Oxidation of Two Ternary Cu–Ni–20 wt.% Cr Alloys at 700–800°C in 1 atm O2

Two ternary Cu–Ni–Cr alloys containing approximately 20 wt.% chromium, but with a different Cu and Ni content, have been oxidized in 1 atm of pure oxygen at 700–800°C. The alloy containing about 60 wt.% nickel (Cu–60Ni–20Cr) was composed of a single solid-solution phase and formed external scales of chromium ocide with an outermost layer containing a mixture of copper and nickel oxides. The alloy comprised of about 40 wt.% nickel (Cu–40Ni–20Cr) contained a mixture of two metal phases and formed complex external scales, containing copper oxide and a nickel–chromium spinel plus a region where islands of the metallic phase richer in chromium surrounded by a thin chromia layer were mixed with oxidized islands rich in copper and nickel, producing a situation out of equilibrium. With time, a very irregular and thin but essentially continuous layer of chromia formed at the base of the mixed internal region for this alloy, producing a gradual decrease of the corrosion rate down to very low values. The oxidation behavior of the two alloys is interpreted in terms of their different microstructure. In particular, the fast initial oxidation of Cu–40Ni–20Cr, associated with the formation of large amounts of copper oxides, is attributed to restrictions in chromium diffusion in the alloy due to the simultaneous presence of two metal phases.     

Oxidation of a Three-Phase Cu–45Ni–30Cr Alloy at 700–800°C Under 1 atm O2

The oxidation of a ternary Cu–Ni–Cr alloy containing approximately 45 wt.% Ni and 30 wt.% Cr has been studied in 1 atm O₂ at 700–800°C. The alloy contains a mixture of three phases: the one with the largest copper and lowest chromium content forms the matrix, the one with an intermediate chromium content has a rather large volume fraction and forms large islands, while the phase richest in chromium forms isolated particles dispersed in the other two phases. At variance with another Cu–Ni–Cr ternary three-phase alloy containing only 20 wt.% Cr and 20 wt.% Ni, which formed complex scales containing mixtures of the oxides of the various components and double oxides, plus an irregular region composed of a mixture of alloy and oxides, the present alloy is able to form protective, external chromia scales. A similar result could be obtained with alloys containing about 20 wt.% Cr, but composed of either a single phase (Cu–60Ni–20Cr) or of a mixture of two phases (Cu–40Ni–20Cr). The need for a larger chromium content for producing chromia scales for three-phase as compared to two-phase Cu–Ni–Cr alloys is attributed to the limitations of the diffusion of the alloy components in the metal substrate imposed by their multiphase nature.     

Oxidation of a quaternary three-phase Cu-20Ni-20Cr-5Fe alloy at 700-900 °C in 1 atm of pure O2

The oxidation of a quaternary Cu-Ni-Cr-Fe alloy containing approximately 20 at.% Ni, 20 at.% Cr and 5 at.% Fe, balance Cu (Cu-20Ni-20Cr-5Fe), was studied at 700-900 °C in 1 atm of pure oxygen. The alloy is composed of a mixture of three phases, where the lightest α phase with the largest Cu content forms the matrix, while the other two, much richer in Cr, form a dispersion of isolated particles. At variance with the ternary three-phase Cu-20Ni-20Cr alloy examined previously, which was unable to form protective chromia scales over the alloy surface even after an extended period of oxidation, the present alloy formed complex external scales containing mixtures of the oxides of the various components plus a deep internal region containing a mixture of alloy and oxide phases. With time, a very irregular and thin but essentially continuous chromia layer formed at the bottom of the mixed internal oxidation region, producing a gradual decrease of the oxidation rate. Thus, the addition of 5 at.% Fe to Cu-20Ni-20Cr alloy is able to decrease the critical Cr content required to form the most stable oxide and promotes the formation of a continuous chromia scale under a lower Cr content in spite of the simultaneous presence of three different phases.     

High-Temperature Oxidation of Two-Phase Nanocrystalline Ag–Cr Alloys in 1 atm O2

Two nanocystalline two-phase Ag–Cr alloys prepared by mechanical alloying and containing approximately 30 and 50 wt.% Cr were oxidized in 1 atm O₂ at 700 and 800°C. Under all conditions, a continuous layer of chromia formed at the surface of the alloys, in spite of the very low solubility of Cr in Ag. A layer of AgCrO₂ also formed externally to the chromia layer. In the case of the Ag–30Cr alloy, some Ag particles were also present on the scale, directly in contact with the gas phase. Moreover, Cr particles dissolved in the subsurface region of the alloy, while internal oxidation of Cr was absent. Ag–Cr alloys prepared by powder metallurgy with coarse grain sizes were able to form an irregular thin chromia layer only at a Cr content of 69 wt.%, while an alloy containing 35 wt.% Cr corroded much more rapidly than the nanocrystalline Ag–30Cr alloy. This difference in the scaling behavior is attributed to the large reduction in the alloy grain size, which favors the dissolution of the Cr-rich particles in a Cr-depleted silver matrix and thus provides a faster supply of chromium from the alloy to the scale.     

The air oxidation of two-phase Cu-Cr alloys at 700–900°C

The oxidation in air of three two phase Cu-Cr alloys with nominal Cr contents of 25, 50, and 75 wt. % was studied at 700–900°C. The alloys corroded nearly parabolically, except at 900°C, when the corrosion rates decreased with time more rapidly than predicted by the parabolic rate law. The corrosion rate decreased for higher Cr contents in the alloy under constant temperature and generally increased with temperature for the same alloy composition. The scales were complex and consisted in most cases of an outermost copper oxide layer free from chromium and an inner layer composed of a matrix of copper oxide or of the double oxide Cu₂Cr₂O₄, often containing particles of chromium metal surrounded by chromia and then by the double oxide. Metallic copper was also frequently mixed with chromia. Cr-rich regions tended to form continuous chromia layers at the base of the scale, especially at the highest temperature. No chromium depletion was observed in the alloy.     

Preparation and Oxidation of Novel Electrodeposited Cu–Ni–Cr Nanocomposites

The poor oxidation resistance of Cu-base alloys limits their applications at high temperatures, and it cannot be improved by conventional Cr alloying because that requires an extremely high Cr content. In this work a chromia scale has been formed on novel Cu-base nanocomposites which contain much less chromium. Cu–Ni–Cr nanocomposites, with the weight percentage ratio of Cu/Ni ≈1 and various amounts of Cr, were produced by co-electrodeposition of Cu–Ni alloy with Cr nanoparticles which subsequently acted as “seeds” for chromia formation. The results of oxidation tests in air at 800°C showed that only 15 wt.% Cr in the nanocomposite was required to form an external chromia scale. Furthermore, the scale consisted only of chromia rather than the duplex scales which form on the conventional alloy even when it contains significantly more chromium.     

The oxidation of Co−Nb alloys under low oxygen pressures at 600–800°C

The oxidation of two Co–Nb alloys containing 15 and 30 wt.% Nb has been studied at 600–800° C in H₂–CO₂ mixtures providing an oxygen pressure of 10^–24 atm at 600°C and 10^–20 atm at 700 and 800°C, below the dissociation pressure of cobalt oxide. At 600 and 700°C both alloys showed only a region of internal oxidation composed, of a mixture of alpha cobalt and of niobium oxides (NbO₂ and Nb₂O₅) and at 700°C also the double oxide CoNb₂O₆, which formed from the Nb-rich Co₃Nb phase. No Nb-depleted layer formed in the alloy at the interface with the region of internal oxidation at these temperatures. Upon oxidation at 800°C a transition between internal and external oxidation of niobium was observed, especially for Co–30Nb. This corrosion mode is associated with the development of a single-phase, Nb-depleted region at the surface of the alloy. The corrosion mechanism of these alloys is examined with special reference to the effect of the low solubility of niobium in cobalt and to the relation between the microstructures of the alloys and of the scales.     

Oxidation behavior of several chromia-forming commercial nickel-base superalloys

Several commercially available Ni-base superalloys were exposed isothermally in air at temperatures between 750° and 1000°C and also under cyclic conditions at 1000°C. The kinetics of oxidation were determined and the scales were analyzed by electron microscopy and X-ray diffraction. Thin adherent chromia-rich scales formed on the alloys at 750°C after 1000 hr. Although Waspaloy showed the lowest weight gain in this test, it also showed the deepest internal corrosion due to oxidation of the grain-boundary carbides. At temperatures up to 1000°C the external scales were also chromia-rich but there was greater internal corrosion. Titanium in the alloys oxidized, diffusing through the chromia scale to form faceted rutile (TiO₂) grains at the surface as well as forming TiO₂ and TiN internally. The amount of rutile at the oxide surface increased with temperature and alloy Ti concentration. Alumina formed as discrete internal oxides below the chromia scale, although Astroloy when oxidized isothermally at 1000°C developed a semicontinuous internal layer of alumina due to its higher Al content. Under cyclic conditions Astroloy formed a thicker, less-protective scale of transition oxides probably due to its lower Cr content.     

The oxidation of two ternary Ni-Cu-10 at.% Al alloys in 1 atm of pure O2 at 800-900 °C

The oxidation of the ternary alloys Ni-45Cu-10Al and Ni-30Cu-10Al has been studied at 800-900 °C under 1 atm O₂. The presence of 10 at.% Al reduces significantly the oxidation rate of the corresponding Cu-Ni alloys during the initial oxidation stages, even before the establishment of a complete Al₂O₃ layer. The weight of individual sample of the two ternary Ni-Cu-10Al alloys at 800 °C increases more rapidly than at 900 °C during the initial oxidation stage. As oxidation proceeds, the weight gain at 800 °C slows down to a degree that the total weight gain after 24 h oxidation at 800 °C is less than that at 900 °C. Due to a faster formation of the Al₂O₃ layer, which suppresses earlier the further oxidation of Cu and Ni, the external region of the scales grown on Ni-45Cu-10Al contain much less Cu and Ni oxides than those grown on Ni-30Cu-10Al. The transition from the internal oxidation to the selective external oxidation of the most reactive component Al in Ni-Cu-Al alloys is favored by higher values of the Al content, of temperature and of the Cu/Ni ratio.     

Oxidation of the three-phase Cu-20Ni-30Cr and Cu-20Ni-40Cr alloys at 700-800 °C in 1 atm O2

The oxidation of two ternary Cu-Ni-Cr alloys containing approximately 30 and 40 at.% Cr, but with a similar Ni content, was studied at 700-800 °C in 1 atm of pure oxygen. Both alloys contain a mixture of three phases, where the phase with the largest copper and lowest chromium content (α) forms the matrix, while the phase with an intermediate content of Ni and Cr (β) and that richest in chromium (γ) are present in the form of particles dispersed in the α matrix. The kinetics of oxidation were rather irregular and presented two approximately parabolic stages which for the alloy with 40 at.% Cr were followed by a final nearly linear stage. Generally, the corrosion rates decreased by increasing the chromium content in the alloy under constant temperature and increased with temperature for a constant alloy composition. The scales formed on the two alloys were rather complex and consisted in most cases of an outermost copper oxide layer followed by a layer containing a mixture of oxides of nickel and copper as well as Cu-Cr and Ni-Cr spinel and finally by an innermost very irregular and convoluted but continuous Cr₂O₃ layer which protruded into the alloy and contained a number of Cu-rich metal islands.     

Comparison of the oxidation of high-sulfur Ni-25Cr-5Al alloys in as-cast and as-sputtered states

To understand the effect of sulfur on the oxidation of nanocrystalline (NC) alloys, a high-sulfur alloy having a chemical composition similar to a coarse-grained (CG) cast alloy of Ni-25Cr-5Al-1S (wt.%) was fabricated using magnetron sputtering. The oxidation of the two alloys in isothermal and cyclic conditions in air at 1000 °C shows that the alumina scale formed on the cast alloy was susceptible to spallation, whereas the alumina scale on the sputtered alloy was intrinsically adhesive. The result indicates that the nanocrystallization of alloys helps to eliminate the detrimental “sulfur effect” on oxidation, through minimizing the segregation of sulfur to the scale/alloy interface.     

Influence of Al Addition on Oxidation Behavior of Cu–Ni–Cr Alloys at 973–1073 K in 1.01 × 10<Superscript>2</Superscript> kPa Pure Oxygen

The oxidation behavior of Cu–20Ni–15Cr–2.5Al and Cu–20Ni–20Cr–2.5Al alloys was studied at 973–1073 K in 1.01 × 10² kPa pure oxygen. The oxidation kinetics exhibited large deviations from the parabolic rate law and were comprised of three or four quasi-parabolic stages. Oxidation rates of the present alloys were much lower than those previously reported for a Cu–20Ni–20Cr alloy. Cu–20Ni–15Cr–2.5Al alloy formed a continuous scale of chromia in contact with the alloy, while at other locations, the scale formed deep protrusions into alloy along β phases. Cu–20Ni–20Cr–2.5Al alloy formed a continuous scale of chromia with a small quantity of light and unoxidized precipitates of α phase, especially at 1073 K. There was a thin layer depleted in Cr beneath the continuous scales of chromia. The addition of 2.5 at.% Al to Cu–Ni–Cr alloy made the diffusion of reactive component Cr become much faster and facilitated the formation of a continuous external scale of chromia for a lower Cr content.     

Influence of a Reactive Element on the Growth of a Thermally Grown Chromia Scale: A Grazing Emission X-ray Fluorescence Study

The oxidation of 55Fe–25Cr–20Ni (wt.%) alloys, with and without added reactive element (RE) Y, were studied using grazing-emission X-ray fluorescence (GEXRF). Samples were studied after isothermal treatments at 750°C in O2 and after cyclic-oxidation treatments. In early-stage oxidation, a Ni-rich scale is formed. The distribution of this early-stage Ni deposit is studied as the scale evolves. The Ni deposit, serving as a marker, remains on the outer scale surface in Y-containing alloys, but is not detectable in scales on Y-free alloys. The results indicate that new chromia scale growth occurs at the outer surface in Y-free alloys but, for Y-containing alloys, new growth occurs away from the outer surface. Thus, a shift in the growth mode is apparently observed at 750°C, consistent with higher-temperature observations. However, unlike the high-temperature measurements, scale-growth rates are not significantly affected by the RE.     

Temperature dependence of oxide scale formation on high-Cr ferritic steels in Ar–H2–H2O

Four high chromium ferritic steels were oxidized in Ar/H₂/H₂O at temperatures between 500 and 900 °C. Polished specimens of all steels formed iron-rich oxides at temperatures below 600 °C, whereas increasing the temperature resulted in local formation of protective chromia scales. As the temperature was raised further, the specimens were totally covered with chromia scales. For the higher chromium steels this was also observed at 900 °C but not for the steel with a chromium content of 16%. The temperature dependence of the oxidation rates is governed by the competing diffusion processes in the alloy and the growing scales.     

The Oxidation of Two Fe-Y Alloys Under Low Oxygen Pressures at 600-800°C

The corrosion of pure yttrium and of two Fe-basealloys containing approximately 15 and 30 wt.% Y wasstudied at 600-800°C in H₂-CO₂mixtures providing an equilibrium oxygen pressure of10^-24 atm at 600°C and 10^-20 atm at 700 and800°C. The corrosion of yttrium under these lowoxygen pressures resulted in the growth ofY₂O₃ scales and presented twoapproximately parabolic stages at 800°C, while at 600-700°C it was faster andnonprotective. The corrosion of the two alloys followedapproximately the parabolic rate law, except for Fe-15Yat 600°C which oxidized nearly linearly. At 600 and700°C, when the gas-phase oxygen pressure was in thefield of stability of iron oxide, the alloys formed alsoa thin external Fe₃O₄ layer, whileat 800°C, when the oxygen pressure was below thestability of FeO, a thin outermost layer of pure iron wasobserved to form. Under all conditions a region ofinternal oxidation formed in the alloy, in which theyttrium-rich phases were transformed into a mixture ofiron metal and oxides, which included double Fe-Yoxides as well as Y₂O₃. Themicrostructure of the internal-oxidation region followedclosely that of the original alloys, which moreover didnot undergo any yttrium depletion. These results are examinedby taking into account the low solubility of yttrium iniron and the presence of intermetallic compounds in thealloys.     

The corrosion of two NiNb alloys under 1 atm O2 at 600–800 °C

The oxidation of two NiNb alloys containing 15 and 30 wt% Nb has been studied at 600–800 °C in pure oxygen under 1 atm O₂ at 600–800 °C. The scales formed on both alloys under all conditions show an external scale, generally duplex, containing an outermost layer of nearly pure NiO and an innermost region of NiO mixed with the double NiNb oxide NiNb₂O₆. Moreover, the samples corroded at all temperatures also show a region of internal oxidation composed of a mixture of alpha nickel and niobium oxides (Nb₂O₅ or/and NbO₂), which formed from both alloy phases Ni₈Nb and Ni₃Nb. No important depletion of niobium was observed in the alloy close to the interface with the zone of internal oxidation, while the depth of this region is generally much higher than measured for the corrosion of the same alloys under low oxygen pressures at the same temperatures. The corrosion mechanism of these alloys is examined with special reference to the effects of the low solubility of niobium in nickel.     

Improving the high-temperature oxidation resistance of a β-γ TiAl alloy by a Cr2AlC coating

A Cr₂AlC coating was deposited on a β-γ TiAl alloy. Isothermal oxidation tests at 700 °C and 800 °C, and thermocyclic oxidation at 800 °C were performed in air. The results indicated that serious oxidation occurred on the bare alloy. Thick non-protective oxide scales consisting of mixed TiO₂ + α-Al₂O₃ layers formed on the alloy surface. The coated specimens exhibited much better oxidation behaviour by forming an Al-rich oxide scale on the coating surface during the initial stages of oxidation. This scale acts as diffusion barrier by effectively blocking the ingress of oxygen, and effectively protects the coated alloys from further oxidation.     

Effect of alloy grain size and silicon content on the oxidation of austenitic Fe-Cr-Ni-Mn-Si alloys in pure O2

Austenitic Fe-18Cr-20Ni-1.5Mn alloys containing 0, 0.6, and 1.5 wt.% Si were produced both by conventional and rapid solidification processing. The isothermal and cyclic oxidation resistance of the alloys were studied at 900°C in pure O₂ to elucidate the role of alloy microstructure and Si content on oxidation properties. The conventionally-processed, large-grained alloy that contained no silicon formed Fe-rich nodules during oxidation. The nodule formation was effectively eliminated by either reducing the alloy grain size by rapid solidification or by adding Si to the alloy. The lowest weight gains were achieved when a continuous silica layer formed between the alloy and the external chromia scale. The formation of the continuous silica layer required a ombination of fine alloy grain size and high Si content. The presence of S in the alloy was found to be detrimental to oxide scale adherence when the silica layer was continuous.     

Oxide scale adhesion and impurity segregation at the scale/metal interface

The chemistry at scale/metal interfaces was studied using scanning Auger microscopy after removal of the scale in ultra-high vacuum using an in situ scratching technique. Al₂O₃ and Cr₂O₃ scales formed between 900°C and 1100°C on Fe-18 wt.% Cr-5 wt.% Al and on Ni-25 wt.% Cr alloys, respectively, were investigated. The adhesion of these scales was determined qualitatively by way of micro-indentation and scratching on the surface oxide. All of the alumina scales fractured to the same degree to expose the metal surface, regardless of the oxidation temperature. The chromia-forming alloy on the other hand, developed more adherent scales at lower oxidation temperatures. About 20 at.% sulfur was found at the metal surface in all cases, and its presence was not only detected on interfacial voids, but also on areas where the scale was in contact with the alloy at temperature. Results from this study clearly demonstrated that sulfur as an alloying impurity does segregate to the scale/alloy interface. However, for alumina scales and chromia scales, the effect of this segregation on oxide adhesion is noticeably different.     

Effect of presulfidation on the oxidation behavior of Co-based alloys. Part I. Presulfidation at sulfur partial pressures above the dissociation pressure of cobalt sulfide

The effects of presulfidation in H₂-H₂S atmospheres of sulfur activity sufficient to form cobalt and chromium sulfides on the oxidation rates of Co-Cr binary alloys containing 0–25 wt.% Cr and Co-25 wt.% Cr alloys containing 0–2 wt.% C have been investigated. Presulfidation increases the oxidation rate, but the effect is not very dramatic. Carbon additions to the Co-25 wt.% Cr alloy progressively increase the oxidation rate, but not to as great an extent as a simple model based on the reduction of the chromium activity in the alloy. Sulfur released from the preformed sulfides by oxidation diffuses into the alloy precipitating fresh sulfides, there appears to be no outward diffusion of sulfur through the oxide scale. These internal sulfides have a liquid-like morphology in cobalt-base alloys when the oxidation is carried out at 1000°C, as compared to 800°C in corresponding nickel-base alloys. When the sulfide layer produced during the presulfidation is thin, so that oxidation destroys the continuous sulfide layer, the subsequent scale morphologies after oxidation exhibit many features similar to samples subjected to hot corrosion in environments containing sodium sulfate.