Scale-metal Interface Research Articles

High-temperature oxidation behavior of laser additively manufactured AlCrCoNiSi-based high-entropy alloys at 1100 ℃

High-temperature oxidation behavior of two laser additively manufactured AlCrCoNiSi-based high-entropy alloys (Al10 and Al15) are systematically studied in this work. These two alloys exhibit similar oxidation behaviors involving initial linear kinetics oxidation and subsequent parabolic kinetics oxidation at 1100 ℃. However, Al10 alloy has a superior oxidation resistance than Al15 alloy according to the oxidation rate constant. α-Al2O3 is the predominant phase of the oxide scales grown on both alloys. Cross-sectional views of the scales show that the scales consist of surface equiaxed grains and underlying columnar grains. For all oxidized alloys, a newly-formed fcc layer forms in the subsurface Al-depletion zone due to the preferential oxidation of aluminum. Al10 alloy exhibits a much thicker newly-formed fcc layer than Al15 alloy after the same oxidation time. This work ascribes the superior oxidation resistance of Al10 alloy to its thick newly-formed fcc layer, which suppresses the outward Al diffusion from the substrate to scale-metal interface by increasing the diffusion distance.

Prevention of High-Temperature Surface Degradation in SiMo Cast Irons by Cr and Al Alloying

High silicon molybdenum-alloyed cast irons (SiMo) with spherical graphite are widely used for cast thin-wall exhaust manifolds, worked in high-temperature oxidation environment; therefore, prevention of surface degradation during service is important for such common application. The base SiMo and two alloyed by 1 pct Cr and 3 pct Al cast irons were tested at high temperatures in air and a combustion atmosphere to study the microstructural processes of surface degradation. It was shown that surface degradation of SiMo cast irons involves several microstructural processes, including the formation of multilayered oxide scale and development of internal porosity, resulting in decarburization of graphite nodules. Monitoring weight change of the specimens and Leco C analysis were used for decoupling the rates of surface oxidation and metal matrix decarburization. SEM/EDX analysis was used to study the phase composition of scale and high-resolution TEM analysis was performed to determine the detailed information about the atomic structure of protection layers formed on scale/metal matrix interface. The results were used to qualify and link different surface degradation mechanisms. In the base SiMo cast iron, the protection efficiency of formed amorphous silica layers is limited by intensive decarburization at temperatures above 700 °C. It was shown that Cr and Al alloying prevents surface degradation and suppresses decarburization by developing thermally stable oxide films on metal-scale interface, thus allowed to increase a safe working temperature up to 800 °C in air and the combustion atmosphere.

Oxidation of porous stainless steel supports for metal-supported solid oxide fuel cells

Oxidation behavior of porous P434L ferritic stainless steel, used for the fabrication of metal-supported solid oxide fuel cells (MS-SOFC), is studied under anodic and cathodic atmospheres. Temperature- and atmosphere-dependence is determined for as-sintered and pre-oxidized stainless steel. Pre-oxidation reduced the long-term oxidation rate. For pre-oxidized samples, the oxidation rate in air exceeds that in humid hydrogen for temperatures above 700 °C. The influence of PrOx, LSCF-SDC, and Ni-SDC coatings is also examined. The coatings do not dramatically impact oxide scale growth. Oxidation in C-free and C-containing anodic atmospheres is similar. Addition of CO2, CH4, and CO to humidified hydrogen to simulate ethanol reformate does not significantly impact oxidation behavior. Cr transpiration in humid air is greatly reduced by the PrOx coating, and a PrCrO3 reaction product is observed throughout the porous structure. A dense and protective chromia-based scale forms on steel samples during oxidation in all conditions. A thin silica enriched oxide layer also forms at the metal-scale interface. In general, the oxidation behavior at 700 °C is found to be acceptable.

STEM investigations of γ-TiAl produced by additive manufacturing after isothermal oxidation

Short-term isothermal oxidation of TiAl 48-2-2 alloy produced by Electron Beam (EBM) was investigated at 750, 800, 850 and 900 °C with special focus on the oxidation kinetics and metal-scale interface evolution. It was shown that at the lowest oxidizing temperature a passivating and thin mixture of alumina and titania is formed with subsurface Z-phase formation. At higher temperatures a porous and non-protective oxide scales formed with subsequent nitridation at the metal-scale interface. Moreover, long-term isothermal oxidation at 750 °C up to 3000 h in 500 h intervals was performed and related to the initial band microstructure of the alloy resulting from EBM process. Internal oxidation was observed on lamellar α2 / γ colonies while α-alumina mixed with TiN and Ti2AlN was found to form on equiaxed γ-TiAl regions. Analytical Scanning Electron Microscopy (STEM) aided with Energy Dispersive Spectroscopy (EDS) and Electron Energy Loss Spectroscopy (EELS) were used for investigations of the oxide scales as well as nitride layers formation at different temperatures.

High temperature oxidation behavior of C103 alloy with boronized and siliconized coatings during 1000 h at 1100 °C in air

The paper presents the results of investigations concerning the formation and high temperature oxidation behavior of silico-boronized coatings on C103 Nb-base alloy. The coatings were obtained by boronizing and subsequent siliconizing using pack cementation method which resulted in the formation of inner NbB2 and outer NbSi2 layers. The coated C103 alloy was subject to isothermal oxidation test at 1100 °C for 1000 h during which the coating provided protection against high temperature oxidation of the base material. The oxidation behavior was studied using SEM and STEM methods and the samples from the metal-scale interface were prepared by Focused Ion Beam method. It was shown that during high temperature exposure the reaction between the NbSi2 and NbB2 layers results in the formation of Nb5Si3 and T2 phases. The oxidation process is hindered at the front of this reaction where mixed oxide scale was found consisting of Nb2O5, NbO2 and borosilicate glass.

Effects of tungsten content on the high-temperature oxidation behavior of Ni-xW-6Cr alloys

The oxidation of Ni-(5–30 wt.%) W-6 wt.% Cr alloys was studied at 850 °C in air, among which Ni-25 W-6Cr alloy shows the best oxidation resistance. Because W acts as secondary getter, an external continuous oxide scale (NiCr2O4) is formed, which is responsible for the excellent oxidation resistance of these alloys. When the W content reach above 25 wt. %, the excessive W can facilitate the growth of CrWO4, preventing the formation of NiCr2O4, and breaking the continuity of the chrome oxide scale. Additionally, NiWO4 formed near the metal-scale interface can weaken the adhesion of oxide scales, causing cracks.

Study of the scale formed on super 304H boiler tube steels after long-term steam oxidation at high temperatures

Steam oxidation of a Super 304H boiler tube was performed at steam temperatures of 650 °C and 700 °C for time durations up to 5000 h. The oxidation scale outcomes which formed on as-received untreated and shot-peened (SP) tubes were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). γ-Grain nanocrystallization on the surface of the SP specimen generated an abundant fraction of grain boundary that guaranteed the formation of a continuous amorphous SiO2 layer at the metal-scale interface that enhanced the formation of dense Cr2O3 and FeCr2O4 in the scale. Scale breakaway along defects in the outer layer of the scale was accelerated by the steam temperature in the untreated specimens, whereas the scale that formed on the SP specimens appeared to be stable.

STEM and APT characterization of scale formation on a La,Hf,Ti-doped NiCrAl model alloy

A thermally grown scale formed on a cast NiCrAl model alloy doped with lanthanum, hafnium, and titanium was examined after isothermal exposure at 1100 °C for 100 h in dry flowing O2 to understand the dopant segregation along scale grain boundaries. The complex scale formed on the alloy surface was composed of two types of substrates: phase-dependent, thin (<250 nm) outer layers and a columnar-grained ∼3.5 μm inner alumina layer. Two types of oxides formed between the inner and outer scale layers: small (3–15 nm) La2O3 and larger (≤50 nm) HfO2 oxide precipitates. Nonuniform distributions of the hafnium, lanthanum, and titanium dopants were observed along the inner scale grain boundaries, with hafnium dominating in most of the grain boundaries of α-Al2O3. The concentration of reactive elements (RE) seemed to strongly depend on the grain boundary structure. The level of titanium grain boundary segregation in the inner scale decreased toward the model alloy (substrate), confirming the fast outward diffusion of titanium. Hafnium was also observed at the metal–scale interface and in the γ′ (Ni3Al) phase of the alloy. High-resolution scanning transmission electron microscopy (STEM) confirmed the substitution of REs for aluminum atoms at the scale grain boundaries, consistent with both the semiconducting band structure and the site-blocking models. Both STEM and atom probe tomography allowed quantification of REs along the scale grain boundaries across the scale thickness. Analysis of the scale morphology after isothermal exposure in flowing oxygen revealed a myriad of new precipitate phases, RE segregation dependence on grain boundary type, and atomic arrangement along scale grain boundaries, which is expected to influence the scale growth rate, stability, and mechanical properties.

Characterization of Si-aluminide coating and oxide scale microstructure formed on γ-TiAl alloy during long-term oxidation at 950 °C

Recently γ-TiAl intermetallics have been successfully applied on low pressure turbine blades of modern jet engines mostly due to their low density, high specific strength and creep resistance, all of which make them excellent alternatives for Ni-based superalloys. However, due to insufficient high temperature oxidation resistance above 800 °C there is the necessity for development of protective coatings that will allow the formation of a thin and slow growing α-alumina oxide scale.The paper presents the results of investigations concerning the behavior and influence of silicon on the substructure of aluminized ɣ-TiAl alloy in the as-deposited state as well as after long-term isothermal high temperature oxidation. The Si-modified aluminide coating was produced by pack cementation method and oxidized isothermally at 950 °C for 3000 h. The detailed microstructural examination was performed using analytical SEM and STEM techniques along with Focused Ion Beam (FIB) technique which was utilized for the preparation of a sample from the metal-scale interface after long-term oxidation test. The addition of silicon to the aluminide coating resulted in the formation of nanometric Ti5Si3 precipitates that were found to bind both Ti and Nb from the alloy. It was demonstrated that the investigated ɣ-TiAl with Si-modified aluminide coating is capable of forming a continuous and uniform α-alumina oxide scale at 950 °C that remains adherent for 3000 h. The oxide scale was found to obey the growth mechanism typical for Ni-based superalloys and consisted of outer equiaxed and inner columnar grains.

Characteristics of oxide scale formed on ferritic stainless steels in simulated reheating atmosphere

The aim of this study is to optimise the stainless steel oxidation behaviours during hot rolling. The high temperature oxidation behaviours of ferritic stainless steels B443NT and B445J1M were studied over the temperature range from 1000 to 1150°C in a humid atmosphere containing 18% water vapour, as measured by a thermogravimetric analyser (TGA). The results indicate that breakaway oxidation occurs at 1090°C for the B443NT steel, which is 60°C lower than that for the B445J1M steel. The occurrence of iron oxide nodules on the steels marks the onset of breakaway oxidation; however, the breakaway oxidation phenomenon of B445J1M is different from that of B443NT due to a compact and continuous Mn–Cr spinel which is formed on the surface of B445J1M. The oxide nodules with regenerated Cr2O3 scale underneath the Fe–Cr spinel display better adhesion without showing pores at the metal–scale interface.

Non-steady state carburisation of martensitic 9–12%Cr steels in CO2 rich gases at 550 °C

Two martensitic steels were reacted with Ar–50%CO2 at 550°C for exposure times up to 150h. Microstructural analyses and glow discharge optical emission spectroscopy revealed that internal carburisation of both steels beneath external oxide scales occurred, in spite of the very low equilibrium carbon activity of the atmosphere. The carbon concentration at the metal–scale interface increased throughout the reaction and the carbon uptake varied approximately linear with time. A model is proposed to describe the non steady-state carburisation kinetics whereby the usual diffusion equation describing carbon movement into the alloy is modified to account for loss of carbon as precipitated carbides.

High Temperature Corrosion of Metallic Materials in Composed Oxidizing Environments

AbstractBasing on actual theoretical approach and experimental results, the mechanism of sulphide formation beneath the oxide scale grown on metals in SO2-O2 atmospheres has been described. It has been shown that in spite of much lower sulphur partial pressure in the oxidizing atmosphere than the dissociation pressure of the sulphide to be formed, the sulphidation process takes place beneath the oxide scale. This, at the first sight, unexpected behavior results from the fact that sulphur is diffusing inwards through the primary oxide scale in the molecular form, i.e. SO2 molecules. Reaching thus metal-scale interface, where the oxygen partial pressure is very low, virtually equal to the dissociation pressure of the oxide forming the scale, SO2 ⇔ O2 + ½S2 equilibrium is shifted to the right, as a result of which the partial pressure of sulphur vapor dramatically increases, reaching the value several orders of magnitude higher than that needed for sulphide formation.Analogous situation is observed during oxidation of chromium steels in CO2-O2 atmospheres. In this case, namely, carburisation process is observed beneath the oxide scale, in spite of the fact that carbon activity in this environment is several order of magnitude lower than that required for chromium carbide formation. This again unexpected situation becomes understandable if one assumes – like in the case of metal oxidation in SO2 containing atmosphere – that carbon is transported through the oxide scale in the form of CO2 molecules.The final conclusion is, that the explanation of the mechanism of sulphide formation beneath the oxide scale on metals and of carburization beneath the oxide scales on steels constitutes the important step forward, leading to the better understanding of high temperature corrosion mechanisms of metallic materials, observed in multicomponent agresive gases.

Influence of Re on the Properties of a NiCoCrAlY Coating Alloy

MCrAlY can serve as stand-alone overlay coatings or bond coats in thermal barrier coating systems, and its properties play a vital role in determining the performance of these coating systems. In order to further understand the behavior of MCrAlY coatings, several NiCoCrAlY model alloys with different levels of Re (0.3 wt%, 6 wt%, and 9 wt%) were investigated. Microstructural observation showed the addition of Re promoted the precipitation of Cr-rich phases, such as α-Cr and σ. The presence of α-Cr lowered the coefficient of thermal expansion (CTE) of the coating alloys, which could reduce the CTE mismatch at the scale-metal interface. The solid solution strengthening effect of Re is responsible for an increase in Rockwell hardness of the coating alloys. But the isothermal oxidation resistance at 1100°C was deteriorated due to the precipitation of brittle α-Cr phase, a phase of inferior oxidation resistance compared with β-NiAI and γ-Ni.

Hydrogen and moisture-induced scale spallation: Cathodic descaling of a single crystal superalloy

Moisture-induced delayed spallation (MIDS) of protective alumina scales at room temperature is a well known phenomenon. One mechanism proposes that water and Al alloy react, produce hydrogen at the scale–metal interface, and enable spallation. To test this mechanism, preoxidized samples of a single crystal superalloy, Rene’N5 + Y, were subjected to standard cathodic hydrogen charging treatments known to produce hydrogen embrittlement in bulk Ni and Ni 3Al alloys. Cathodic hydrogen charging, at <1 mA and an estimated −0.45 V SCE, stripped the scales at the oxide-metal interface, resulting in an initial loss of ∼3 mg/cm 2 and little additional change with time. This was supported by macro-photos and SEM of the spalled surface. On the other hand, anodic polarization at <1 mA produced less, but steady, linear weight loss (0.3 mg/cm 2), primarily by anodic dissolution of the metal. Hydrogen charging was thus shown to be detrimental to the alumina scale–metal bond, supporting the hydrogen factor in MIDS. These and other MIDS results show remarkable similarities to embrittlement of Ni subject to hydrogen charging at similar potentials and varying amounts of interfacial (grain boundary) sulfur segregation. The MIDS phenomenon is also discussed in terms of comparative static corrosion fatigue characteristics. It is not necessarily related to other diverse moisture effects occurring at high temperature.

Finite element analysis of oxidation induced metal depletion at oxide–metal interface

In many situations, chromium depletion and voiding below oxide/alloy interface accompany with the selective oxidation of Cr–contained alloys. At the same time, stresses are induced at the oxide/alloy interface, in the metal and in the oxide. The combination of depletion and stress generation initiates interfacial crack-like voids which eventually lead to spallation of the oxide scale. In this paper, a continuum thermodynamic model which accounts for stress-diffusion interaction in the oxidation of Cr–Fe alloys is carried out via a two-dimension finite element implementation. For the purpose of generating depletion areas, grain areas and grain boundary areas in the alloy are defined via random Voronoi tessellation. Periodic boundary condition is also employed. The model predicts that the depletion of chromium mostly occurs at the fast diffusion area (grain boundary), and large normal tensile stresses are generated at both metal-scale interface and alloy grain boundaries. Once the depletion-induced voids reaches a certain level at the interface, concentration of stress could induce further propagation of interfacial crack.

Stress–oxidation interaction in selective oxidation of Cr–Fe alloys

Metal oxidation is sustained by continuous diffusion of ions in the scale layer. At the same time, stresses are induced in the scale layer and the base metal due to internal oxidation and volumetric change when metal ions are oxidized. Such stress may in return affect the diffusion of ions in the oxide layer, thus changing the oxidation kinetics. In this paper, a continuum thermodynamic model is developed to account for such stress–diffusion interaction in the oxidation of Cr–Fe alloys. The model predicts that the compressive stress in the scale layer has a rather nonlinear distribution across the layer thickness with its maximum at the metal–scale interface. Such stress significantly slows down the rate of oxidation. Consequently, the growth kinetic is not strictly parabolic. It is found that the distribution of stress and diffusive ions in the scale layer can be normalized by the oxidation rate constant so that, with proper scaling, the numerical solutions given in this paper are applicable to any value of the rate constant.

Subsurface microstructural changes in a cast heat resisting alloy caused by high temperature corrosion

A cast HP ModNb alloy (Fe–25Cr–35Ni–1Nb, wt.%) was oxidised and carburised in CO–CO 2 corresponding to a C = 0.1 and p O 2 = 3 × 10 −16 atm at 1080 °C. Formation of an external, chromium-rich oxide scale led to depletion of this metal in a deep alloy subsurface zone. Within that zone, secondary chromium-rich carbides dissolved, primary carbides oxidised, solute silicon and aluminium internally oxidised, and extensive porosity developed. Pore volumes correspond to the difference between metal loss by scaling and metal displacement by internal oxidation, assuming the scale–metal interface to be fixed. The pores are concluded to be Kirkendall voids.

The role of interface structure in oxidation reactions

The dynamics of the scale–metal interface exhibits an asymmetry in behavior in oxidation and other scaling reactions depending on whether the mechanism is one of cation or anion-diffusion. For the cation case, the growth step is at the free surface and line defects at the metal–scale interface serve to annihilate vacancies in the scaling mechanism. For the anion case, the growth step is at the scale–metal interface. Disconnections and dislocations provide the growth sites for the reaction and both their topological and elastic properties influence the mechanism. Scaling reactions to form nickel oxide and molybdenum disulfide are considered as examples. Conditions favoring scale fracture are discussed. The results are related to the reactive element effect and to the significance of the Pilling–Bedworth ratio.

The Influence of H&lt;sub&gt;2&lt;/sub&gt;O on Iron Oxidation at 600°C: A Microstructural Study

The oxidation of iron in dry O2 and in wet O2 (40% H2O) has been studied at 600°C. The oxide microstructure was investigated by SEM/EDX, FIB and XRD. Iron forms a layered scale in dry and wet oxygen at 600°C. The scale consists of a top hematite layer, a middle magnetite layer and a wüstite layer close to the scale metal interface. All three layers grow with time, but with different growth rates, the overall growth being approximately parabolic. The presence of water vapour increases the rate of oxidation and affects the evolution of the oxide microstructure. The higher rate of oxidation in the presence of water vapour is due to an increased growth rate of the magnetite layer and, especially, of the hematite layer, while the growth of the wüstite layer is not affected. It is suggested that water vapour influences grain boundary transport in the hematite layer.

Effect of minor elements and a Ce surface treatment on the oxidation behavior of an Fe–22Cr–0.5Mn (Crofer 22 APU) ferritic stainless steel

This paper details the oxidation behavior of coupons from three different Crofer 22 APU sheets. Oxidation tests were conducted at 800 ∘ C in moist air for 2300 h. The major features of the scales were similar in all three coupons, with a continuous Cr–Mn spinel layer forming at the gas–oxide interface and a Cr 2O 3 layer forming at the oxide–metal interface. Also, at the Cr 2O 3–metal interface, large Cr–Mn oxide particles formed in the base metal. Metal extrusions in the oxide scale were observed in the vicinity of these Cr–Mn oxide particles. All three alloys had internal oxide particles forming in the base metal beneath the oxide scale, and the extent of this internal oxidation was related to the Al and Si-content in the coupons (as also reported by Piron-Abellan J, Huczkowski P, Ertl S, Shemet V, Singheiser L, Quadakkers W. In: Fuel cell seminar, 2004. San Antionio, TX, November 1–5, 2004). Of note, discontinuous Ti-oxide and Mn-oxide particles formed above the outer spinel layer on the coupons with relatively high Al and Si contents. The effectiveness of rare-earth surface modification on improving oxidation resistance of this alloy was demonstrated. Pack cementation-like processes were used to treat the surface of the coupons with Ce. A continuous Cr–Mn outer oxide layer and a continuous inner Cr 2O 3 layer formed on the surface of the treated coupons during oxidation. However, the thickness of the scales and amount of internal oxidation were significantly reduced with the treatment, leading to the decrease in oxidation rate. The amount of internal oxidation was also reduced with the treatment. The treatment appeared to suppress the formation of SiO 2 at the scale–metal interface in high Si containing alloys. ASR measurements indicate that the treatment decreases the electrical resistance, which is a benefit for SOFC interconnect application.