Cr-rich Oxide Layer Research Articles

Cr doping and heat treatment effect on core–shell Ni nanocluster film

Core–shell nickel (CS-Ni) and 5 at.% chromium-doped nickel (CS-Ni5Cr) nanocluster (NC) films, prepared by a nanocluster deposition system, were studied for heat treatment (HT)-induced structural, physical, and magnetic property alterations. Understanding the HT influence and oxidation mechanism at nanoscale can make these nanomaterials potential candidates for applications that involve stainless steel alloys. The contribution of Cr doping in altering the microstructural and relative oxidation kinetics was investigated in detail before and after the HT. The oxidation mechanism describes that the cation diffusivity increases following the doping of 5 at.% of Cr in Ni, which makes the oxidation rate of Ni5Cr HT higher than that of Ni-HT. At a temperature of 600 °C, a dramatic change was observed in surface morphology with many island-like nanostructures on the surface of Ni5Cr. The interface structure of the Cr-rich oxide layer plays a key role in the islands formation via agglomeration of NCs. The as-prepared and HT samples were analyzed by transmission electron microscopy, atomic force microscopy, magnetic force microscopy, energy-dispersive spectroscopy, and vibrating sample magnetometer to provide an insight on the effectiveness of chromium-doped nickel film.

Effects of distribution and growth orientation of precipitates on oxidation resistance of Cu–Cu12–[Crx/(12+x)Ni12/(12+x)]5 alloys

Abstract

The Role of Grain Boundaries during the Initial Oxidation Stages in Cu-Added Austenitic Stainless Steel at 700 °C Studied at the Atomic Scale

The early stages of the oxidation behavior (≤ 12 h, in air with 20% water vapor at 700°C) of Cu-added austenitic stainless steels with various grain sizes was investigated using atom probe tomography (APT), transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), electron probe microanalyzer (EPMA) and X-ray diffraction (XRD). A thin Cr-rich oxide layer is formed over the entire surface of a small-grained (8 µm) sample, which is similar to that formed near grain boundaries of medium-grained (17 µm) and large-grained (27 µm) samples. Oxidation of all samples proceeds via the lattice and grain boundary diffusion of Cr, leading to the formation of a protective Cr-rich oxide first at the grain boundaries and then via lateral growth toward the grain interiors. APT and TEM studies clearly reveal that within 4 μm of the grain boundaries, the oxide layer exhibits a duplex-layer structure consisting of a thin Fe-rich (Fe,Cr)3O4 oxide (~55 nm) above and a protective Cr2O3 oxide (~40 nm) below as a diffusion barrier. In contrast, further away from the grain boundaries, a non-protective Fe-rich (Fe,Cr)3O4 oxide (~160 nm) and a Cr-rich (Fe,Cr)3O4 oxide (~40 nm) are formed as the outer and inner layers, respectively. The oxidation kinetics was studied in terms of mass gain measurements. The critical grain size (< 8 µm) to prevent the formation of fast-growing, non-protective Fe-rich oxides is discussed based on the experimental findings.

Photoelectrochemical and XPS characterisation of oxide layers on 316L stainless steel grown in high-temperature water

Passive films on AISI 316L stainless steel were grown by exposure in high temperature (300 °C and 150 bar) water. X-ray photoelectron spectroscopy was employed to study their composition as a function of immersion time. A photoelectrochemical investigation, supported by electrochemical and impedance measurements, allowed to get information on the solid-state properties of the investigated layers. The experimental results suggest the formation of a stratified layer with an outer iron-rich layer and an inner Cr-rich oxide layer, whose relative thickness and composition are dependent on the immersion time.

A Comparative Study on the Oxidation of Fe-25Cr-7Ni-4Mo and Fe-25Cr-7Ni-2Mo-4W Steels

Stainless steel materials (FeCr and FeCrNi-based alloys) are employed in a wide range of modern applications due to their ability to withstand corrosive environments while maintaining good mechanical properties. Their corrosion resistance originates from Cr-rich oxide layer which serves as a barrier against ion diffusion between the alloy and the ambient phase. Custom steel grades can be designed for specific applications by optimizing their properties throughout alloy composition [1].

Corrosion behavior of F82H exposed to high temperature pressurized water with a rotating apparatus

The present study reports the corrosion behavior of a reduced-activation ferritic martensitic steel F82H exposed to high temperature pressurized water for 28 and 100h using a rotating disk apparatus at rotation speeds of 500 and 1000rpm at a temperature of 573K under a water pressure of 15MPa with <20ppb dissolved oxygen. Static tests without rotation increased the weight of sample with time, whereas rotating tests decreased the weight of sample with the exposure time and rotating speed. Oxide particles in the size of several micron meters on the surface of static samples were not observed on the rotated samples which became covered with a thin Cr-rich oxide layer. These results suggest an occurrence of erosion–corrosion and/or flow-accelerated corrosion of F82H under the rotating condition.

The effect of Mn1.5Co1.5O4 coatings on the development of near surface microstructure for Haynes 230 oxidized at 800 °C in air

The effect of reactively sintered Mn1.5Co1.5O4 (MCO) coatings on the development of surface microstructure for Haynes 230 (H230) oxidized in air at 800°C has been studied using a combination of thermo-gravimetric analysis and electron microscopy techniques. The bare alloy exhibits a parabolic rate constant of 8.8×10−9mg2cm−4s−1, and forms a two-layer oxide scale with a continuous chromia layer and a thinner discontinuous MnCr2O4 overlayer. For the MCO-coated H230, the reduction step of the reactive sintering process converts the MCO coating to a mixture of Co and MnO with a thin Cr-rich oxide layer at the interface with the alloy substrate. Following the re-oxidation step, there is a 200nm chromia layer and a 400nm cubic spinel reaction layer (RL) between the alloy and the MCO. These layers thicken to 800nm and 1.2μm, respectively after 1000h oxidation. These observations are compared to our previous studies of MCO-coated Crofer 22 APU, and the implications for long-term SOFC performance are discussed.

EFFECTS OF TEMPERATURE ON LECTROCHEMICAL CORROSION OF DOMESTIC NUCLEAR-GRADE 316L STAINLESS STEEL IN Zn-INJECTED AQUEOUS ENVIRONMENT

The effects of temperature(7) on oxide films for 316L stainless steel(316L SS) in borated and lithiated water without and with Zn injection were investigated by in situ potentiodynamic polarization curves and electrochemical impedance spectra(EIS).The protective property of oxide films in the Zn-free/Zn-injected solution degraded with increasing temperature.With increasing temperature,the structure of the oxide film varied from a single layer to double layers,the Cr-rich oxide layer played a key role on retarding further oxidation.Compared to the Zn-free case,the corrosion rate decreased significantly in the Zn-injected solution due to the formation of compact Zn-bearing oxide films,but the instinct growth mechanism of the oxide films remained unchangeable.The solubilities and structure model of oxides were proposed to discuss the formation mechanism of oxide films on316L SS in the hydrothermal solution.

Preparation and performances of Co–Mn spinel coating on a ferritic stainless steel interconnect material for solid oxide fuel cell application

Ferritic stainless steels have become the candidate materials for interconnects of intermediate temperature solid oxide fuel cell (SOFC). The present issues to be solved urgently for the application of ferritic stainless steel interconnects are their rapid increase in contact resistance and Cr poisoning. In the present study, a chloride electrolyte suspension has been developed to electro-deposit a Co–Mn alloy on a type 430 stainless steel, followed by heat treatment at 750 °C in argon and at 800 °C in air to obtain Co–Mn spinel coatings. The experimental results indicate that an adhesive and compact Co–Mn alloy layer can be deposited in the chloride solution. After heat treatment, a complex coating composed of an external MnCo2O4 layer and an inner Cr-rich oxide layer has been formed on 430SS. The coating improves the oxidation resistance of the steel at 800 °C in air, especially in wet air, and inhibits the outward diffusion of Cr from the Cr-rich scale. Moreover, a low contact resistance has been achieved with the application of the spinel coatings.

Initiation of Metal Dusting Corrosion in Conversion of Natural Gas to Syngas Studied under Industrially Relevant Conditions

The initial stages of metal dusting corrosion and carbon formation were studied under syngas at industrially relevant conditions. A set of Inconel 601 alloy coupons were preoxidized in 10% steam in N2 at 540 °C for 6 h, followed by exposure to carburizing (ac >1) syngas mixtures. The exposure was performed at 550, 650, or 750 °C at 20 bar for 20 h under a CO/H2/CO2/H2O/Ar mixture of composition 20/25/15/10/30 or 50/25/15/10/0 (%). The resulting surfaces and carbonaceous products were then characterized by means of optical imaging and light optical microscopy, scanning electron microscopy (SEM), depth profile analysis by Auger electron spectroscopy under ion-sputtering and transmission electron microscopy/energy dispersive spectroscopy (TEM/EDS). Little or no carbon formed at 550 °C, and the alloy matrix remained reasonably intact, with some smaller changes occurring within the Cr rich oxide layer covering the underlying metallic bulk. The higher carbon formation occurring under the exposure at 650 and 750...

Creep and Oxidation Behaviors of Alloy 617 in Air and Helium Environments at 1173 K

Abstract Creep and oxidation behaviours of Alloy 617 in air and helium (He) environments at 1173K were comparatively investigated under different applied stress levels. There were no large differences in the shapes of the creep curves between the air and He environments. Creep rupture time in the He environment was shorter than that in air. The outer Cr-oxide thickness of the air specimens was thicker in short-tested duration than that of the He specimens. However, in the long- tested duration over 3,000 h, the Cr-oxide thickness in the He environment was larger than in air. It was found that creep rupture life was closely related to the thickness of the outer Cr-oxide layer, because the form of the outer Cr-rich oxide layer brings about the Cr-depleted region which may deteriorate material strength or creep life.

Grain-Size Effects on the High-Temperature Oxidation of Modified 304 Austenitic Stainless Steel

The high-temperature oxidation behavior of modified 304 austenitic stainless steels in a water vapor atmosphere was investigated. Samples were prepared by various thermo mechanical treatments to result in different grain sizes in the range 8–30 μm. Similar Σ3 grain boundary fraction was achieved to eliminate any grain-boundary characteristics effect. Samples were oxidized in an air furnace at 700 °C with 20 % water vapor atmosphere. On the fine-grained sample, a uniform Cr2O3 layer was formed, which increased the overall oxidation resistance. Whereas on the coarse-grained sample, an additional Fe2O3 layer formed on the Cr-rich oxide layer, which resulted in a relatively high oxidation rate. In the fine-grained sample, grain boundaries act as rapid diffusion paths for Cr and provided enough Cr to form Cr2O3 oxide on the entire sample surface.

Mechanism of breakaway oxidation of Fe–Cr and Fe–Cr–Ni alloys in dry and wet carbon dioxide

Model Fe–Cr and Fe–Cr–Ni alloys were exposed to Ar–CO2 and Ar–CO2–H2O gas mixtures at 650°C. While all alloys initially formed protective Cr2O3 scales, nucleation and growth of iron-rich oxide nodules resulted in some cases in breakaway oxidation. The conditions leading to departure from the protective stage are discussed in terms of Cr2O3 thermodynamic and kinetic stability. The morphological and compositional evolutions accompanying nodule development were examined. The influence of carbide precipitation on alloy chromium diffusion and the ability of the alloy to form and maintain Cr-rich oxide layers was investigated.

Influence of CO2 purity on the oxidation of a 12Cr ferritic – martensitic steel at 550°C and importance of the initial stage

It has been shown that ferritic – martensitic (F/M) steels with 12 wt% Cr can form a protective oxide layer depending on the CO2 purity from the first stages of oxidation. The impurities which could explain this difference could be O2. In the case where O2 concentration in the gas phase was significant (>ppm), a thick duplex oxide layer was observed, whereas, if the O2 concentration was low (<ppm), a Cr rich oxide layer about hundred times thinner was formed. These results strongly suggested that O2 reacted before CO2 on the steel surface and that the protective oxide layer was formed by reactions between chromium and O2.

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.

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.

Corrosion of 9Cr Steel in CO2 at Intermediate Temperature I: Mechanism of Void-Induced Duplex Oxide Formation

Under CO2 exposure at an intermediate temperature, typically 550 °C, 9Cr–1Mo steel forms a duplex oxide scale made of an outer magnetite layer and an almost-as-thick inner Fe–Cr rich spinel oxide layer. It is proposed that the inner Fe–Cr spinel layer grows according to a mechanism involving void formation at the oxide/metal interface. The driving force for pore formation is the outward magnetite growth: iron vacancies are injected at the oxide/metal interface then condense into voids. The fresh metallic surface made available is then oxidized by CO2, which diffuses fast through the scale. The physical aspects, the integrity and the nature of the scale are shown to be very dependent on the oxygen potential existing in the environment.

Oxidation behavior and mechanism of powder metallurgy Rene95 nickel based superalloy between 800 and 1000 °C

The oxidation behaviors of powder metallurgy (PM) Rene95 Ni-based superalloy in the temperature range of 800–1000 °C are investigated in air by virtue of isothermal oxidation testing, X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. The results show that the oxidation kinetics follows a square power law as the time extends at each temperature. The oxidation layers are detected to be composed of Cr 2O 3, TiO 2 and a small amount of NiCr 2O 4. The cross-sectional morphologies indicate that the oxidation layer consists of three parts: Cr-rich oxide layer, Cr and Ti duplex oxide layer, and oxidation affected zone. Theoretical analyses of oxidation kinetics and thicknesses of oxidation layers confirm that the activation energy of oxidation of PM Rene95 superalloy is 165.32 kJ mol −1 and the oxidation process is controlled by diffusions of oxygen, Cr, and Ti. Accordingly, a diffusion-controlled mechanism is suggested to understand the oxidation behaviors of PM Rene95 superalloy at elevated temperatures.

A review of the oxidation behaviour of structural alloys in steam

<title/>The focus of this review is the state of knowledge of the oxidation behaviour in steam of alloys with potential for use as pressure parts in steam boilers. The rate of oxide growth on steam-touched surfaces and the characteristics of that oxide are of increasing interest as the quest for improvements in cycle efficiency leads to progressively higher operating temperatures and pressures. The consequences of increased rate of growth of these oxides are of concern because of implications for tube overheating and oxide exfoliation. Mitigation of such problems requires a mechanistic understanding of the influences of alloy composition and microstructure, and especially of the evolution with time of specific scale structures. Similarly, the relative effects of factors such as time, temperature and operating parameters must be understood. The oxidation behaviour of the class of ferritic steels that forms the bulk of the heat transfer surface in steam boilers is of particular importance since alloys in the range 9-12%Cr (% in alloy compositions signifies weight percentage, unless indicated otherwise) are close to a transition from oxidation behaviour based on relatively thick Fe-based scales to the formation of much thinner, Cr-rich oxides. For austenitic steels protective behaviour in steam depends critically on the rapid development of a continuous Cr-rich oxide layer, otherwise oxide growth rates similar to the ferritic steels may result. Understanding the interplay among compositional and microstructural requirements for strengthening and oxidation resistance, and their influence on the rate and mode of scale evolution is key to the most effective application of these alloys. The oxidation behaviour of high-temperature Ni-based alloys in steam has received relatively little attention, but the broad range of alloying additions considered, compared to austenitic steels, has the potential to contribute in different ways to the scale morphologies and oxidation behaviour. Underlying these interests is the apparently significant contribution to oxide growth in steam from inward transport of oxidant species that likely involve hydrogen. The particular species involved and their roles in the oxidation process are expected to exert a large influence on the oxide morphologies developed, while the fate of any hydrogen released in the alloy is a further topic of particular interest.

High-Temperature Corrosion of Steel in an ESEM With Subsequent Scale Characterisation by Raman Microscopy

Stainless steels used as materials for technical processes at elevated temperatures are protected against high temperature corrosion by the formation of a Cr-rich oxide layer