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1.
Cao  Z. Q.  Niu  Y.  Gesmundo  F. 《Oxidation of Metals》2001,56(3-4):287-297
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.  相似文献   

2.
Gesmundo  F.  Niu  Y.  Viani  F.  Douglass  D. L. 《Oxidation of Metals》1998,49(1-2):147-167
The oxidation of three Cu-Cr alloys containing25, 50, and 75 wt.% Cr and having a two-phasemicrostructure has been studied at 700-900°C inH2-CO2 mixtures under10-19 atm O2, i.e., below thestability of the copper oxides. At variance with theresults obtained by oxidizing the same materials in air,the alloys corroded quite slowly and formed onlyexternal chromium-oxide scales rather than showinginternal oxidation of chromium. In view of the quitesmall solubility of chromium in copper, lower than thecritical value calculated for the transition from theinternal to the external oxidation of chromium, this result is attributed mainly to a supply ofchromium from the small Cr-rich particles present in thetwo-phase eutectic mixture. The corrosion kineticsfollowed the parabolic rate law to a reasonableapproximation, but the rate constants changed ratherirregularly as functions of temperature and alloycomposition. The scaling rates were generally largerthan those measured for pure chromium under the sameoxygen pressure, but much smaller than those of the same alloysin air. These results are examined with specialreference to the two-phase nature of thealloys.  相似文献   

3.
Zeng  C. L.  Rizzo  F. C.  Monteiro  M. J.  Wu  W. T. 《Oxidation of Metals》1999,51(5-6):495-506
The oxidation of Fe-Y alloys containing 2 and 5at.% Y and pure iron has been studied at 600-800°Cin air. The oxidation of pure iron follows the parabolicrate law at all temperatures. The oxidation of Fe-Y alloys at 600°C approximatelyfollows the parabolic rate law, but not at 700 and800°C, where the oxidation goes through severalstages with quite different rates. The oxide scales on Fe-2Y and Fe-5Y at 700 and 800°C arecomposed of external pure Fe oxides containingFe2O3,Fe3O4, and FeO, with FeO being themain oxide and an inner mixture of FeO andYFeO3. The scales on Fe-2Y and Fe-5Y at 600°C consist ofFe2O3,Fe3O4, andY2O3, with a minor amount of FeO.Significant internal oxidation in both Fe-Y alloysoccurred at all temperatures. The Y-containing oxidesfollow the distribution of the original intermetalliccompound phase in the alloys. The effects of Y on theoxidation of pure Fe are discussed.  相似文献   

4.
Ti-23Al, Ti-50Al and Ti-50Al-2Nb (at.%) wereoxidized in air at 900°C for times up to 1130 hr.The resulting oxide scale structures were analyzed ingreat detail by metallographic and microprobe investigations and theAl2O3 structures in the complexoxide scales were correlated with the course of thethermogravimetric curves. It appears that in order toachieve long-term protective behavior of the scales, it is necessary to stimulate theformation of a thin Al2O3 barrierat the scale-metal interface and not at a position inthe outer part of the scale. The Nb effect seems to bemostly due to this stimulation of anAl2O3 layer at theinterface.  相似文献   

5.
Zhao  Z. L.  Niu  Y.  Gesmundo  F.  Wang  C. L. 《Oxidation of Metals》2000,54(5-6):559-574
Two nanophase Ni-base alloys containing 50 and 25 at.% Ag prepared by mechanical alloying, denoted Ni–50Ag and Ni–25Ag were oxidized in air at 600 and 700°C for 24 hr. Ni–50Ag underwent internal oxidation of nickel, associated with the formation of a continuous outermost layer of silver metal with scaling rates larger than those for pure nickel. On the contrary, Ni–25Ag formed a continuous NiO layer surmounted by a discontinuous silver layer and internal oxidation was suppressed. The oxidation rate of Ni–25Ag decreased with time much more rapidly than predicted by the parabolic rate law during the initial stage and eventually became parabolic, with rate constants much lower than those for the oxidation of pure nickel. These results are attributed to the two-phase nature and, particularly, to the very small grain size of the two alloys.  相似文献   

6.
The internal oxidation of Ni in dilute Cu?CNi alloys exposed to 1 atm. O2 at 900 °C, and thus in the presence of external CuOx scales, is absent for Ni levels below a critical value, extremely close to that required for the simultaneous equilibrium between the alloy and the oxides of the two components. For Ni contents not largely exceeding this limit the internal oxidation of Ni deviates from the classical behavior described by Wagner, approaching that predicted by Morral et al. for the precipitation of compounds with limited thermodynamic stability.  相似文献   

7.
Four ternary Cu–Zn–Al alloys containing 5 or 10 at.% Zn and 2 or 4 at.% Al plus an alloy containing 2 at.% Al and 15 at.% Zn have been oxidized at 800 °C in 1 atm O2, and their behavior has been compared with that of the corresponding binary Cu–Zn and Cu–Al alloys. For the alloy containing 4 at.% Al, which is already able to form external alumina scales, the addition of Zn is only effective in reducing the mass gain during the fast, initial-oxidation stage. Conversely, the addition of 15 at.% Zn to Cu–2Al is able to prevent the formation of external scales containing mixtures of the Cu and Al oxides, resulting in the formation of external alumina scales after an initial stage of faster rate, producing a limited third-element effect. Finally, the addition of Al to both Cu–5Zn and Cu–10Zn is able to prevent the internal oxidation of Zn, producing a kind of reversed third-element effect. Possible mechanisms for these effects are examined on the basis of general treatments concerning the scaling behavior of ternary alloys.  相似文献   

8.
Niu  Y.  Yan  R. Y.  Fu  G. Y.  Wu  W. T.  Gesmundo  F. 《Oxidation of Metals》1998,49(1-2):91-114
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.  相似文献   

9.
The oxidation behavior of FeCoNi, FeCoNiCr, and FeCoNiCrCu equi-molar alloys was studied over the temperature range 800–1000 °C in dry air. The ternary and quaternary alloys were single-phase, while the quinary alloy was two-phase. In general, the oxidation kinetics of the ternary and quinary alloys followed the two-stage parabolic rate law, with rate constants generally increasing with temperature. Conversely, three-stage parabolic kinetics were observed for the quaternary alloy at T 900°C. The additions of Cr and Cu enhanced the oxidation resistance to a certain extent. The scales formed on all the alloys were triplex and strongly dependent on the alloy composition. In particular, on the ternary alloy, they consist of an outer-layer of CoO, an intermediate layer of Fe3O4, and an inner-layer of CoNiO2 and Fe3O4. Internal oxidation with formation of FeO precipitates was also observed for this alloy, which had a thickness increasing with temperature. The scales formed on the quaternary alloy consisted of an outer layer of Fe3O4 and CoCr2O4, an intermediate layer of FeCr2O4 and NiCr2O4, and an inner layer of Cr2O3. Finally, the scales formed on the quinary alloy are all heterophasic, consisting of an outer layer of CuO, an intermediate-layer of CuO and Fe3O4, and an inner-layer of Fe3O4, FeCr2O4, and CuCrO2. The formation of Cr2O3 on the quaternary alloy and possibly that of CuCrO2 on the quinary alloy was responsible for the reduction of the oxidation rates as compared to the ternary alloy.  相似文献   

10.
The oxidation behavior of three Fe–Si alloys containing approximately 5, 9 and 13 at.% Si has been studied at 600–800 °C under 1 atm O2. Fe–5Si and Fe–9Si followed multi-stage parabolic kinetics, while Fe–13Si showed more complex kinetics at all temperatures. The increase in Si content resulted in a transition from the internal oxidation of Si beneath an external scale of iron oxides to the exclusive external formation of silica. The critical contents required for the transitions between the various possible oxidation modes of the binary Fe–Si alloys were calculated and compared with the experimental results. A thermodynamic mechanism for the formation of Fe-rich oxide nodules observed in the oxidation of Fe–5Si and Fe–9Si has been proposed.  相似文献   

11.
《Intermetallics》2000,8(3):293-298
The oxidation of a cobalt-based alloy containing 15 wt% yttrium (Y) as well as of pure yttrium has been studied in 1 atm of pure oxygen at 600–800°C. The alloy always corrodes faster than pure cobalt and follows the parabolic rate law only approximately, forming an external scale of cobalt oxide which in its inner section contains particles of yttrium oxide. Beneath the external scales, a region of internal oxidation of yttrium having a depth much larger than predicted using the known solubility and diffusivity of oxygen in pure cobalt is also present. This anomalous depth is attributed to enhanced oxygen diffusion in the alloy along preferential paths associated with the localized internal oxidation of yttrium. No yttrium depletion is observed in the alloy beneath the front of internal oxidation. The scaling behavior of this alloy is interpreted by taking into account the limited solubility of yttrium in cobalt and the presence of the intermetallic compound Co17 Y2.  相似文献   

12.
Zeng  C.L.  Li  M.C.  Liu  G.Q.  Wu  W.T. 《Oxidation of Metals》2002,58(1-2):171-184
The oxidation of pure Ni and three Ni–Ti alloys containing 5, 10, and 15 wt.% Ti over the temperature range 650–850°C in air was studied to examine the effect of titanium on the oxidation resistance of pure nickel. Ni–5Ti is a single-phase solid solution, while the other two alloys consisted of nickel solid solution (-Ni) and TiNi3. The oxidation of Ni–Ti alloys at 650°C follows an approximately parabolic rate law and produces a decrease in the oxidation rate of pure Ni by forming an almost pure TiO2 scale. At higher temperatures, Ni–Ti alloys also follow an approximately parabolic oxidation, and their oxidation rates are close to or faster than those of pure Ni. Duplex scales containing NiO, NiTiO3 and TiO2 formed. Some internal oxides of titanium formed, especially at 850°C. In addition, the two-phase structure of Ni–10Ti and Ni–15Ti was transformed into a single-phase structure beneath the scales.  相似文献   

13.
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.  相似文献   

14.
Gesmundo  F.  Niu  Y.  Oquab  D.  Roos  C.  Pieraggi  B.  Viani  F. 《Oxidation of Metals》1998,49(1-2):115-146
The air oxidation of three Fe-Cu alloyscontaining 25, 50, and 75 wt.% Cu has been studied at600-800°C. The oxidation followed the parabolic lawonly approximately with rates lower than those of thepure constituent metals. The scales were alwayscomposed of an inner layer containing a mixture ofcopper metal and iron oxide and of an outer oxide layerwhose composition depended on the copper content of the alloy. For the two alloys richer in ironthe external layer was composed mostly of iron oxideswith some copper-rich particles which oxidized only inthe external-scale zone. For the alloy richest in copper the external layer contained a complexmixture of iron oxides, copper particles and doubleFe-Cu oxides surmounted by an outermost copper-oxidelayer. No significant iron depletion was observed in the alloys beneath the region of internaloxidation. The peculiar scale microstructure observedfor these alloys is considered mainly as a consequenceof their two-phase microstructure and of the limited solubilities of the two components in oneanother.  相似文献   

15.
To clarify the oxidation mechanism of Cu2O to CuO, Cu2O oxidation was studied at 600–1050 °C under 1atm O 2. The Cu2O specimens were prepared through completely oxidizing 99.99999 and 99.5 pure copper at 1000°C in an Ar + 1 O 2 atmosphere. The oxidation kinetics of Cu2O specimens prepared from both purity levels followed the logarithmic law, not the parabolic law or the cubic law as reported in the literature. The activation energy for Cu2O oxidation is relatively high in the lower-temperature range, but becomes very small or even negative at higher temperatures. The logarithmic oxidation rate law can be explained by Davies et al.s model related to grain-boundary diffusion in the oxide layers. The very small or negative activation energies in the higher-temperature range can be attributed to the very small thermodynamic driving force and the fast lateral growth of CuO grains related to a sintering effect. The influence of small amount of impurities is also discussed.  相似文献   

16.
Susan  D. F.  Marder  A. R. 《Oxidation of Metals》2002,57(1-2):131-157
The oxidation of nickel-matrix/aluminum-particle composite coatings was studied using thermogravimetric (TG) analysis in air at 800°C for up to 100 hr. Long-term oxidation behavior was investigated with furnace exposures up to 2000 hr. The coatings were applied to nickel substrates by the composite electrodeposition technique and vacuum heat treated for 3-hr at 825°C prior to oxidation testing. The heat-treated coatings contained a two-phase (Ni)+(Ni3Al) microstructure and the overall coating composition was approximately 7 wt.% Al. Also examined were uncoated nickel substrates and bulk Ni–Al alloys containing 6.2, 9.0, and 14 wt.% Al. For all samples, mass-gain kinetics were obtained from thermogravimetric (TG) experiments and furnace exposures and the composition and morphology of the oxidation products were examined using optical microscopy, scanning-electron microscopy (SEM), electron-probe microanalysis (EPMA), and X-ray diffraction (XRD). An outer NiO layer and an inner -Al2O3 layer formed on the composite-coating surface. The addition of a small amount of Si (about 1–2 at.%) was found to have little effect on Ni–Al composite-coating oxidation behavior. The Ni–Al coatings behave similarly to bulk + (Ni3Al) or single-phase (Ni3Al). In addition, at lower temperatures, such as 800°C, the coatings benefit from a small grain size that enhances Al diffusion to the surface to form the protective alumina layer. Based on oxidation kinetics and morphology, a critical Al content of about 6 wt.% was found, below which internal oxidation and higher oxidation mass gains were observed.  相似文献   

17.
The oxidation of four Ni–10Cr–ySi–4Al alloys has been studied in 1 atm O2 at 900 and 1000 °C to examine the effects of various Si additions on the behavior of the ternary alloy Ni–10Cr–4Al, which during an initial stage formed external NiO scales associated with an internal oxidation of Cr + Al, later replaced by the growth of a chromia layer at the base of the scale plus an internal oxidation of Al. The addition of 2 at.% Si was able to prevent the oxidation of nickel already from the start of the test, but was insufficient to form external alumina scales at 1000 °C, while at 900 °C alumina formed only over a fraction of the alloy surface. At 1000 °C the addition of 4 at.% Si produced external chromia scales plus a region of internal oxidation of Al and Si, a scaling mode which formed over a fraction of the alloy surface in combination with alumina scales also by oxidation at 900 °C. Conversely, the presence of about 6 at.% Si produced external alumina scales over the whole sample surface at 900 °C, but only over about 60 % of the alloy surface at 1000 °C. The changes in the oxidation modes of the ternary Ni–10Cr–4Al alloy produced by Si additions have been interpreted by extending to these quaternary alloys the mechanism of the third-element effect based on the attainment of the critical volume fraction of internal oxides needed for the transition to the external oxidation of the most-reactive-alloy component, already proposed for ternary alloys.  相似文献   

18.
Chen  R. Y.  Young  D. J.  Blairs  S. 《Oxidation of Metals》2000,54(1-2):103-120
The corrosion behavior of Fe–22Mo–10Al (a/o, atom %),Fe–20.5Mo–15.7Al, and Fe–10Mo–19Al was examined inflowing H2/H2S gases of 4 Pa sulfur partial pressureat 900°C. Al2O3 was stable on all the alloys inthe atmospheres investigated. Fe–22Mo–10Al andFe–20.5Mo–15.7Al reacted slowly, following the parabolic ratelaw. Multilayered reaction products were formed on these alloys and it isuncertain which layer(s) provided the protection. Fe–10Mo–19Alreacted even more slowly, exhibiting two-stage parabolic kinetics. Duringthe early stage of this alloy's reaction, a preferential reaction zone,consisting of an oxide mixture, possibly Al2O3+FeAl2O4,and nonreacting Fe3Mo2, provided the protection. Duringthe later reaction stage, the formation of a continuous, externalAl2O3 layer further decreased the alloy reaction rate.  相似文献   

19.
Three powder metallurgy (PM) TiAl alloys with a fully lamellar structure were oxidized isothermally and cyclically between 800 and 1000°C in air in order to find the effect of W on the oxidation behavior of Ti–48Al–2Cr–2Nb alloys. The alloys oxidized parabolically during isothermal oxidation. Tungsten improved the isothermal and cyclic oxidation resistance. The oxide scales consisted primarily of an outer TiO2 layer, an intermediate Al2O3-rich layer, and an inner TiO2-rich layer. The alloying elements of Cr, Nb, and W tended to segregate in the lower part of the scale owing to their thermodynamic nobility. In the vicinity of the scale/matrix interface, TiN and Ti2AlN coexisted.  相似文献   

20.
《Corrosion Science》1963,3(3):161-IN8
Cr was oxidized in 1 atm of oxygen at 980, 1090 and 1200°C for periods up to 100 hr. Surface preparation has a large effect on scaling; electropolished Cr oxidizes rapidly. Non-uniform oxide layers form exhibiting nodule growth, blistering, wrinkling and multilayered ballooning. These and other observations indicate that compressive stresses develop during film thickening. This suggests that anion as well as cation diffusion takes part in the growth process and that new oxide forms within the oxide layer. The resulting continuous plastic deformation is considered in interpreting the oxidation kinetics. Best values of the rate constants are derived from measurement of layer thickness at selected areas on the metallographic cross-section.. Moisture did not affect the rate. Cr is at least as oxidation resistant as Fe-25 Cr alloy.  相似文献   

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