D. J. Young

On the determination of copper surface area by reaction with nitrous oxide

Abstract The chromatographic technique for determining the specific copper surface area of catalysts by reaction with nitrous oxide has been investigated. Promotion of bulk oxidation of copper in catalysts containing oxides of chromium, zinc and aluminium led to an overestimation of the surface copper atoms. The application of a single pulse of nitrous oxide in excess of that required to oxidize an the surface copper and a temperature of 90°C has been shown to provide a reliable measurement of specific copper surface areas.

Oxidation properties of transition metals

The transition metals when exposed to oxygen at low and intermediate temperatures form thin, protective oxide films of up to some thousands of Angstroms in thickness. At higher temperatures, thick (> 1/z) oxide scales are generally formed. The kinetics of the high-temperature process are well established, whilst those of the low-temperature reaction are a matter of contention in the literature. Our discussion reflects this difference by incorporating a detailed consideration of lowtemperature kinetics. Oxidation kinetics are, of course, determined by the reaction mechanism. The mechanism is in turn determined by the properties of the solid oxide reaction product. It should therefore be possible to predict the kinetics of an oxidation reaction from the appropriate properties of the solid oxide or, conversely, to determine those properties from kinetic measurements. This discussion therefore commences with a review of the relevant structural, thermodynamic and transport properties of transition metal oxides. A confrontation of theory and experiment for both the lowand hightemperature r6gimes then follows, and both the successes and the inadequacies of oxidation theories are pointed out. Finally, the model employed for the oxide product is extended to include line defects and internal surfaces, and the importance of these parameters on transition metal oxidation is assessed.

Role of Water Vapor in Chromia-Scale Growth at Low Oxygen Partial Pressure

The oxidation behavior of pure chromium and ODS-Cr alloys in Ar-H2-H2O and Ar-O2-H2O was studied at 1000°C. At high oxygen potentials, the addition of H2O to the gas had negligible effect on the scaling behavior. However, at low oxygen potentials, when the pH2O/pH2 ratio was held constant, the oxidation rate increased with water partial pressure. Increasing values of pH2O/pH2 led to more rapid rates. At fixed pH2O values, the rate increased with increasing pH2. Compact scales were formed under all conditions. In addition Cr2O3 blades grew on the scale surface when pure chromium was reacted with H2O/H2 mixtures, but not in reaction with O2/H2O. These blades did not form when Y2O3 dispersion-strengthened material was reacted. A model, in which oxide growth was sustained by diffusion of chromium vacancies and adsorption of H2O on oxide exposed to low oxygen-activity gas led to the formation of hydroxyl species, explained most of the complex effects of gas composition on scale growth and blade formation. However, it failed to account for the observed increase in scaling rate with pH2 at fixed pH2O. The latter effect is ascribed to alteration of an additional contribution to diffusion from chromium interstitials.

The oxidation of iron-chromium-manganese alloys at 900C

The oxidation of nine ternary iron-chromium-manganese alloys was studied at 900°C in an oxygen partial pressure of 26.7 kPa. The manganese concentration was set at 2, 6, and 10 wt. %, and chromium at 5, 12, and 20 wt. %. The scales formed on the low-chromium alloys consisted of (Mn,Fe)2O3, α-Fe2O3, and Fe3O4. These alloys all exhibited internal oxidation and scale detachment upon cooling. The scales formed on the higher-chromium alloys were complicated by nodule formation. Initially, these scales had an outer layer of MnCr2O4 with Cr2O3 underneath, adjacent to the alloy. With the passage of time, however, nodules formed, and the overall reaction rate increased. This tendency was more marked at higher manganese contents. Although these alloys contained a high chromium content, the product chromia scale usually contained manganese. It was concluded that the presence of manganese in iron-chromium alloys had an adverse effect on the oxidation resistance over a wide range of chromium levels.

High-temperature corrosion of Cr2O3-forming alloys in CO-CO2-N2 atmospheres

The corrosion of Fe−28Cr, Ni−28Cr, Co−28Cr, and pure chromium in a number of gas atmospheres made up of CO−CO2(−N2) was studied at 900°C. In addition, chromium was reacted with H2−H2O−N2, and Fe−28Cr was reacted with pure oxygen at 1 atm. Exposure of pure chromium to H2−H2O−N2 produced a single-phase of Cr2O3. In a CO−CO2 mixture, a sublayer consisting of Cr2O3 and Cr7C3 was formed underneath an external Cr2O3 layer. Adding nitrogen to the CO−CO2 mixture resulted in the formation of an additional single-phase layer of Cr2N next to the metal substrate. Oxidizing the binary alloys in CO−CO2−N2 resulted in a single Cr2O3 scale on Fe−28Cr and Ni−28Cr, while oxide precipitation occurred below the outer-oxide scale on Co−28Cr, which is ascribed to the slow alloy interdiffusion and possibily high oxygen solubility of Co−Cr alloys. Oxide growth followed the parabolic law, and the rate constant was virtually independent of oxygen partial pressure for Fe−28Cr, but varied between the different materials, decreasing in the order chromium >Fe−28Cr>Ni(Co)−28Cr. The formation of an inner corrosion zone on chromium caused a reduction in external-oxide growth rate. Permeation of carbon and nitrogen through Cr2O3 is thought to be due to molecular diffusion, and it is concluded that the nature of the atmosphere affects the permeability of the oxide.

Methanol synthesis over Raney copper-zinc catalysts: I. Activities and surface properties of fully extracted catalysts

The activity of fully extracted Raney copper-zinc catalysts for the methanol synthesis reaction, and the associated physical and chemical properties of these catalysts, have been examined. The Raney catalysts were prepared by leaching a series of AlCusZn alloys containing approximately 50 wt% Al and differing CuZn ratios with aqueous NaOH until complete reaction had taken place. Hydrogenation of a mixture of carbon monoxide and carbon dioxide showed that Raney catalysts prepared from alloys containing approximately 50 wt% Al, 30–36 wt% Cu, and 14–20 wt% Zn had the greatest activity for methanol synthesis. The active component for these Raney catalysts was found to be metallic copper and the activity exhibited a maximum for catalysts containing approximately 97 wt% copper. The residual zinc in these catalysts was found to provide a promotional effect to catalytic activity. The surface areas of the catalysts increased from 17 to 39 m2g−1 with increasing zinc content of the precursor alloy. The catalysts exhibited a narrow pore size distribution with the pore radius decreasing with increasing alloy zinc level. Carbon monoxide and hydrogen adsorption were used to determine the nature of the catalyst surface.

Internal oxidation and carburisation of heat-resistant alloys

The commercial alloys Nicrofer-HT, Alloy 800 and Type 304 stainless steel have been exposed under thermal cycling conditions to CO–CO2 gas mixtures at temperatures of 650–750 °C. Thermal cycling led to repeated scale spallation which accelerated chromium depletion from the alloy subsurface regions. Subsequent dissolution of carbon and oxygen into the alloys led to extensive internal precipitation of carbides and oxides. The large volume fractions of carbide and oxide left small quantities of iron–nickel-rich metal. The in situ oxidation of internal carbides in the stainless steel led to large volume expansions and the development of mechanical stress. This was increased during thermal cycling, leading to disintegration of the surface regions. Temperature and surface treatment were both found to be significant factors in the resistance of alloys to the CO–CO2 atmosphere.

Factors Affecting Chromium Carbide Precipitate Dissolution During Alloy Oxidation

Ferrous alloys containing significant volumefractions of chromium carbides were formulated so as tocontain an overall chromium level of 15% (by weight) buta nominal metal matrix chromium concentration of only 11%. Their oxidation at 850°C inpure oxygen led to either protectiveCr2O3 scale formation accompaniedby subsurface carbide dissolution or rapid growth ofiron-rich oxide scales associated with rapid alloy surface recession, which engulfedthe carbides before they could dissolve. Carbide sizewas important in austenitic alloys: an as-castFe-15Cr-0.5C alloy contained relatively coarse carbides and failed to form aCr2O3 scale, whereas the samealloy when hot-forged to produce very fine carbidesoxidized protectively. In ferritic alloys, however, evencoarse carbides dissolved sufficiently rapidly to provide the chromium flux necessary to formand maintain the growth of a Cr2O3scale, a result attributed to the high diffusivity ofthe ferrite phase. Small additions of silicon to theas-cast Fe-15Cr-0.5C alloy rendered it ferritic and led toprotective Cr2O3 growth. However,when the silicon-containing alloy was made austenitic(by the addition of nickel), it still formed aprotective Cr2O3 scale, showing that the principal function of silicon was inmodifying the scale-alloy interface.

Metal Dusting of Fe-Cr and Fe-Ni-Cr Alloys under Cyclic Conditions

Metal dusting is the disintegration of alloys into carbon and metal particles during high-temperature exposure to carbon-bearing gases. Model Fe–Cr and Fe–Ni–Cr alloys were studied to test the hypothesis that M3C formation is necessary for metal dusting to occur. The alloys were exposed to a 68% CO–26% H2–6% H2O gas mixture at 680°C (ac=2.9) under thermal cycling conditions. Equilibrium calculations predicted the formation of M3C at the surface of Fe–25Cr, but not Fe–60Cr. All compositions were expressed in w/o, weight percent. Alloys of Fe–25Cr with 2.5, 5, 10, and 25 w/o nickel additions were also exposed to the same conditions to study the role of nickel in destabilizing the precipitation of M3C and, hence, altering the resistance to metal dusting. Metal dusting was observed on all the alloys except Fe–60Cr. For Fe–25Cr, Fe–25Cr–2.5Ni, and Fe–25Cr–5Ni, the carbonization and dusting process was localized, and its incidence decreased in Fe–25Cr–2.5Ni, consistent with the increased destabilization of M3C precipitation. However, Fe–25Cr–10Ni and Fe–25Cr–25Ni both underwent extensive dusting in the absence of protective Cr2O3 formation. The carbon deposits formed consisted of carbon filaments, which contained particles at their tips. These were shown by electron diffraction to be exclusively Fe3C in Fe–25Cr, Fe–25Cr–2.5Ni, and Fe–25Cr–5Ni, and a mixture of austenite and (Fe,Ni)3C in Fe–25Cr–10Ni and Fe–25Cr–25Ni.

Alloy phase transformations driven by high temperature corrosion processes

Abstract Heat-resistant alloys survive high temperature exposure by growing protective oxide scales. This process involves the selective oxidation of an alloy constituent and necessarily leads to changed alloy compositions in the subsurface regions. This paper examines several examples of phase transformations driven by selective oxidation processes. Depending on the alloy system, the phase transformations concerned involve either the formation of a new phase or the dissolution of an existing one. For example, aluminium depletion from γ-TiAl leads to formation of Z-Ti 50 Al 30 O 20 as a result of simultaneous inward oxygen diffusion. As another example, selective oxidation of aluminium from α-Cr+β-NiAl alloys leads to formation of γ-Ni at the alloy surface. Inward diffusion of nickel then causes an α+β→γ+β phase transformation. In the case of depletion causing phase dissolution, the examples of metal silicide dissolution in Ni–Si and Co–Si alloys and chromium carbide dissolution in Fe–Cr–C alloys are shown to occur under diffusion control during selective formation of either SiO 2 or Cr 2 O 3 .