Environmental Effects on Orthorhombic Alloy Ti-22Al-25Nb in Air Between 650 and 1000°C

The environmental behavior of an orthorhombictitanium-aluminide alloy, Ti-22Al-25Nb, was studied indry and humid air between 650 and 1000^°Cby scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction.Microhardness measurements were performed after exposureto gage hardening due to nitrogen and oxygen ingress.The parabolic rate constant of Ti-22Al-25Nb was of the same order as conventional titanium alloys andTi₃ Al-base titanium aluminides at and below750^°C. Between 800 and1000^°C, the oxidation resistance ofTi-22Al-25Nb was as good as -TiAl base aluminides;however, the growth rate changed from parabolic tolinear after several tens of hours at 900 and1000^°C. The mixed oxide scale consistedof TiO₂, AlNbO₄, andAl₂O₃ with TiO₂ beingthe dominant oxide phase. Underneath the oxide scale, anitride layer formed in the temperature rangeinvestigated and, at 1000^°C, internal oxidation was observed below thislayer. In all cases, oxygen diffused deeply into thesubsurface zone and caused severe embrittlement.Microhardness measurements revealed that Ti-22Al-25Nbwas hardened in a zone as far as 300 m belowthe oxide scale when exposed to air at900^°C for 500 hr. The peak hardnessdepended on exposure time and reached five times theaverage hardness of the bulk material under the aboveconditions.     

High temperature corrosion mechanisms of certain new TiAl-based intermetallic alloys in an aggressive H2/H2O/H2S environment at 850 °C

High temperature corrosion behaviour of three TiAl-based intermetallic alloys - Ti-44Al-8Nb-1B, Ti-46Al-8Nb-1B and Ti-48Al-2Nb-2Cr-1B (at.%) - was studied in an environment of H₂/H₂S/H₂O yielding p_S2 ∼ 6.8 × 10⁻¹ Pa and p_O2 ∼ 1.2 × 10⁻¹⁵ Pa potentials at 850 °C. The kinetic results obtained by a discontinuous gravimetric method indicate that increase in Al and Nb concentrations led to enhanced high temperature corrosion resistance, the corrosion resistance decreasing in the order: Ti-46Al-8Nb-1B > Ti-44Al-8Nb-1B > Ti-48Al-2Nb-2Cr-1B. The scale development studies using SEM, TEM, EDX, WDS and XRD confirmed the formation of a multilayered scale on all materials. An outer layer consisting of TiO₂ existed beneath which an Al₂O₃ layer was present. Then a layer of TiO₂ formed again, below which an Al-enriched NbAl₃ was observed. A TiS layer was found beneath the NbAl₃ layer. The formation of TiS led to the development of a NbAl₃ band between the multilayered scale and the substrate.     

Mechanism of isothermal oxidation of the intel-metallic TiAl and of TiAl alloys

The oxidation behavior of Ti36Al, Ti35Al-0.1C, Ti35Al-1.4V-0.1C, and Ti35 Al-5Nb-0.1C (mass-%) in air and oxygen has been studied between 700 and 1000°C with the major emphasis at 900°C. Generally an oxide scale consisting of two layers, an outward- and an inward-growing layer, formed. The outward-growing part of the scale consisted mainly of TiO₂ (rutile), while the inward-growing part is composed of a mixture of TiO₂ and -Al₂O₃. A barrier layer of Al₂O₃ on TiAl between the inner and the outer part of the scale was visible for up to 300 hr. Under certain conditions, the Al₂O₃ barrier dissolved and re-precipitated in the outer TiO₂ layer. This shift leads to an effect similar to breakaway oxidation. Only the alloy containing Nb formed a longlasting, protective Al₂O₃ layer, which was established at the metal/scale interface after an incubation period of 80–100 hr. During this time, Nb was enriched in the subsurface zone up to approximately 20 w/o. The growth of the oxide scale on TiAl-V obeyed a parabolic law, because no Al₂O₃ barrier layer formed; large Al₂O₃ particles were part of the outward-growing layer. A brittle 2-Ti₃Al-layer rich in O formed beneath the oxide scale as a result of preferential Al oxidation particularly when oxidized in oxygen. Oxidation in air can lead also to formation of nitrides beneath the oxide scale. The nitridation can vary between the formation of isolated nitride particles and of a metal/Ti₂AlN/ TiN/oxide, scale-layer system. Under certain conditions, nitride-layer formation seemed to favor protective Al₂O₂₃ formation at the metal/scale interface, however, in general nitridation was detrimental with the consequence that oxidation was generally more rapid in air than in oxygen.     

The influence of microstructure on the isothermal and cyclic-oxidation behavior of Ti-48Al-2Cr at 800°C

The isothermal and cyclic-oxidation behavior of the intermetallic Ti48Al-2Cr (at. %) alloy were studied at 800°C in air. Emphasis was placed on the effect of microstructures, in a range relevant for practical applications; i.e., duplex, near gamma, nearly lamellar, and fully lamellar; obtained by various heat treatments. The oxidation kinetics of the intermetallic alloy showed initially the formation of a relatively protective oxide scale. After an exposure time of about 10 hr the oxidation rate increased significantly, due to a Loss of protectivity of the oxide scale. The growth rate of the oxide scale, as well as its composition, structure and morphology showed no major relation to the microstructure of the base material. The oxidation of Ti-48Al-2Cr in air, initially resulted in the formation of -Al₂O₃, TiO₂ (rutile), Ti₂AlN, and TiN. After Longer exposure times, the mixed-oxide scale with an alumina-rich Layer at the outside was overgrown by the fast-growing TiO₂, responsible for the rapid kinetics. Using¹⁸O/¹⁵N experiments some mechanistic aspects were discussed in relation to the existence of a nitrogen-rich Layer near the scale/alloy interface. Thermal-cyclic-oxidation experiments up to 3000 1-hr cycles showed that spallation of the oxide scale initiated after about 175 1-hr cycles. Also in this case the growth rate of the oxide scale as well as its composition, structure and morphology showed no major relation to the microstructure of the base material.     

Hot-Corrosion Behavior of TiAl-Base Intermetallics in Molten Salts

The hot-corrosion behavior of Ti-50Al,Ti-48Al-2Cr-2Nb, and Ti-50Al10Cr alloys was investigatedin (Na, K)₂SO₄ andNa₂SO₄ + NaCl melts. TiAlintermetallics showed much better hot-corrosionresistance in (Na, K)₂SO₄ at900°C than the Ni-base superalloy K38G. Two types ofcorrosion products formed on Ti-50Al: some areas werecovered with a continuous Al2O3 scale, whereas otherareas formed a mixed Al₂O₃ +TiO₂ scale; TiS existed at the scale-alloyinterface. A mixed Al₂O₃ +TiO₂ scale formed on Ti-48Al-2Cr 2Nb, and nosulfide was found beneath the scale. An adherentAl₂O₃ scale, however, formed onTi-50Al-10Cr, which provided excellent hot-corrosionresistance. All three alloys suffered severe hotcorrosion in Na₂SO₄ + NaCl meltsat 850°C. A mixed Al₂O₃ +TiO₂ scale formed on all three alloys andmany voids and pits existed in the scale-alloy interface. Thehot-corrosion mechanisms are discussed.     

Oxidation behavior of an Fe-38Ni-13Co-4.7Nb-1.5Ti-0.4Si superalloy at high temperature in Ar-H2O atmospheres

The oxidation of an Fe-38Ni-13Co-4.7Nb-1.5Ti-0.4Si superalloy (Incoloy 909 type alloy), was investigated at temperatures between 1000 K and 1400 K in Ar-(1, 10%)H₂0 atmosphere using metallographic, electron probe microanalysis, and X-ray diffraction techniques. The oxide scales consist of an external scale and an internal scale which has an intergranular scale (above 1200 K) and an intergranular scale. The oxide phases in each scale are identified as-Fe₂,O₃ (below 1200 K) or FeO (above 1300 K) and CoO · Fe₂O₃ and FeO · Nb₂O₅, respectively. The morphologies, the oxide phases and the oxidation rates do not depend on the partial pressure of H₂O in the range between one and ten percent in Ar gas. The rate constants for the intergranular-scale formation in this alloy are about one-tenth as large as those in Fe-36%Ni alloy reported previously. At all the temperatures the scales grow according to a parabolic rate law and the apparent activation energies for the processes are estimated.     

Cyclic-oxidation behavior of TiAl and of TiAl alloys

The cyclic-oxidation behavior of (in w/o) Ti-36Al, Ti-35Al-0.1C, Ti-35Al-1.4V-0.1C and Ti-35Al-5Nb-0.1C was studied between 800 and 1000° C in air. A few experiments were also performed in oxygen. Scale spallation after oxidation in air occurs during cooling on TiAl, TiAl-C, and TiAl-V at or close to the metal/scale interface when a critical scale thickness has been achieved. This process repeats and can lead to a stratified scale. These three materials form scales composed of an inward-growing fine-grain mixture of TiO₂-Al₂O₃ and an outward-growing coarse-grain TiO₂ layer or TiO₂+Al₂O₃ mixture. The TiAl-Nb alloy had a significantly different behavior. The scale on this material grew very slowly because a protective Al₂O₃ layer formed at the metal/scale interface. This behavior resulted in much better resistance to spallation because the critical scale thickness was reached only after a much longer time, and is different from the behavior of the other three alloys. Oxidation in air leads to slight nitridation of the subsurface zone beneath the scale. In comparison to oxidation in air, oxidation in oxygen improves the cyclicoxidation behavior. Whereas the scale formed in air was uniformly thick over the entire surface, the scale grown in oxygen varied locally in structure and thickness. A large fraction of the surface was covered with a thin Al₂O₃ layer, while the remaining part formed a two-layer scale similar to that formed in air. The results are discussed briefly in the light of a recently published model for scale spallation under compressive stress, however, quantitative estimations are not possible due to a lack of relevant data.     

Oxygen exchange in oxidation of an Fe-20Cr-10Al alloy in ∼10 mbar O2/H2O-gas mixtures at 920°C

The oxidation of an alumina forming Fe-200-10Al alloy was studied in situ. The gas phase components in isotopically labeled 10 mbar O₂/H₂O-gas mixtures were measured from a virtually closed system at 920°C. Oxidation rates and oxygen-exchange rates were measured and related to each other. From the exchange rates the dissociation rates of O₂ and H₂O were calculated. These dissociation rates were in the early stage of oxidation found to be the same as the oxidation rate. The rate of oxygen exchange with the oxide can exceed the oxidation rate. From this follows that in analysis of a two-stage oxidation in^16,16O₂/^18,18O₂ the exchange rate of O between the oxide and gas phase has to be considered. The oxides formed in H₂O containing gas had a H/O-ratio of 0.1. Vacuum annealing of the alloy before oxidation increased the oxidation rate by a factor of 2.     

Ti-6Al-4V合金高温氧化行为及显微组织研究

分析了Ti-6Al-4V合金在900 _oC,930 _oC,960 _oC温度下的氧化行为及表层显微组织变化。在0.5-24 h时间内,氧化层不断增厚,越靠近表面氧化层越疏松。氧含量在氧化层/富氧层界面的5 μm内发生急剧下降,进入富氧层后缓慢下降直到稳定。氧化层中以TiO₂为主,同时也出现了Al₂O₃。富氧层中的a相含量远远高于基体内部,其晶粒尺寸也发生长大。富氧层深度与热暴露时间的关系可用对数函数描述,通过线性回归分析计算出了O在富氧层中的扩散激活能为206 kJ/mol。     

A comparative study on titania layers formed on Ti, Ti-6Al-4V and NiTi shape memory alloy through a low temperature oxidation process

For improving the bioactivity and biocompatibility of metals for medical applications, anatase titania layers were synthesized on Ti, Ti-6Al-4V and NiTi shape memory alloy (SMA) using the H₂O₂-oxidation and hot water aging treatment method at 80 °C. The thickness of the titania layers on Ti, Ti-6Al-4V and NiTi SMA was 7.43 ± 0.93 μm, 3.14 ± 0.38 μm and 4.04 ± 0.25 μm, respectively. X-ray diffraction (XRD) and transmission electron microscopic (TEM) analysis indicated that the titania layers formed were poorly crystalline anatase. Fourier transform infrared spectroscopy (FTIR) suggested that abundant Ti-OH functional groups were produced on titania, which could improve bioactivity of the metals. In addition, the titania layer formed on Ti substrate was shown to contain more molecularly chemisorbed water and Ti-OH functional groups than those on Ti-6Al-4V and NiTi SMA. Atomic force microscopic (AFM) results showed that the surface roughness values of metal samples depended on the scanning size and that surface roughness of samples significantly increased after the H₂O₂-oxidation and hot water aging treatment for all three metals. Compared to Ti-6Al-4V and NiTi SMA, the H₂O₂-treated and aged Ti samples exhibited the roughest surface. The wettability of samples was evaluated through water contact angle measurements. After the H₂O₂-oxidation treatment, the three metals exhibited high hydrophilicity. The bonding strength of titania layers on Ti, Ti-6Al-4V and NiTi was also investigated. Potentiodynamic polarization tests indicated that the corrosion resistance of H₂O₂-treated and aged Ti, Ti-6Al-4V and NiTi SMA was significantly improved due to the titania layer formation.     

Sulfidation behavior of Fe-25Cr-20Ni-1.5Al2O3 in simulated coal combustion/gasification environments

The effect of minor addition of -Al₂O₃ dispersoids on the sulfidation behavior of Fe-25Cr-20Ni was investigated over a range of pO₂, 1.13×10^–20 to 1.18×10 ****^–22 atm. at constant pS₂=1.22×10^–8 atm. Fe-25Cr-20Ni and Fe-25Cr-20Ni 1.5 Al₂O₃ with and without preformed oxide scales were exposed to bioxidant gas mixtures H₂/H₂O/H₂S/Ar at 700° C. Both isothermal and cyclic exposures were included. Scales were characterized by a combination of several surface analytical tools. A remarkable improvement in sulfidation resistance is observed in Fe-25Cr-20Ni-1.5Al₂O₃ under the conditions investigated here. This is attributed to the ability of the alloy to form and maintain a predominantly Cr₂O₃ scale with reduced Fe-diffusion and content. Possible scientific reasons for such improvement are discussed. The base alloy, Fe-25Cr-20Ni, fails to develop and retain such a Cr₂O₃ scale and undergoes sulfidation within a few minutes of exposure. The scale breakdown process by sulfidation is explained qualitatively. Experimental evidence suggests that sulfur in the environment enhances Fe-diffusion and content in the scale.     

Spark anodizing behaviour of titanium and its alloys in alkaline aluminate electrolyte

Spark anodizing of titanium, Ti-6Al-4V and Ti-15V-3Al-3Cr-3Sn in alkaline aluminate electrolyte produces highly crystalline anodic films consisting mainly of Al₂TiO₅ with α- and γ-Al₂O₃ as minor oxide phases, irrespective of substrate composition. However, the apparent efficiency for film formation decreases in the following order: Ti-6Al-4V, titanium and Ti-15V-3Al-3Cr-3Sn. A large amount of aluminium species are incorporated from the electrolyte, probably by plasma-chemical reaction, and become distributed throughout the film thickness. This distribution indicates that the electrolyte penetrates near to the film/substrate interface through the discharge channels. Thus, the outwardly migrating aluminium ions under a high electric field can be present even in the inner part of the anodic films. Voids are developed at the film/substrate interface, particularly on the vanadium-containing alloys, reducing the adhesion of the anodic film to the substrate.     

Effect of enamel coating on oxidation and hot corrosion behaviors of Ti-24Al-14Nb-3V alloy

The effect of enamel coating on the isothermal and cyclic oxidation at 900 °C in air and on the hot corrosion resistance of Ti-24Al-14Nb-3V in both 85% Na₂SO₄+15%K₂SO₄ and 15%NaCl+85% Na₂SO₄ molten mixed salts at 850 °C was investigated. The results indicated that Ti-24Al-14Nb-3V alloy exhibited poor oxidation resistance due to the formation of nonprotective Al₂O₃+TiO₂+AlNbO₄ scales and poor hot corrosion resistance due to the spallation of scales formed in molten Na₂SO₄+K₂SO₄ and NaCl+Na₂SO₄. Enamel coating suppressed the migration of oxygen and corrosive ions into the substrate to improve the oxidation and hot corrosion resistance of Ti-24Al-14Nb-3V alloy. However, the dissolution of oxides components of the coating into the molten salts degraded enamel coating and the degradation of the coating involved a process by which Cl⁻ anion penetrated into the substrate through voids in the coating to accelerate corrosion of Ti-24Al-14Nb-3V alloy.     

Oxidation characteristics of Ti3Al-Nb alloys and improvement in the oxidation resistance by pack aluminizing

The oxidation behavior of three T_i3-Al-Nb alloys: Ti-25Al-11Nb, Ti-24Al-20Nb, and Ti-22Al-20Nb was investigated in the temperature range of 700–900°C in air. The uncoated alloy Ti-25Al-11Nb showed the lowest weight gain with nearly parabolic oxidation rate; while the other two alloys had much higher weight gain, accompanied by excessive oxide scale spalling. The scale analysis, using XRD, SEMIEDAX, and AES revealed that the scale was a mixture of TiO₂, Al₂O₃, and Nb₂O₅ with the outer layer rich in TiO₂. The effect of variation in Al and Nb content on the oxidation behavior is discussed. A decrease in Al content of the alloy adversely affects the oxidation resistance; and it seems that a Nb content as high as 20 at.% is also not beneficial. Hence these alloys, especially Ti-24Al-20Nb and Ti-22Al-20Nb, should not be used in the as-received condition above 750°C. An attempt was made to improve the oxidation resistance of these alloys by pack aluminizing which led to the formation of an Al rich TiAl₃ surface layer doped with Nb. The coating process was gaseous-diffusion controlled with a parabolic Al deposition rate. The weight gains for the aluminized alloy specimens oxidized at 900°C in air were much lower than that of the uncoated specimens. The weight gains were further decreased in the case of Si-modified aluminized specimens. The scale analysis revealed an alumina-rich scale with some amount of titania doped with Nb. The improvement in the oxidation resistance of the pack-aluminized alloys at 900°C is attributable to the formation of the alumina-rich oxide scale. The addition of Si to the aluminizing pack seems to promote further the growth of an alumina-rich scale by lowering the oxygen partial pressure in the system.     

High-Temperature Oxidation of Ti-48Al-2Nb-2Cr and Ti-25Al-10Nb-3V-1Mo

The short-term oxidation behavior of a-TiAl alloy (Ti-48Al-2Nb-2Cr) was compared andcontrasted to that of an₂-Ti₃Al base(Ti-25Al-19Nb-3V 1Mo) alloy. Oxidation ofTi-25Al-10Nb-3V-1Mo was found to occur at a moderate rate at 800°C, in aN₂ + 20% O₂ environment. A largeincrease in the oxidation rate occurred above thistemperature. This large weight increase was attributedto a breakdown in the protective oxide scale on the surface of the₂ intermetallic alloy, therebypermitting rapid diffusion of oxygen and nitrogen to thesurface of the intermetallic. The oxidation rate of thisalloy at 1200°C was not significantly higher thanthe oxidation rate at 1000°C. In contrast, theoxidation rate of Ti-48Al-2Nb-2Cr remained low up to1200°C. At this temperature, a significant increasein oxidation was observed and was attributed to acceleratedoxygen diffusion through the ₂ phaseand increased solubility of oxygen in the gamma phase ofthe intermetallic microstructure. This weight increaseoccurred despite the fact that at 1200°C, theintegrity of the oxide layer formed on the surface ofthis alloy was maintained. The results of this studyillustrate the need for developing protectiveenvironmental coatings tailored to the individualintermetallic alloy.     

Cracking and Healing of Oxide Scales on Ti-Al Alloys at 900°C

The behavior of oxide scales on Ti-50Al andTi-50Al-2Nb (at.%) was investigated in constantstrain-rate tensile tests at 900°C in air. Thestrain rates ranged between 1 × 10^-9sec^-1 and 3 × 10^-4sec^-1. The tests were accompanied byacoustic-emission measurements in order to detectscale-cracking processes during deformation. Thecritical strains to scale cracking amounted to 0.12-0.5%for the scales on TiAl and 0.17-0.58% for Ti-50Al-2Nb. Thesevalues were found to depend strongly on the size of thepores in the scales and, by using a fracturemechanics-based model, the results for the criticalstrains could be condensed into a narrow scatterband,which is independent of the applied strain rate. Healingof scale cracks was found for strain rates below 1.5× 10^-6 sec^-1 (Ti-Al) and 1.9× 10^-4 sec^-1 (Ti-Al-Nb), respectively. It turned out that the healingprocess is dominated by TiO₂ growth. In alater healing stage, the originalAl₂O₃ barrier is, however,restored in the scale on Ti-50Al. For Ti-50Al-2Nb, an Al₂O₃ layer is found onthe former scale-crack contours. The healing process isalso described by a quantitative model. As a generalconclusion from the investigations, it turned out thatcritical strains to oxide-scale cracking can be estimated fromjust simple oxidation experiments without sophisticatedmechanical testing if the microstructural parameters ofthe scale are determined quantitatively as a function of oxidation time by metallographicmeans.     

Enhancing Biocompatibility and Corrosion Resistance of Ti-6Al-4V Alloy by Surface Modification Route

Titanium (Ti) and its alloys are widely used as candidate materials for biomedical implants. Despite their good biocompatibility and corrosion resistance, these materials suffer from corrosion after implantation in biological environments. The aim of this research work is to study the effect of two coatings on biocompatibility and corrosion behavior of Ti-6Al-4V biomedical implant material. Hydroxyapatite (HA) and hydroxyapatite/titanium dioxide (HA/TiO₂) coatings were thermal-sprayed on Ti-6Al-4V substrates. In the latter case, TiO₂ was used as a bond coat between the substrate and HA top coat. The corrosion behavior of coated and un-coated samples in Ringer’s solution was studied by potentiodynamic and linear polarization techniques. Before and after corrosion testing, XRD and SEM/EDS techniques were used for the analysis of phases formed and to investigate microstructure/compositional changes in the coated specimens. The cellular response was analyzed by the MTT (microculture tetrazolium) assay. The results showed that both the HA, as well as, the HA/TiO₂ coatings significantly increased the corrosion resistance of the substrate material. The HA coating was found to be more biocompatible as compared to the un-coated and HA/TiO₂-coated Ti-6Al-4V alloy.     

High temperature oxidation of Ti−48%Al−2%W intermetallics

Powder metallurgically produced Ti-48% Al-2%W alloys were oxidized between 800 and 1050°C in air. The W-addition was quite effective in providing isothermal and cyclic oxidation resistance. The alloys oxidized parabolically up to 1050°C during isothermal oxidation, with small weight gains. The scales were adherent up to 900°C during cyclic oxidation. Oxide scales consisted primarily of an outer TiO₂ layer, an intermediate Al₂O₃ layer, and an inner (TiO₂+Al₂O₃) mixed layer. Tungsten was present below the intermediate Al₂O₃ layer. and also at the scale-matrix interface as W-enriched compounds. Below the oxide scale, a Ti₃Al zone containing some W and O existed.     

Effect of Nb on oxidation behavior of high Nb containing TiAl alloys

The isothermal oxidation behavior of Ti-45Al-8Nb and Ti-52Al-8Nb alloys at 900 °C in air was investigated. The early oxidation behaviors were studied by using XPS and AES. And the microstructure and the composition of the oxidation scale were studied by using XRD and SEM. The results show that the oxidation behavior of TiAl alloy is significantly improved by Nb addition. Nb substitutes for Ti in TiO₂ as a cation with valence 5, and thus to suppress TiO₂ growth. The (Ti,Nb)O₂-rich layer is a dense and chemically uniform which is more protective than the TiO₂ layer. Nb addition also lowers the critical Al content to form an external alumina. Nb₂Al phase is formed in the metallic matrix at the oxide–metal interface on the high Nb containing TiAl alloys.     

Hydrogen and Yttria in Chromium: Influence on the High-Temperature Oxidation Kinetics in O2, Oxide-Growth Mechanisms and Scale Adherence

The effects of hydrogen and Y_2O_3 on high-temperature oxidation of Cr in 20 mbar O₂ have been studied at 900°C. Oxidation- and O₂-dissociation rates were determined from gas-phase measurements. Hydrogen in Cr leads to breakdown of the oxide scale. The oxide scale on Cr–1%Y₂O₃ charged with hydrogen for 4 hr (resulting in Cr–1%Y₂O₃ with approximately 10 ppm hydrogen) is adherent to the metal substrate. The oxidation rate is similar for Cr with 1 ppm hydrogen and Cr–1%Y₂O₃ with 1 ppm hydrogen, but significantly lower for 4-hr H-charged Cr–1%Y₂O₃. The oxidation rate of Cr–5%Fe–1%Y₂O₃–25 ppm H is also lower than the oxidation rate of Cr–5%Fe–1%Y₂O₃–1 ppm H. This indicates that unless hydrogen is present, there are virtually no effects of the addition of 1% Y₂O₃ to Cr. Using labeled oxygen, ^16,16O₂ and ^18,18O₂, was found that at 900°C the dissociation rate of O₂ is higher on Y₂O₃ than on Cr₂O₃. It is suggested that the well-known improvement of the oxidation resistance of Cr as an effect of additions of Y or Y₂O₃ is related to an increase in the dissociation rate of O₂ and that there is a synergetic effect in combining Y₂O₃ and hydrogen.