The oxidation of nickel-chromium alloys in atmospheres containing sulphur dioxide

The simultaneous attack of oxygen and sulphur on a range of NiCr alloys containing 2–30%Cr has been investigated between 500 and 790°C in Ar-10% SO₂ atmospheres. Reaction kinetics were followed using an automatic recording balance and scale structures were examined by metallography, scanning electron microscopy and microprobe analysis.Compared with pure Ni, the addition of up to 5%Cr at 500°C, or 15%Cr at 700°C, has little effect on kinetics. At higher Cr levels (8–10% at 550°C or 15–20% at 700°C) initial protective behaviour is followed by a period of rapid attack. Even with Cr contents as high as 30% the protection shown is not as good as in the absence of sulphur from the atmosphere. The failure to form protective oxides is thought to be due primarily to the penetration of initially formed Cr₂O₃ scales by sulphur, possibly by solution and diffusion, leading to the formation of duplex (oxide + sulphide) scales which grow rapidly by nickel ion migration.     

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     

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.     

The effects of yttrium ion implantation on the oxidation of nickel-chromium alloys. I. The microstructures of yttrium implanted nickel-chromium alloys

Yttrium ions of 150 keV energy were implanted into the alloys Ni-20Cr, Ni-4Cr, and into nickel. The microstructures were then characterized using transmission electron microscopy, selected area channeling patterns and back-scattered electron images. Low yttrium fluences between 1×10¹⁴ and 5× 10¹⁵ Y⁺/cm² did not alter the microstructures of Ni-20Cr. However, fluences of 1×10¹⁶, 5×10¹⁶, and 7.5×10¹⁶ caused the crystalline structures of the alloy to be replaced by an amorphous phase. Fluences of 7.5×10¹⁶ Y⁺/cm² also rendered Ni-4Cr and nickel amorphous. Self-ion implantation experiments on Ni-20Cr did not cause the amorphous phase to form. The depth distribution of elements in Ni-20Cr following yttrium ion implantation (7.5× 10¹⁶ Y⁺/cm²) was determined by Auger electron spectroscopy. This showed in addition to the added yttrium a surface depletion in nickel concentration and a simultaneous enrichment in chromium concentration. At approximately 500 Å, the chromium concentration is approximately 32 at.%. This depletion/enrichment zone extends throughout the implanted layer. Annealing the Ni-20Cr implanted with 7.5×10¹⁶ Y⁺/cm² in vacuum for one hour at 600°C resulted in the recrystallization of Ni-Cr solid solution and the formation of very fine grains of Y₂O₃. Annealing at 800°C for 5 minutes showed recrystallized Ni-Cr, Y₂O₃, and an additional phase or phases.     

The internal oxidation of Nb-Hf alloys

The internal oxidation of some binary Nb-Hf and several commercial Nb alloys containing Hf was studied at 1568 and 1755°C in oxygen pressures ranging from 5×10 ^–5 to 1×10^–3 torr.The reaction kinetics were linear, suggesting that diffusion of oxygen in the substrate was not rate-controlling. The dependence of the reaction rate on oxygen pressure was linear also. Well-defined reaction fronts were observed at higher pressures and the lower temperature, whereas ill-defined fronts occurred at lower pressures and at the higher temperature. The solubility product was much higher than normally encountered in Wagnerian-type behavior and gave rise to varying solute content across the internal-reaction zone. The solute-concentration profiles (EPMA/WDS) of the matrix between particles exhibited a sigmoidal shape for well-defined reaction fronts, whereas the profiles showed a gradual decrease in solute with distance near the front for ill-defined fronts, dropping fairly abruptly at the metal/gas interface. The solute concentration never reached zero at the surface for any condition studied. In contrast to classical, Wagnerian behavior, solute continued to precipitate out after the reaction zone had passed, leading to a variation in the mole fraction of oxide in the zone. SEM/EDXA and XRD showed that precipitation occurred by the formation of precursors (Hf-rich regions surrounded by Hf-depleted regions), followed by precipitation of tetragonalHfO₂,which in some cases transformed to monoclinicHfO₂ and subsequently coarsened. The precipitate morphology varied with solute concentration, temperature, oxygen pressure, and location within the reaction zone. High temperature and high oxygen pressure favored a Widmanstätten structure, whereas low temperature and low oxygen pressure favored a spheroidal precipitate structure. Widmanstätten plates were observed to spheroidize at longer times, suggesting that the interfacial energy between particles and matrix was very high. The presence of a small amount of Y (0.11 w/o in C129) always resulted in spheroidal particles. It appears that Y markedly increased the particle/matrix interfacial energy. Microhardness profiles showed decreasing values with distance into the sample for some conditions and alloys but increasing values in other cases. Hardness increases in the substrate in advance of the interface showed that oxygen activity did not reach zero at the reaction front, once again contrary to classical behavior but consistent with high solubility products of the oxide. Results are analyzed in terms of oxygen-trapping by reactive solutes as noted in the literature for both lattice-parameter measurements and oxygen diffusivity studies.     

The high-temperature oxidation of some oxidation-resistant copper-based alloys

The oxidation behavior of Cu-2Be, Cu-5Al, Cu-8Al, Cu-3Si, Cu-2Al-2Si, Cu-2.5Al-2.5Si, Cu-4.5Al-2Si, Cu-7.5 Al-2Si, Cu-6.5Al-4Si, and Cu-4.5Al-5Si, in the temperature range 100–800°C, in air, has been investigated by gravimetric measurements and by electron microscopical examination of stripped oxide films. Most of the alloys showed considerable resistance to oxidation. This was given mainly by a thermally grown film of -alumina on the Cu-Al and Cu-Al-Si alloys and by a beryllia film on the Cu-Be alloy. Other oxide phases, principally copper oxides, were also found to grow on the alloys and these are described. Silicon additions to Cu-Al alloys are found to improve their oxidation resistance, although no crystalline oxides containing silicon were observed in the oxide films stripped from the Cu-Al-Si alloys.     

The development of internal and intergranular oxides in nickel-chromium-aluminium alloys at high temperature

The development of internal oxides, intergranular oxides and internal voids in Ni-15.1Cr-1.1Al and Ni-28.8Cr-1.0Al during oxidation in 1 atm oxygen at 1000° to 1200°C has been studied. In both cases, the formation of an external Cr₂O₃-rich scale causes vacancies to be generated in the alloy due to the different diffusion rates of chromium towards the alloy-scale interface and of nickel back into the bulk alloy. At 1000°C, condensation of these vacancies at the alloy grain boundaries facilitates formation of intergranular oxides while, at 1200°C, the vacancies condense to give voids in the grains and grain boundaries. Internal oxides are formed at both temperatures. The internal and intergranular oxides are mainly α-Al₂O₃, although some Cr₂O₃-rich oxides are produced near the alloy-scale interface. Possible mechanisms for the development of the internal and intergranular oxides in these alloys are discussed and related to the observed oxide morphologies and compositions.     

Accelerated oxidation,internal oxidation,intergranular oxidation,and pesting of intermetallic compounds

The compounds MoSi₂, NiAl, and NbAl₃ all form protective oxide films, particularly at high temperatures where the diffusion of Si or Al is more rapid and, for the case of MoSi₂, the transient oxides evaporate. However, at low temperatures, all three can undergo accelerated oxidation. The mechanisms of degradation are unique to the particular compound although there are some similarities. The accelerated oxidation of MoSi₂ occurs at temperatures below 600°C by the rapid growth of Mo oxides which prevent development of a continuous silica film. Internal or intergranular oxidation does not occur. If the specimen contains cracks or pores, the rapid oxidation in these defects leads to fracture of the specimen or pesting. The accelerated oxidation of NiAl occurs at temperatures below 1000°C at reduced oxygen partial pressures as the result of internal oxidation and rapid intergranular oxidation. The intergranular oxidation does not lead to pesting. Special circumstances are required for the accelerated oxidation of NiAl as it does not appear to occur in flowing gases unless sulfur is present. The accelerated oxidation of NbAl₃ also occurs at temperatures less than 1000°C and at reduced oxygen partial pressures and takes the form of intergranular oxidation of Al. The intergranular oxidation results in pesting of NbAl₃. The phenomena of accelerated oxidation, internal oxidation, intergranular oxidation, and pesting have not been investigated in detail for most other intermetallic compounds but one or more of these phenomena seems to afflict most aluminides and silicides.     

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.     

The role of yttrium in high-temperature oxidation behavior of Ni-Cr-Al alloys

Oxidation of five nickel-chromium-aluminum alloys with yttrium additions of between 0.005 and 0.7wt.% has been studied in the temperature range 800–1200°C in oxygen at pressures of 1, 10, and 720 Torr. The yttrium additions improve the oxidation behavior of the nickel-chromium-aluminum alloys, and generally an addition of 0.1 or 0.2 wt.% yttrium gives the best improvement in the oxidation resistance. In this case, the oxidation kinetics indicate asymptotic approach toward zero scale growth with time. This is suggested to be caused by the formation of subgrains in the alloy which (a) provide enhanced diffusion of aluminum to the surface and (b) increase the number of oxide nucleation sites. Preferred oxidation of aluminum occurs, resulting in the formation of an -Al ₂O₃ layer. Additions of more than 0.3 wt.% yttrium result in preferential grain boundary oxidation and a convoluted alloy-oxide interface. This effect, key-on effect along with the formation of an aluminum and yttrium double oxide, produces increased oxide adherence for these alloys.Part of the work was carried out during the author's stay at Aerospace Research Laboratories, Wright-Patterson Air Force Base, Dayton, Ohio 45433, USA. It was sponsored in part by the Air Force Materials Laboratory, Research and Technology Division, AFSC, through the European Office of Aerospace Research, OAR, U.S. Air Force.     

Internal nitridation of nickel-chromium alloys

The nitriding behavior of nickel-chromium alloys was studied in ammonia-hydrogen mixtures over the range of 700–900°C. Nitridation rates decreased with increasing chromium content, but the critical amount of chromium for transition from internal nitridation to continuous-nitride film formation was found to be much greater than the critical value to form a continuous-Cr ₂O₃ film during oxidation. In general, internal-nitridation rates were found to obey the parabolic rate law. Parabolic rate constants and activation energies for the diffusion of nitrogen were measured. Very fine precipitates formed at the lowest temperature, increasing in size with increasing temperature. The precipitate number density was found to vary within the internally nitrided zone, decreasing with distance from the gas/metal surface. The precipitate morphology changed also with temperature and distance, becoming Widmanstätten at higher temperatures and/or increasing distance within the zone. CrN formed for all exposure conditions. No Cr ₂ N was detected under any conditions studied.     

Kinetics and mechanism of high-temperature oxidation of dilute cobalt-chromium alloys

Kinetics of oxidation of Co-Cr alloys containing 0.4%–15% Cr was studied as a function of temperature (1273–1573 K) and oxygen pressure (4 × 10²–10⁵Pa). The oxidation process was found to be approximately parabolic and faster than that for pure cobalt. The scales are double-layered and consist of a compact outer CoO layer and a porous inner layer containing CoO slightly doped by chromium and spinel CoCr₂O₄. The oxidation mechanism was investigated by means of platinum markers and the¹⁸O isotope. The scale on the alloys containing less than 1% Cr grows exclusively by outward diffusion of cobalt, while that on the alloys containing more chromium—with a significant contribution of inward oxygen transport from atmosphere. This transport is not a lattice diffusion, but proceeds presumably through microfissures resulting from the secondary process of perforating dissociation of the outer scale layer.     

Corrosion of nickel-chromium alloys, stainless steel and niobium at supercritical water oxidation conditions

Results of immersion tests of UNS N06625 (alloy 625), UNS S31609 (alloy 316 L), Ni-20Cr alloy and Nb coupons exposed to oxygenated ammoniacal sulphate solution at supercritical water oxidation (SCWO) conditions are presented. The corrosion behavior of the alloy 316 L (UNS S31603) SCWO reactor tubing is also presented under the same conditions. Immersion coupons corroded at a rate not exceeding 40 mm yr⁻¹ while the reactor tube itself corroded at a rate of between 160-1500 mm yr⁻¹, depending on which length of the reactor is considered to have corroded. Morphological and chemical analysis of the oxides present on the coupon samples suggest that iron oxides, which had initially precipitated on the corroding alloy 316 tube surface, were removed by heat flux-driven fluid mechanical action and transported through the reactor where they deposited on the coupons. Niobium was resistant to corrosion at the tested conditions.     

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 of iron-chromium-nickel alloys containing 0–30% chromium

The oxidation behavior of iron-chromium-nickel alloys containing 0–30% chromium has been determined for oxidation at 1000°C in static pure oxygen atmospheres. Particular emphasis has been placed on the correlation between the kinetics of oxidation and the morphologies and compositions of the scales produced. Maximum oxidation resistance was associated with the formation of chromic oxide scales on alloys containing greater than 20% chromium. The loss of an oxide species from these scales by volatilization may limit the usefulness of alloys protected by chromic oxide scales to a temperature less than 1000°C.Research supported by the Broken Hill Proprietary Co., Ltd., Postgraduate Research Scholarship.Unless otherwise specified, all compositions referred to in this paper are in weight percent.     

The high-temperature oxidation of Ni-20%Cr alloys containing various oxide dispersions

The oxidation behavior of Ni-20%Cr alloys containing approximately 3 vol.% Y₂O₃, ThO₂, and A1₂O₃ as dispersed particles has been examined in the temperature range 900 to 1200° C in slowly flowing oxygen at 100 Torr. The results show that the oxidation behavior of the Y₂O₃-, ThO₂-, Al₂O₃-, and Ce0₂-containing alloys is very similar and that some anomalies in the behavior of the ThO₂-containing alloy might be explained by the slower rate of chromium diffusion in this coarse-grained alloy. Two Al₂O₃-containing alloys were studied. One with a relatively coarse dispersoid size behaved in a manner analogous to a dispersion-free Ni-30% Cr alloy at 1100°C. The other alloy contained a dispersion of fine Al₂O₃ particles and behaved exactly like the Y₂O₃-containing alloy at 1000 and 1100°C, but at 1200° C oxidized at a faster rate. It has been shown that the adherent scales on dispersion-containing alloys have a stabilized fine grain size, whereas the nonadherent scales on dispersion-free alloys undergo grain growth.This work has been supported by the Naval Air Systems Command under Contract No. N00019-72-C-0190.     

Mechanism of internal oxidation in silver alloys

The internal oxidation mechanism in silver alloys was studied by residual resistivity, gravimetric, and calorimetric measurements, and by transmission electron microscopy. If the flux, of oxygen atoms is very large with respect to the oxide formation, the mechanism of internal oxidation includes two stages. The first one is the fixation of oxygen in the form of oxidized elementary species, including only one solute atom. These species are stable and keep a certain mobility. The second one is the coalescence process of these elementary species with the formation of the first clusters. When the flux of oxygen atoms is slowed down, the oxide formation mechanisms are more complicated. The initially formed species can include more than one solute atom because of their diffusion. The two stages are not separate.