Decarburization and internal oxidation in a commercial-grade nickel

A commercial-grade nickel containing small amounts of carbon, manganese, and silicon was exposed to air for periods up to 288 hr at 1050°C to study the effect of oxidation on the formation of oxides of these impurity elements. Exposure of nickel to air led to decarburization. The maximum amount of decarburization occurred during the initial period of air exposure and the loss in carbon was more in the metal with a smaller section size. Decarburization in the metal produced voids in the oxide scale due to the formation of CO₂ gas. It has been shown further that CO and/or CO₂ gas bubbles, which form in high purity nickel on grain boundaries during exposure to air at elevated temperatures, cannot exist in commercial-grade nickel where manganese is present as an impurity. Instead, oxides of manganese form in the grain boundaries as well as in the matrix. This is because manganese oxide is more stable than CO or CO₂ gas.     

Das Oxydationsverhalten von Nickel-Vanadium-Legierungen in Luft

The oxidation behaviour of nickel-vanadium alloys in air With oxidation tests carried out on pure nickel in air at 1000°C, a simple oxide film of NiO is formed. With the oxidation of nickel-vanadium alloys, however, several layers of different composition are formed. the outermost layer contains mainly NiO and a small content of nickel vanadate, Ni(VO₃)₂. Below it is a second oxide layer which has, on the outside, a strong concentration of vanadium. On the side facing the metal, this second layer has a low content of either metal, and is porous. This is followed by an inner oxidation zone which projects into the matrix in the form of conic islands with concentrations of V₂O₃. In the temperature range from 800 to 1200 °C, the scale constants indicating the reactions of the nickel-vanadium alloys are of an order of magnitude above that of unalloyed nickel. The oxidation reactions obey parabolic laws for the formation of the outer NiO layers with nickel and of NiO and Ni(VO₃)₂ with the nickel-vanadium alloys. The growth of the inner oxidation zones is governed by a logarithmic law. The activation energy of the oxidation in air, for nickel and for the nickel-vanadium alloys investigated, is of the order of magnitude of 50kcal/Mol.     

The isothermal oxidation of Fe<Subscript>3</Subscript>Al−4%Cr−(0, 0.5, 1, 2%)Mo alloys at 1000°C

The air oxidation characteristics of Fe₃Al-4%Cr-(0, 0.5, 1, 2%)Mo alloys at 1000°C were studied using TGA, XRD, EPMA, and TEM/EDS. Molybdenum increased the oxidation resistance of Fe₃Al-4%Cr alloys. The whole Al₂O₃ grains that formed on the alloy surface contained a small amount of dissolved Fe ions. The Al₂O₃ grains next to the oxide-matrix interface additionally contained a small amount of dissolved Cr and Mo ions. Beneath the thin but non adherent Al₂O₃ layer, an Al-depleted, Fe-enriched matrix zone formed due to the consumption of Al in the scale.     

The oxidation of dilute iron-silicon alloys ([Si] ⩽ 1 wo) in carbon dioxide

The manner in which silicon, present as a minor alloy constituent, modifies the oxidation of iron in CO₂/1 %CO at 500°C has been studied. Increasing amounts of silicon progressively reduce the oxidation rate within the range $\text{[}\text{Si}\text{] = 0–1}\text{w}\text{o}$ and a variety of physical techniques have been used to examine oxidized specimens in pursuit of the origins of this beneficial influence. The scale forming on the alloys is composed of two layers in each of which iron is present as Fe₃O₄. The inner layer of scale contains silicon at approximately the same level (on a vol. % basis) as the original metal while the outer scale appears to contain no silicon. The boundary between the two layers is also marked by an abrupt change in grain size and in texture of the Fe₃O₄. At the boundary between the alloy and the scale there develops a thin layer of non-ferrous material in which the concentration of silicon is increased by more than an order of magnitude above that in the bulk alloy. This layer also includes a substantial accumulation of carbon which is thought to derive from carbon oxide gases which have penetrated to the base of the scale before taking part in the oxidation reaction. The observation of the layer of non-ferrous material between the alloy and the scale constitutes a qualitative agreement with theoretical studies which indicate that the reduction in oxidation rate conferred by the presence of silicon in the alloy is at least partly due to the impeded passage of Fe³⁺ ions from the alloy into the scale.     

The influence of silicon and yttrium on isothermal scaling of an austenitic Fe-Cr-Ni alloy (IN 519) at 1000 °C

When present in austenitic Fe-Cr-Ni alloys, both silicon and yttrium influence scaling behaviour during oxidation tests in air at high temperatures. The former promotes the formation and maintenance of a continuous Cr₂O₃ scale and the latter improves scale adhesion. During the isothermal exposure of nominally Fe + 24%Cr + 24%Ni east alloys at 1000°C a silicon content above 0.8% reduces the rate of scaling by providing additional sites for the lateral growth of the Cr₂ O₃ layer. Yttrium gives rise to the formation of Fe₉Y particles which disrupt the continuity of the Cr₂O₃ scale. The beneficial influence of silicon dominates the potentially detrimental effect of yttrium in alloys containing both silicon and yttrium.     

Effect of carbide volume fraction on the oxidation of austenitic Fe‐Cr‐C alloys

A series of Fe‐15Cr‐(2‐3)Mo alloys (compositions in weight percent) was produced with different carbon concentrations, to control the distribution of chromium between matrix metal and M₂₃C₆ precipitates. The alloys were oxidized in the austenitic state at 850°C in pure oxygen, with and without a pre‐oxidation treatment at low oxygen potential, where no iron oxide could form. Protective, chromia‐rich scaling took place if the chromium concentration at the metal‐scale interface was high enough. This concentration was controlled by the original alloy matrix chromium concentration, and whether or not a high diffusivity ferrite zone developed at the surface by decarburization. Ferrite zone formation was assisted by pre‐oxidation at low oxygen potentials. The value of the carbides as suppliers of additional chromium was demonstrated by comparison with the oxidation performance of carbide‐free alloys of corresponding matrix chromium levels. However, because dissolution of the coarse carbides could be slow, alloys with high volume fractions of large carbides were unsuccessful.     

The oxidation of Iron-Chromium-Manganese alloys at 900°C

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, -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.     

The effect of silicon and manganese on the oxidation mechanism of Ni-20 Cr

Additions of 3% silicon or manganese to Ni-20 Cr reduced the oxidation rate, whereas additions of 1% had little effect. Three percent silicon alloys formed an inner scale of SiO₂, and 3% manganese alloys formed an inner spinel layer of essentially pure MnCr₂O₄. The experimentally determined solid-state growth rate of NiCr₂O₄ was about 1000 times slower than the growth rate for Cr₂O₃. It has been established that the protective layer on Ni-20 Cr (Nichrome alloys) is the spinel and not Cr₂O₃ as previously postulated. The mechanism for scale growth is discussed for Ni-20 Cr alloys.This work was performed at Stanford Research Institute, Menlo Park, Calif. and was supported by the National Aeronautics amd Space Administration, Contract NAS 3-11165.     

Coking and Dusting of Fe–Ni Alloys in CO–H2–H2O Gas Mixtures

Metal dusting of Fe–Ni alloys was investigated in a CO–H₂–H₂O–Ar gas corresponding to a _C = 19.6 at 650 °C. Thermogravimetric analysis showed that increasing the nickel content in the alloy decreased the initial rate of carbon uptake. A uniform Fe₃C scale formed on pure iron, a layer with mixed structures of Fe₃C, γ and α-Fe developed on ferritic Fe–5Ni, and small amounts of Fe₃C developed at the surface of an austenite layer grown on two-phase (α + γ) Fe–10Ni. At nickel levels above 10%, no carbide appeared. These observations are shown to be broadly consistent with local equilibrium according to the Fe–Ni–C phase diagram. However, the failure of higher nickel austenitic alloys to form the (Fe,Ni)₃C expected at high carbon activities indicates a barrier to nucleation and growth of this phase. Graphite deposition was catalysed by (Fe,Ni)₃C on ferritics and by the metal itself on austenitics. The rates of carbon deposition on Fe–60Ni corresponded to the existence of three parallel and independent paths: the synthesis gas, the Boudouard and the carbon methanation reactions.     

The scale constituents and spalling characteristics of NiFe (0–60%) alloys oxidized in air at 800–1200°C

The spalling behaviour of scales on NiFe alloys containing 0, 2, 10, 20, 30, 40, 50 and 60% Fe oxidized in air at 900, 1000, 1100 and 1200°C for periods up to 165 h have been investigated. The phases present and their relative amounts in the scales formed at 1200°C have been determined. Spalling was most severe on the Ni30%Fe alloy, which had a scale consisting of 30%N_x Fe_2?x O₄ and 70% Ni_1?x Fe_xO.     

Decarburization of 60Si2MnA in Atmospheres Containing Different Levels of Oxygen,Water Vapour and Carbon Dioxide at 700–1000 °C

The decarburization behaviour of 60Si2MnA in atmospheres containing 0–21% O₂,?<?20 ppm–17%H₂O, and with or without 8%CO₂, at 700–1000 °C, was investigated. The new findings of the current study were: (a) severe decarburization was associated with the formation of wüstite (FeO) scale on the steel surface, (b) the carbon activity at the steel–FeO interface was most likely determined by the reaction equilibrium between FeO and dissolved carbon in steel, (c) when a ferrite layer was able to form, the decarburization tendency was determined by the relative carbon permeability (defined as the product of carbon concentration difference at the two interfaces of the ferrite layer and carbon diffusivity) through the ferrite layer, and therefore, (d) the decarburization tendency at 800 °C was greater than those at 700 and 900 °C as the relative carbon permeability at 800 °C was the greatest. If FeO was absent when heating in dry O₂-containing gases, however, possibly as a result of the formation of a SiO₂ layer at the steel surface, decarburization was very much alleviated or avoided. At 1000 °C, the decarburization tendency was alleviated even when FeO was able to form because formation of a ferrite layer was not possible and carbon diffusivity in austenite was much lower than that in ferrite. A preformed oxide scale was effective in providing decarburization protection only when the steel was exposed to dry O₂-containing atmospheres.

     

Änderungen der Struktur und der chemischen Zusammensetzung von Rohrwerkstoffen bei der Hochtemperaturpyrolyse

Changes of structure and chemical composition of tubing materials for high temperature pyrolysis The changes of structure and chemical composition of the surface layers of radiation tubes for hydrocarbon pyrolysis furnaces made of Cr25Ni35Nb steel were investigated after longterm service. It was found that carburization of the tube wall occurs where the primarily formed M₂O₃ oxide film with high chromium content is damaged by local spalling. The M₂O₃ oxide film spalling or deterioration is usually attributed to reduction by carbon from the pyrolysis medium. Our own investigations showed M₂O₃ oxide film deterioration to be caused by residual stresses generated during the growth of the zone beneath the oxide film, which is internally oxidised and affected by intercrystal line corrosion. During further oxidation negative effects including the dangerous carburization and furnace life is reduced by the formation of M₃O₄ oxide. This oxide contains less chromium and deteriorates the protection. The regeneration of the M₂O₃ oxide film having a high chromium content is not possible because of the lower chromium, silicon and manganese depletion in the surface-near zone, which prevents M₃O₄ oxide formation.     

Metal dusting of binary iron aluminium alloys at 600 °C

In this work experiments on metal dusting of binary iron aluminium alloys with 15, 26 and 40 at.% Al were performed in strongly carburising CO‐H₂‐H₂O gas mixtures at 600 °C. The mass gain kinetics was measured using thermogravimetric analysis (TGA). The carburised samples were characterised by means of light optical microscopy (LOM), scanning electron microscopy (SEM), X‐ray diffraction (XRD) and X‐ray photoelectron spectroscopy (XPS). It was found that the mass gain kinetics depends on the CO content of the gas mixtures and on the Al content of the alloys. With decreasing carbon activity the carburisation reaction kinetics decreases and the onset of metal dusting is retarded for increasing time periods. With increasing Al content of the alloys the carburisation reaction is slower and metal dusting sets on at later times. The samples were not pre‐treated for the formation of a protective oxide scale. By X‐ray Photoelectron Spectroscopy (XPS) analyses of the carburised iron aluminium samples it was found that the formation of Al₂O₃ layers has taken place in the CO‐H₂‐H₂O gas atmospheres. Needle‐ or plate‐like κ‐phase (Fe₃AlC_x) precipitates close to the surface of the carburised Fe‐15Al sample were detected by means of XRD and LOM. The coke on top of the carburised samples mainly consists of filamentous carbon with metal particles at their tips.     

COSP: A computer model of cyclic oxidation

A computer model useful in predicting the cyclic oxidation behavior of alloys is presented. The model considers the oxygen uptake due to scale formation during the heating cycle and the loss of oxide due to spalling during the cooling cycle. The balance between scale formation and scale loss is modeled and used to predict weight change and metal loss kinetics. A simple uniform spalling model is compared to a more complex random spall site model. In nearly all cases, the simpler uniform spall model gave predictions as accurate as the more complex model. The model has been applied to several nickel-base alloys which, depending upon composition, form Al₂O₃ or Cr₂O₃ during oxidation. The model has been validated by several experimental approaches. Versions of the model that run on a personal computer are available.     

On the effect of nickel on the corrosion-electrochemical behavior of iron-nickel alloys

The pH-potential diagram of the Ni−H₂O system is refined, and similar diagrams of the Fe−Ni- and X18H10 alloy-H₂O systems are constructed for a temperature of 25°C. It is shown that nickel may enter into a mixed, magnetite-base spinel (Fe_1-x Ni_x)Fe₂O₄ (in the case of iron-nickel alloys) or into an iron-chromite base spinel (Fe_1-x Ni_x)(Cr_2-y Fe_y)O₄ (iron-chromium-nickel alloys), thus participating in the passivation of alloys.     

Metal dusting of nickel-base alloys

Metal dusting, the disintegration of metallic materials into fine metal particles and graphite was studied on nickel, Fe Ni alloys and commercial Ni-base alloys in CO H₂ H₂O mixtures at temperatures between 450–750°C. At carbon activities a_c > 1 all metals can be destroyed into which carbon ingress is possible, high nickel alloys directly by graphite growth into and in the material, steels via the intermediate formation of instable carbide M₃C. Protection is possible only by preventing carbon ingress. Chromium oxide formation is the best way of protection which is favoured by a high chromium concentration of the alloy and by a surface treatment which generates fast diffusion paths for the supply of chromium to the surface. The metal dusting behaviour of Alloy 600 is described in detail. A ranking of the metal dusting resistance of different commercial nickel-base alloys was obtained by exposures at 650°C and 750°C.     

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.     

Comparison of isothermal and cyclic oxidation behavior of twenty-five commercial sheet alloys at 1150°C

Twenty-five commercial nickel-, iron-, and cobalt-base sheet alloys incorporating chromium or chromium and aluminum additions for oxidation resistance were tested at 1150°C in air for 100 hr in both isothermal and 1-hr cyclic furnace exposures. The alloys were evaluated by sample specific weight change, by type of scale formed, by amount and type of spall, and by sample thickness change and microstructure. In isothermal steady-state oxidation, four types of controlling oxides were observed depending on alloy composition: NiO, Cr₂O₃-chromite spinel, ThO₂-blocked Cr₂O₃, and Al₂O₃-aluminate spinel. The latter three types are considered protective. In the Cr₂O₃-forming alloys, however, scale vaporization is a critical factor in determining the parabolic scaling rate based on paralinear oxidation. In cyclic oxidation the alloys which form Cr₂O₃-chromite spinel scales were degraded severely when sufficient chromite spinel developed to trigger spalling. The cyclic behavior of the other three types of alloys does not differ greatly from their isothermal behavior. If chromite spinel formation is minimal, the thinner the oxide formed, the less the tendency to spall. Factors contributing to a thin scale are low isothermal scaling rates; reactive element additions, such as thorium, lanthanum, and silicon; and scale vaporization. Scale vaporization may, however, lead to catastrophic oxidation at high gas velocities or low pressures or both. A tentative mass-balance approach to scale buildup, scale vaporization, and scale spalling was used to calculate the critical oxidation parameter—the effective metal thickness change. In general, this calculated thickness change agrees with the measured change to within a factor of 3 if a correction is made for grain boundary oxidation. The calculated thickness change parameter was used to rate the oxidation resistance of the various alloys under isothermal or cyclic conditions. The best alloys in cyclic furnace oxidation tests were either Al₂O₃-aluminate spinel formers or Cr₂O₃ formers with ThO₂ blockage.     

High temperature oxidation of Ti−48%Al−2%W intermetallics

Powder metallurgically produced Ti-48% Al-2%W alloys were oxidized between 800 and 1050°C in air. The W-addition was quite effective in providing isothermal and cyclic oxidation resistance. The alloys oxidized parabolically up to 1050°C during isothermal oxidation, with small weight gains. The scales were adherent up to 900°C during cyclic oxidation. Oxide scales consisted primarily of an outer TiO₂ layer, an intermediate Al₂O₃ layer, and an inner (TiO₂+Al₂O₃) mixed layer. Tungsten was present below the intermediate Al₂O₃ layer. and also at the scale-matrix interface as W-enriched compounds. Below the oxide scale, a Ti₃Al zone containing some W and O existed.     

Modification of the oxidation behavior of high-purity austenitic Fe-14Cr-14Ni by the addition of silicon

The oxidation behavior of Fe-14Cr-14Ni (wt.%) and of the same alloy with additions of 1 and 4% silicon was studied in air over the range of 900-1100° C. The presence of silicon completely changed the nature of the oxide scale formed during oxidation. The base alloy (no silicon) formed a thick outer scale of all three iron oxides and an internally oxidized zone of (Fe,Cr,Ni) spinels. The alloy containing 4% silicon formed an outer layer of Cr₂O₃ and an inner layer of either (or possibly both) SiO₂ and Fe₂SiO₄. The formation of the iron oxides was completely suppressed. The oxidation rate of the 4% silicon alloy was about 200 times less than that of the base alloy, whereas the 1% silicon alloy exhibited a rate intermediate to the other two alloys. The actual ratio of the oxidation rates may be less than 200 due to possible weight losses by the oxidation of Cr₂O₃ to the gaseous phase CrO₃. The lower oxidation rate of the 4% silicon alloy was attributed to the suppression of iron-oxide formation and the presence of Cr₂O₃, which is a much more protective scale.