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.     

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.     

Oxidation Behavior of Graded NiCrAlYRe Coatings at 900, 1000 and 1100 °C

A graded NiCrAlYRe coating was prepared by combining arc ion plating (AIP) with chemical vapor deposition (CVD) aluminizing. Quasi-isothermal oxidation tests of the graded NiCrAlYRe coating and the conventional NiCrAlYRe coating were performed in air at 900, 1000 and 1100 °C for up to 1000, 1000 and 200 h, respectively. The results showed that the graded NiCrAlYRe coating exhibited better long time oxidation resistance than the conventional NiCrAlRe coating. This favorable oxidation behavior was attributed to the rapid formation of a protective α-Al₂O₃ scale and a sufficient Al reservoir. The structures and morphologies of oxide scales varied under different oxidation conditions. θ-Al₂O₃ was observed on both coatings during oxidation at 900 °C, however, the graded coating showed more favorable conditions for θ-Al₂O₃ to grow than the conventional coating. For the graded coating, phase transformation from θ-Al₂O₃ to α-Al₂O₃ resulted in a sharp decrease in the parabolic rate constant k_p between 900 and 1,000 °C.     

Steam Oxidation Behavior of Advanced Steels and Ni-Based Alloys at 800 °C

This publication studies the steam oxidation behavior of advanced steels (309S, 310S and HR3C) and Ni-based alloys (Haynes^® 230^®, alloy 263, alloy 617 and Haynes^® 282^®) exposed at 800 °C for 2000 h under 1 bar pressure, in a pure water steam system. The results revealed that all exposed materials showed relatively low weight gain, with no spallation of the oxide scale within the 2000 h of exposure. XRD analysis showed that Ni-based alloys developed an oxide scale consisting of four main phases: Cr₂O₃ (alloy 617, Haynes^® 282^®, alloy 263 and Haynes^® 230^®), MnCr₂O₄ (alloy 617, Haynes^® 282^® and Haynes^® 230^®), NiCr₂O₄ (alloy 617) and TiO₂ (alloy 263, Haynes^® 282^®). In contrast, advanced steels showed the development of Cr₂O₃, MnCr₂O₄, Mn₇SiO₁₂, FeMn(SiO₄) and SiO₂ phases. The steel with the highest Cr content showed the formation of Fe₃O₄ and the thickest oxide scale.     

Oxidation Behavior of Tribaloy T-800 Alloy at 800 and 1,000 °C

The oxidation behavior of Co-based Tribaloy T-800 alloy has been studied isothermally in air at 800 and 1,000 °C, respectively. The results showed that the oxidation mechanism was dependent on the exposure temperature. The oxidation of the alloy followed subparabolic oxidation kinetics at 800 °C. The oxide scale at this temperature exhibited a multi-layered structure including an outer layer of Co oxide, a layer composed of complex oxide and spinel, a nonuniform Mo-rich oxide layer, an intermediate mixed oxides layer and an internal attacked layer with different protrusions into Laves phase. During 1,000 °C exposure, it followed linear kinetics. The oxidation rendered a relatively uniform external Cr-rich oxide layer coupled with a thin layer of spinel on the top surface and voids at local scale/alloy interface and intergranular region together with internal Si oxide at 1,000 °C.     

Hot Corrosion Behavior of Superalloy IN718 at 550 and 650 °C

This investigation was undertaken to evaluate oxidation and hot corrosion behavior of the Fe-Ni-based superalloy IN718, at 550 and 650 °C, to explore its performance as turbine engine components under marine environment. Uncoated and different salt-coated samples (100 wt.% NaCl, 75 wt.% Na₂SO₄ + 25 wt.% NaCl, and 90 wt.% Na₂SO₄ + 5 wt.% NaCl + 5 wt.% V₂O₅) were exposed in air at 550 and 650 °C under cyclic heating and cooling for 100 h. Weight gain was studied for both uncoated and salt-coated samples. X-ray diffraction, scanning electron microscopy, and electron dispersive spectroscopy were used to characterize the oxidation and corrosion products. A possible mechanism of corrosion, based on the corrosion compounds, is discussed. The variation in weight gain with time showed a parabolic growth of oxides. Coating with NaCl was found to be detrimental both at 550 °C as well as 650 °C. On the other hand, the salt mixture of NaCl and Na₂SO₄ had no effect at 550 °C; however, it was detrimental at higher temperature of 650 °C. Coatings of salt mixture of Na₂SO₄, NaCl, and V₂O₅ caused very slow oxidation at both the temperatures. Increase in thickness of salt coating was observed to enhance the rate of hot corrosion. Among the three types of salt coatings, the coating of NaCl was found to be most damaging both at 550 and 650 °C.     

Surface Condition of New γ–γ′ Co-Al-Mo-Nb and Co-Al-W Cobalt-Based Superalloys After Oxidation at 800 °C

A new type γ–γ′ Co-Al-Mo-Nb Co-based superalloys were developed due to limitations of basic Co-Al-W superalloys, related to tungsten alloying. The present study aims to characterization of new γ–γ′ Co-10Al-5Mo-2Nb (at.%) cobalt-based superalloy performance in terms of the high-temperature exposure under cyclic conditions, with particular regard to surface condition. Specimens were tested in cycles of high-temperature exposition (25, 50, 75, 100 and 150 h) in air environment at 800 °C. Detailed analysis of oxidized surfaces by scanning electron microscopy, energy-dispersive spectroscopy and x-ray diffraction was made at various intervals during testing. The cyclic oxidation behavior of new alloy was compared to the basic Co-9Al-9W (at.%) Co-based superalloy.     

Oxidation of AM60B Mg Alloys Containing Dispersed SiC Particles in Air at Temperatures Between 400 and 550 °C

AM60B magnesium alloys, with and without dispersed SiC particles, were oxidized between 400 and 550 °C in air. The scales generated consisted primarily of MgO and a small amount of Mg₃N₂ formed by the outward diffusion of cations (Mg, Al, Mn) and the inward diffusion of anions (N, O). The SiC particles were stable in the AM60B alloy during oxidation and increased its oxidation resistance to a certain extent. However, given the predominance of the non-protective MgO as the main oxide, the SiC_p/AM60B composites were inevitably destroyed as oxidation progressed.     

Oxidation Behavior of Sputtered DD98M Nanocrystalline Coating at 1000 °C

Increasing the efficiency of coal fired steam power plants is an important contribution towards clean coal power. In fact, new ferritic steels are expected to withstand 325 bar and 650 °C. Moreover, in order to facilitate CO₂ capture oxygen can be used instead of air for combustion (oxycombustion) so that no NO_X emissions are produced. Boiler components, such as superheater tubes, are exposed to both steam and fireside corrosion and at higher temperatures, ferritic steels corrode at very fast rates under both atmospheres. A solution can be found in the use of protective coatings, a number of which, applied by techniques capable of depositing said coatings both on the inner and outer surfaces of tubes, are being studied within nationally and European funded projects. In particular, two new Ni and Cr modified aluminide coatings deposited on P92 by non-line-of-sight hybrid processes have been produced and the preliminary results of on-going laboratory testing, both under oxycombustion model atmospheres as well as under pure steam at 650 °C are promising, in particular those exhibited by the Cr enriched aluminide coating. Moreover, results obtained in a pilot oxycombustion boiler operated by CIUDEN in Leon, Spain are also shown.     

Hot Corrosion Behavior of Some Superalloys in a Simulated Incinerator Environment at 900 °C

Incinerators are being used to burn solid waste of all types. This burning of waste creates a very aggressive environment at extremely high temperature. This environment attacks the various components of the incinerators. Some studies have been reported regarding behavior of steels in simulated incinerator environment at 550 °C. In present work superalloys Superco 605, Superni 600, and Superni 718 have been subjected to cyclic oxidation in 40 wt.% K₂SO₄ + 40 wt.% Na₂SO₄ + 10 wt.% KCl + 10 wt.% NaCl environment at 900 °C under cyclic condition. Weight change measurements have been done and weight change has been plotted against the numbers of cycles. The oxide scales formed on the surface of the corroded superalloys have been characterize by FESEM, EDS, XRD, cross-sectional analysis, and x-ray mapping. The nickel-based superalloys Superni 600 and Superni 718 indicated better resistance to corrosion in the above environment whereas Superco 605 lead to massive weight gain.     

The Effect of Pre-Oxidation Treatment on the High-Temperature Oxidation of Co–Re–Cr Model Alloys at Laboratory Air

The aim of a pre-oxidation treatment at low oxygen partial pressure is to promote the formation of a Cr₂O₃ (or Al₂O₃) scale in such a way that the oxide layer can reliably prevent the contact of oxygen with the metallic substrate also at high oxygen partial pressure. In this study, the pre-oxidation treatment was applied to the two alloys Co–17Re–23Cr and Co–17Re–30Cr (at.%). Pre-oxidation of the Co–Re–xCr alloys was found to be non-protective for the metallic substrate at high oxygen partial pressure despite the formation of a Cr₂O₃ layer. Different kinds of Cr₂O₃ scale damage were observed depending on the Cr concentration. The Cr₂O₃ scale formed on the alloy Co–17Re–23Cr loses its protective properties as a result of cation transport by lattice and grain boundary diffusion, while the Cr₂O₃ scale formed on the alloy Co–17Re–30Cr degraded as a consequence of scale cracking. In addition, the effect of alloying with Si was investigated and found to be promising.     

High-Temperature Corrosion Behaviour of Aluminized-Coated and Uncoated Alloy 718 Under Cyclic Oxidation and Corrosion in NaCl Vapour at 750 °C

To evaluate the suitability of HR3C and 22Cr–25Ni–2.5Al AFA steels as the heat-resistant alloys, the oxidation behavior of them was investigated in air at 700, 800, 900 and 1000 °C. The evolution of oxide layer on the surface and subsurface was investigated using a combination of compositional/elemental (SEM, EDS) and structural (XRD, GDOES) techniques. A dense and continuous Cr₂O₃ healing layer on the HR3C was formed at the temperature of 700 or 800 °C, but the Cr₂O₃ oxide film on HR3C was unstable and partly converted into a less protective MnCr₂O₄ with the increase in temperature to 900 or 1000 °C. The composition and structure of oxide film of 22Cr–25Ni–2.5Al AFA steels are significantly different to the HR3C alloys. The outer layer oxides transformed from Cr₂O₃ to Al-containing oxides, leading to a better oxidation resistance at 700 or 800 °C compared to HR3C. Further, the oxide films consist of internal Al₂O₃ and AlN underneath the outer loose layer after 22Cr–25Ni–2.5Al AFA oxidized at 900 or 1000 °C. It can be proved that the internal oxidation and nitrogen would make 22Cr–25Ni–2.5Al AFA steels have worse oxidation resistance than HR3C alloys at 900 or 1000 °C.     

Simulation of Fe-Cr-X Alloy Exposed to an Oxyfuel Combustion Atmosphere at 600 °C

In coal-fired power plants using oxyfuel combustion process with carbon capture and sequestration, instead of air, a mixture of oxygen and recirculated flue gas is injected in the boiler. A series of steels were exposed to CO₂-SO₂-Ar-H₂O gas mixtures at 600 °C for 1000 h to compare their high temperature corrosion behavior. During the corrosion process, carburization, decarburization and recrystallization were observed underneath the oxide scale depending on the gas mixture and alloy composition. The conditions that lead to carburization are not yet completely understood, but decarburization can be simulated using thermodynamic and kinetic models. In this work, the results of these simulations are compared with measured values for one of the alloys that displayed a decarburized region. Since the mobility of carbon in the scale is not known, two strategies were adopted: simulation of alloy-atmosphere contact; and estimation of the carbon flux to produce the observed decarburization. The second approach might give an insight on how permeable to carbon the scale is.     

Oxidation Comparison of Alumina-Forming and Chromia-Forming Commercial Alloys at 1100 and 1200 °C

21 Commercial alumina-forming and chromia-forming high-temperature alloys were tested at 1100 and 1200 °C for up to 1000 h. Exposure was performed in air using 100-h cycles on ~2-mm-thick test coupons placed in highly sintered alumina crucibles. Visual appearance, gross mass gain and amount of spallation were monitored after every 100 h. Investigation of topography and cross sections was performed in LOM and SEM with EDS at breakaway oxidation or at 1000 h depending on what occurred first. In order to investigate the earlier part of the oxidation process in more detail, separate samples from all materials were exposed under the same conditions for a single 100-h cycle and were then investigated in the same manor.     

Improved Oxidation Resistance of a New Aluminum-Containing Austenitic Stainless Steel at 800 °C in Air

The oxidation resistance of austenitic stainless steels modified with various aluminum contents was investigated. The weight gain per unit area is in parabolic relation to oxidation time, and the oxidation rate significantly decreases with increased aluminum content. Outer layer oxides of austenitic stainless steel transform from Cr₂O₃ to a composite oxide layer comprising Cr and Al, and more dense Al-containing oxides formed with increasing the added Al contents. Since the diffusion of element Al is enhanced and the diffusion of element Cr is inhibited, the oxides enriched in Al dramatically contribute to the improved oxidation resistance of austenitic stainless steels at high temperature. The possible oxidation mechanisms are also proposed based on microstructural observations.     

Short-time Oxidation Behavior of Low-carbon, Low-silicon Steel in Air at 850–1,180 °C—III: Mixed Linear-and-Parabolic to Parabolic Transition Determined Using Local Mass-Transport Theories

Under certain assumptions, Levich derived an equation for calculating gas-phase mass transport rate. This equation is commonly used to calculate the linear oxidation rate constant when the oxidation kinetics is controlled by gas phase transport and has been used in this study to calculate oxidation kinetics of steel at 850–1,180 °C in flowing air. In theory, at the early stage, two kinetics components, one linear and another parabolic, simultaneously operate at different locations of the sample, contributing to the overall kinetics and therefore, the oxidation kinetics should show an initial mixed linear-and-parabolic pattern, followed by a purely parabolic stage. However, the actual gas flow pattern in a laboratory tube furnace does not meet the requirements for deriving the Levich’s equation and as a result, significant discrepancies are found between the calculated and experimental results. To resolve the discrepancy, experimental conditions should be modified to meet the gas flow conditions required for deriving the Levich’s equation. Alternatively, a more appropriate equation under the current gas flow conditions in a laboratory tube furnace should be derived.     

Effect of Ti (Macro-) Alloying on the High-Temperature Oxidation Behavior of Ternary Mo–Si–B Alloys at 820–1,300 °C

The aim of the present investigation was to gain an initial understanding of the effect of (macro-) alloying with Ti on the oxidation behavior of Mo–Si–B alloys in the ternary phase region of Mo_ss–Mo₃Si–Mo₅SiB₂ at 820–1,300 °C. Motivated by recent studies and thermodynamic calculations, the alloy compositions Mo–9Si–8B–29Ti (at.%) and Mo–12.5Si–8.5B–27.5Ti (at.%) were selected and synthesized by arc-melting. Compared to the reference alloy Mo–9Si–8B, superior initial oxidation rates at 1,100–1,300 °C as well as a significant density reduction by nearly 18 % were observed. Due to enhanced initial evaporation of MoO₃ and mainly inward diffusion of oxygen, a borosilicate-rutile duplex scale with a continuous TiO₂ phase had formed. Detailed investigations of the oxidation mechanism by SEM, EDX, XRD and confocal micro-Raman spectroscopy indicated that Ti alloying is promising with regard to further improvement of the oxidation resistance as well as the strength-to-weight ratio of Mo–Si–B alloys.