The oxidation of Fe-19.6Cr-15.1Mn stainless steel

The oxidation behavior in air of Fe-19.6Cr-15.1Mn was studied from 700 to 1000°C. Pseudoparabolic kinetics were followed, giving an activation energy of 80 kcal/mole. The scale structure varied with temperature, although spinel formation occurred at all temperatures. At both 700 and 800°C, a thin outer layer of -Mn₂O₃ formed. The inner layer at 700°C was (Fe,Cr,Mn)₃O₄, but at 800°C there was an intermediate layer of Fe₂O₃ and an inner layer of Cr₂O₃ + (Fe, Cr,Mn)₃O₄. Oxidation at 900°C produced an outer layer of Fe₃O₄ and an inner layer of Cr₂O₃+(Fe,Cr,Mn)₃O₄. Oxidation at 1000°C caused some internal oxidation of chromium. In addition, a thin layer of Cr₂O₃ formed in some regions with an intermediate layer of Fe₃O₄ and an outer layer of (Fe,Mn)₃O₄. A comparison of rates for Fe₃O₄ formation during oxidation of FeO as well as for the oxidation of various stainless steels, which form spinels, gave good agreement and strongly suggests that spinel growth was rate controlling. The oxidation rate of this alloy (high-Cr) was compared with that of an alloy previously studied, Fe-9.5Cr-17.8Mn (low-Cr) and was less by about a factor of 12 at 1000°C and by about a factor of 100 at 800°C. The marked differences can be ascribed to the destabilization of wustite by the higher chromium alloy. No wustite formation occurred in the high-Cr alloy, whereas, extensive wustite formed in the low-Cr alloy. Scale structures are explained by the use of calculated stability diagrams. The mechanism of oxidation is discussed and compared with that of the low-Cr alloy.     

Oxidation of an Fe-12% Ni alloy in oxygen at 700–1000°C

The oxidation of an Fe-12% Ni alloy has been studied at 700–1000°C using thermogravimetric, metallographic, and electron probe microanalysis techniques. At all temperatures parabolic kinetics were observed and the activation energy for the process was 48±4 kcal mole^–1. At 700°C Fe₃O₄ and Fe₂O₃ were present in the external scale and scaling was accompanied by a progressive Ni enrichment of the underlying alloy. When the Ni-enriched zone contained 50–60% Ni, this metal entered the spinel phase, eventually leading to the formation of Ni_xFe_3–xO₄ where x had a value of 0.24 close to the alloy and <0.01 close to the Fe₂O₃ phase boundary. At higher temperatures (900–1000°C) Ni entered the spinel phase very early in the oxidation process. There was a buildup in Ni concentration in the Ni_xFe_3–xO₄ phase to x values of 0.4 and at 900°C only this corresponded to a transition to a lower parabolic rate of oxidation. The internal oxide phase was identified as Ni_0.7Fe_2.3O₄. The mechanism of oxidation of the alloy is discussed in the light of present knowledge concerning the Fe-Ni-O system.     

Strain determination in thermally-grown alumina scales using fluorescence spectroscopy

By exploiting the strain dependence of the ruby luminescence line, we have measured room-temperature residual strains in thermally-grown alumina scales. Measurements were made on two alloys Fe-5Cr-28Al and Fe-18Cr 10Al (at.% bal. Fe), oxidized between 300–1300°C. Significantly different levels of strain buildup were observed in scales on these alloys. Results on similar alloys containing a dilute reactive element (RE) are also presented. Scales formed on RE-containing alloys (Zr or Hf) could support significantly higher strains at T 1000°C. Strain relief associated with spallation thresholds is readily observed. In early-stage oxidation, the evolution of transition phases is monitored using Raman and fluorescence spectroscopies. The fluorescence technique also provides a sensitive probe of early-stage formation of -Al₂O₃. It appears that, in the presence of Cr₂O₃ or Fe₂O₃, the -phase of Al₂O₃ can form at anomalously low temperatures.     

Oxygen exchange in oxidation of an Fe-20Cr-10Al alloy in ∼10 mbar O2/H2O-gas mixtures at 920°C

The oxidation of an alumina forming Fe-200-10Al alloy was studied in situ. The gas phase components in isotopically labeled 10 mbar O₂/H₂O-gas mixtures were measured from a virtually closed system at 920°C. Oxidation rates and oxygen-exchange rates were measured and related to each other. From the exchange rates the dissociation rates of O₂ and H₂O were calculated. These dissociation rates were in the early stage of oxidation found to be the same as the oxidation rate. The rate of oxygen exchange with the oxide can exceed the oxidation rate. From this follows that in analysis of a two-stage oxidation in^16,16O₂/^18,18O₂ the exchange rate of O between the oxide and gas phase has to be considered. The oxides formed in H₂O containing gas had a H/O-ratio of 0.1. Vacuum annealing of the alloy before oxidation increased the oxidation rate by a factor of 2.     

The effect of particle size of alumina dispersions on the oxidation resistance of Ni-Cr alloys

The oxidation behavior of Ni-Cr alloys with various chromium concentrations and particle sizes of a dispersion of 10 vol.% Al₂O₃ was observed in 1 atm of oxygen at 1000°C. This study was intended to determine the critical chromium concentration to form a protective Cr₂O₃ oxide layer for different Al₂O₃ particle sizes. The oxidation rate of Ni-Cr alloys containing 10 vol.% Al₂O₃ followed a parabolic rate law and a Cr₂O₃ protective layer continuously formed when the oxidation rate decreased rapidly. Times to form a continuous and protective Cr₂O₃ layer during the initial oxidation shortened as the size of the dispersion decreased. The critical chromium concentration to form a protective Cr₂O₃ layer in the oxide scale was 69 wt.% and was related strongly to the particle size of the Al₂O₃ dispersion.     

Sulfidation behavior of Fe-25Cr-20Ni-1.5Al2O3 in simulated coal combustion/gasification environments

The effect of minor addition of -Al₂O₃ dispersoids on the sulfidation behavior of Fe-25Cr-20Ni was investigated over a range of pO₂, 1.13×10^–20 to 1.18×10 ****^–22 atm. at constant pS₂=1.22×10^–8 atm. Fe-25Cr-20Ni and Fe-25Cr-20Ni 1.5 Al₂O₃ with and without preformed oxide scales were exposed to bioxidant gas mixtures H₂/H₂O/H₂S/Ar at 700° C. Both isothermal and cyclic exposures were included. Scales were characterized by a combination of several surface analytical tools. A remarkable improvement in sulfidation resistance is observed in Fe-25Cr-20Ni-1.5Al₂O₃ under the conditions investigated here. This is attributed to the ability of the alloy to form and maintain a predominantly Cr₂O₃ scale with reduced Fe-diffusion and content. Possible scientific reasons for such improvement are discussed. The base alloy, Fe-25Cr-20Ni, fails to develop and retain such a Cr₂O₃ scale and undergoes sulfidation within a few minutes of exposure. The scale breakdown process by sulfidation is explained qualitatively. Experimental evidence suggests that sulfur in the environment enhances Fe-diffusion and content in the scale.     

Long-term high-velocity oxidation and hot corrosion testing of several NiCrAl and FeCrAl base oxide dispersion strengthened alloys

Several oxide dispersion strengthened (ODS) alloys have been tested for cyclic, long-term, high gas-velocity resistance to oxidation at 1100°C and hot corrosion at 900°C. Both nominally Ni-16Cr-4Al and Fe-20Cr-4.5Al ODS alloys were subjected up to 2500 cycles, where each cycle consisted of 1 hr in a hot, Mach 0.3 combusted gas stream followed by a 3-min quench in an ambient temperature, Mach 0.3 air blast. For comparison to existing technology, a coated superalloy was simultaneously tested. The ODS iron alloy exhibited clearly superior behavior, surviving 3800 oxidation and 2300 hot corrosion cycles essentially unscathed. While the ODS nickel alloys exhibited adequate oxidation resistance, the long-term hot corrosion resistance could be marginal, since the best life for such alloys under these conditions was only 1100 cycles. However, the hot corrosion resistance of the ODS Ni-base alloys is excellent in comparison to that of traditional superalloys.     

Electronic transport in thermally grown Cr2O3

Electrical conductivity of thermally grown Cr₂O₃ has been measured as a function of temperature and over a range of oxygen partial pressures from that of air to that of the Cr/Cr₂O₃ equilibrium. The conductivity showed p-type behavior over the range of the present investigation. At temperatures above 1000°C, the conductivity values were independent of oxygen partial pressure and indicated intrinsic semiconductor behavior. The mobility of holes, determined by measuring conductivity at fixed compositions (i.e., fixed in Cr_2-O₃), increased with temperature. This behavior can be attributed to hopping-type conduction. For 10^–5, the activation energy for hole hopping was 0.248 eV, and the calculated hole mobilities were 5.4x10^–2 and 2.4x10^–1 V/cm² · s at 500 and 1000°C, respectively. The oxidation kinetics of Cr were determined by measuring the electrical conductivity and electromotive force across the oxide layer at 875°C. The result agreed well with the oxidation data obtained in thermogravimetric tests.     

The high-temperature oxidation resistance of Co-Al alloys

Most Ni and Co-base alloys used for high-temperature service rely on the production of a compact, stable Cr₂O₃ scale for their oxidation resistance. However, as operating temperatures have risen above 900–950° C, the loss of Cr₂O₃ as the volatile CrO₃ has led to an inadequate life span of these alloys, particularly in rapidly flowing, turbulent gas streams. As a result of this, it has been necessary to examine the possibility of using Al₂O₃ as the protective scale. Al₂O₃ has a lower growth rate than Cr₂O₃, it is nonvolatile, and, unlike Cr-containing systems, it is less likely to form compound oxides such as spinels. In this study, the amount of Al which must be present in the Co-Al system to form a continuous layer of Al₂O₃ in the temperature range 800–1000° C has been determined. The quantity was found to rise from about 7–10 wt. % at 800° C to 10–13 wt. % at 900° C and 13 wt. % at 1000° C. Notice has also been taken of the abilities of the alumina-forming alloys to re-form a protective oxide in the event of spalling, blistering, or any other disruptions of the scale, and some cyclic-oxidation checks have been conducted on the Co13Al alloy at 900 and 1000° C.This work has been partly supported by the Science Research Council and one of us (G.N.I.) wishes to thank them for the award of a Science Research Council Research Studentship     

Improved Oxide Spallation Resistance of Microcrystalline Ni-Cr-Al Coatings

Microcrystalline Ni-20Cr-3Al coatings weredeposited on Ni-20Cr-3Al substrates by unbalancedmagnetron-sputter deposition. The grain size of thecoatings was varied by using different Ar pressures.Cyclic-oxidation testing was performed at 1100°C. It wasfound that (1) an external-Al₂O₃ scale formed oncoating A (4.7 m thick, 50 nm grain size); (2) anexternal Cr₂O₃ scale and internalAl₂O₃ oxide formed on coating B (14 m thick, 500 nm grain size);and (3) an outer layer scale ofCr₂O₃ +NiCr₂O₄ and interior layer ofAl₂O₃ formed on the as-cast alloy.Extensive spallation of the Cr₂O₃+ NiCr₂O₄ scale took place on the as-cast alloy, but no obviousspallation occurred on the two coatings. Improvement ofthe spallation resistance of the scale is explained byeffective diffusional creep of the coatings and the micropegging effect of the inward-grownoxides.     

The Effect of Lanthanum on the Scales Developed on Thin Foils of Fe-20Cr-5Al at Very High Temperatures

A study has been undertaken of the effects oflanthanum on the oxidation of thin foils of Fe-20Cr-5Alin air at 1150°C. The addition of lanthanum causesthe time to breakaway to increase from about 24 hr for Fe-20Cr-5Al to over 400 hr. Oxidationof the lanthanum-containing alloy occurs in threestages. During the first stage, an-Al₂O₃ layer is establishedand thickens with time until the aluminum in the foil is depleted sufficiently for alayer of Cr₂O₃ to become stableand develop at the scale-alloy interface. This continuesto thicken at a relatively slow rate until breakawayoccurs. The main emphasis in the present paper has been anexamination and analysis of the scale established on thelanthanum-containing alloy in cross section in theanalytical transmission electron microscope (TEM), after an exposure period that coincides with thesecond stage of oxidation, prior to breakaway. The scaleat that time consists of three layers. The outer layeris composed of equiaxed Al₂O₃grains. The intermediate and inner layers consist of columnarAl₂O₃ grains and equiaxedCr₂O₃ grains, respectively.Numerous voids are observed in the oxide grainboundaries and at the intermediate-inner layerinterface. Lanthanum segregates in the oxide grain boundaries andits concentration increases toward the outermost surfaceof the scales. These results are consistent with thedynamic segregation model to account for the effects of reactive elements on thegrowth of Al₂O₃ scales.     

Oxidation of Fe-3 wt.% Cr alloy

The oxidation behavior and the oxide microstructure on Fe-3 wt. % Cr alloy were investigated at 800°C in dry air at atmospheric pressure. Two distinct oxidation rate laws were observed: initial parabolic oxidation was followed by nonparabolic growth. The change in the oxidation kinetics was caused by microchemical and microstructural developments in the oxide scale. Several layers developed in the oxide scale, consisting of an innermost layer of (Fe,Cr)₃O₄ spinel, an intermediate layer of (Fe,Cr)₂O₃ sesquioxide, and two outer layers of Fe₂O₃ hematite, each with different morphologies. Wustite (Fe_1–xO) and distorted cubic oxide (-(Fe,Cr)₂O₃) were observed during the iniital parabolic oxidation only.     

Kinetics and mechanisms of the oxidation of cobalt at 600–800°C

Two-phase layered scales comprising CoO and Co ₃O₄ formed on cobalt during oxidation at 600°, 700°, and 800°C and at oxygen partial pressures in the range 0.001–1 atm. The kinetics, which were obtained by thermogravimetric analysis, obeyed a parabolic rate law after an initial, non-parabolic stage of oxidation. The monoxide consisted of relatively large grains (10 ) and the spinel comprised small grains (3 ) for all conditions of oxidation. Grain boundary diffusion of cations played a significant role in the growth of the spinel layer. Thermogravimetric data and the steady-state ratio of the oxide layer thicknesses were employed to calculate the rates of thickening of the individual oxide layers and the rate of oxidation of CoO to Co₃O₄.     

Oxidation-sulfidation behavior of iron-chromium-nickel alloys

Oxidation-sulfidation studies of Fe-Cr-8Ni alloys with 4, 12, and 22 wt. % Cr were conducted at 750 and 875°C in multicomponent gas mixtures that contained CO, CO₂, CH₄, H₂, and H₂S. The reaction processes resulted in parabolic kinetics. A chromium concentration in the range 0–12 wt. % in the alloy had a negligible effect on the parabolic rate constant; however, the rate constant for the alloy with 22 wt. % Cr was significantly lower. For a given sulfur partial pressure, the oxygen partial pressures required for the formation of a continuous oxide layer in an Fe-22Cr-8Ni alloy were 10² to 10³ times those calculated for Cr-Cr₂O₃ equilibrium at temperatures of 875 and 750° C, respectively.Work supported by the U.S. Energy Research and Development Administration.On visiting appointment from the Academy of Mining and Metallurgy, Cracow, Poland through a fellowship of the International Atomic Energy Agency.     

Transient-Alumina Transformations During the Oxidation of Magnetron-Sputtered CoCrAl Nanocrystalline Coatings

The thermally-grown alumina formed at 1000, 1100, and 1200°C on magnetron-sputtered, nanocrystalline CoCrAl coatings, with and without yttrium, has been characterized using photostimulated-luminescence spectroscopy. The measurements enable the evolution of initially-formed transient alumina to its stable, phase to be followed, and in particular, the effect of yttrium on the transformation. Yttrium retards the transformation from gamma to theta alumina and also its subsequent transformation to alumina. The retardation of the transformation decreases with increasing oxidation temperature until at 1200°C the transformation is complete within minutes. The presence of yttrium in the coatings also affects the residual stress in the thermally-grown oxide. For samples oxidized at 1100 and 1200°C the residual stress is 0.3GPa higher in the oxide on the Y-containing coating, whereas the residual stresses are the same after oxidation at 1000°C.     

Improvement of the application of an electrochemical method for the determination of transport properties of an alumina scale. Part I: Alumina scale on aβ-NiAl alloy

An electrochemical method based on analyzing the potential-current curves, plotted at various applied P_{O 2} for an alumina scale developed at 1100°C on a -NiAl alloy, was used to determine transport parameters in the scale. The variation of V_o vs P_{O 2} indicates that the scale consists of an inside zone characterized by an ionic-transport number t_i0.4 and an outside part with t_i0.1. This leads to a value of the oxygen pressure at the NiAl/Al₂O₃ interface 2×10^–27 atm, not far from the equilibrium value. The variation of the oxygen chemical potential inside the scale was determined, showing a steep variation of µ_O in the middle of the scale. The calculated oxidation constant is close to the experimental one, which would indicate that mainly charged species are responsible for the alumina scale growth.     

Oxidation properties of Fe-5Cr-4Al (wt.%) alloys in oxygen at temperatures 1000°C–1320°C

Oxidation kinetics of a parent Fe-5Cr-4Al alloy subjected to two types of anneals were investigated at temperatures ranging from 1000°C to 1320°C. The alloy annealed at 850°C exhibited a rapid transient oxidation stage associated with growth of nodules containing iron oxides and internal precipitation of -Al₂O₃ in the alloy beneath these nodules. The nodules nucleated and grew from sites located in the regions of the alloy grain boundaries during the period of rapid alloy grain growth. Nodular growth virtually ceased when a continuous -Al₂O₃ film formed at the nodule-alloy interface. The alloy subjected to anneal at 1000°C and at the reaction temperature to stabilize the alloy grain size tended upon oxidation to form a protective -Al₂O₃, layer by parabolic kinetics at temperatures to 1250°C. If this alloy was oxidized in stages at 1000°C, a protective -Al₂O₃ scale was formed up to 1320°C. The temperature coefficient of the parabolic oxidation kinetics was consistent with diffusion processes at boundaries of the -Al₂O₃ grains playing an essential role during growth of this protective oxide layer.     

A scanning electron microscope study of the surface morphology of TD-NiCr oxidized at 800°C to 1200°C

Scanning electron microscopy was used to study the surface morphology of oxides formed on TD-NiCr at 800, 1000, and 1200° C for times up to 64 hr. Surfaces abraded with 120 grit oxidized to form Cr ₂O₃ which gradually developed large crystals of Cr₂O₃ (R~5 m). The shape of the crystals suggested vapor growth. Surfaces polished through 0.5 m diamond paste and oxidized had NiO on the surface except over grain boundaries and scratches which had Cr₂O₃ covering them. The NiO was faceted but showed little evidence of vapor growth. Wire samples of TD-NiCr 0.005 in. diam were oxidized at 1200°Cfor up to 50 hr at pressures from 10^–5 to 700 Torr. The only oxide formed was Cr₂O₃. Extensive large surface crystals ofCr₂O₂ resulting from vaporization and condensation were observed at pressures of 10Torr and above, but not at lower pressures. Humid air oxidation altered the oxide morphology markedly.     

Water-vapor-effect on the oxidation of Fe-21.5 wt.%Cr-5.6 wt.%Al at 1000°C

Fe-21.5 wt. %Cr-5.6 wt. %Al oxidation, at 1000°C, in dry or wet oxygen shows that steam has an influence on the oxide-scale growth mechanism. Steam modifies the kinetics of early-stage oxidation. In dry oxygen, an initial fast linear regime is observed during one hour. Under wet conditions, weight-gain curves follow the same parabolic regime over the entire oxidation test. The scale structure strongly depends on the presence of steam in the gaseous environment. With dry oxygen, the scale is composed mainly of-Al₂O₃ after the initial formation of-Al₂O₃ identified by ESCA and RHEED. The kinetics transient stage corresponds to the necessary time for the internal part of the initial-Al₂O₃ scale to transform into a continuous-Al₂O₃ diffusion barrier. Under wet oxygen conditions, transient oxides are identified as (Mg, Fe) (Cr, Al)₂O₄, MgAl₂O₄ (orthorhombic), Al₂O₃ (hexagonal), these oxides transform into MgAl₂O₄ (cubic), Cr₃O₄, Fe₂O₃,-Al₂O₃, with time. When water vapor does not change drastically oxidation kinetics, the induced presence of iron and chromium in the oxide scale could be responsible for weakening the protectiveness of alumina scales.     

A solid-state EMF study of the Fe-S-O and Co-S-O ternary systems

The equilibrium oxygen potentials of the two-phase equilibria Fe₃O₄/Fe_1–xS, Co_1–xS/Co₃S₄, Co₃S₄/CoS₂, Co_1–xS/CoO, and CoO/CoSO₄ were measured as a function of temperature. A solid-state emf technique using calcia-stabilized zirconia (CSZ) solid electrolyte was used. These equilibria were studied atP _SO21 atm; the equilibrium Co_1–xS/CoO was also studied atP _SO20.1 atm. Two emf cell designs were used for the measurements atP _SO21 atm andP _SO21 and 0.1 atm, respectively. The homogeneity range of FeS in equilibrium with Fe₃O₄ and that of Co_1–xS in equilibrium with CoO at 1073 K andP _SO21 atm were measured by electron microprobe analysis.