Evolution of Oxide Film of T91 Steel in Water Vapor Atmosphere at 750 °C

In order to investigate the evolution of oxide film on T91 steel, oxidation tests were conducted in water vapor atmosphere at 750 °C. The phase compositions and microstructures of the oxide scales for early stage oxidation were investigated by using glancing angle XRD and SEM equipped with EDS. The results showed that during the initial oxidation stage Cr-rich oxide film formed and then it covered the sample surface rapidly. The initial Cr-rich oxide film was mainly composed of FeCr₂O₄, (Fe,Cr)₂O₃ and Fe₂O₃. This oxide film acted as a barrier against outward diffusion of iron and inward diffusion of oxygen. During the initial oxidation stage, chromium in the sample surface was consumed gradually, and then a large amount of iron ions penetrated the oxide film and diffused rapidly to the sample surface, resulting in forming an outer “non-protective” Fe₂O₃ layer.     

Optical spectroscopic and electrochemical characterization of oxide films on a ferritic stainless steel

Colored oxide films that form on ferritic stainless steel in a high-temperature, oxidizing environment and correspond to different chemical compositions can cause a deterioration of pitting resistance and corrosion performance. Herein, optical spectroscopic and electrochemical techniques have been used to reveal the relationship between color, chemical composition, and corrosion resistance of oxide films formed in the temperature range from 400°C to 800°C for 30 min and at 800°C for 10, 20, 30, and 60 min. The substrate with a thin and dense passivation film leads to a low pitting potential but high corrosion resistance. Oxide films of yellowish or brownish color formed below 600°C are mainly iron oxides, which correspond to low corrosion resistance. No passivation characteristics can be observed for polarization curves of oxide films formed at 500°C and 600°C. The color of oxide films varies from blue to dark gray with the increase of oxidation time at 800°C. Corrosion resistance changes with different proportions of Fe₃O₄, Cr₂O₃, and FeCr₂O₄. The gray oxide films formed at 800°C for 30 min exhibit the lowest pitting susceptibility and the highest corrosion resistance.     

Cyclic oxidation of AISI 316L stainless steel – influence of water vapour between 800 and 1000°C

At 800 and 900°C, 10 vol.-%H₂O in air has little effect on the AISI 316L stainless steel oxidation under isothermal and cyclic conditions. The oxide scale is composed of Cr₂O₃ with a small amount of Mn_1·5Cr_1·5O₄ at the external interface. Results show that water molecules or protons can modify the diffusion process in the scale and lower the oxidation rate. At 1000°C, a deleterious effect of water vapour on the scale structure is observed. In situ X-ray diffraction was used to analyse the oxide formation on AISI 316L specimens during isothermal oxidation at 1000°C in moist air. Results show that the breakaway oxidation is due to the iron oxidation starting after 31 h oxidation. This leads to an external Fe₂O₃ scale growth and an internal multilayered FeCr₂O₄ scale formation. In wet air, thermal cycling conditions lead to continuous weight losses at 1000°C, whereas the scale remains adherent in dry air.     

Surface modifications of oxide layer formed in isothermal high temperature conditions of AISI 304 stainless steel

Abstract

Chromia forming steels are excellent candidates to resist to high temperature oxidising atmospheres because they form protective oxide scales. To understand the oxidation mechanisms of the AISI 304 stainless steel in air at 800°C, in situ X-ray diffraction (XRD) has been used not only during high temperature oxidation, but also during and after cooling. The in situ XRD analyses carried out during high temperature AISI 304 steel oxidation in air at 800°C reveal the growth of iron containing oxides such as haematite Fe₂O₃ and iron chromite FeCr₂O₄, after 35 h of the oxidation test, whereas the initial nucleation of the oxide layer shows the single growth of chromia. Iron containing oxides develop over the initial layer and these oxides appear to be poorly adherent and spall off during cooling between 200 and 50°C. Protection against high temperature oxidation would be increased when the initial nucleation of manganese spinel compound is delayed in the oxide scale.     

Oxidation behaviour of Super 304H stainless steel in supercritical water

Oxidation of Super 304H steel in supercritical water at 600°C and 25?MPa was investigated, for the oxidation time of up to 1000?h. The oxidation kinetics approximately followed a parabolic law. The composition and microstructure of the oxide were investigated using the SEM equipped with EDS, EBSD and XRD. The phase compositions of the oxide evolve with increasing time. Cr₂O₃ was present at the initial stage and not detected while Fe₂O₃ appeared at longer time. EBSD indicated that continuous Cr₂O₃ formed at the interface of oxide and matrix for HR3C at 600°C for 40?h but only scattered Cr₂O₃ can be observed for 200?h. After the formation of a thin Fe–Cr–Ni oxide film on the Super 304H steel surface, the Fe-rich outer nodular oxide begins to grow and forms a continuous layer after 1000?h. The growth process of Super304H steel in supercritical water was discussed.     

KCl-Induced Corrosion of a 304-type Austenitic Stainless Steel in O2 and in O2 + H2O Environment: The Influence of Temperature

The oxidation of the 304-type (Fe18Cr10Ni) austenitic stainless steel was investigated in the temperature range 400–600 °C in 5% O₂ and 5% O₂ + 40% H₂O. Exposure time was up to 1 week. Prior to exposure, the polished samples were coated with 0.1 mg/cm² KCl. Uncoated samples were also exposed and used as references. The oxidized samples were analyzed by gravimetry and by ESEM/EDX, XRD, IC and AES. The results show that KCl is strongly corrosive. Corrosion is initiated by the reaction of KCl with the chromia-containing oxide that normally forms a protective layer on the alloy. This reaction produces potassium chromate particles, leaving a chromium-depleted oxide on the alloy surface. At 500 and 600 °C this results in rapid oxidation, resulting in the formation of a thick scale consisting of a mixture of hematite, spinel oxide ((Fe,Cr,Ni)₃O₄) and K₂CrO₄. The thick scale is poorly protective and permeable to e.g. chloride ions. The KCl-induced corrosion of alloy 304L in dry O₂ and in an O₂ + H₂O mixture increases strongly with temperature in the range 400–600 °C. The strong temperature dependence is explained partly by the temperature dependence of the chromate-formation reaction and partly by the ability of the chromium-depleted oxide to protect the alloy at low temperature. At 400 °C, the oxide was still protective after 168 h.     

The characterization by raman spectroscopy of oxide scales formed on a 20Cr25NiNb stabilized stainless steel

Laser Raman spectroscopy (LRS) has been used to examine the composition of 0.3–2 μm thick scales formed on the 20Cr25NiNb stabilized stainless steel during oxidation, at 600–950°C, in CO₂ and in CO₂ + 4%CO + 350 vpm CH₄ + 300 vpm H₂O + 400 vpm H₂. All the scales contained iron rich spinel oxides, except in carbon dioxide at 600°C, where Fe₂O₃ predominated. This difference was responsible for the greater attack of the stainless steel at 600°C in carbon dioxide than in the mixed gas environment. Increasingly at ≥800°C the spinel composition was modified by manganese incorporation and Cr₂O₃ was also a major component of the scales formed in both environments. The principal scale constituents identified by LRS accord both with thermodynamic predictions and current understanding of scale development on the 20/25/Nb stainless steel.     

High-Temperature Oxidation Behavior of HASTELLOY C-4 in Steam

A commercial superalloy HASTELLOY C-4, was studied to evaluate its resistance to oxidation at elevated temperatures. Specimens were exposed to steam from 600° to 1200°C for 1–400 hr. The reaction kinetics of oxidation were determined, and the morphology of the oxide scales was investigated by optical and scanning-electron microscopy. It was observed that C-4 exhibited excellent oxidation resistance at all temperatures under study. This behavior is due to the formation of a compact and adherent oxide. The oxides formed during oxidation were identified by X-ray diffraction. The oxides observed were mainly Cr₂O₃, NiCr₂O₄, NiCrMnO₄, FeCr₂O₄ and (Cr, Fe)₂O₃.     

Corrosion behavior of an alumina forming austenitic steel exposed to supercritical carbon dioxide

Microstructure characterization of corrosion behavior of an alumina forming austenitic (AFA) steel exposed to supercritical carbon dioxide was conducted at 450–650 °C and 20 MPa. At low temperature and short exposure times, the oxidation kinetics were parabolic and the oxide scales were mainly composed of protective and continuous Al₂O₃ and (Cr, Mn)-rich oxide layers. As the temperature and exposure time increased, the AFA steel gradually suffered breakaway oxidation and its oxide scales showed a multilayer structure mainly composed of Fe₃O₄, (Cr, Fe)₃O₄, NiFe/FeCr₂O₄/Cr₂O₃/Al₂O₃, FeCr₂O₄/Al₂O₃, and NiFe/Cr₂O₃/Al₂O₃, in sequence. The corrosion mechanism based on the microstructure evolution is discussed in detail.     

Cyclic Oxidation of P91 by Thermogravimetry and Investigation of Integrity of Scale by “Transient-Mass-Gain” Method

The growth and degradation of the oxide scale on modified 9Cr–1Mo ferritic steel was studied at 1123 K using a thermogravimetric balance by employing the “transient-mass-gain method” in conjunction with the adaptation of a cyclic-oxidation procedure. The total duration of the oxidation was 1000 h. The experiment revealed that the cracking of the scale was initiated when the average thickness was 72 μm. Spallation occurred when the average thickness was 75 μm. The rate of spallation was found to be enhanced as the scale thickens and attained a higher rate after 90 μm. The rate constants for the different stages of oxidation were found to be different. The specimen was examined by SEM, EDS and XRD. The scale morphology revealed outwardly protruded growth, a uniform adherent oxide layer and a spalled region. Four oxide phases were identified; Cr₂O₃, Fe₂O₃, (FeCr)₂O₃ and FeCr₂O₄. The spall contained more (FeCr)₂O₃ whereas the adherent scale was more FeCr₂O₄.     

Water Vapour Effect on Ferritic 4509 Steel Oxidation Between 800 and 1000?��C

The 4509 alloy (Fe?C18Cr?CNb?CTi) was oxidised in dry and wet air in the 800?C1000 °C temperature range. Results showed that the formation of a chromia layer acts as a good diffusion barrier under isothermal conditions at 800 and 900 °C, under 7.5 vol.% water vapour and dry air. Nevertheless, a breakaway is generally observed at 1000 °C, under wet air 7.5 vol.% H₂O. It is proposed that the oxidant H⁺/OH^? species react at the internal interface with iron in the chromium-depleted alloy zone. Wüstite reacts with Cr₂O₃ to form FeCr₂O₄. Outward iron diffusion leads to Fe₃O₄ and Fe₂O₃ formation. The chromia scale was consumed by reaction with wüstite, but chromia also internally forms owing to a chromium oxidation process with the inner chromium-rich alloy area.     

Effect of Aluminizing and Nitridation on the Oxidation Behavior of AISI 316LN SS

The application of stainless steel at temperatures above 973 K is rendered difficult by the chromium loss due to generation of volatile CrO₃ species and the consequent reduction in the capacity to maintain protective scale. We have attempted a method to circumvent this problem by modifying the surface. The surface was modified by three techniques each suiting a particular application. The first two methods involved aluminizing the surface by physical vapor deposition as the initial step. The pre-treatments were carried out in two ways viz. (i) high temperature diffusion-annealing and (ii) laser annealing of the surface. In the third method the specimen was modified by ion-nitriding. The oxidation behavior of these modified steels was compared with stainless steel in the as-received condition to exactly gauge the effect of surface modification. The oxidation experiment was carried out at 1123 K for 3.6 Ms. The oxidation was interrupted at specific time intervals to examine the mass change of the specimen and mass of the spalled oxide. The results indicated that aluminizing followed by heat-treatment and laser-treatment showed significant improvement in the adherence of the scale. On the other hand the bare and nitrided specimens showed similar behavior characterized by intermittent spallation and poor adherence. Nitriding resulted in the increase in the surface hardness. The post-oxidation examinations were carried out using SEM, EDAX and GIXRD. The uncoated specimen showed the presence of uniform oxide layer and contained oxides of iron and chromium. The aluminum-coated specimen showed the presence of adherent scale. The surface showed a nodular structure due to the formation of pits and zones of enhanced oxidation. The major part of the oxide was alumina with small fractions of chromium and iron oxides. The nodular region was enriched in the oxide of iron. The analysis of the surface by GIXRD revealed the nature of different phases formed on the surface. The oxide on the oxidized bare metal was Cr₂O₃, aluminized (non-oxidized) AlFe and Al₂Fe, aluminized and oxidized Al₂O₃ and FeCr₂O₄, aluminized and laser-treated Al₂O₃, FeCr₂O₄, Fe₂O₃ and Fe₃O₄, nitrided Fe₂O₃ and FeCr₂O₄. In the light of the above results the mechanism of scale failure has been proposed.     

Effect of Pretreatment of Steel 10 in a Saturated Aqueous NH4F on Its High-Temperature Oxidation in Air

Pretreatment of steel 10 in a saturated aqueous solution of NH₄F substantially affects its high-temperature (250–850°C) oxidation in air. During the oxidation, the pretreated steel becomes covered with a heat-resistant film mainly consisting of Fe₂O₃ or Fe₂O₃ + Fe₃O₄; however, the film can either promote or impede steel oxidation depending on the isothermal exposure time.     

Investigation of the Evolution of the Oxide Scale Formed on 310 Stainless Steel Oxidized at 600 °C in Oxygen with 40% Water Vapour Using FIB and TEM

Detailed microstructure investigations were performed on oxide scales formed on 310 stainless steel exposed isothermally at 600 °C to O₂ with 40% water vapour for 1–336 h. FIB microscopy was used to study the evolution of the surface morphology and to prepare cross-section TEM thin foils of the oxide scales. The foils were investigated by analytical transmission electron microscopy. The results showed that a thin protective base oxide scale had formed after 1 h. Due to Cr loss from the oxide scale through water vapour induced Cr evaporation, local breakaway oxidation occurs, resulting in the formation of oxide nodules. The development of these nodules depends on whether a new Cr-rich healing layer is formed or not. A model for the evolution of the oxide scale is proposed based on the results regarding the composition and distribution of various phases in the oxide scale and subjacent steel.     

Short Term Oxidation Behavior of Si-Free Low-Cr Alloy Sputtered Coating

A sputtered coating of a low-Cr alloy without Si was deposited on the cast alloy with the same composition. The short term (100 h) oxidation behavior of the sputtered coating and the cast alloy was evaluated in air at 800 °C. The results indicated that the sputtered coating exhibited a higher oxidation resistance than the cast alloy. It was found that the mass gain of the cast alloy increased continuously with oxidation time and was higher than that of the sputtered coating, which demonstrated only a slight increase in mass gain with oxidation time after 5 h thermal exposure. During the initial thermal exposure of 0.5 h, the oxide scale formed on the cast alloy consisted of Fe₂O₃ and (Fe,Co,Cr)₃O₄ spinel with a small amount of Cr. However, (Fe,Co,Cr)₃O₄ spinel and Fe₂O₃ were thermally grown on the sputtered coating. After oxidation for 100 h, the oxide scale formed on the cast alloy consisted of Co₃O₄ and (Fe,Co)₃O₄ with internal oxide of Cr, while a double-layer oxide consisting of an outer (Fe,Co,Cr)₃O₄ spinel layer and an inner Cr₂O₃ layer was developed on the sputtered coating.     

High Temperature Oxidation of the Austenitic (35Fe27Cr31Ni) Alloy Sanicro 28 in O2 + H2O Environment

The present study investigates the high temperature oxidation of alloy Sanicro 28 (35Fe27Cr31Ni) in 5% O₂ and in 5% O₂ + 40% H₂O. Polished steel coupons were isothermally exposed in a tube furnace at 600, 700 and 800 °C for up to 168 h. The samples were investigated by gravimetry, grazing angle X-ray diffraction (XRD), Auger electron spectroscopy (AES), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy/energy dispersive X-rays (STEM/EDX). The results show that the material forms a protective scale in both environments. The scale is duplex. The inner part of the scale consists of corundum type chromium-rich (Cr _x Fe_1?x )₂O₃, and the outer layer consists of spinel type oxide. Chromia is lost from the protective oxide by vaporization of CrO₂(OH)₂ in O₂ + H₂O environment. The capacity of Sanicro 28 to suffer chromia vaporization without forming a rapidly growing iron-rich oxide is attributed to its high Cr/Fe ratio. The spinel formed at the oxide/gas interface could in addition be beneficial for oxidation behavior in wet oxygen because it may slow down chromia evaporation.     

Oxidation Behavior of HVAF-Sprayed NiCoCrAlY Coating in H<Subscript>2</Subscript>–H<Subscript>2</Subscript>O Environment

Isothermal oxidation behavior of an HVAF-sprayed NiCoCrAlY coating on AISI 304L was studied in an Ar–10 %H₂–20 %H₂O environment at 600 °C. Techniques such as BIB/SEM, EDS, and XRD were used to comprehensively characterize the coating and the coating/substrate interface to investigate the oxidation mechanisms. Results were also compared with those obtained from an uncoated AISI 304L substrate. The alumina-forming NiCoCrAlY coating was found to exhibit superior oxidation behavior due to the formation of a slow-growing and protective Al₂O₃ scale, while the chromia-forming bare 304L substrate lost its protective capability due to the formation of a duplex [Fe₃O₄ on (Fe,Cr)₃O₄ spinel oxide] corrosion product layer.     

Cyclic Oxidation Kinetics and Oxide Scale Morphologies Developed on the IN600 Superalloy

The oxidation behavior of the IN600 Ni?CCr?CFe superalloy was investigated in air at temperatures ranging from 750 to 950 °C, for up to 12 cycles. Oxidation kinetics and oxide scale morphologies were examined using weight gain measurements, SEM-EDS, and X-ray diffraction. The cyclic oxidation kinetic results suggested that the oxidation behavior of the IN600 alloy approximately followed a sub-parabolic rate and the scaling process was controlled by the formation of a chromia scale. At 850 °C, SEM-EDS observations indicated that the formed oxide scale was primarily composed of Cr₂O₃, and the internal oxidation of Cr and Ti occurred. At 950 °C, a fast initial stage with high weight gain was observed, followed by a steady-state stage with gradual weight gain. Additionally, a considerable change in the oxidation kinetic occurred. SEM-EDS observations and XRD results indicated that the external scale was relatively thick with a localized porous, preferential adherent, and a complex oxide scale was developed. This complex oxide scale consisted of an outermost thin layer composed of MnCr₂O₄?CCr₂O₃ mixed together with a small amount of isolated TiO₂, an intermediate relatively thick layer, composed of Cr₂O₃, and an innermost discrete layer formed at the scale/alloy interface, which enriched by Ni/NiO mixed with Ti-, Al-, and Fe-oxides. Finally, only the Al alloying element was internally oxidized to form Al₂O₃ fingers, which create a discrete and narrow internal oxidation zone. Al oxide was observed as a dark area and primarily grows along the alloy grain boundaries in the vicinity of the inward chromia pegs.     

High-Temperature Oxidation of Fe3Al and Fe3Al–Zr Intermetallics

The oxidation behavior of Fe₃Al and Fe₃Al–Zr intermetallic compounds was tested in synthetic air in the temperature range 900–1200 °C. The addition of Zr showed a significant effect on the high-temperature oxidation behavior. The total weight gain after 100 h oxidation of Fe₃Al at 1200 °C was around three times more than that for Fe₃Al–Zr materials. Zr-containing intermetallics exhibited abnormal kinetics between 900 and 1100 °C, due to the presence and transformation of transient alumina into stable α-Al₂O₃. Zr-doped Fe₃Al oxidation behavior under cyclic tests at 1100 °C was improved by delaying the breakaway oxidation to 80 cycles, in comparison to 5 cycles on the undoped Fe₃Al alloys. The oxidation improvements could be related to the segregation of Zr at alumina grain boundaries and to the presence of Zr oxide second-phase particles at the metal–oxide interface and in the external part of the alumina scale. The change of oxidation mechanisms, observed using oxygen–isotope experiments followed by secondary-ion mass spectrometry, was ascribed to Zr segregation at alumina grain boundaries.     

Effect of Chromium Content on the Oxidation Behaviour of Ferritic Steels for Applications in Steam Atmospheres at High Temperatures

Environments containing water vapour are common in many industrial processes, such as power generation systems. Hence, long-term oxidation (1000 h) of P-91 and AISI 430 was studied at 650 and 800 °C, in 100% H₂O atmosphere. The oxidation resistance of the AISI 430 is better than that of the P-91, due to the formation of protective phases on the surface. At 650 °C, a scale composed of Fe₃O₄, Fe₂O₃ and (Fe,Cr)₃O₄ is formed on P-91, although at 800 °C the scale is mainly composed of Fe₃O₄ and (Fe,Cr)₃O₄. On the other hand, on AISI 430 the scale is composed mainly of (Fe,Cr)₂O₃ at 650 °C, and at 800 °C a layer of Cr₂O₃ is formed and remains owing to the higher diffusion rate of Cr at this temperature than at 650 °C, the latter of which compensates the Cr depletion by the degradation of the chromia scale.