The influence of silicon on the high-temperature oxidation of nickel

An investigation of the oxidation of nickel-silicon alloys has been carried out in order to ascertain the mode of development of partially or fully protective SiO₂ layers. The addition of 1% Si has little effect on the oxidation rate of nickel at 1000°C but is sufficient for partial-healing layers of amorphous SiO₂ to be established. These layers are incorporated into the inner part of the duplex NiO scale but do not react with the oxide to form a double oxide. Increasing the silicon concentration to 4% or 7% facilitates the development of apparently continuous amorphous SiO₂ layers at the base of the NiO scale, resulting in reduced rates of oxidation. However, these layers develop imperfections, possibly microcracks resulting from oxide growth stresses, and are unable to prevent some continued transport of Ni²⁺ ions into the NiO scale and oxygen into the alloy, particularly for Ni-4% Si. Although the formation of SiO₂-healing layers can reduce the rate of oxidation of nickel, they provide planes of weakness that result in considerable damage under the differential thermal contraction stresses during cooling. In particular, severe scale spalling occurs for Ni-4% Si and Ni-7% Si as failure occurs coherently within the SiO₂ layer.     

The high-temperature oxidation resistance of iron-silicon-aluminum alloys

Silicon or chromium can be used as an oxygen getter in iron-aluminum alloys to prevent the internal oxidation of aluminum. This suppresses the formation of the iron oxide nodules that tend to destroy binary iron-aluminum alloys during high-temperature oxidation. Alloys of iron containing aluminum and silicon in varying proportions were heated in flowing air for 50 hr at 1093°C. Of the alloys tested, one containing 6% aluminum and 1 % silicon was the most resistant to oxidation.     

The high-temperature oxidation resistance of Co-Al alloys

Most Ni and Co-base alloys used for high-temperature service rely on the production of a compact, stable Cr₂O₃ scale for their oxidation resistance. However, as operating temperatures have risen above 900–950° C, the loss of Cr₂O₃ as the volatile CrO₃ has led to an inadequate life span of these alloys, particularly in rapidly flowing, turbulent gas streams. As a result of this, it has been necessary to examine the possibility of using Al₂O₃ as the protective scale. Al₂O₃ has a lower growth rate than Cr₂O₃, it is nonvolatile, and, unlike Cr-containing systems, it is less likely to form compound oxides such as spinels. In this study, the amount of Al which must be present in the Co-Al system to form a continuous layer of Al₂O₃ in the temperature range 800–1000° C has been determined. The quantity was found to rise from about 7–10 wt. % at 800° C to 10–13 wt. % at 900° C and 13 wt. % at 1000° C. Notice has also been taken of the abilities of the alumina-forming alloys to re-form a protective oxide in the event of spalling, blistering, or any other disruptions of the scale, and some cyclic-oxidation checks have been conducted on the Co13Al alloy at 900 and 1000° C.This work has been partly supported by the Science Research Council and one of us (G.N.I.) wishes to thank them for the award of a Science Research Council Research Studentship     

Oxide sublimation during the high-temperature oxidation of nickel alloys with protective coatings

It is shown that the oxidation of heat-resistant alloys is accompanied by a sublimation of the oxides MoO₃, WO₃, Al₂O₃, and CrO₃. The determinant in the sublimate is the molybdenum content, exceeding that in scale by a factor of 50–100. Protective coatings diminish the sublimation, rather than fully eliminate it. To diminish the molybdenum oxide sublimation decisively, it is expedient to alloy the coating with magnesium or magnesium oxide. Original Russian Text ? E.G. Ivanov, 2008, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2008, Vol. 44, No. 4, pp. 415–418.     

Effect of boron and hafnium on the corrosion resistance of high-temperature nickel alloys

Conclusion Introduction of 0.1% B and 1% Hf into low-carbon corrosion-resistant nickel alloys makes it possible to increase their resistance to high-temperature salt corrosion. In this case, it becomes possible to increase the heat resistance of the alloys while maintaining the high corrosion resistance as a result of the increase in the titanium and aluminum content.Central Scientific-Research Institute of Corrosion of Metals "Prometei" St. Petersburg. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 4, pp. 17–20, April, 1992.     

The role of nickel in the high-temperature oxidation of Fe-Cr-Ni alloys in oxygen

The oxidation of several largely austenitic Fe-Cr-Ni alloys in 1 atm oxygen at 800–1200°C has been studied thermogravimetrically, metallographically, and in detail by electron probe micro analysis. Fe-Cr-Ni alloys of this type are protected by Cr₂O₃-healed scale, which thickens slower than on the corresponding binary Fe-Cr and Ni-Cr alloys, presumably because nickel and iron ions dope the Cr₂O₃ more effectively together than singly and/or because the alloy composition and ability to absorb cation vacancies are such as to produce a smaller vacancy activity gradient or level in the scale, or voids within it. The scale adhesion, as on Ni-Cr alloys, is generally good after long times, at least partly due to the convoluted alloy-oxide interface, in some cases to large intergranular Cr₂O₃-rich stringers, and possibly to the general specimen mechanical properties. Nonprotective stratified scale development is relatively unusual and often produces nickel-rich, alloy-particle-containing nodules, as on Fe-Ni alloys. Careful selection of ternary and more complex alloys with appropriate alloy interdiffusion coefficients and oxygen solubilities and diffusivities should permit development of materials with the best compromise between ease of Cr₂O₃ establishment, avoidance of breakaway, and readiness of scale healing.     

On the high-temperature oxidation of nickel

This paper summarizes on some of the extensive experimental data and corresponding models suggested to account for the oxidation mechanism of Ni in the temperature range 500-1400 °C. In addition it reports on in-house experimental data from investigations related to the oxidation of high-purity Ni from 500 to 1300 °C in the oxygen pressure range 1×10⁻⁴-1 atm based on TG, measurements of surface kinetics, two-stage oxidation, scanning electron microscopy, atomic force microscopy, secondary ion mass spectroscopy etc. The main part of this paper focuses on the more complex models suggested to account for experimental observations of the oxidation kinetics and the oxide morphology below 1000 °C.     

TiA1基合金离子渗碳与高温抗氧化性研究

研究了辉光离子渗碳处理对TiA1基合金的渗层组织、表面硬度和抗高温氧化性的影响。实验显示。渗碳处理可在TiA1基合金的表面形成由碳化物和过渡层组成的复合相结构，经不同渗碳处理的试样，其表面硬度和抗高温氧化性均得到明显提高。     

TiAl基合金离子渗碳与高温抗氧化性研究

研究了辉光离子渗碳处理对TiAl基合金的渗层组织、表面硬度和抗高温氧化性的影响。实验显示,渗碳处理可在TiAl基合金的表面形成由碳化物和过渡层组成的复合相结构,经不同渗碳处理的试样,其表面硬度和抗高温氧化性均得到明显提高。     

The influence of reactive-element coatings on the high-temperature oxidation of pure-Cr and high-Cr-content alloys

The influence of various reactive-element (RE) oxide coatings (Y₂O₃, CeO₂, La₂O₃, CaO, HfO₂, and Sc₂O₃) on the oxidation behavior of pure Cr, Fe–26Cr, Fe–16Cr and Ni–25Cr at 900°C in O₂ at 5×10^–3 torr has been investigated using the¹⁸O/SIMS technique. Polished samples were reactively sputtercoated with 4 nm of the RE oxide and oxidized sequentially first in¹⁶O₂ and then in¹⁸O₂. The effectiveness of each RE on the extent of oxidation-rate reduction varied with the element used. Y₂O₃ and CeO₂ coatings were found to be the most beneficial, whereas Sc₂O₃ proved to be ineffective, for example, for the oxidation of Cr. SIMS sputter profiles showed that the maximum in the RE profile moved away from the substrate-oxide interface during the early stages of oxidation. After a certain time the RE maximum remained fixed in position with respect to this interface, its final relative position being dependent on the particular RE. The position of the RE maximum within the oxide layer also varied with the substrate composition. For all coatings¹⁸O was found to have diffused through the oxide to the substrate-oxide interface during oxidation, the amount of oxide at this interface increasing with increasing time. The SIMS data confirm that for coated substrates there has been a change in oxidegrowth mechanism to predominantly anion diffusion. The RE most probably concentrates at the oxide grain boundaries, generally as the binary oxide (RE) CrO₃. Cr³⁺ diffusion is impeded, while oxygen diffusion remains unaffected.     

On modeling the oxidation of high-temperature alloys

Oxidation of high-temperature alloys represents complex, strongly coupled, non-linear phenomena which include: (i) diffusion of oxygen in the alloy; (ii) an oxidation reaction in which the reaction product causes substantial permanent, anisotropic volumetric swelling; (iii) high-temperature elastic–viscoplastic deformation of the base alloy and the oxide; and (iv) transient heat conduction. We have formulated a continuum-level chemo-thermomechanically coupled theory which integrates these various nonlinear phenomena. We have numerically implemented our coupled theory in a finite-element program, and have also calibrated the material parameters in our theory for an Fe–22Cr–4.8Al–0.3Y heat-resistant alloy experimentally studied by Tolpygo et al. Using our theory, we simulate the high-temperature oxidation of thin sheets of FeCrAlY and show that our theory is capable of reproducing the oxide thickness evolution with time at different temperatures, the permanent extensional changes in dimensions of the base material being oxidized and the development of large compressive residual stresses in the protective surface oxide which forms. As an application of our numerical simulation capability, we also consider the oxidation of an FeCrAlY sheet with an initial groove-like surface undulation, a geometry which has been experimentally studied by Davis and Evans. Our numerical simulations reproduce (with reasonable accuracy) the shape-distortion of the groove upon oxidation measured by these authors. This example has obvious ramifications for delamination failure of a ceramic topcoat on a thermally grown oxide layer in thermal barrier coatings.     

Microstructure,oxidation resistance and high-temperature strength of γ′ hardened Pt-base alloys

Starting from the base composition with 12 at.% Al, 6 at.% Cr, 5 at.% Ni and Pt balance, precipitation hardened Pt–Al–Cr–Ni alloys with additions of 2 at.% Mo, Re, Ru and W, respectively, were investigated. After homogenization heat treatment and air cooling, all five-component alloys show a bimodal distribution of γ′ precipitates. Along with a fraction of fine γ′ precipitates embedded in a Pt-rich matrix, some coarse primary γ′ particles are observed. Mo, Re and W additions increase strength above 1000 °C, whereas Ru has no beneficial effect on strength. W increases the γ′ volume fraction most effectively, however, the oxidation resistance is worsened dramatically by W additions. The Re-containing alloy shows the best resistance against γ′ growth and coarsening during long-term ageing.     

显微组织对Cu—Cr—Ni合金高温氧化行为的影响

研究了两种单／双相Cu-Cr-Ni合金的高温氧化行为。结果表明,合金氧化动力学偏离抛物线规律,其瞬时抛物线速率常数随时间延长而降低。两种合金表面氧化膜的结构差别较大,单相合金表面形成－连续的Cr2O3层,双相合金表面氧化膜外层是一边疆的CuO层,Ni和Cr的氧化发生在合金内部,这种合金与氧化物共存的混合内氧化与经典的内氧化明显不同,氧化层最里面形成了一连续的CrO3膜,抑制了合金的进一步氧化。     

The high-temperature oxidation behavior of vanadium-aluminum alloys

The oxidation behavior in air of pure vanadium, V-30Al, V-30Al-10Cr, and V-30Al-10Ti (weight percent) was investigated over the temperature range of 700–1000° C. The oxidation of pure vanadium was characterized by linear kinetics due to the formation of liquid V₂O₅ which dripped from the sample. The oxidation behavior of the alloys was characterized by linear and parabolic kinetics which combined to give an overall time dependence of 0.6–0.8. An empirical relationship of the form: W/A=Bt + Ct^1/2 + D was found to fit the data well, with the linear contribution suspected to be from V₂O₅ formation for V-30Al and V-30Al-10Cr, and a semi-liquid mixture of V₂O₅ and Al₂O₃ for V-30Al-10Ti. The parabolic term is presumed related to the formation of a solid mixture of V₂O₅ and Al₂O₃ for V-30Al and V-30Al-10Cr, and TiO₂ for V-30Al-10TiThe addition of aluminum was found to reduce the oxidation rate of vanadium, but not to the extent predicted by the theory of competing oxide phases proposed by Wang, Gleeson, and Douglass. This was attributed to the formation of a liquid-oxide phase in the initial stages of exposure from which the alloys could not recover. Ternary additions of chromium and titanium were found to decrease the oxidation rate further, with chromium being the most effective. The oxide scales of the alloys were found to be highly porous at 900° C and 1000° C, due to the high vapor pressure of V₂O₅ above 800° C.     

The oxidation performance of modern high-temperature alloys

The high-temperature oxidation resistance of an alloy is a key design criterion for components in a variety of industrial applications, such as advanced gas turbines, industrial heating, automotive, waste incineration, power generation and energy conversion, chemical and petrochemical processing, and metals and minerals processing. The importance of correctly assessing the long-term oxidation behavior of high-temperature alloys is illustrated. As applications move to higher temperatures, new alloys are needed. In this paper, the oxidation performance of three newly developed alloys, an alumina-forming Ni-Fe-Cr-Al alloy, a γ′-strengthened Ni-Cr-Co-Mo-(Al+Ti) alloy, and a nitride-strengthened Co-Cr-Fe-Ni-(Ti+Nb) alloy is presented. Author’s note: All compositions reported in this article are in weight percent.     

The high-temperature oxidation behavior of Co-Re-Cr-based alloys

Co-Re-Cr-based model alloys have been developed for high-temperature applications beyond 1,200°C. The purpose of the present investigation is to gain an insight into the oxidation mechanisms of the model Co-Re-Cr alloys and to find ways to improve oxidation resistance of this class of materials. The first generation of this class of alloys showed a rather poor oxidation resistance during exposure to laboratory air. As a consequence of the lack of protectiveness of the oxide layer, the vaporization of rhenium oxide takes place during oxidation. It has been found that Si stabilizes the Cr₂O₃ scale, enhancing the oxidation resistance significantly.