N₂ produced a single-phase of Cr₂O₃. In a CO–CO₂ mixture, a sublayer consisting of Cr₂O₃ and Cr₇C₃ was formed underneath an external Cr₂O₃ layer. Adding nitrogen to the CO–CO₂ mixture resulted in the formation of an additional single-phase layer of Cr₂N next to the metal substrate. Oxidizing the binary alloys in CO–CO₂–N₂ resulted in a single Cr₂O₃ scale on Fe–28Cr and Ni–28Cr, while oxide precipitation occurred below the outer-oxide scale on Co–28Cr, which is ascribed to the slow alloy interdiffusion and possibily high oxygen solubility of Co–Cr alloys. Oxide growth followed the parabolic law, and the rate constant was virtually independent of oxygen partial pressure for Fe–28Cr, but varied between the different materials, decreasing in the order chromium >Fe–28Cr>Ni(Co)–28Cr. The formation of an inner corrosion zone on chromium caused a reduction in external-oxide growth rate. Permeation of carbon and nitrogen through Cr₂O₃ is thought to be due to molecular diffusion, and it is concluded that the nature of the atmosphere affects the permeability of the oxide.     

The effect of an Al2O3 dispersion on the adhesion of Al2O3 scales on aluminized Co-10% Cr alloys

The beneficial effect of dispersions of reactive-metal oxide particles on the adhesion of Cr₂O₃ and Al₂O₃ scales formed on heat-resisting alloys is wellknown. It has been shown that an Al₂O₃ dispersion in an alloy can improve the adhesion of a Cr₂O₃ scale, and it is of particular interest in assessing the various theoretical proposals for the effect to determine whether such a dispersion can affect the adhesion of an Al₂O₃ scale. In this investigation, a Co–10% Cr–1 % Al alloy was first internally oxidized to form an Al₂O₃ dispersion. This alloy was then aluminized so that on subsequent oxidation an Al₂O₃ scale developed. It was shown that the dispersion did indeed improve the scale adhesion. The implications of this result are discussed.     

The influence of grain-boundary segregation of Y in Cr2O3 on the oxidation of Cr metal

The oxidation behavior at 900°C of pure Cr and Cr implanted with 2×10¹⁶ Y ions/cm² was studied. The kinetics of oxidation were measured thermogravimetrically and manometrically. The mechanisms of oxide growth were studied using¹⁸O-tracer oxidation experiments, and the composition and microstructure of the oxide scales were characterized by TEM and STEM. Segregation of Y cations at Cr₂O₃ grain boundaries was found to be the critical factor governing changes in the oxidation behavior of Cr upon the addition of Y. In the absence of Y, pure Cr oxidized by the outward diffusion of cations via grain boundaries in the Cr₂O₃ scale. When Y was present at high concentration in the scale, as when Cr implanted with 2×10¹⁰ Y ions/cm² was oxidized, anion diffusion predominated. It is concluded that strain-induced segregation of Y at grain boundaries in the oxide reduced the cation flux along the grain boundaries. The rate of oxidation was reduced because the grain-boundary diffusivity of cations became lower than the grain-boundary diffusivity of the anions, which then controlled the rate of oxidation. Changes in the relative rates of Cr³⁺ and O^2– transport, as well as a solute-drag effect exerted by Y on the oxide grain boundaries, resulted in changes in the microstructure of the oxide.     

High temperature oxidation of chromium nitrides

Chromium nitride powders oxidized to Cr₂O₃ noticeably above 400°C. Bulk chromium nitrides that were manufactured by sintering were oxidized between 900 and 1100°C in atmospheric air. The Cr₂O₃ layer that formed on bulk chromium nitrides having pores was relatively dense, and grew primarily via the inward transport of oxygen. The Cr₂O₃ layer to some degree deterred the nitrogen evolution from bulk chromium nitrides.     

Formation of Al2O3 Scales on Single-Phase RuAl Produced by Reactive Sintering

The oxidation behavior of single-phase RuAl produced by powder metallurgy combined with arc melting was investigated. Oxidation was conducted at 1000°C; oxide scale growth and phase formation were studied using scanning-electron microscopy (SEM) and x-ray diffraction (XRD). A dense protective scale with an Al–depleted sublayer was formed during oxidation. The oxide scale is the stable -Al₂O₃. The oxide-scale morphology shows the presence of whiskers, with a needle-like form, which suggests that the growth of the oxide scale is produced by outward diffusion of Al. At the beginning, oxidation follows a parabolic law, but, after 100 hr of oxidation; the growth rate is slower than expected from a parabolic law.     

Comparison of Oxidation Behavior and Electrical Properties of Doped NiO- and Cr2O3-Forming Alloys for Solid-Oxide, Fuel-Cell Metallic Interconnects

The goal of this paper was to determine if NiO-forming alloys are a viable alternative to Cr₂O₃-forming alloys for solid-oxide fuel-cell (SOFC) metallic interconnects. The oxide-scale growth kinetics and electrical properties of a series of Li- and Y₂O₃-alloyed, NiO-forming Ni-base alloys and La-, Mn-, and Ti-alloyed Fe–18Cr–9W and Fe–25Cr base ferritic Cr₂O₃-forming alloys were evaluated. The addition of Y₂O₃ and Li reduced the NiO scale growth rate and increased its electrical conductivity. The area-specific-resistance (ASR) values were comparable to those of the best (lowest ASR) ferritic alloys examined. Oxidation of the ferritic alloys at 800°C in air and air+10% H₂O (water vapor) indicated that Mn additions resulted in faster oxidation kinetics/thicker oxide scales, but also lower oxide scale ASRs. Relative in-cell performance in model SOFC stacks operated at 850°C indicated a 60–80% reduction in ASR by Ni+Y₂O₃, Ni+Y₂O₃, Li, and Fe–25Cr+La,Mn,Ti interconnects over those made from a baseline, commercial Cr₂O₃-forming alloy. Collectively, these results indicate that NiO-forming alloys show potential for use as metallic interconnects.     

Ti、Cr元素在Cr_2O_3薄膜中的扩散及其对摩擦学性能的影响

为了研究Ti和Cr元素在Cr_2O_3薄膜中的扩散行为及其对摩擦学性能的影响,对Cr_2O_3薄膜在1 000℃大气环境下进行不同保温时间的退火处理。利用扫描电子显微镜、三维轮廓仪、X射线衍射仪以及透射电镜对薄膜的表面形貌、成分以及结构进行分析,采用球盘式摩擦磨损试验机对Cr_2O_3薄膜的摩擦学性能进行研究。结果表明:在1 000℃下Inconel 718基材中的Ti和Cr会沿着Cr_2O_3薄膜的晶界扩散至薄膜表面,与大气中的氧反应生成由Cr_2O_3和Cr_2Ti_7O_(17)组成的复合相,并在薄膜表面晶界处堆积形成类网状凸起结构。随着保温时间的增加,这种类网状凸起结构的高度也越大。该结构可以显著降低薄膜的摩擦因数以及磨损率。当保温时间只有1 min时,薄膜的平均摩擦因数为0.35,磨损率为5.7μm~3/Nm。而当保温时间达到240 min时,薄膜的摩擦因数降至0.24,磨损率降至3.2μm~3/Nm。     

The influence of grain-boundary segregation of Y in Cr2O3 on the oxidation of Cr metal. II. Effects of temperature and dopant concentration

The oxidation behavior of pure Cr and Cr implanted with Y was studied as a function of temperature (900 and 1025°C) and ion-implantation dose (1×10¹⁵ and 2×10¹⁶ Y ions/cm²). The microstructures of the Cr₂O₃ scales were affected by both of the variables studied. Yttrium ions segregated at the grain boundaries in the Cr₂O₃ scales formed on the implanted alloys and the concentration of Y at the grain boundaries decreased with a decrease in the dose of implanted Y. The mechanism of growth of the Cr₂O₃ scales was altered by the presence of the Y ions at the Cr₂O₃ grain boundaries only when a critical concentration of Y at the grain boundaries was exceeded.     

High temperature oxidation of a (Cr2N−Mo2S3)/Cr double-layered coating deposited on steel by D.C. magnetron sputtering

A coating consisting of (Cr₂N−Mo₂S₃) overlay coating and an underlying Cr coating was deposited on a steel substrate by D.C. magnetron sputtering. The oxidation characteristics of the deposited double-layered coating were studied at temperatures ranging from 400 to 900 °C in air. The oxidation product was primarily Cr₂O₃. The unreacted coating beneath the oxide scale had some dissolved oxygen, sulfur, and iron. Oxidation of the coating occurred via complex routes such as the outward diffusion of chromium and nitrogen from Cr₂N and iron from the substrate, and the inward transport of oxygen from air, chromium from Cr₂N, and S from Mo₂S₃. This counter diffusion of various ions occurred easily via fine crystallites that constituted the coating, which had some solubility of S, O, and Fe.     

Effect of oxide grain structure on the high-temperature oxidation of Cr

Cr was oxidized in 1 aim O₂ at 980, 1090, and 1200°C. ElectropolishedCr and some orientations of etched Cr oxidize rapidly and develop compressive stress in the growing Cr₂O₃; other orientations oxidize slowly, apparently free of stress. SEM examination of fracture sections shows that the thick oxide is polycrystalline whereas the thin oxide on etched Cr is monocrystalline. It is deduced that the monocrystalline oxide grows by lattice diffusion of cations outward, and the polycrystalline layer by the two-way transport of cation diffusion outward and anion diffusion inward along oxide grain boundaries. The consequent formation of oxide within the body of the polycrystalline layer generates compressive stress and leads to wrinkling by plastic deformation. The activation energy for oxidation of Cr by cation lattice transport is 58 kcal/mole. Polycrystalline Cr₂O₃ forms on Fe-26Cr alloy, whether electropolished or etched; oxidation is accordingly rapid and accompanied by compressive stress.