Relation between impurities and oxide-scale growth mechanisms on Ni-34Cr and Ni-20Cr alloys. I. Influence of C,Mn, and Si

The oxidation behavior of Ni-Cr alloys (34 and 20 wt.% Cr) was investigated between 850 and 1200°C in oxygen for a maximum duration of about 70 hr. The oxide-growth mechanism is a diffusion process controlled by either outward diffusion of chromium in Cr₂O₃ (Ni-34Cr alloy) or by an increase in grain size (Ni-20Cr alloy). In the case of the Ni-34Cr alloy, low values of chromium diffusion were found for the growth of Cr₂O₃ by taking into account the general equation of Wagner. The influence of impurities (Si, C, Mn, Ni) diffusing from the underlying alloy is analyzed because of their doping effect in the outer oxide scales.     

The effect of silicon and manganese on the oxidation mechanism of Ni-20 Cr

Additions of 3% silicon or manganese to Ni-20 Cr reduced the oxidation rate, whereas additions of 1% had little effect. Three percent silicon alloys formed an inner scale of SiO₂, and 3% manganese alloys formed an inner spinel layer of essentially pure MnCr₂O₄. The experimentally determined solid-state growth rate of NiCr₂O₄ was about 1000 times slower than the growth rate for Cr₂O₃. It has been established that the protective layer on Ni-20 Cr (Nichrome alloys) is the spinel and not Cr₂O₃ as previously postulated. The mechanism for scale growth is discussed for Ni-20 Cr alloys.This work was performed at Stanford Research Institute, Menlo Park, Calif. and was supported by the National Aeronautics amd Space Administration, Contract NAS 3-11165.     

High temperature corrosion of Ni-20Cr and Ni-15Cr-8Fe alloys in sulfur dioxide

Two nickel-base alloys, Ni-20Cr and Ni-15Cr-8Fe, in the form of wire specimens, have been exposed to 100 mbar of sulfur dioxide between 550 and 850°C. For Ni-20Cr, an outer Cr₂O₃ layer formed only at the beginning of the reaction, but very quickly Ni₃S₂ grew preferentially at the exterior by outward diffusion of nickel. The reaction rate is regulated by an external interfacial process. A barrier effect was noted near 645°C associated with the formation of NiCr₂O₄; a new acceleration takes place above 680°C. The external growth of Ni₃S₂ is attributed to the low radius of curvature of the samples. For Ni-15Cr-8Fe, the reaction mechanism is rather similar, except that no barrier effect occurred. A protective Cr₂O₃ layer formed above 800°C in both cases.     

Microstructure and anti-oxidation behavior of laser clad Ni-20Cr coating on molybdenum surface

A Ni-20Cr coating was deposited on a molybdenum substrate by laser cladding. The observation of the microstructure by SEM demonstrates that the coating is free of cracks and pores, and metallurgically bonded to the substrate. XRD and EDS analysis results show that some dilution occurs at the coating/substrate interface and that Mo combines with Ni-20Cr, to form a Ni-Cr-Mo alloy coating with slight oxidation. The oxidation behavior of the coating indicates that the laser clad Ni-20Cr coating can effectively prevent oxidation of molybdenum at 600 °C in air. The oxide scale formed on the coating surface by oxidation in air is composed of NiO, Cr₂O₃ and NiMoO₄.     

Effect of Y2O3 dispersoids in 80Ni-20Cr alloy on the early stages of oxidation at low-oxygen potential

Specimens of a 80Ni-20Cr type alloy, with and without Y₂O₃ dispersoid particles, were oxidized at 1000°C in H₂/H₂O mixtures where the partial pressure of oxygen (P _{O 2}) was varied between 10₃ and 10₂₄ atm. Oxide particles nucleated homogeneously on both alloys, and preferential nucleation on dispersoid particles at the surface was not observed. Continuous Cr₂O₃ films formed slightly faster at aP _{O 2} of 10^–21 atm on the alloy containing the dispersoid, but the difference was negligible at higher pressures. Oxidation atP _{O 2}=10^-19 and 10^–21 atm involved both the formation of Cr₂O₃ and the evaporation of chromium. Thin films of -Al₂O₃ were observed on both alloys after oxidation atP _{O 2}.     

The oxidation of Ni-20 wt. % Cr-2ThO2

The oxidation of TD NiCr (Ni-20 wt. % Cr-2ThO₂) has been investigated between 900 and 1200°C, and the oxidation behavior is compared with Ni-30 wt. % Cr and Co-35 wt. % Cr alloys. All alloys develop Cr₂O₃ scales but the weight changes obtained for the NiCr and CoCr alloys show an increase with time whereas above 1000° C the TD NiC shows a progressive loss in weight from the evaporation of CrO₃ from the scale, and the reaction products appear to be formed mainly at the alloy-scale interface. However, no mechanism for its formation has been established.     

The Effects of Reactive-Element,Ion-Implantation-Induced Amorphous Layers on the Oxidation of Co-12Cr and Ni-12Cr Alloys

Nickel-chromium (Ni-12Cr, wt.%) andcobalt-chromium (Co-12Cr, wt.%) alloys were ionimplanted with 150 keV yttrium to fluences that rangedbetween 2 × 10¹⁶ and 1 ×10¹⁷ ions/cm². The influence ofthe implantation on the microstructure of the alloy wasdetermined. The effect of the highest dose implantationon the alloys' oxidation response at 1000°C, 48 hrwas measured. Both alloys contained an amorphous surface phase as a result of this fluence and one ofthe effects of oxidation was to recrystallize theamorphized alloy in the first few minutes of oxidation.The lower doses of 2 × 10¹⁶ions/cm² were sufficient to cause amorphization of both the Ni-12Cr andthe Co-12Cr. The implantation reduced the isothermalmass gain by a factor of 25% for the Ni-12Cr, but had anegligible effect on the Co-12Cr alloy. Short-term oxidation experiments at 600°C showed viatransmission electron microscopy that, in the absence ofthe yttrium implant, the Ni-12Cr alloy forms NiO in thefirst minute of oxidation and the Co-12Cr alloy forms CoO and CoCr₂O₄.The implanted Ni-12Cr, on the other hand (1 ×10¹⁷Y⁺/cm²), formsrecrystallized Ni-Cr, Y₂O₃, andNiO in the near-surface region, while the implantedCo-12Cr alloy forms CoO, CoCr₂O₄, and a recrystallized intermetallic alloy fromthe amorphized region.     

Oxidation of Fe-3 wt.% Cr alloy

The oxidation behavior and the oxide microstructure on Fe-3 wt. % Cr alloy were investigated at 800°C in dry air at atmospheric pressure. Two distinct oxidation rate laws were observed: initial parabolic oxidation was followed by nonparabolic growth. The change in the oxidation kinetics was caused by microchemical and microstructural developments in the oxide scale. Several layers developed in the oxide scale, consisting of an innermost layer of (Fe,Cr)₃O₄ spinel, an intermediate layer of (Fe,Cr)₂O₃ sesquioxide, and two outer layers of Fe₂O₃ hematite, each with different morphologies. Wustite (Fe_1–xO) and distorted cubic oxide (-(Fe,Cr)₂O₃) were observed during the iniital parabolic oxidation only.     

Oxidation behavior of Ni-Cr-1ThO2 alloys at 1000 and 1200°C

The oxidation behavior of Ni-13.5-33.7Cr-1ThO₂ alloys in flowing oxygen at 150 Torr was investigated in the temperature range 1000–1200°C. Gravimetric measurements of the oxidation kinetics have been combined with microstructural studies of the reacted samples in order to evaluate the reaction mechanisms. The oxide products formed on the alloys were a function of Cr content, sample surface preparation, reaction time, and temperature. The presence of ThO₂ appears to produce two effects during alloy oxidation. First, enhanced Cr diffusion to the alloy surface results in rapid formation of a Cr₂O₃ subscale beneath NiO on Ni-13.5Cr-1ThO₂ and selective oxidation of Cr for Ni-22.6Cr-1ThO₂. Second, the mechanism of formation of Cr₂O₃ is apparently different from that for simple Ni-Cr alloys, resulting in about an order of magnitude reduction in the Cr₂O₃ growth rate. The oxidationvaporization of Cr₂O₃ to CrO₃ becomes rate controlling for the higher Cr alloys after only a few hours of exposure at 1200°C.     

Effect of internal oxidation pretreatments and Si contamination on oxide-scale growth and spalling

Internal oxidation pretreatments carried out in quartz capsule with a Rhines pack were found to have a profound effect on the subsequent oxidation behavior of alloys. Specimens of Co-15 wt.% Cr, Co-25 wt.% Cr, Ni-25 wt.% Cr, and Ni-25 wt.% Cr-1 wt.% Al were tested at 1100°C after pre-oxidation treatments. Even without the development of internal oxide particles, pretreated binary CoCr and NiCr alloys oxidized with significantly lower rates. Selective oxidation of chromium was observed on the non-Cr₂O₃-forming Co-base alloys, whereas on the Cr₂O₃-forming Ni-base alloys, elimination of base-metal oxide, reduction in the Cr₂O₃ growth rate, and better scale adhesion were found. These effects were more apparent with pre-oxidation temperatures greater than 1000°C and with longer pretreatment times. Contaimination of Si from the quartz is believed to be the cause.     

Intergranular oxidation and internal void formation in Ni-40% Cr alloys

Internal void formation and intergranular oxidation behaviour have been studied during the oxidation of two Ni-40Cr alloys in 1 atm oxygen at 1000° to 1200°C. The development of an external Cr₂O₂ scale causes vacancies to be generated in the alloy at the alloy-scale interface as chromium diffuses into the scale, and others to be generated in the alloy due to the different diffusion rates of chromium towards the interface and of nickel back into the bulk alloy. At 1200°C, internal void formation results from condensation of such vacancies at inclusions in the grains and at the grain boundaries. The intergranular oxidation observed at 1000°C, 1100°C and to a lesser extent. 1200°C results from preferential condensation of vacancies to form voids in the alloy grain boundaries. Significant depletion of chromium in the alloy adjacent to the scale facilitates the supply of oxygen from the scale and its penetration into the alloy grain boundaries to form intergranular oxide. Such intergranular oxide develops deep into the alloy following diffusion of this oxygen through a porous network in the oxide, which arises because of the vacancy condensation, and oxidation of chromium at the tip of the intergranular penetration.     

The effects of yttrium ion implantation on the oxidation of nickel-chromium alloys. II. Oxidation of yttrium implanted Ni-20Cr

Yttrium ions were implanted into an alloy of nickel containing 20 wt.% chromium. The oxidation weight gains of the implanted alloy, as well as unimplanted Ni-20Cr, were measured in 1 atm. of oxygen at temperatures between 815°C and 1100°C. Though a yttrium fluence of 2×10¹⁵ ions/cm² increased the weight gained at a given time for the implanted alloy compared to the unimplanted alloy, a fluence of 7.5×10¹⁶ ions/cm² considerably retarded the oxidation process. This retardation was caused both by a reduction in the duration of transient oxidation and by a reduction in the rate of parabolic oxidation that follows the transient stage. Examination of oxidized specimens revealed that the higher fluence yttrium implantation caused an earlier formation of Cr₂O₃, thus shortening the time during which NiO forms and contributing to the reduced time and weight gain during transient oxidation. This accelerated formation of Cr₂O₃ is explained by reference to the yttrium-implantation-induced amorphous phase present on the alloys, the formation of small grains of nickel-chromium solid solution within this amorphous phase, and the presence of NiCr₂O₄ at the oxide/gas interface. Potential causes of the reduced rates of parabolic oxidation are also discussed.     

The influence of superficially applied oxide powders on the high-temperature oxidation behavior of Cr2O3-forming alloys

The effects of superficially applied CeO₂, mixed rare earth oxides, Co₃O₄, and Cr₂O₃ powders on the isothermal and cyclic oxidation of Ni-Cr alloys and the effects of CeO₂ and MgO powders on the isothermal oxidation of Fe-25 wt.% Cr have been studied over the temperature range 940–1150°C in pure oxygen and dry air. The rates of oxidation of both the Ni- and Fe-base alloys were markedly reduced by the application of CeO₂ powder. The presence of CeO₂ also improved the scale adherence and resulted in marked changes in the oxidation morphology. The presence of Co₃O₄ or Cr₂O₃ powders on Ni-Cr alloys or MgO on Fe-Cr also produced changes in the oxidation morphology but did not decrease the rate of oxidation. These results are interpreted in terms of the influence of the oxide powders on the development of scale microstructure and their effectiveness in decreasing grain boundary transport in Cr₂O₃.This paper is based in part on the Ph.D. thesis of G. M. Ecer (1975) and in part on the M.S. thesis of R. B. Singh (1977).     

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.     

The high-temperature oxidation of cobalt-21 wt.% chromium-3 vol.% Y2O3 alloys

Alloys of Co-21 wt. % Cr-3 vol. % Y ₂ O ₃ have been prepared by a mechanical alloying method, and oxidized in oxygen at 100 Torr in the temperature range 900–1200°C. The general effects of the dispersed oxide phase are similar to those reported for nickel-base alloys: the selective oxidation of chromium to form a continuous protective Cr ₂ O ₃ scale is promoted; the rate of growth of Cr ₂ O ₃ is reduced compared to dispersoid-free alloys; the adhesion of the Cr ₂ O ₃ is greatly improved; and the scale-forming reaction is probably at the scale-metal interface in the alloys containing the dispersoid, whereas it is at the scale-oxygen interface in dispersoid-free alloys. This last point has not been positively demonstrated. The improvement in adhesion is of particular significance, since the scales on cobalt-base alloys are prone to spallation, and it has been possible to study the mechanism of adhesion in more detail. It appears that in dispersoid-free material the metal recedes from the scale-metal interface, leaving the scale supported on the tops of metal peaks but this does not happen in the alloy containing the dispersoid, either because the growth direction of the scale has been changed, or because of changes in the substrate grain size. In general, the observations support the model proposed in an early study for the oxidation of Ni-20 wt.% Cr alloys containing oxide dispersions.This work has been supported by the Naval Air Systems Command under Contract No. N00019-71-C-0079.     

The oxidation of Ni-70 wt.%Cr in oxygen between 1073 and 1473°K

Oxidation of the relatively simple, two-phase alloy Ni-70 wt.%Cr in oxygen between 1073 and 1473°K results in the formation of a Cr₂O₃ scale containing less than O.5 wt.% Ni in solid solution. The oxidation kinetics are irreproducible for an initial period, which is brief at 1073 and 1273°K but much more pronounced at 1473°K, both in duration and degree. This behavior is associated with the failure of the protective Cr₂O₃ scale. However, after longer periods a compact layer of Cr₂O₃ becomes established under isothermal conditions and results in a change to more reproducible kinetics, especially at 1073 and 1273°K. Oxidation causes chromium depletion and the formation of a single-phase zone which separates the scale and the two-phase bulk alloy. The depth of Cr₂O₃ internal oxide coincides with this zone. The oxidation behavior is compared with that of more Ni-rich, single-phase Ni-Cr alloys, with particular reference to the effects of the constitution of the underlying alloy and the integrity of the protective oxide.     

The effect of cerium on the oxidation of Ni-50Cr alloys

The oxidation behavior of Ni-50Cr alloys with minor cerium additions was studied between 800 and 1100° Cin oxygen, air, and oxygen at reduced partial pressures. Optical and scanning electron metallography, X-ray diffraction, and electron-probe microanalysis techniques were used to characterize the changes in scale and substrate morphology and to identify the oxidation products. Platinum markers were used to determine the direction of ionic transport. The effects of cold work, initial alloy phase distribution, and cyclic oxidation were also studied. The Cr ₂ O ₃ scales on the cerium-containing alloys grew while being largely separated from the metal substrate. Oxidation rate, oxide grain growth, and the tendency of scales to spall on cooling were reduced substantially with increasing alloy cerium content. The first two effects are suggested to result from the interaction of cerium ions and cerium oxide particles with oxide grain boundaries in reducing grain-boundary diffusion and oxide-boundary mobility. The third is suggested to result from the thinner, finer-grained scales formed on the Ce-containing alloys.This work is based on a portion of a thesis by G. M. Ecer submitted to the University of Pittsburgh in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Metallurgical and Materials Engineering.Formerly graduate student. Department of Metallurgical and Materials Engineering, University of Pittsburgh.     

Chemical and phase composition of nanosize oxide and passive films on Ni-Cr alloys. I. XPS studies of films obtained by alloy oxidation in air

XPS data for thin (less than 100 nm) oxide films obtained by oxidation of Ni-4 Cr and Ni-12.5 Cr alloys at 500°C (0.5 h) are discussed. Thermodynamic analysis of 3 NiO + 2 Cr = Cr₂O₃ + 3 Ni solid-phase reaction is given, in which both the Gibbs energy change in the thermochemical process and the change in the interface energy at the alloy-oxide film boundary are taken into account.