The oxidation of a Co–15 wt% yttrium alloy in 1 atm O2 at 600–800°C

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.     

The Sulfidation of a Co-15 wt.% Ce Alloy in H2-H2S Mixtures at 600-800°C

The sulfidation behavior of a Co-Ce alloycontaining approximately 15 wt.% Ce has been studied at600-800°C in H₂-H₂S mixturesproviding a sulfur pressure of 10^-8 atm, butalso of 10^-7 atm at 800°C. At 600 and 700°C the alloy corrodes moreslowly than pure cobalt, but more rapidly than purecerium. At 800°C under 10^-8 atmS₂, which is below the stability of thecobalt sulfides, the alloy corrodes quite slowly, but under 10^-7 atmS₂ it corrodes very rapidly and practicallyat the same rate as pure cobalt. The sulfidationkinetics are generally irregular, except for a few casesof nearly parabolic behavior. The sulfidation of the alloy produces duplexscales, containing an outermost layer of practicallypure cobalt sulfide and an inner complex layer where thetwo components are simultaneously present. Cerium is not able to diffuse out of thealloy-consumption region, where it forms a ceriumsulfide mixed with cobalt sulfide and an innermostregion where cerium sulfide is mixed with cobalt metal.The cobalt sulfide forms a continuous network which allowsthe growth of the external CoS_y layer, eventhough at rates reduced with respect to pure cobalt.Thus, a cerium content of 15 wt.% is not sufficient toprevent the sulfidation of the base metal. Theseresults as well as the details of the microstructure ofthe scales grown on the alloy are interpreted by takinginto account the limited solubility of cerium in the base metal and the presence in the alloy ofan intermetallic compound rich in cerium.     

The Corrosion of Co-15 wt.% Y at 600-800°C in Sulfidizing-Oxidizing Atmospheres

The corrosion of Co-15 wt.% Y has been studiedat 600-800°C inH₂-H₂S-CO₂ mixturesproviding a sulfur pressure of 10^-8 atm at600-800°C and of 10^-7 atm at 800°Cand an oxygen pressure of 10^-24 atm at 600°C and of10^-20 atm at 700-800°C. The corrosionrates in such sulfidizing-oxidizing atmospheres werecompared with those of pure cobalt and yttrium. Theaddition of yttrium to cobalt is only slightly beneficial, sincefor a yttrium content of 15 wt.% the corrosion rate isreduced quite significantly with respect to pure cobaltat 800°C under 10^-7 atm S₂,only to a limited extent at 600°C, and even slightlyincreased at 700°C. Moreover, the alloy corrodesconsiderably more rapidly than pure yttrium at800°C, when the latter behaves protectively. At 600 and 700°C, yttrium exhibitedbreakaway behavior, while the alloy corroded morerapidly than yttrium at short times, but more slowly atlong times. Under all conditions, except at 800°Cunder 10^-8 atm S₂, the alloy formsan external layer of cobalt sulfide overlying anintermediate region of very complex compositioncontaining a mixture of the compounds of the two metalsand an innermost region of internal attack containing compoundsof yttrium with both oxygen and sulfur. Thus, cobalt canstill diffuse through the intermediate region to formthe outer cobalt-sulfide layer at nonnegligible rates. The scaling behavior of the Co-15% Yalloy is discussed by taking into account the limitedsolubility of yttrium in cobalt as well as the presenceof an intermetallic Co-Y compound in thealloy.     

The Air Oxidation of Two-Phase Fe-Cu Alloys at 600-800°C

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.     

Oxygen transport in growing nickel oxide scales at 600–800°C

The effect of oxidation conditions on the penetration of oxygen into growing nickel oxide scales has been examined using oxygen isotope tracers and SIMS analysis. Depth profiles and cross-section images of oxide scales show that the proportion of oxygen penetration is increased as the temperature is reduced. At 600°C, the amount of oxygen tracer transported to the scale-metal interface is considerable, approaching that observed for a conventional duplex microstructure. The formation of healing oxide along numerous fissures through the oxide is directly observed in SIMS images. These fissures seem to coincide with grain boundaries in the preexisting oxide. As the oxidation temperature is increased, the oxygen transport becomes less uniform, some regions of the scale showing little or no transport. Reduction in the oxygen pressure reduces the amount of oxygen penetrating the scale.This work has been carried out as part of the longer-term research within he Underlying Research Programme of AEA Technology.     

The Oxidation of Fe-2 and 5 at.% Y Alloys at 600-800°C in Air

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.     

Oxidation mechanism of cobalt based alloy at high temperatures (800–1100°C)

Abstract

A cobalt based Phynox alloy has been oxidised in the 800–1100°C temperature range. Kinetic results show that the parabolic behaviour is followed under isothermal conditions. The scale growth mechanism of cobalt based Phynox alloy in air is consistent with a growth mechanism limited by the diffusion process in a growing Cr₂O₃ oxide scale. Thermal cycling tests show that the best scale adherence is found at 1000°C. This temperature permits a rapid chromium supply from the substrate to form a continuous chromia scale. A keying effect at the internal interface is promoted by the presence of silicon and molybdenum. At 900°C, CoCr₂O₄ cobalt containing oxide formation is favoured and leads to a bad scale adherence. At 1100°C, thermal cycling conditions lead to scale spallation and chromium depletion. Then, important weight losses are registered corresponding to the oxidation of cobalt and molybdenum to induce CoCr₂O₄ and CoMoO₄ formation.     

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.     

Air-oxidation of a Co-based amorphous ribbon at 400–600 °C

The oxidation behavior of a commercial Co₆₉B₁₂Si₁₂Fe₄Mo₂Ni₁ amorphous ribbon (Co6-AR) was studied over the temperature range of 400–600 °C in dry air. The results showed that virtually no oxidation occurred at 400 °C. On the other hand, the oxidation kinetics of the Co6-AR alloy at 450–600 °C generally followed a multi-stage parabolic-rate law, and the parabolic-rate constants (k_p values) tend to increase with increasing temperature. It was found that the oxidation rates of the glassy alloy are slower than those of pure Co, indicative of a better oxidation resistance. An exclusive scale of CoO was observed after the oxidation of the glassy alloy in the temperature range of interest, and several crystalline phases formed on the substrate beneath the scale, consisting of pure Co (both FCC and HCP structures), Co₃B, Co₂Si, CoFe, and Co₂B (absent at 450 °C), which indicated the occurrence of crystallization.     

Tensile Deformation of Iron Oxides at 600–1250°C

Tensile tests of virtually pure FeO, -Fe₃O₄, and -Fe₂O₃ were performed at 600–1250°C at strain rates of 2.0×10^–3–6.7×10^–5 s^–1 under controlled gas atmospheres. Mechanical properties and deformation/fracture behavior were investigated. For -Fe₂O₃, brittle fracture resulted at 1150–1250°C and the fracture strain was below 4.0% at a strain rate of 2.0×10^–4 s^–1. -Fe₃O₄ deformed plastically above 800°C. Steady-state deformation was indicated at 1200°C; elongation of 110% was obtained. Plastic deformation observed at 800 to 1100°C was considered to result from dislocation glide. Using TEM, the Burgers vector of dislocations observed in deformed -Fe₃O₄ was determined to be <110>, its slip system was estimated to be {111}<110>. FeO deformed plastically above 700°C. Steady-state deformation became predominant above 1000°C. Elongation of 160% was obtained at 1200°C. Strain rates of FeO at 1000°C and 1200°C were proportional to the fourth power of the saturated stress, indicating that plastic deformation was affected by dislocation climb.     

The corrosion mechanism of Fe-based superalloy in molten NaCl-52 wt-%MgCl2 at 520°C

NaCl-52?wt-%MgCl₂ is a good thermal storage medium at medium–high temperature. But the corrosion of chlorate on metal is serious and the mechanism is unclear. In this paper, the corrosion kinetics curves of Fe and three kinds of Fe-based superalloys were measured by the immersion salt corrosion method at 520°C. The microstructure and composition on the surface and cross-section were characterised by a scanning electron microscopy with EDS analysis and X-ray diffraction. The results show that corrosion kinetics curves obey linear law, and the average mass loss rate of Fe is the lowest of all. After corrosion for 20?h, the main composition on Fe surface was mainly magnesia (MgO) and Fe. Shell structure appeared on the surface of three alloys, the composition of shell was MgO, while matrix had Fe, Ni and its compounds. After corrosion for 160?h, the surfaces of four samples became loose, and they generated different corrosion products. The cross-sectional morphology and line scanning analysis results show that Fe corrosion is relatively mild, while Fe-based alloys presented obvious corrosion layer, and the content of Fe and Cr near corrosion layer decreased. The corrosion mechanism of Fe-based superalloys mainly involves oxidation–chlorination.     

Oxidation behavior of a Zr–Cu–Al–Ni amorphous alloy in air at 300–425 °C

The oxidation behavior of Zr–30Cu–10Al–5Ni bulk metallic glass and its crystalline counterpart was studied over the temperature range of 300–425 °C in dry air. In general, the oxidation kinetics of both amorphous and crystalline alloys followed a two- or three-stage parabolic rate law at T⩾350 °C, while at 300 °C the amorphous alloy oxidized following a linear behavior. The oxidation rate constants for the amorphous alloy are slightly higher than those for the crystalline alloy at 350–400 °C. The scale formed on the amorphous alloy consists of mainly tetragonal-ZrO₂ at 300 °C, while a mixture of monoclinic-ZrO₂ (m-ZrO₂) and tetragonal-ZrO₂ (t-ZrO₂) and some CuO were detected at higher temperatures. The scale formed on the crystalline alloy, on the other hand, consists of mainly Al₂O₃, some tetragonal-ZrO₂, and a slight amount of monoclinic-ZrO₂ at 300 °C. At higher temperatures, the crystalline alloy consists of mainly monoclinic-ZrO₂, some CuO and Cu₂O, and limited tetragonal-ZrO₂. It is suggested that the formation of Al₂O₃ (at 300 °C) and CuO/Cu₂O (at 350-400 °C) on the crystalline alloy is responsible for the reduced oxidation rates as compared with those of amorphous alloy.     

Air Oxidation of FeCoNi-Base Equi-Molar Alloys at 800–1000°C

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.     

The Oxidation of Two Ternary Ni–Cu–5 at.%Al Alloys in 1 atm of Pure O2 at 800–900°C

The oxidation of a Ni-rich and a Cu-rich single-phase ternary alloy containing about 5at.% aluminum has been studied at 800 and 900°C under 1atm O₂. The behavior of the Ni-rich alloy is similar to that of a binary Ni–Al alloy with a similar Al content at both temperatures, with formation of an external NiO layer coupled to the internal oxidation of aluminum. The Cu-rich ternary alloy shows a larger tendency to form protective alumina scales, even though its behavior is borderline between protective and non-protective. In fact, at 800°C, after an initial stage of fast reaction during which all the alloy components are oxidized, this alloy is able to develop a continuous layer of alumina at the base of the scale which prevents the internal oxidation of aluminum. On the contrary, at 900°C the innermost alumina layer undergoes repeated rupturing followed by healing, so that internal oxidation of Al is only partly eliminated. As a result, the corrosion kinetics of the Cu-rich ternary alloy at 900°C are much faster than at 800°C and very similar to those of pure copper and of Al-dilute binary Cu–Al alloys. Possible reasons for the larger tendency of the Cu-rich alloy to form external alumina scales than the Ni-rich alloy are examined.     

Characterization of oxide structures formed on nickel-chromium alloy during low pressure oxidation at 500–600°C

Low pressure oxidation studies of Ni-18%Cr alloy were carried out at temperatures of 500–600°C for very brief periods. Detailed XPS, AES, SEM, and TEM studies identified four stages in the initial oxidation. These are: (1) formation of a mixed nickel-chromium oxide overlayer; (2) growth of submicron-sized oxide nodules; (3) development of dark hole-like patches on the surface; and (4) growth of second generation oxide nodules. Both types of nodules consist primarily of a nickel structure depleted in oxygen. Their formation appears to result from a very rapid outward movement of nickel from localized defects in the metal. The dark patches result from the presence of a chromium oxide-rich underlayer, which appears to form by a lateral migration of chromium from adjacent oxide/metal interface regions and from grain boundaries.     

Corrosion behaviour of IN625 in superheated steam at 800°C

Concerns with greenhouse gas emissions and the uncertainty of long-term supply of fossil fuels have resulted in renewed interest in nuclear energy as an essential part of the energy mix for the future. Canada is currently undertaking the design of Generation IV supercritical water-cooled reactor (SCWR) with improved higher thermodynamic efficiency and considerable plant simplification. The identification of appropriate materials for in-core and out-of-core components to contain the SCWR coolant is one of the major challenges for the design of Canadian SCWR. This study is carried out to evaluate the oxidation/corrosion behaviours of Inconel 625 (IN625) under superheated steam (SHS) at a temperature of 800°C (0.1?MPa) for up to 3000?hours. The results show that chromium-based oxide forms on the surface after exposure in SHS for 800, 2000 and 3000?hours. The oxide formation leads to initial weight increase followed by weight loss after extended exposure, likely due to chromium oxide dissolution. No localised oxide spallation was observed on all samples examined. Despite the slight weight reduction, a dense and adhered oxide layer, consisting primarily of Cr₂O₃ with some spinel (Ni(Cr,Fe)₂O₄), remained on IN625 after 3000?hours in SHS.     

The influence of dissolved oxygen concentration on the corrosion of grey cast iron in water at 50°C

The formation of corrosion scales has been studied on grey cast iron in flowing water at 50°C as a function of O₂ concentrations from 0.1 to 3.95 ppm O₂. Below 1.0 ppm O₂, nodular scales form containing Fe₃O₄ and a green rust, GR. At higher O₂ concentrations, a continuous scale eventually forms, consisting of a porous subscale of Fe₂O₄ + GR overlaid with a compact crust of Fe₃O₄ + GR and a thin surface layer of γ-FeOOH. ‘Chimneys’ oriented in the water flow direction grow out of the crust. γ-FeOOH is reduced to Fe₃O₄ which becomes the principal constituent of the scale. Scales on cast iron components from central heating systems closely resemble those found in the present work.     

Improvement of isothermal oxidation resistance of a γ'-strengthened Co-Al-W-Mo-Ta-B alloy at 800℃via doping Ce

Isothermal oxidation resistance,oxide scale evolution and failure mechanism of Ce-doped Co-Al-W-MoTa-B alloy(0.01 at%,0.05 at%,0.10 at% and 0.20 at% Ce)exposed at 800℃ were compared.The 0.01 Ce and 0.05 Ce alloys were consisted of γ/γ' coherent micro structure,while the κ-Co_3 W compound precipitated at the grain boundary of the 0.1 OCe and 0.20 Ce alloys in addition to the γ/γ'microstructure.The oxidation kinetics curves of the Cedoped alloys exhibited a parabolic time dependence on the weight gain.With an increasing nominal Ce content,the weight gain of the Co-Al-W-Mo-Ta-B alloys monotonically decreased.An oxide scale composed of a dense and uniform outer Co_3 O_4+CoO layer,a middle CoAl_2 O_4 and CoWO_4 compound layer and an inner Al_2 O_3 layer.The excellent oxidation resistance of 0.2 Ce alloy was mainly attributed to a shorter incubation stage for the formation of the continuous and protective Al_2 O_3 layer and the thickest Al_2 O_3 layer during entire oxidation process.     

The Sulfidation of a Cu-15 vol.% Nb Alloy in H2-H2S Mixtures at 400-600°C

The corrosion of a Cu-15 vol.% Nb alloy byH₂-H₂S mixtures has been studiedunder 10^-12 atm S₂ at 400 and500°C and under 10^-10 atm S₂at 500 and 600°C. The alloy is a two-phase mixtureof the two terminal solid solutions and is composed of a Cu-richmatrix containing particles of the Nb-rich phaseelongated parallel to the sample surface and isolatedmainly from each other. The alloy corroded at ratessimilar to those of pure copper at 400 and 500°Cunder 10^-12 atm S₂, but moreslowly than pure copper at 500 and 600°C under10^-10 atm S₂. The scales wereduplex, containing an external layer of pure coppersulfide and an inner very porous region composed of amixture of sulfides of the two metals in which, however,the core of the large Nb particles was stilluncorroded.     

Deformation of Iron Oxides upon Tensile Tests at 600–1250°C

Tensile tests of virtually pure FeO, -Fe₃O₄, and -Fe₂O₃ were performed at 600–1250°C at strain rates of 2.0×10^–3–6.7×10^–5 s^–1 under controlled gas atmospheres. Mechanical properties and deformation/fracture behavior were investigated. For -Fe₂O₃, brittle fracture resulted at 1150–1250°C, and the fracture strain was below 4.0% at a strain rate of 2.0×10^–4 s^–1. Oxide of -Fe₃O₄ deformed plastically above 800°C. Steady-state deformation was indicated at 1200°C; elongation of 110% was obtained. Plastic deformation observed at 800–1100°C was considered to result from dislocation glide. Using TEM, burgers vector of dislocation observed in deformed -Fe₃O₄ was determined to be 110, and its slip system was estimated to be {111}<110>. Oxide of FeO deformed plastically above 700°C. Steady-state deformation became predominant above 1000°C. Elongation of 160% was obtained at 1200°C. Strain rates of FeO at 1000 and 1200°C were proportional to the fourth power of the saturated stress, indicating that the plastic deformation was affected by dislocation climb.