Oxidation of Low-Carbon,Low-Silicon Mild Steel at 450–900°C Under Conditions Relevant to Hot-Strip Processing

The oxidation behavior of a low-carbon, low-silicon mild steel was investigated in ambient air at 450–900°C to simulate steel strip oxidation during finishing hot rolling and coiling. Oxide scales developed at 880–900°C for a very short time (12 sec) had a structure similar to that formed on pure iron, but with a greater thickness ratio between the magnetite and wüstite layers. However, the scale structure after oxidation for a longer period (200 sec) at 900°C deviated significantly from that reported for pure iron. This difference was attributed to the loss of scale–steel adhesion at some locations. Oxide scales formed in the range of 580–700°C after oxidation for more than 2 hr also differed from those reported for pure iron. The scale structures were irregular, comprising mainly hematite and magnetite with no or very little wüstite, while the thickness ratio of these two layers differed considerably at different locations. The scale formed at 450–560°C was relatively uniform with a two-layered (hematite and magnetite) structure; however, the thickness ratio of these two scale layers varied for different oxidation temperatures and different oxidation durations. It was also found that limited oxygen supply (zero air flow) improved the scale–steel adhesion, and substantially reduced the relative thickness of the hematite layer. Continuous-cooling experiments proved that significant growth of the hematite layer, as well as the entire scale layer, may occur if the steel is cooled slowly through the temperature range 600–660°C, and even more significantly through the range 660–720°C.     

Oxide-Scale Structures Formed on Commercial Hot-Rolled Steel Strip and Their Formation Mechanisms

The oxide-scale structure developed on commercial hot-rolled steel strip at the mid-coil position was examined. The initial oxide scale after rolling and cooling on the run-out table had a three-layer (hematite, magnetite, and wustite) structure; the thickness was found to be a function of the finishing temperature. From this initial structure, various final scale structures developed after coiling, depending on the coiling temperature, oxygen availability, and cooling rate. For relatively low coiling temperatures (e.g., at 520°C), the final scale structure comprised an inner magnetite/iron mixture layer, an outer magnetite layer, and, at regions away from the center, a very thin outermost hematite layer. For higher coiling temperatures (e.g., in the range of 610 to 720°C), a two-layer hematite/magnetite structure was observed at the edge regions, whereas at the center regions, these two layers were absent and the entire scale layer comprised a mixture of the wustite-transformation products, i.e., a mixture of proeutectoid magnetite, magnetite+iron eutectoid, and a certain amount of retained wustite. At regions between the edges and the center, the oxide structures were similar to those developed at low coiling temperatures (<570°C), i.e., an inner layer comprising a mixture of the wustite-transformation products, an intermediate magnetite layer and at regions near the edges, an outermost hematite layer. In addition, two distinct structures were observed on strips with a coiling temperature of 720°C. One structure comprised a very thick hematite layer (3–5 m) formed near the edges (within 10–20 mm from the edges), while the other structure comprised a substantial amount of retained wustite formed at the center regions. The formation mechanisms of various oxide scale structures are discussed.     

To the Low-Temperature Passivity of Iron at the Gaseous Oxidation

The low-temperature passivation of the oxide growth on iron is studied with the use of digital ellipsometry at a temperature of 300°C and an oxygen pressure from 10^–3 to 1 mmHg. The maximum increment in the oxide thickness as a function of the oxygen pressure P _a–p is observed in an hour of exposure, which indicates the active–passive transition. This passivity of iron and other metals can be caused by the multilayer and multiphase structure of the oxide film formed. As the oxygen pressure is increased, an external protective hematite layer appears on iron, and the inner quickly growing magnetite layer has no time to reach its limiting thickness. Shortening of the period of a rapid growth of magnetite, which is observed upon the increase in the oxygen pressure at the same exposure, results in the maximum of the summary thickness of the layer at a certain pressure P _a–p. Hematite over the magnetite layer is usually formed as laterally spreading islets, the coalescence of which sharply decelerates the oxidation. In the time–pressure–oxide thickness plot, the areas of the low-temperature passivation of iron can be distinguished in wide ranges of temperature and pressure. The electrophysical treatment in the range of the active–passive transition sharply intensifies the oxidation of iron-based alloys and leads to the formation of layers with a substantial thickness and protective ability.     

Kinetics of oxidation of ferrous alloys by super-heated steam

The kinetics of the oxidation of ferrous alloys in steam (10–60 kPa) at 450–550°C have been studied by measuring both the rate of hydrogen emission and the amount of metal oxidized. Excellent agreement has been found between the amount of metal oxidized calculated from both the total mass of hydrogen produced in the reaction and the thickness of the oxide layer formed; rate constants calculated from the rate of hydrogen emission, the mass of hydrogen produced as the reaction proceeds, and the oxide formed agree within experimental error. The rate of oxidation of a 9%Cr-1%Mo alloy at 501°C was found to be independent of the partial pressure of the steam. For this alloy, the activation energy agreed with literature values obtained at higher temperatures and pressures. The effect of the chromium and silicon content on the oxidation rates is compared. The rate constants are compared with theoretical calculations, assuming that the rate is determined by diffusion of iron in the magnetite lattice. For the 9%Cr-1%Mo alloy, the parabolic rate constant and activation energy are in excellent agreement with values calculated using Wagner's theory. The experimental rate constants are greater for the alloys containing smaller amounts of chromium; diffusion of iron along magnetite grain boundaries may be the dominant mechanism.     

Effect of creep on the oxidation characteristics of Fe-Si alloys at 973–1073 K

Studies of the simultaneous creep and oxidation of Fe-1Si and Fe-4Si alloys at a constant tensile stress of 16 N· mm^–2 at 973–1073 K have shown that scales formed at oxygen partial pressures of 20–1013 mbar were thicker by a factor of 2 than those formed on uncrept specimens. Scales on uncrept alloys comprised alternate layers of wustite and fayalite, whereas scales on crept alloys exhibited an additional external layer of magnetite. Only intergranular oxidation (fayalite) was observed in uncrept alloys, but crept alloys showed both intra- and intergranular oxidation (silica). Uniquely nodular scales were formed only on the Fe-4Si alloy on crept and uncrept specimens. Oxidized, uncrept Fe-1Si showed a fine-grained ferrite substrate which was absent in the crept alloy. It is believed that oxide growth stresses stimulated a recrystallization process.     

The oxidation of Ni-Nb alloys at low-oxygen pressures at 600–800°C

The oxidation of two Ni–Nb alloys containing 15 and 30 wt.% Nb has been studied at 600–800° C in H₂–CO₂ mixtures providing an oxygen pressure of 10^–24 atm at 600° C and 10^–20 atm O₂ at 700 and 800° C, these pressures being less than the dissociation pressure of nickel oxide. The scales formed on both alloys at 600 and 700° C show only a region of internal oxidation composed of a mixture of alpha nickel and niobium oxides (Nb₂O₅ or/and NbO₂), which formed from both the metal phases present, i.e., Ni₈Nb and Ni₃Nb. Only small, or even no, Nb depletion was observed in the alloys close to the interface with the zone of internal oxidation at these temperatures. On the contrary, samples of both alloys corroded at 800° C produced a continuous external scale of niobium oxides without internal oxidation. The corrosion mechanism of these alloys is examined with special reference to the effect of the low solubility of niobium in nickel.     

Oxidation of an Fe—19 wt. % Ni alloy in CO2 at 700–1000°C

The oxidation of an Fe—19.34 wt. % Ni alloy in dry CO₂ has been studied at 700—1000°C using thermogravimetry, metallography, and EPMA. Weight gains for oxygen consumption followed a linear-parabolic-linear sequence at all temperatures. During the initial linear stage the scale consisted mainly of magnetite and the activation energy of 133±25 kJ · mole^–1 is considered to be due to dissociation of CO₂ into CO and adsorbed oxygen on the outer magnetite surface. During the parabolic oxidation stage a continuous Ni-rich layer containing 70% Ni forms a barrier to the diffusion which has an activation energy of 192±79 kJ · mole^–1. The breakdown of the barrier layer causes a return to linear kinetics with an activation energy of 138±42 kJ · mole^–1 for dissociation of CO₂ on the outer surface. During the final linear stage there is pronounced general and intergranular subscale formation. Detailed information is presented of the Ni redistribution and concentrations during oxidation and its correlation with the kinetics and morphology.     

The oxidation of Co−Nb alloys under low oxygen pressures at 600–800°C

The oxidation of two Co–Nb alloys containing 15 and 30 wt.% Nb has been studied at 600–800° C in H₂–CO₂ mixtures providing an oxygen pressure of 10^–24 atm at 600°C and 10^–20 atm at 700 and 800°C, below the dissociation pressure of cobalt oxide. At 600 and 700°C both alloys showed only a region of internal oxidation composed, of a mixture of alpha cobalt and of niobium oxides (NbO₂ and Nb₂O₅) and at 700°C also the double oxide CoNb₂O₆, which formed from the Nb-rich Co₃Nb phase. No Nb-depleted layer formed in the alloy at the interface with the region of internal oxidation at these temperatures. Upon oxidation at 800°C a transition between internal and external oxidation of niobium was observed, especially for Co–30Nb. This corrosion mode is associated with the development of a single-phase, Nb-depleted region at the surface of the alloy. The corrosion mechanism of these alloys is examined with special reference to the effect of the low solubility of niobium in cobalt and to the relation between the microstructures of the alloys and of the scales.     

Kinetics of wustite-fayalite scale formation on iron-silicon alloys

A superficial duplex scale consisting of an inner wustite-fayalite conglomerate and an outer layer of wustite was formed on an iron-1.5wt.% silicon alloy upon its exposure in carbon dioxide-carbon monoxide atmospheres at 890and 1000°C. Variations in the oxidation curves before onset of linear reaction kinetics could be correlated with appearance of fayalite bands in the scales. The linear rate constants for the overall reaction kinetics showed a proportional dependence on the pressure of carbon dioxide due to reaction control by surface reaction steps for dissociation of carbon dioxide and incorporation of chemisorbed oxygen into the wustite lattice. Growth of the wustite-fayalite conglomerate was not dependent upon the oxidizing potential of an atmosphere. Wustite growth involved iron migration while the growth of the inner wustite-fayalite conglomerate layer involved inward migration of oxygen resulting from a dissociative reaction of wustite at its inner surface.This work forms part of a research project sponsored by the American Iron and Steel Institute and the National Research Council of Canada.     

Thickness of the oxide layers formed during the oxidation of iron

The theory of the growth of two oxide layers by Yurek et al. has been applied to the oxidation of iron at 1100° C. The theoretical parabolic rate constants for the simultaneous growth of the two oxide layers were calculated from radioactive tracer diffusion coefficients for wustite and magnetite. Good agreement was found between the theoretical and experimental values of the ratio of scale thicknesses.     

Review of the High-Temperature Oxidation of Iron and Carbon Steels in Air or Oxygen

This paper reviews previous studies on iron and steel oxidation in oxygen or air at high temperatures. Oxidation of iron at temperatures above 700°C follows the parabolic law with the development of a three-layered hematite/magnetite/wüstite scale structure. However, at temperatures below 700°C, inconsistent results have been reported, and the scale structures are less regular, significantly affected by sample-preparation methods. Oxidation of carbon steel is generally slower than iron oxidation. For very short-time oxidation, the scale structures are similar to those formed on iron, but for longer-time oxidation, because of the less adherent nature, the scale structures developed are typically much more complex. Continuous-cooling conditions, after very short-time oxidation, favor the retention of an adherent scale, suggesting that the method proposed by Kofstad for deriving the rate constant using continuous cooling or heating-oxidation data is more appropriate for steel oxidation. Oxygen availability has certain effects on iron and steel oxidation. Under continuous cooling conditions, the final scale structure is found to be a function of the starting temperature for cooling and the cooling rate. Different scale structures develop across the width of a hot-rolled strip because of the varied oxygen availability and cooling rates at different locations.     

Mechanism of Iron Oxide Scale Reduction in 5%H<Subscript>2</Subscript>–N<Subscript>2</Subscript> Gas at 650–900 °C

The reduction behaviour of the oxide scale on hot-rolled, low-carbon steel strip in 5%H₂–N₂ gas at 650–900 °C was studied. In general, the reduction rate of the oxide scale at the centre location was more rapid than that at the near-edge location. In both cases, the reduction rates at 650 °C were extremely low and the rates increased with increased temperature, reaching their maxima at 850 °C. Arrhenius plot of the rate constant derived from the early parabolic stage revealed that the reduction mechanism at 650–750 °C differed from that at 750–850 °C, with the former being oxygen diffusion in α-Fe and the latter most likely iron diffusion in wustite. In all cases, a thin iron layer formed on the scale surface within a very short time and then the thickness of this layer remained essentially unchanged, while the scale layer was gradually reduced via outward migration of the inner wustite–steel interface, as a result of inward iron diffusion through the wustite layer to that interface. More rapid oxygen diffusion through the thin surface iron layer than the oxygen supply rate through interface reaction was believed to result in a lower oxygen potential at the outer iron–wustite interface, thus providing a driving force for iron to diffuse through the wustite layer. The inner wustite–iron interface became undulating initially; then with the rapid advance of some protruding sections, some parts of the wustite layer were reduced through first, and finally the remaining wustite islands were reduced to complete the reduction process. Porosities were generated when wustite islands were reduced due to localized volume shrinkage. Higher oxygen concentrations in the scales of the near-edge samples were believed to be responsible for their slower reduction rates than those of the centre location samples.     

The oxidation properties of U-16.6 at.% Nb-5.6 at.% Zr and U-21 at.% Nb

The fundamental oxidation characteristics of two U-base alloys, U-16.6 at.% Nb-5.6 at.% Zr and U-21at.% Nb, in the temperature range 500–1000° C in oxygen at 0.05 Torr are described. Both alloys undergo large dimensional changes during oxidation at temperatures above 650° C due to stresses generated in the oxide during oxidation. Oxidation rate curves for both alloys were determined at 100° C intervals between 500 and 1000° C; the activation energy for the process is shown to be small. The morphology of the oxide scale formed on the two alloys is complex and is described in detail. Stresses estimated at 10⁶ psi are shown to develop in oxidizing specimens, and a mechanism for the generation of these stresses is proposed.Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation.     

Primary stage of Fe-Cr-C alloy oxidation at 1100°C

The oxidation of Fe-Cr-C alloys containing 13% Cr and carbon ranging from 0.15 to 1.63% was studied at 1100°C for 3 hr. It was found that the oxidation rate increased with increasing carbon content in the alloys. The measured time variation of CO₂ evolution showed that, during the first period of oxidation (0–40 min), a compact surface layer of FeO formed, which prohibited the free transfer of CO₂ into the streaming oxidation atmosphere. The measurable CO₂ evolution started between 40 and 50 min, and the most rapid evolution occurred in the interval t=60–90 min of oxidation. This effect corresponds well to scale-layer damage and to the maximum O₂ absorption. The measurements carried out by the combined thermobalance-Chromatograph equipment were supplemented by metallographic investigation of the oxide layers. The results enabled us to interpret the primary stage of the oxidation of Fe-Cr-C alloys and to discuss the relations between the cation diffusivities in the individual oxide sublayers, i.e., in wustite, magnetite, and hematite.     

Oxide scales characterization of micro-alloyed steel at high temperature

A micro-alloyed steel was oxidized at different temperature between 900 °C and 1200 °C in 80%N₂–15%CO₂–5%O₂ atmosphere. As compared with the pure iron, an additional layer between the matrix and the oxide scale was detected in the micro-alloyed steel. The micro-alloying elements, mainly Si and Mo, are found accumulated in the subscale in the form of Laves phases. As the Laves phases can impede the diffusion of iron ion and electron across the subscale, the micro-alloyed steel shows a lower oxidation rate than that of pure iron. Moreover, the existence of the subscale results in a different thickness ratio of hematite, magnetite and wustite in the micro-alloyed steel as compared with that in pure iron. It is also found that the surface roughness of oxide scale significantly depends on the content of hematite.     

Oxygen Transport during the High Temperature Oxidation of Pure Nickel

The high temperature oxidation of nickel has been investigated in air under atmospheric pressure in the temperature range 600–900°C. The oxidation kinetic curves deviate from the parabolic law for temperatures over 800°C. The observation of scale morphologies and the use of two stage oxidation experiments under ¹⁶O₂/¹⁸O₂ atmospheres showed that oxygen transport through the NiO scale had to be taken into consideration during the oxidation process. Despite the main outward diffusion of Ni species through the oxide scale, the inward oxygen diffusion at lower temperatures (<800°C) or the oxygen transport, probably as molecular species, via pores or micro-cracks were found to play a major role in the formation of duplex oxide scales, made of small equiaxed oxide grains at the metal/oxide interface overgrown by larger columnar grains at the gas/oxide interface. Oxygen diffusion coefficients into thermally grown NiO scales were determined and compared to the values of Ni diffusion coefficients from the literature.     

Study of the oxidation kinetics of vanadium carbide

The oxidation of an oxycarbide of vanadium, VO_0.6C_0.7, and of a vanadium carbide, VC_0.98, was studied athermally up to temperatures of 800° C and isothermally between 400 and 580° C at oxygen pressures ranging from 10^–2 to 1 atm. The oxycarbide followed the parabolic rate law below 450° C with V₂O₅ forming as the only reaction product. The activation energy was 49 kcal/mole. VC_0.98 did not form an oxide in this temperature range, but rather dissolved oxygen, the activation energy being 26.6 kcal/mole. No oxygen pressure dependence on the kinetics was found for either sample in this temperature range. Both samples followed the cubic rate law during oxidation in the range of 500–580° C during which V₂O₅ formed. There was a P^1/3 dependence and the activation energy was the same for both materials, 51 kcal/mole. The cubic rate law and the positive pressure dependency (rather than an anticipated negative dependency) were attributed to an electric field associated with oxygen ions chemisorbed on a thin layer of V₂O₅.     

The oxidation resistance of a sputtered,microcrystalline TiAl-intermetallic-compound film

The oxidation behavior of a cast TiAl intermetallic compound and its sputtered microcrystalline film was investigated at 700–900°C in static air. At 700°C, both the cast alloy and its sputtered microcrystalline film exhibited excellent oxidation resistance. No scale spallation was observed. However, at 800–900°C, the oxidation kinetics for the cast TiAl alloy followed approximately a linear rate law, which indicates that it has poor oxidation resistance over this temperature range. The poor oxidation resistance of TiAl was due to the formation of an Al₂O₃+TiO₂ scale which spalled extensively during cooling. Nevertheless, the sputtered, TiAl-microcrystalline film exhibited very good oxidation resistance. The oxidation kinetics followed approximately the parabolic rate law at all temperatures. Although the composition of the scales was the same as that of scales formed on the cast alloy, the scales formed on the sputtered microcrystalline-TiAl film are adherent strongly to the substrate. No scale spallation was found at 700–850°C, while a small amount of spallation was observed only at 900°C. This indicates that microcrystallization can improve the oxidation resistance of the TiAl alloy.     

Influence of Small Amounts of Impurities on Copper Oxidation at 600–1050°C

In order to study the influence of small amount of impurities on the copper oxidation kinetics, the oxidation was examined at 600–1050°C in 0.1 MPa oxygen atmosphere using 99.5% (2N) and 99.9999% (6N) pure copper specimens. The influence of impurities has been discussed considering the roles of the nonprotective CuO layer, the impurity layer at the Cu₂O–Cu interface, and the diffusion of copper in the Cu₂O layer. The nonprotective CuO layer for 2N copper can greatly enhance copper oxidation. However, the impurities concentrated at the region near the Cu₂O–Cu interface for 2N copper can slow oxidation. Contrary to the presence of metallic impurities, such as Ni, Sb, and Pb, the nonmetallic elements As and Se dissolved in Cu₂O have a deleterious influence on the outward diffusion of copper. Grain-boundary diffusion in Cu₂O can somewhat contribute to 2N copper oxidation at 850–1050°C, but its effect in enhancing oxidation at 600–800°C is weaker than the effect of the impurity layer at the Cu₂O–Cu interface in impeding oxidation.     

High temperature deformation of oxide scale

Samples of ultra low carbon steel were oxidized in a chamber designed to control the thickness of the scale layer prior to their deformation by plane strain compression. The specimens were reheated within the temperature range of 950-1150 °C for different periods of time to vary the scale thickness; compression was conducted at temperatures that vary from 650 to 1150 °C. Metallographic analyses were conducted on deformed and undeformed scales to evaluate the integrity of the oxide crust that is made almost exclusively of wustite. It was found that the integrity of the crust depends on the temperature at which deformation is carried out and on the amount of deformation imparted. It is concluded that thin scales exhibit a plastic behaviour when deformed at temperatures above 900 °C and a brittle behaviour at temperatures below 700 °C; mixed behaviour was observed within this temperature range, as the scale was found to resist limited amounts of deformation.