Corrosion of 9Cr Steel in CO2 at Intermediate Temperature III: Modelling and Simulation of Void-induced Duplex Oxide Growth

9Cr–1Mo steel forms in CO₂ at 550?°C a duplex oxide layer containing an outer magnetite scale and an inner Fe–Cr rich spinel scale. The inner spinel oxide layer is formed according to a void-induced oxidation mechanism. The kinetics of the total oxide growth is simulated from the proposed oxidation model. It is found that the rate limiting step of the total oxide growth is iron diffusion through high diffusion paths such as oxide grain boundaries in the inner Fe–Cr rich spinel oxide layer. In the proposed oxidation model, a network of nanometric high diffusion paths through the oxide layer allows the very fast supply of CO₂ inside pores formed at the oxide/metal interface. Its existence is demonstrated to be physically realistic and allows explaining several observed physical features evolving in the oxide layer with time.     

Etching of oxide scales on chromium ferritic steels

Abstract

The oxide scale formed on chromium ferritic steels (21/4–9% Cr) by reaction with water/steam in the temperature range 200–570°c consists of an inner Fe/Cr spinel layer and an outer layer of magnetite. These layers are often indistinguishablefrom one another when the scale is sectioned and viewed under an optical microscope. Possible etching techniques, able to reveal the two layers (often without affecting the metal surface), have been investigated. Successful techniques are listed and the mechanisms involved are discussed in relation to current knowledge of oxide dissolution and passivation of metal surfaces.     

Corrosion of 9Cr Steel in CO2 at Intermediate Temperature II: Mechanism of Carburization

In parallel to the formation of a duplex oxide scale, 9Cr–1Mo steel carburizes strongly under CO₂ at 550?°C and this carburization accelerates with time. It is observed that an increase of the total CO₂ pressure in the environment from 1 to 250 bars induces a higher carbon deposition in the inner Fe–Cr rich spinel oxide layer. In order to explain this phenomenon, modelling of the carburization process was carried out. A mechanism involving gas diffusion of CO₂ and CO through the oxide layer, the Boudouard reaction and carbon diffusion through the metallic substrate is proposed.     

Modelling of the oxide scale formation on Fe-Cr steel during exposure in liquid lead-bismuth eutectic in the 450–600 °C temperature range

Previous studies showed that the oxidation of T91 (Fe-9Cr martensitic steel) in liquid Pb-Bi eutectic leads to the formation of a duplex oxide layer containing an inner Fe_2.3Cr_0.7O₄ spinel layer and an upper magnetite layer. The magnetite layer is easily removed by the Pb-Bi flow when the oxygen concentration is low and the flow velocity is high. This phenomenon is not currently understood. The magnetite layer growth rate is limited by the iron diffusion in the oxide layer lattice. The Fe-Cr spinel layer grows in the nanometric cavities formed at the Fe-Cr spinel /T91 interface by the outwards diffusion of iron. Due to this mechanism the growth rate of the Fe-Cr spinel layer is linked to that of magnetite. A modelling of this mechanism is presented. The modelling is in agreement with the experimental data in the case of a high oxygen concentration. However, the calculated oxide scale thicknesses are systematically lower than the experimental values in the case of a low oxygen concentration when the iron diffusion only occurs via interstitials in the oxide scale. Consequently, the estimation of the iron diffusion coefficient, when diffusion occurs via interstitials, is not reliable. To have a better estimation of this iron diffusion coefficient in the Fe-Cr spinel, when diffusion occurs via interstitials, a fit is done using experimental data coming from the European DEMETRA project. Although this evaluation is only based on a fit on the experimental data, it permits to estimate the oxide layer growth kinetics in case of the formation of the described duplex oxide layer, for each oxygen concentration (leading to a vacancy and/or an interstitial diffusion), each temperature between 450 and 620 °C and each hydrodynamic flow. This model shows that the hydrodynamic flow affects the corrosion rate only by the removal of the upper magnetite layer leading to an increase of the oxygen concentration at the spinel/magnetite interface. The oxidation mechanism is thus neither changed by the Pb-Bi flow nor by the oxygen concentration. However, the oxygen concentration modifies the iron diffusion process in the oxide lattice.     

The mechanism of corrosion of Fe-9%Cr alloys in carbon dioxide

Oxide layers have been grown on Fe-9% Cr, Fe-9% Cr-0.3% Si, and Fe 9% Cr-0.6% Si alloys in carbon dioxide at 853 °K. It is known that such oxides are duplex, the outer layer being magnetite, formed by iron transport. The inner layer is Fe-Cr spinel but little is known about its growth mechanism so this has been investigated using oxygen-18 as a tracer. Oxides were grown first in C¹⁶O₂ and then in C¹⁸O₂ and the distribution of oxygen-18 in the scale measured using nuclear techniques. For all the alloys used, significant amounts of oxygen-18 were observed within the inner layer in addition to growth of¹⁸O-rich magnetite at the outer surface. The two possibilities of the oxygen-18 being present as a consequence of isotopic exchange or because new oxide had formed within the spinel layer are discussed. Our conclusion is that it is very unlikely that significant isotopic exchange had occurred in any part of the scale, and we deduce that at least a substantial amount of the oxygen-18 in the inner layer was deposited as a result of new oxide formation within that layer. The results also indicate that the location of growth sites within the inner layer differed between the alloys.     

Investigation of Isothermal and Cyclic Oxidation of Plasma-Nitrided Hot-Work Tool Steel at Elevated Temperatures

The oxidation of a plasma-nitrided, hot-work tool steel at temperatures that cover a range of operations from post-plasma-nitriding oxidation to steel thixoforging processing was investigated. Thermal exposure at 500 °C led to the formation of a thin Fe?CCr spinel layer and an even thinner outermost layer of hematite. The former is the only oxide that grew on samples exposed to oxygen-lean conditions at 750 °C. A thick, multi-layered oxide scale formed on the surface when the plasma nitrided hot-work tool steel was held at 750 °C under atmospheric conditions. In this scale, the outermost hematite layer and the inner Fe?CCr spinel were separated by a magnetite layer. The oxide scale produced during thermal cycling at 750 °C was also multi-layered with an identical oxide scale configuration to that formed during isothermal exposure at 750 °C. The hematite layer, which retained its integrity during isothermal exposure at 750 °C, suffered small cracks that were instrumental in its fracture and spallation during thermal cycling. The distinct feature resulting from cyclic oxidation, however, was the wide gap that formed along the magnetite?Cspinel interface. Thermal expansion mismatch produced compressive stresses which in turn led to buckling of the magnetite layer and to its detachment; while, the spinel layer adhered to the tool steel substrate and survived throughout thermal cycling. Enrichment of nitrogen and the subsequent precipitation of N₂ gas were also believed to have contributed to the gap formation. Formation of such a gap poses a serious threat to the integrity of the oxide scale and was shown to be responsible for the spallation of the magnetite layer upon thermal cycling.     

Mechanism of Magnetite Seam Formation and its Role for FeO Scale Transformation

The phase transformation behavior of a thermally grown oxide scale of FeO on pure-Fe and an Fe–2wt%Au alloy was investigated. Particular attention was paid to formation of a magnetite seam, which is the Fe₃O₄ layer formed at the FeO/alloy interface at an initial stage of the phase transformation, since it has important effects on the overall phase transformation of FeO scale. A thin Au(Fe) layer was found to develop on the Fe–Au alloy at the FeO/alloy interface after 32 min of oxidation at 750 °C in air. This Au(Fe) layer prevented formation of a magnetite seam and accelerated the FeO eutectoid reaction. The Au(Fe) layer acted as a “chemical diffusion barrier” for inward diffusion of Fe from the FeO to the alloy substrate across the FeO/alloy interface and prevented magnetite seam formation.     

Oxide-formation characteristics in the early stages of oxidation of Fe and Fe-Cr alloys

Electron microscopy investigations have been conducted on the oxides formed on Fe and Fe-Cr alloys at elevated temperatures (700–800?C) and at low oxygen partial pressures (～10^?3Pa). Oxide nucleation and growth on chromium-rich iron alloys are significantly different from that for pure iron. On pure Fe and on Fe-3%Cr, wustite and magnetite particles nucleate and grow out of the surface, while on the higher Cr-containing alloys (≥9 wt. % Cr) the spinel oxide (Fe, Cr)₃O₄ nucleates and grows into the metal. The differences in oxide formation in the early stages of oxidation are explained in terms of the diffusion of different species being rate-controlling and in terms of rapid diffusion, for example, at the metal-oxide interface.     

Oxidation mechanism of an Fe-9Cr-1Mo steel by liquid Pb-Bi eutectic alloy at 470 °C (Part II)

This paper is the second part of a global study on the oxidation process of an Fe-9Cr-1Mo martensitic steel (T91) in static liquid Pb-Bi. It focuses on the growth mechanism of a duplex oxide scale. The oxide layer has a duplex structure composed of an internal Fe-Cr spinel layer and an external magnetite layer. The magnetite layer grows by iron diffusion until Pb-Bi/oxide interface whereas the Fe-Cr spinel layer grows, at the metal/oxide interface, inside the space kept “available” by the iron vacancies accumulation due to iron outwards diffusion for magnetite formation. This growth mechanism is close to the “available space model”. However, this model is completed by an auto-regulation process based on oxygen supply.     

KCl Induced Corrosion of a 304-type Austenitic Stainless Steel at 600°C; The Role of Potassium

The influence of KCl on the oxidation of the 304-type (Fe18Cr10Ni) austenitic stainless steel at 600°C in 5% O₂ and in 5% O₂ + 40%H₂O is investigated in the laboratory. The samples are coated with 0.1 mg/cm² KCl prior to exposure. Exposure time is 1–168 h. Uncoated samples are exposed for reference. The oxidized samples are analyzed by ESEM/EDX, XRD and AES. The results show that small additions of potassium chloride strongly accelerate high temperature corrosion, the oxide thickness being up to two orders of magnitude greater after exposure in the presence of KCl. The rapid corrosion is initiated by the formation of potassium chromate through the reaction of KCl with the protective oxide. Chromate formation is a sink for chromium in the oxide and leads to a loss of its protective properties. The resulting rapidly growing scale consists of an outer hematite layer with embedded K₂CrO₄ particles and an inner layer consisting of spinel oxide, (Fe,Cr,Ni)₃O₄. Little or no chlorine is found in the scale or at the scale/metal interface.     

Pre-oxidation Effect on Oxidation Behavior of F91 in Carbon Dioxide at 550 °C

Upon exposure to CO₂ at 550 °C, F91 tends to form rapidly growing scales consisting of an outer Fe oxide and an inner Fe–Cr spinel oxide. A comparative study has been carried out between the pre-oxidized and non-pre-oxidized F91, to determine the influence of pre-oxidation upon the oxidation behavior of F91 in CO₂. Formation of a rapidly growing scale and carburization could be inhibited by a pre-oxidation treatment in air prior to oxidation in CO₂. Although during exposure to CO₂, a fast growing scale still would form locally, pre-oxidation changed its structure. Effects of pre-oxidation time on the oxidation resistance in CO₂ are investigated.     

Oxidation behaviour of Nimonic 263 in high-temperature supercritical water

The oxidation tests of the Nimonic 263 alloy exposed to deaerated supercritical water at 600–700°C under 25?MPa were carried out for up to 1000?h. Oxidation rate increased with an increase in temperature. The microstructure and phase composition of oxide scale were analysed by scanning electron microscopy/energy dispersive X-ray spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. It can be seen that a complex oxide structure formed on the surface of Nimonic 263 including an outer layer of Ni–Fe/Ni–Cr spinel oxide, Ni/Co hydroxide and TiO₂ and an inner layer of a mixture of NiCr₂O₄ and Cr₂O₃ while the innermost layer is made up of Cr₂O₃. The MoO₃ can be observed at 600°C but disappeared with the increasing temperature. The growth mechanism of oxide scale was discussed.     

High-Temperature Oxidation Behavior of SIMP Steel at 800 °C

In this work, the high-temperature oxidation behavior of SIMP and commercial T91 steels was investigated in air at 800 °C for up to 1008 h. The oxides formed on the two steels were characterized and analyzed by XRD, SEM and EPMA. The results showed that the weight gain and oxide thickness of SIMP steel were rather smaller than those of T91 steel, that flake-like Cr₂O₃ with Mn_1.5Cr_1.5O₄ spinel particles formed on SIMP steel, while double-layer structure consisting of an outer hematite Fe₂O₃ layer and an inner Fe–Cr spinel layer formed on T91 steel, and that the location of the oxide layer spallation was at the inner Fe–Cr spinel after 1008 h, which led the ratio between the outer layer and the inner layer to decrease. The reason that SIMP steel exhibited better high-temperature oxidation resistance than that of T91 steel was analyzed due to the higher Cr and Si contents that could form compact and continuous oxide layer on the steel.     

The mechanism of oxide film formation on Alloy 690 in oxygenated high temperature water

Oxide films formed on Alloy 690 exposed to 290 °C water containing 3 ppm O₂ were investigated. It was found that Cr rich oxides form initially through solid-state reactions. Ni–Fe spinels gradually develop on surface layer by precipitation with increasing immersion time. Initially formed Cr rich oxides react with outwards diffusing Ni and Fe to form small spinel particles which then vanish gradually. An inner layer develops from oxide/matrix interface through inward diffusion of oxidant. Cr is preferentially oxidized and tends to dissolve into solution. The resultant inner layer consists of predominant NiO which cannot serve as a protective barrier layer.     

Structural mechanism of scale for 12Cr–W–Mo–Co steels with enhanced oxidation resistance

The phase constituent, morphologies, layer structures of the scale of 12Cr–W–Mo–Co heat resistant steel (HRS) formed in dry air and air with 10% vapour were systematically investigated. The interface between the scale and ferritic/martensitic matrix of this HRS was also studied. For the scale formed in air, single particle- and sheet-shaped oxide layer, which are composed of (Fe, Co, Cr)₂O₃, were formed. The scale combines with steel matrix via coherent or semi-coherent structure. For the scale formed in air with 10% vapour, the oxides take the shape of particulate. Layering phenomenon has been observed, i.e. the external layer is composed of (Fe, Co)₂O₃/(Fe, Co)₃O₄, the internal layer with spinel (Fe, Co, Cr)₃O₄ and the transition layer with Cr rich and Cr poor regions. The interface between the transition layer and the matrix is tight and steady, but the region linking the internal scale and transition layer is shaky and brittle. Both the oxidation processes in air and air with 10% vapour are considered to be controlled by diffusion mechanism.     

Reactive molecular dynamics study of the oxidation behavior of iron-based alloy in supercritical water

A theoretical study of the oxidation behavior of iron-based alloy in the supercritical water (SCW) has been carried out based on ReaxFF force-field molecular dynamics simulation. An atomic model has been proposed to simulate the initial chemisorption reactions and atoms diffusion behavior across the oxide layer. Simulation results imply that Cr addition has an important effect on the oxidation behavior of iron-based alloy. In the initial stage of oxidation, H₂O prefers to adsorb on the Cr atom, and some species in the form of Cr(OH)₄ are observed on the FeCr alloy surface. Once an initial oxide layer is formed, further oxidation is controlled by the migration of vacancy. The O vacancies are formed at the oxide/FeCr alloy interface and migrate toward the steam, whereas Fe vacancies are formed at the oxide/steam interface and migrate toward the FeCr alloy. Attributed to the stronger binding energy of O–Cr bond than O–Fe bond, the Cr diffusivity in the oxide is less than Fe atoms. Thus, double oxide layers, including the inner Fe–Cr–O layer and outer Fe–O layer, are formed on the FeCr alloy, which is in good agreement with previous experimental observation.     

Oxidation mechanism of a Fe-9Cr-1Mo steel by liquid Pb-Bi eutectic alloy (Part I)

This paper is the first part of a global study on the oxidation process of a Fe-9Cr-1Mo martensitic steel (T91) in static liquid Pb-Bi. It focuses on the oxygen transport mode across the oxide scale. The oxide layer has a duplex structure composed of an internal Fe-Cr spinel layer and an external magnetite layer. Oxygen 18 tracer experiments are performed: they show that the magnetite layer grows at the Pb-Bi/ oxide interface whereas the Fe-Cr spinel layer grows at the metal/oxide interface. Oxygen seems to diffuse across the oxide scale dissolved inside nanometric lead penetrations called nano-channels. Specific experiments are performed to characterize the nano-channels.     

In Situ Oxide Growth Characterization of Mn-Containing Ni–25Cr (wt%) Model Alloys at 1050 °C

The oxidation behaviour of four model alloys with the composition Ni–25Cr–xMn (with x = 0, 0.5, 1 and 1.5 wt%) was investigated at 1050 °C in air by thermogravimetry and by in situ observations in an environmental scanning electron microscope (ESEM). The addition of manganese modifies the oxidation rate of Ni–25Cr alloys by (1) increasing the parabolic constant k _p compared to that for Ni–25Cr and (2) lowering the short-time oxidation rate. Regardless of the Mn concentration, in situ ESEM observations indicated the formation of spinel crystallites from the very beginning of the oxidation process. The size of the spinel crystallites was directly linked to the initial Mn concentration. The obtained results suggested that the formation of the spinel layer at the top of the chromia oxide scale rather than at the metal–oxide interface as thermodynamically expected must be attributed to the higher diffusion rate of Mn than Cr both in the lattice and at the grain boundary.     

Oxidation of Fe-8Cr-14Ni-Al-Si-Mn from 700 to 1000°C

This paper reports an investigation into reducing the Cr concentration in commercial-grade stainless steels while maintaining oxidation protection at elevated temperatures. Aluminum and Si were added as partial substitute alloy elements to enhance the reduced operation protection resulting from Cr concentration reduced by approximately 50 pct of that found in stainless steels. The goal of this study was to determine the oxidation mechanism of such an Fe, Al-Si alloy: Fe-8Cr-14Ni-1Al-3.5Si-1Mn. During the initial oxidation period the protection resulted from a thin film of Al₂O₃ over an Fe and Cr spinel. Long-term oxidation protection resulted from the gradual formation of a Cr sesquioxide (Cr₂O₂) inner oxide layer. Eventually an outer oxide layer formed that was a mixed composition spinel of Cr and Mn (MnO · Cr₂O₃). The Al₂O₃, which was part of the original protective layer flaked off early in the oxide testing, and the aluminum oxide that formed later appeared as an internal oxide precipitate.     

Corrosion characteristics of T91 steel in lead–bismuth eutectic with different oxygen concentrations at 500°C

Corrosion tests of T91 were carried out in the stagnant lead–bismuth eutectic (LBE) containing 10^?6, 10^?7, and 10^?8 wt% oxygen at 500°C for up to 2000 h, respectively. The dependence of corrosion characteristics on oxygen concentrations in LBE was obtained. The results show that the oxide-scale structure changes from a three-layer magnetite/spinel/internal oxidation zone (IOZ) scale under the oxygen concentration of 10^?6 wt%, to the formation of a two-layer spinel/IOZ scale under the oxygen concentrations of 10^?7 and 10^?8 wt%. In addition, with the decrease of the oxygen concentration, the Cr/Fe ratio in the Fe–Cr spinel slightly increases in a linear rule while the thicknesses of the oxide layers gradually decrease.