The Effect of H<Subscript>2</Subscript> and H<Subscript>2</Subscript>O on the Oxidation of 304L-Stainless Steel at 600 °C: General Behaviour (Part I)

The effect of p(H₂O) and p(H₂) on the oxidation of 304L stainless steel at 600 °C has been investigated in the present study. The samples were analysed by means of X-ray diffraction, Auger spectroscopy, and scanning electron microscopy equipped with energy dispersive spectroscopy. The results showed that at fixed p(H₂), the corrosion rate increased considerably with increasing p(H₂O). At fixed p(H₂O), the corrosion rate decreased slightly with increasing p(H₂). Duplex oxide scales formed during the exposure in all environments. The outer and inner layer consisted of Fe₃O₄ and (Fe, Cr)₃O₄, respectively. The latter was mainly in the form of internal oxidation. The Cr-rich oxide formation was observed at the initial oxidation process before oxide breakdown. The Auger analysis also suggested the presence of Cr-rich oxide layer just after the breakaway oxidation. The results indicated that the rate-determining step in the corrosion attack is surface controlled or diffusion controlled through an oxide layer with fixed thickness over time.     

Effect of Water Vapor on the Oxidation Behavior of Fe–1.5Si in Air at 1073 and 1273 K

The oxidation behavior of Fe–1.5Si was investigated at 1073 and 1273 K in air, air–H₂O, Ar–H₂O, O₂–H₂O, and O₂ atmospheres. The extent of corrosion in atmospheres containing H₂O increased rapidly after an incubation period of slow oxidation, the incubation period becoming shorter in the order, O₂–H₂O, air–H₂O, and Ar–H₂O. With increasing H₂O contents in air–H₂O, the incubation time decreased. During the incubation period, oxidation was slow, because of the formation of an inner Si-rich oxide layer and a Pt marker was located between the external Fe₂O₃ (Fe₃O₄ included) and an inner Si-rich oxide layer. During the rapid oxidation, the inner FeO+Fe₂SiO₄ layer thickened and a Pt maker was at the interface between an external Fe-oxide and an inner FeO+Fe₂SiO₄ layer. Observations of scale cross sections indicated that voids made channels along the boundaries of columnar FeO crystals, suggesting transport of water molecules. The Si-rich oxide layer changed into an FeO+Fe₂SiO₄ mixture due to penetration of water molecules. A combined process of perforating dissociation and transport of water molecules is suggested to be the cause of the rapid growth of the inner FeO+Fe₂SiO₄ layer.     

Kinetics of H2O2 reaction with oxide films on carbon steel

The nature of carbon steel surfaces in 0.01 M borate solutions (pH 10.6) have been characterized using a range of electrochemical techniques and ex situ analyses such as Raman and Auger spectroscopy. Their subsequent behaviour on exposure to 10⁻³ M H₂O₂-containing solutions has also been studied. The anodically oxidized carbon steel surfaces have been characterized according to three regions: (I) the potential range <−0.5 V (vs SCE), when the surface is active and covered by Fe^II/Fe^III oxide/hydroxide; (II) the potential range −0.5 V to 0.0 V when the surface is passivated by an outer layer of Fe^III oxide/hydroxide over the inner layer of Fe^II/Fe^III oxide/hydroxide; and (III) potentials >0 V when further growth of the underlying layer appears to lead to minor film breakdown/restructuring. The addition of H₂O₂ to films grown in the passive region or above (II and III) leads initially to a degradation of the outer layer allowing increased growth of the inner layer. Subsequently, the outer passivating layer is repaired and passivity re-established. These changes appear to be confirmed by Raman spectroscopy.     

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.     

Sulfur Segregation to Al2O3–FeAl Interfaces Studied by Field Emission-Auger Electron Spectroscopy

Using a 30-nm field-emission Auger spectroscopy probe, the segregation of sulfur to a growing oxide–metal interface was studied. The interfaces were formed by the oxidation of a Fe–40at.% Al alloy at 1000°C for various times. Both the oxide and the alloy sides of the interface were examined after spalling the surface Al₂O₃ layer in ultra-high vacuum. Results were compared with similar studies performed using conventional AES and related to scale development and the interface microstructure. Sulfur started to segregate to the interface only after a complete layer of -Al₂O₃ developed there, its concentration then increased slowly with further oxidation until reaching a level close to half a monolayer. Higher amounts were observed on interfacial-void surfaces, where Al and S cosegregated. The study showed that sulfur segregation to oxide–alloy interfaces depended on the type of interface, indicating possible relationships between segregation energies and interface microstructure.     

Influence of the prior oxide film on the growth and structure of oxides formed on Fe---26Cr alloys at 600°C

A Fe---26 Cr alloy has been oxidized at 600°C in 5 × 10⁻³, 5 × 10⁻² and 5 × 10⁻¹ torr oxygen to examine the influence of the prior oxide film on the growth and structure of oxides formed at high temperature. Different prior oxides were produced either by electropolishing or by annealing the electropolished specimen in vacuum at 600°C. Auger electron spectroscopy (AES) showed the Cr content of the prior oxide film to be increased from 50 to 95% during annealing, and electron diffraction indicated a change in oxide structure from amorphous to crystalline. At 5 × 10⁻³ torr, electropolished Fe---26 Cr oxidizes faster than the vacuum annealed specimens because the amorphous prior oxide gives rise to a finer-grained cubic oxide with more grain boundary easy diffusion paths for cation transport. From AES and electron back-scattering Fe⁵⁷ Mössbauer spectroscopy it is concluded that this cubic oxide is a duplex layer of inner γ-Cr₂O₃ and outer Fe₃O₄. The oxidation rate slows markedly when nucleated α-Fe₂O₃ covers the cubic oxide. With increased oxidation time Fe₃O₄ converts to α-Fe₂O₃ and the γ-Cr₂O₂ to α-Cr₂O₃. Annealed Fe---26 Cr oxidizes slower primarily because of a lower cation transport through a coarser-grained cubic oxide rather than because of a higher Cr content in the prior oxide. α-Fe₂O₃ nucleates at an earlier stage in the oxidation and essentially stifles the reaction. The extent of Cr incorporation into any of the Fe oxides produced in 5 × 10⁻³ torr oxygen is small ( 5%). Increasing the oxygen pressure from 5 × 10⁻³ to 5 × 10⁻² and 5 × 10⁻¹ torr has little effect on the mechanism of oxidation of vacuum annealed Fe---26 Cr, except that the overall extent of oxidation is less because of earlier α-Fe₂O₃ formation and, after a few hours of oxidation, up to 20% Cr is incorporated into the α-Fe₂O₃ lattice. On electropolished Fe---26 Cr at 5 × 10⁻² and 5 × 10⁻¹ torr oxygen nodules of α-Cr₂O₃ form and continue to grow both at grain boundaries and within the grains. Possible mechanisms for this nodule formation, which is exclusive to electropolished specimens oxidized at the higher pressures, are considered.     

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.     

Early oxidation behaviors of Mg–Y alloys at high temperatures

The early oxidation behaviors of Mg–Y alloys (Y = 0.82, 1.09, 4.31 and 25.00 wt.%) oxidized in pure O₂ have been investigated at high temperatures. The results showed that the oxidation behaviors of the Mg–Y alloys (Y = 4.31 and 25.00 wt.%) obeyed a parabolic law, while that of the Mg–Y (Y = 0.82 and 1.09 wt.%) exhibited both parabolic and linear kinetics depending on the oxidation temperature. Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses indicated that an oxide film with a single structure composed of MgO and Y₂O₃ had formed. Moreover, the higher the oxidation temperature was, the thicker the oxide film was. Finally, the corresponding oxidation mechanism has been discussed, and the improved oxidation resistance of the Mg–Y alloys can be due to the formation of a continuous Mg-dissolving Y₂O₃ protective film.     

Characterization of the initial oxide formed on annealed and unannealed 20Cr-25Ni-Nb-stabilized steel in 50 torr CO2 at 973 K

The oxidation of annealed and unannealed 20Cr-25Ni-Nb (wt.%) steel in 50 torr CO₂ at 973 K has been studied in situ using X-ray photoelectron spectroscopy with a view to the characterization of the chemical composition and nature of the oxides formed. The oxide first formed on the annealed steel is shown to be iron rich. Analysis of the bulk oxide using a variety of different spectroscopic techniques including X-ray diffraction, energy dispersive X-ray analysis, and scanning Auger microscopy showed that virtually all of the oxide scale formed after 100hr is a spinel of the type (Fe)Fe_2–x)Cr_xO₄ and is composed of an outer, iron-rich layer covering an iron/chromium-rich layer. By contrast, the oxide first formed on the unannealed steel is chromium rich, and is shown to be patchy consisting of a mixture of different oxides. This layer changes on further oxidation to develop into a manganese-iron-chromium spinel, which is present as the major oxide phase after 100 hr. The reasons for these differences are discussed, and it is argued that a major influence on oxidation behavior is the presence of cold work in the unannealed steel enhanc ing the diffusion of chromium to the surface.     

Inherent oxidation protection of Fe-5Cr-15Ni-2Si-4.5Mo

The oxidation mechanism of Fe-5Cr-15Ni-2Si-4.5Mo alloy was investigated in order to determine the role of Si and Mo in providing oxidation resistance. It was determined that the oxidation protection in the temperature range 750–950°C resulted from formation of a continuous oxide sublayer of SiO ₂ (or possibly Fe ₂ SiO ₄).Molybdenum formed an intermetallic Fe ₂ Mo _1–x Si _x that eventually diffused out into the grain boundaries and formed a protective barrier to the oxidation process. The mechanism behind the improved oxidation is the formation of a SiO ₂ layer at the metal-oxide interface that retards the outward diffusion of Fe. It was also established that the oxidation mechanism was controlled by an activation energy equal to that of Fe ³⁺ ions diffusing through SiO ₂.     

High-Temperature Oxidation Behavior of ODS–Fe3Al

The high-temperature oxidation behavior of an oxide dispersion-strengthened (ODS) Fe₃Al alloy has been studied during isothermal and cyclic exposures in oxygen and air over the temperature range 1000 to 1300°C. Compared to commercially available ODS–FeCrAl alloys, it exhibited very similar short-term rates of oxidation at 1000 and 1100°C, but at higher temperatures the oxidation rate increased because of increased scale spallation. Over the entire temperature range, the oxide scale formed was -Al₂O₃, with the morphological features typical of reactive-element doping and was similar to those formed on the ODS–FeCrAl alloys. Although initially this scale appeared to be extremely adherent to the Fe₃Al substrate, an undulating metal–oxide interface formed with increasing time and temperature, which led to cracking of the scale in the vicinity of surface undulations accompanied by a loss of small fragments of the full-scale thickness. In some instances, the surface undulations appeared to have resulted from gross outward local extrusion of the alloy substrate. Similar features developd on the FeCrAl alloys, but they were typically much smaller after a given oxidation exposure. The ODS–Fe₃Al alloy has a significantly larger coefficient of thermal expansion (CTE) than typical FeCrAl alloys (approximately 1.5 times at 900°C) and this appears to be the major reason for the greater tendency for scale spallation. The stress generated by the CTE mismatch was apparently sufficient to lead to buckling and limited loss of scale at temperatures up to 1100°C, with an increasing amount of substrate deformation at 1200°C and above. This deformation led to increased scale spallation by producing an out-of-plane stress distribution, resulting in cracking or shearing of the oxide.     

Corrosion behavior of Hastelloy C-276 in supercritical water

The corrosion behavior of a nickel-based alloy Hastelloy C-276 exposed in supercritical water at 500–600 °C/25 MPa was investigated by means of gravimetry, scanning electron microscopy/energy dispersive X-ray spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. An oxide scale with dual-layer structure, mainly consisting of an outer NiO layer and an inner Cr₂O₃/NiCr₂O₄-mixed layer, developed on C-276 after 1000 h exposure. Higher temperature promoted oxidation, resulting in thicker oxide scale, larger weight gain and stronger tendency of oxide spallation. The oxide growth mechanism in SCW seems to be similar to that in high temperature water vapor, namely solid-state growth mechanism.     

Continuous and cyclic oxidation of T91 ferritic steel under steam

The oxidation behaviour of T91 ferritic steel in steam has been studied under isothermal and non-isothermal conditions within a temperature range between 575 and 700 °C. Isothermal treatments resulted in parabolic oxidation kinetics. Three clearly defined oxide layers constituted the oxide scales. The innermost layer was a (Fe,Cr)₃O₄. The intermediate layer was porous magnetite (Fe₃O₄) followed by a compact thinner layer of hematite (Fe₂O₃). Under non-isothermal conditions the oxide scales were irregular and evidently cracked. An increase of the oxidation temperature produces an acceleration of the oxidation process, causing an increase of the oxide scale thickness that depends on the temperature increase and the exposure time.     

A study of the growth of iron oxides by Mössbauer spectroscopy

The rate of oxidation of Fe₃O₄ to α-Fe₂O₃ in 1 atm of oxygen has been measured at 592°, 629° and 667°C by Mössbauer transmission spectroscopy. Taken together with our earlier measurements, the data lead to an activation energy for α-Fe₂O₃ growth of 44 kcal mole⁻¹ for the temperature interval 475–670°C. That the Mössbauer technique should prove useful in the study of reactions in oxide scales containing wüstite is also demonstrated.     

S-Induced Destabilization of Aluminum Oxide at the Fe(poly)–S–Al2O3 Interface

Auger measurements reveal that, under UHV conditions, interfacial sulfurinduces the destabilization of an aluminum oxide overlayer at theFe–Al₂O₃ interface at temperatures above400 K. One monolayer deposition of Al onto Fe/S results in the insertion ofAl at the Fe–S interface. Exposure of Fe–Al–S to oxygenat 300 K gives rise to the complete oxidation of the aluminum adlayer asevidenced by the disappearance of the Al⁰ Auger signal and thestoichiometric formation of the aluminum oxide. When the resultingFe–S–Al₂O₃ is annealed progressively tohigher temperatures between 400 and 900 K, analysis of the Auger spectrashows a dramatic decline in the Al/O Auger intensity ratio. This declineis accompanied by the appearance of a small signal due to Al⁰,which maintains a constant intensity as the total Al signal (due mainly toAl³⁺) decreases. The appearance of the Al⁰ Augersignal accompanied by the attenuation of the Al³⁺ signalsignifies the chemical conversion of Al³⁺ into Al⁰,probably followed by diffusion of Al into the bulk. The possibility ofalumina dewetting and island formation, however, cannot be ruled out onthe basis of the present data. In the absence of interfacial sulfur, the alumina–Fe interface is stable to 900 K.     

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.     

Microscopic Features in the Transition from External to Internal Oxidation in an Fe–6 mol.% Si Alloy Annealed Under Various H2O–H2 Atmospheres

The occurrence of external or internal oxidation in Fe–Si alloys is strongly affected by oxidation conditions. In the present study, X-ray diffraction, Auger electron spectroscopy, X-ray photoelectron spectroscopy, secondary-ion mass spectrometry, and glow-discharge optical-emission spectrometry were used for characterizing the microscopic features of oxide layers formed on the (011) surface of an Fe–6 mol.% Si alloy. The starting materials were annealed at 1473 K under dry hydrogen gas and were subsequently annealed at 1123 K under a 75% H₂–25% N₂ atmosphere with various partial pressures of water vapor. The results show that the microscopic morphology and elemental distribution in oxide layers strongly depend on oxidation conditions. The surface was found to become rough by annealing in higher partial pressures of water vapor. This phenomenon may be induced by internal oxidation. Corresponding to the morphological changes of the surface, changes in the distribution of alloying elements have systematically been characterized in surface layers. These experimental results are discussed in conjunction with thermodynamic data on oxidation of elements.     

The growth and structure of oxide films on Fe. II. Oxidation of polycrystalline Fe at 240–320°C

The influence of surface pretreatment and metal orientation on the oxidation of coarse-grained polycrystalline Fe has been studied at 240 to 320°C in 5×10^–3 Torr O₂ using electron diffraction, electron microscopy, and Mössbauer spectroscopy to complement kinetic data. Consistent with previous studies on Fe single crystals, differences in oxidation kinetics for surfaces covered with an electropolish film from those with a similar thickness prior oxide formed by dry oxidation at room temperature are interpreted in terms of differing densities of leakage paths in the oxide layers. The more complex kinetics for electropolished polycrystalline Fe are a result of the leakage path density, the degree of oxide separation, and the extent of -Fe₂O₃ formation varying with substrate orientation. Where adherent Fe₃O₄ layers are formed on polycrystalline and single-crystal Fe surfaces, the parabolic rate constants give an activation energy which is consistent with a previous value of 32 kcal · mole^–1, suggesting that at these low temperatures the transport mechanism for magnetite growth is cation diffusion via easy diffusion paths in the oxide.     

TEM Study on the Internal Oxidation and Nitriding of Dual Phase Fe–Mn–Al–C Alloy AfterNaCl-Induced Corrosion

A dual-phase Fe–Mn–Al–C alloy was oxidized at 750°C in air with 2 initial mg/cm² NaCl deposits. After 9 hr of exposure a fine-void zone was observed in the middle subscale and a coarse-void layer in the inner subscale. The fine-void zone formed due mainly to the interaction between selective oxidation of manganese and the oxidation of metal chlorides, while the formation of the coarse-void layer was caused by chlorination. The product remaining in the fine-void zone was mostly Al₂O₃, and the structure of the substrate in this zone is oxidation-induced ferrite, where nitriding of aluminum occurs forming AlN. Fine Fe₃O₄ particles fill in the coarse voids and the structure of the substrate in this layer is secondary austenite. The mechanism of the formation and growth of the internal oxidation and nitriding in the void zones of the subscale are discussed.     

High-Temperature Oxidation of Ni Coated with La2O3 by Atomic-Layer Chemical-Vapor Deposition (ALCVD)

The effects of superficial (30–100 nm) La₂O₃ surface coatings on the oxidation kinetics of Ni from 700 to 1100°C in air and the oxide morphology of the NiO scales have been investigated. The parabolic rate constant is lower than for uncoated Ni by a factor of 5 to 10. The oxide morphology changes with the La₂O₃ coatings: The oxide scale consists of an outer fine-grain layer with an inner region of coarser, but still equiaxed, grains. SIMS shows that the majority of the La remains at the surface where a highly oxygen-defective spinel, La₂Ni₄O₇, was found by TEM. Two-stage oxidation followed by SIMS profiling reveals that the oxide growth occurs inside the scales.