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  • Initial Oxidation Behavior of Ferritic Stainless Steel Interconnect with Sputtered NiFe 2 Alloy Coating
    Oxid. Met. (IF 1.805) Pub Date : 2020-01-18
    Qingqing Zhao, Shujiang Geng, Gang Chen, Fuhui Wang

    Abstract Initial oxidation behavior of ferritic stainless steel with a sputtered NiFe2 coating for solid oxide fuel cells interconnect application was investigated in air at 800 °C to understand the transformation process from the NiFe2 alloy coating to NiFe2O4 spinel layer. The results indicated that the NiFe2 coating was initially converted to a surface scale with a layered structure consisting of a top Fe2O3 layer followed by NiFe2O4 mid-layer and an NiO inner layer. Cr oxide started to form at scale/steel interface before the coating was completely oxidized. The preferential growth orientation of Fe2O3 on the surface varied with time during the initial oxidation stage. Steel preoxidation prior to coating not only accelerated the oxidation of NiFe2 coating and the growth of NiFe2O4 layer, but also suppressed the orientated growth of Fe2O3. The surface scales on the coated steels were electrically conductive. The oxidation mechanism of the coated steels is discussed.

    更新日期:2020-01-21
  • Oxygen Diffusion Modeling in Titanium Alloys: New Elements on the Analysis of Microhardness Profiles
    Oxid. Met. (IF 1.805) Pub Date : 2020-01-10
    Nicolas Vaché, Daniel Monceau

    Abstract This study focuses on the diffusion of oxygen in titanium alloys during high-temperature oxidation. In particular, the model used to obtain thermokinetic coefficients from microhardness profiles was investigated. A literature review shows that microhardness profiles are modeled by a simple error function in the same way as oxygen concentration profiles obtained by microprobe analysis (EPMA). The analysis of literature shows that the hypothesis of a linear relationship between microhardness and oxygen content is not true over the entire oxygen concentration range and that a parabolic relationship is empirically more accurate. A new modeling equation taking into account this parabolic law is proposed as well as a simplified and easier to use form. The relative error of the diffusion coefficients obtained using the simplified equation was then determined. This new model was applied to the experimental microhardness profile of a Ti-6242s sample oxidized at 625 °C. The resulting oxygen diffusion coefficient is in excellent agreement with the one determined from EPMA profile using the classic error function model. Finally, other data from literature were analyzed with the new model to obtain an Arrhenius diagram of oxygen diffusivity in Ti-64 alloy between 550 and 850 °C. This diagram gives thermokinetic coefficients \(D_{0} = 1.1 \times 10^{ - 5} { \exp }\left( {\frac{{ - 191\,{\text{kJ/mol}}}}{RT}} \right)\) that are close to those reported for pure α-Ti in the temperature range 550–850 °C.

    更新日期:2020-01-11
  • Precipitation Behavior of Laves Phase in the Vicinity of Oxide Film of Ferritic Stainless Steel: Selective Oxidation-Induced Precipitation
    Oxid. Met. (IF 1.805) Pub Date : 2020-01-06
    Liangliang Wei, Liqing Chen, Houlong Liu, Liqiang Han, Na Gong, R. D. K. Misra

    Anti-corrosion behavior of Fe–Cr alloys at elevated temperature is greatly influenced by the protective nature of oxide scale formed on them. However, precipitates formed underneath the scale can affect the diffusion of reactive elements, growth mechanism and failure mechanism of oxide film. Here, we elucidate the precipitation mechanism of Laves phase in the vicinity of oxide film during oxidation at 950–1050 °C in air. Two types of automotive exhaust manifold-used ferritic stainless steels were designed by replacing Mo with different contents of W and adding small content of Ce. By systematic investigations of the precipitates in the vicinity of steel/oxide interface, we conclude that Laves phase precipitated near the oxide film is related to the selective oxidation of chromium, which is dramatically different from conventional precipitation mechanism. Nb was localized at the interface due to rejection from the growing Cr2O3 scale, which promoted the formation of Laves phase. The experimental steels were characterized by excellent oxidation resistance and Ti–rich oxide formed in the internal oxidation zone that hindered the further diffusion of oxygen.

    更新日期:2020-01-06
  • Cathodoluminescence Analysis for the Nondestructive Evaluation of Silica Scale on an Iron-Based Alloy
    Oxid. Met. (IF 1.805) Pub Date : 2019-12-21
    Susumu Imashuku, Kazuaki Wagatsuma

    Information on the composition, morphology, and thickness of surface oxide scale helps to control the performance of heat-resistant alloys. Currently, there is a lack of adequate analytical methods that allow the rapid and nondestructive evaluation of these properties. In this study, we demonstrate a nondestructive method for identifying silica (SiO2) scale and evaluating its morphology and thickness within 1 min by acquiring the cathodoluminescence (CL) images and spectra of SiO2 scale on an Fe–5%Si alloy heated to 900 °C. SiO2 scale emitted yellow–orange, violet, or red luminescence, whereas the other scale products that form on Fe–Si alloys, such as FeO, Fe3O4, Fe2O3, and Fe2SiO4, did not. Thus, we can identify SiO2 scale and observe its morphology on the basis of luminescent color in the CL image. The thickness of SiO2 scale can be correlated to the intensity of the CL peak at 645 nm. Therefore, the acquisition of CL images and spectra is a novel analytical method, which allows one to control the performance of SiO2-forming Fe–Si alloys.

    更新日期:2019-12-21
  • The Effect of Shot Peening on Steam Oxidation of 304H Stainless Steel
    Oxid. Met. (IF 1.805) Pub Date : 2019-12-20
    J. Matthew Kurley, Bruce A. Pint

    Abstract Shot peening is currently utilized in coal-fired power plant components to mitigate scale exfoliation issues from steamside oxidation of austenitic stainless steel superheater and reheater tubing. To focus on commercially available material, this study exposed quarter-ring specimens cut from two different commercially shot peened type 304H tubes and exposed to 1 bar steam for up to 15,000 h at 550°, 600°, 625°, and 650 °C. Specimens were removed at increments to characterize the oxide thickness and microstructure. The shot peened inner surface generally retained a thin, protective Cr-rich scale with occasional Fe-rich oxide nodules at 550–625 °C. The increased oxidation resistance from shot peening began to degrade at 650 °C after as little as 5,000 h. Cut and polished faces of these specimens formed thick, Fe-rich oxides similar to polished 304H coupons. Surprisingly, the mechanically machined outer surface of the tube specimens performed similarly to the shot peened inner diameter, suggesting it had sufficient cold work to achieve a similar benefit. Electron backscatter diffraction and Vickers-hardness measurements were used to characterize post-exposure changes in underlying microstructure and mechanical properties, respectively, imparted by shot peening.

    更新日期:2019-12-21
  • Growth and Exfoliation Behavior of the Oxide Scale on 316L and T91 in Flowing Liquid Lead–Bismuth Eutectic at 480 °C
    Oxid. Met. (IF 1.805) Pub Date : 2019-12-17
    S. J. Tian

    Corrosion tests of the 316L and T91 steels were performed in flowing lead–bismuth eutectic (LBE) with a flowing velocity of 0.3 m/s at 480 °C up to 3000 h. The microstructure and growth kinetics of the oxide scale and metal dissolution for both steels were experimentally determined. The results showed that a thin Fe–Cr spinel layer is formed on the 316L surface, while the oxide scale on the T91 surface consists of magnetite, Fe–Cr spinel and internal oxidation zone. At the first 1500 h, the growth of the oxide layer follows a linear law and a parabolic law for the 316L and T91, respectively. With the increase in exposure time, the oxide scale on both steels partly spalls and subsequently continues to grow in situ. Compared with the T91, although the oxide scale on the 316L surface is thinner, more steel constituents dissolve into the LBE.

    更新日期:2019-12-18
  • Pulse Electrodeposition of Cobalt/Zirconia Coatings: Oxidation and Electrical Performance of Ferritic Stainless Steel Interconnects
    Oxid. Met. (IF 1.805) Pub Date : 2019-12-10
    Fatemeh Saeidpour, Morteza Zandrahimi, Hadi Ebrahimifar

    To enhance oxidation resistance, avoid Cr evaporation and preserve suitable electrical behavior of Crofer 22 APU stainless steel as the interconnect of solid oxide fuel cells, Co/ZrO2 composite coatings were applied on Crofer 22 APU using the pulse electroplating technique. Moreover, isothermal and cyclic oxidation measurements were used to investigate oxidation resistance. In addition, oxidation rates were estimated and microstructures of the coatings were investigated before and after the oxidation tests. According to the results, the coated specimen had a lower oxidation rate constant than the uncoated specimen after 500 h of oxidation. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy and X-ray diffraction analysis showed that the oxide scale formed on the coated steel after oxidation was composed of two layers. The results also showed that the inner layer is thinner and contains Cr and O, while the outer layer is composed of Co, Mn, Cr and O. Furthermore, ZrO2 particles were also found in the outer oxide layer, following the oxidation tests. The Co/ZrO2-coated steel has a thinner inner oxide layer and a lower oxidation rate than the uncoated steel. Additionally, the area-specific resistance (ASR) of the steels with and without coating was also estimated as a function of temperature and time. The results showed that the growth rate of Cr2O3 layer decreased after applying the Co/ZrO2 composite coating, which resulted in lower ASR value. Moreover, after 500 h of oxidation at 800 °C, the ASR value of the Co/ZrO2-coated and uncoated steels was 13 mΩ cm2 and 27.8 mΩ cm2, respectively.

    更新日期:2019-12-11
  • Decarburization of 60Si2MnA in Atmospheres Containing Different Levels of Oxygen, Water Vapour and Carbon Dioxide at 700–1000 °C
    Oxid. Met. (IF 1.805) Pub Date : 2019-12-07
    Yisheng R. Chen, Xuanxuan Xu, Yu Liu

    The decarburization behaviour of 60Si2MnA in atmospheres containing 0–21% O2, < 20 ppm–17%H2O, and with or without 8%CO2, at 700–1000 °C, was investigated. The new findings of the current study were: (a) severe decarburization was associated with the formation of wüstite (FeO) scale on the steel surface, (b) the carbon activity at the steel–FeO interface was most likely determined by the reaction equilibrium between FeO and dissolved carbon in steel, (c) when a ferrite layer was able to form, the decarburization tendency was determined by the relative carbon permeability (defined as the product of carbon concentration difference at the two interfaces of the ferrite layer and carbon diffusivity) through the ferrite layer, and therefore, (d) the decarburization tendency at 800 °C was greater than those at 700 and 900 °C as the relative carbon permeability at 800 °C was the greatest. If FeO was absent when heating in dry O2-containing gases, however, possibly as a result of the formation of a SiO2 layer at the steel surface, decarburization was very much alleviated or avoided. At 1000 °C, the decarburization tendency was alleviated even when FeO was able to form because formation of a ferrite layer was not possible and carbon diffusivity in austenite was much lower than that in ferrite. A preformed oxide scale was effective in providing decarburization protection only when the steel was exposed to dry O2-containing atmospheres.

    更新日期:2019-12-07
  • Effect of Cyclic Reaction on Corrosion Behavior of Chromium-Containing Alloys in CO 2 Gas at 650 °C
    Oxid. Met. (IF 1.805) Pub Date : 2019-12-02
    Xuteng Xi, Charlie Kong, Jianqiang Zhang

    In this work, seven commercial alloys (602CA, 310SS, 253MA, F321, F316L, 800H and 304SS) were investigated in Ar–20% CO2 gas at 650 °C under a cyclic condition (1-h reaction and 0.25-h cooling in each cycle) for up to 310 cycles. The corrosion behavior of these alloys in isothermal reaction condition was also carried out for a purpose of comparison. The results showed that nickel-based 602CA alloy stayed protective in both isothermal and cyclic reaction conditions by forming a thin protective alumina scale. However, alloys F321, F316L, 800H and 304SS all formed thick multilayered oxides with external iron-rich oxides and internal spinel oxides in all reaction conditions. Alloys 310SS and 253MA behaved protective in isothermal reaction condition but formed pitting corrosion in cyclic reaction condition. The high corrosion resistance of 310SS and 253MA was attributed to the high Cr content and the effect of other alloying elements, e.g., Si and Mn, forming additional oxide layers to enhance chromia protection. Cyclic reaction created stress on oxide scale during cooling and heating which accelerated the initiation of breakaway corrosion of these alloys. Carburization due to CO2 reaction was identified for F321, F316L and 304SS, but not for other alloys because of the formation of highly protective alumina or chromia scales.

    更新日期:2019-12-02
  • Oversight or New Insight? Comments on Several Recent Papers Studying High-Temperature Oxidation of Si-Containing Steels
    Oxid. Met. (IF 1.805) Pub Date : 2019-11-26
    Yisheng R. Chen

    Recently, a group of researchers conducted experimental studies on the high-temperature oxidation behaviour of Si-containing steels and published a series of papers in several international journals. Many of the research “findings” in these papers were inconsistent and contradictory with previous results. Based on the different findings, it was claimed that they arrived at “new insights” to the oxidation behaviour of Si-containing steels and discovered a “new method” to reduce the oxide scale amount generated during reheating of Si-containing steels. After examining the experimental methods used, it was revealed that most of the “new findings” were likely the direct consequence of the use of a very low gas flow rate (30 mL/min) in their experiments. This resulted in an extremely low oxygen supply rate when the oxygen concentration in the experimental gas was low, which in turn limited the reaction rate that could be possibly achieved by gas–steel reaction if abundant gas had been made available. The kinetics results and scale thickness/structures thus produced were therefore misleading, which also led to the erroneous interpretation of the results. This paper comments on the main results presented in these papers and briefly discuss several fundamental issues related to the design of steel oxidation experiments.

    更新日期:2019-11-27
  • Magnetic Property Changes of CoNiCrAlY Coating Under Cyclic Oxidation and Hot Corrosion
    Oxid. Met. (IF 1.805) Pub Date : 2019-11-25
    Kh. Rahmani

    In this paper, a non-destructive method for evaluation of a CoNiCrAlY coating applied to gas turbine blades has been studied. The specimens from cast Ni-base superalloy IN738LC were coated with Co–34Ni–18.6Cr–8.7Al–0.5Y by low pressure plasma spray (LPPS) technique. The XRD peaks indicated that CoNiCrAlY coatings are composed of β-NiAl and γ-Co (Ni, Cr) matrix with small amounts of Al2O3 and γ′-Ni3Al. The surfaces of the coated specimens were covered with thin salt mixture of Na2SO4 + l0 wt% NaCl. Afterward, the specimens were maintained at 850 °C for 24 h, cooled to room temperature (as one cycle) and then reintroduced in the furnace. The magnetic susceptibility of coating, as one of the magnetic properties, was periodically determined after each cycle. The results showed that the magnetic susceptibility of Co–34Ni–18.6Cr–8.7Al–0.5Y coating is related to the phase compositions and microstructures. The magnetic susceptibility typically increases with depletion of β-NiAl phase and decrease in the coating thickness during cyclic oxidation and hot corrosion. Therefore, measurement of magnetic susceptibility can be employed as a non-destructive test (NDT) method to estimate the thickness or residual lifetime of CoNiCrAlY coatings.

    更新日期:2019-11-26
  • Reactive Element Effects in High-Temperature Alloys Disentangled
    Oxid. Met. (IF 1.805) Pub Date : 2019-11-23
    Vedad Babic, Christine Geers, Itai Panas

    Reactive elements—REs—are decisive for the longevity of high-temperature alloys. This work joins several previous efforts to disentangle various RE effects in order to explain apparently contradicting experimental observations in alumina forming alloys. At 800–1000 °C, “messy” aluminum oxy-hydroxy-hydride transients initially formed due to oxidation by H2O which in turn undergo secondary oxidation by O2. The formation of the transient oxide becomes supported by dispersed RE oxide particles acting as water equivalents. At higher temperatures, electron conductivity in impurity states owing to oxygen vacancies in grain boundaries (GBs) becomes increasingly relevant. These channels are subsequently closed by REs pinning the said vacancies. The universality of the emerging understanding is supported by a comparative first-principles study by means of density functional theory addressing RE(III): Sc2O3, Y2O3, and La2O3, and RE(IV): TiO2, ZrO2, and HfO2, that upon reaction with water, co-decorate a generic GB model by hydroxide and RE ions. At 100% RE coverage, the GB model becomes relevant at both temperature regimes. Based on reaction enthalpy ΔHr considerations, “messy” aluminum oxy-hydroxy-hydride transients are accessed in both classes. Larger variations in ΔHr are found for RE(III)-decorated alumina GBs as compared to RE(IV). For RE(III), correlation with GB width is found, increasing with increased ionic radius. Similarly, upon varying RE(IV), minor changes in stability correlate with minor structural variations. GB decorations by Ce(III) and Ce(IV) further consolidate the emerging understanding. The findings are used to discuss experimental observations that include impact of co-doping by RE(III) and RE(IV).

    更新日期:2019-11-26
  • Oxidation of 60Si2MnA Steel in Atmospheres Containing Different Levels of Oxygen, Water Vapour and Carbon Dioxide at 700–1000 °C
    Oxid. Met. (IF 1.805) Pub Date : 2019-11-21
    Yisheng R. Chen, Yu Liu, Xuanxuan Xu

    The oxidation behaviour of the spring steel 60Si2MnA in atmospheres containing 0–21% (volume per cent) O2, < 20 ppm (part per million) to 17%H2O, with some containing 8%CO2, at 700–1000 °C was investigated. The oxide scale thicknesses formed in both 17%H2O–N2 and dry O2-containing atmospheres were less than 6 μm after 20 min of oxidation, significantly smaller than those formed in atmospheres containing both oxygen and water vapour, and the scale structures developed in the three different scenarios were also very different. The scale formed in 17%H2O–N2 contained wustite only, the scale formed in dry O2-containing atmospheres comprised primarily hematite and some magnetite, and that in O2–H2O mixtures developed a multi-layered structure, generally with an innermost Fe2SiO4 + FeO layer, followed by FeO/Fe3O4/Fe2O3 layers towards the scale surface. A preformed oxide scale and the presence of 8%CO2 in the atmosphere had little effects on further steel oxidation. The mechanisms of forming different scale structures are discussed.

    更新日期:2019-11-21
  • The Influence of KCl and HCl on the High-Temperature Oxidation of a Fe-2.25Cr-1Mo Steel at 400 °C
    Oxid. Met. (IF 1.805) Pub Date : 2019-11-11
    E. Larsson, J. Liske, A. Persdotter, T. Jonsson, J. -E. Svensson, L. -G. Johansson

    The influence of alkali- and chlorine-containing compounds on the corrosion of superheater alloys has been studied extensively. The current paper instead investigates the corrosive effects of KCl and HCl under conditions relevant to waterwall conditions. A low-alloy (Fe-2.25Cr-1Mo) steel was exposed to KCl(s), 500 vppm HCl(g) and (KCl + HCl) in the presence of 5%O2 and 20% H2O at 400 °C. The results indicate that alloy chlorination by KCl occurs by an electrochemical process, involving cathodic formation of chemisorbed KOH on the scale surface and anodic formation of solid FeCl2 at the bottom of the scale. The process is accompanied by extensive cracking and delamination of the iron oxide scale, resulting in a complex, convoluted scale morphology. Adding 500 vppm HCl to the experimental environment (KCl + HCl) initially greatly accelerated the formation of FeCl2 at the scale/alloy interface. The accelerated alloy chlorination is attributed to HCl reacting with KOH at the scale surface, causing the cathodic process to be depolarized. A rapid slowing down of the rate of chlorination and corrosion in KCl + HCl environment was observed which was attributed to the electronically insulating nature of the FeCl2 layer which forms at the bottom of the scale, disconnecting the anodic and cathodic regions.

    更新日期:2019-11-11
  • Oxidation Behavior of Oxide Dispersion-Strengthened W–Ni Alloys
    Oxid. Met. (IF 1.805) Pub Date : 2019-11-09
    V. R. Talekar, A. Patra, S. K. Sahoo

    Oxidation behavior (at 1000 °C for 10 h) of 1.0 wt% oxide dispersion-strengthened (nano-Y2O3, Al2O3, La2O3 dispersed) mechanically alloyed (10 h) and sintered (1400 °C, 1500 °C, 2 h) W–Ni alloys has been investigated. Oxidation significantly constrains the application window of W and therefore needs to be counteracted by suitable alloying and dispersion. W–Ni–Y2O3 alloy possesses excellent oxidation resistance at the high sintering temperatures. Moreover, W–Ni–Al2O3 sintered at 1400 °C exhibited no spallation or blistering after 10 h of oxidation. The enhanced oxidation resistance of W–Ni–Y2O3 alloy is attributed to superior densification, oxide-scale adhesion with the matrix phase, reduced volatilization of WO3 owing to NiWO4 and enhanced concentration of Y ions enriched with oxide. The investigation will provide a strategy to fabricate oxidation-resistant alloys by oxide dispersion for high-temperature applications.

    更新日期:2019-11-11
  • Thermal Durability of YSZ/Nanostructured Gd 2 Zr 2 O 7 TBC Undergoing Thermal Cycling
    Oxid. Met. (IF 1.805) Pub Date : 2019-10-19
    M. Bahamirian, S. M. M. Hadavi, M. Farvizi, A. Keyvani, M. R. Rahimipour

    Inferior fracture toughness (KIC), low thermal expansion coefficient (CTE), thermochemical incompatibility with bond coat and growth of thermally grown oxide (TGO) are the main destruction factors in conventional Gd2Zr2O7 (GZO) thermal barrier coating (TBC) undergoing thermal cycling. In this paper, to overcome these drawbacks, nanostructured GZO and an intermediate conventional YSZ layer between top and bond coats were used in the TBC system. Single-layer nanostructured GZO and dual-layer YSZ/nanostructured GZO coatings were deposited with atmospheric plasma spray technique on Ni-based superalloy (IN738LC) substrates as a topcoat with a CoNiCrAlY bond coat. Oxidation behaviour of samples was studied and compared in the cyclic mode at 1100 °C for 4 h in each cycle. Also, CTE and KIC of the nanostructured GZO and intermediate YSZ layers were investigated. The results indicated that the oxidation life of dual-layer YSZ/nanostructured GZO coating was 3.2 times more than that of single-layer nanostructured GZO. The microstructural analysis indicated that the growth rate of TGO was considerably slower for dual-layer coating due to reduced oxygen infiltration and cracks propagation. Also, nanostructured topcoat improved the thermochemical compatibility and mechanical properties in TBCs.

    更新日期:2019-11-04
  • Growth Kinetics and Isothermal Oxidation Behavior of Aluminide Pack Coatings on a Multiphase Mo–Si–B Alloy
    Oxid. Met. (IF 1.805) Pub Date : 2019-07-23
    Kwangsu Choi, Wonchul Yang, Kyeong-Ho Baik, Youngmoo Kim, Seong Lee, Joon Sik Park

    Refractory metal alloys, such as Mo–Si–B systems, can extend high-temperature capability more than commercial Ni-based superalloys, but Mo–Si–B alloys require surface coatings to improve their poor oxidation resistance. In this study, aluminide coating layers were created on Mo–3Si–1B (wt%) alloys by pack cementation with NH4Cl, Al and Al2O3 powder. The aluminide coating layers consisted of MoAl4, Mo3Al8, and precipitates. The growth kinetics of the coating layers were estimated by identifying diffusion behaviors. The aluminide coating layer growth constant (k0) was estimated as ~ 741.3 μm/h0.5, and the activation energy (Q) for the growth of the diffusion coating layer was evaluated as ~ 44.2 kJ/mol for the examined coating temperatures of 800, 900 and 1000 °C. The thicknesses of the coating layers calculated by an estimated kinetic equation were compared with the experimental results. The coating layer successfully protected the Mo–Si–B substrate during isothermal oxidation at 1400 °C under an air atmosphere. The growth kinetics of the coated layer and oxidation behaviors were discussed in terms of microstructural analyses.

    更新日期:2019-11-04
  • Effects of Oxidation and Alumina Addition on the Physical and Mechanical Properties of Ti/Al 2 O 3 Composites Prepared by Semi-powder Metallurgy Method
    Oxid. Met. (IF 1.805) Pub Date : 2019-07-27
    Fahamsyah H. Latief, Naser A. Alsaleh, Nashmi Alrasheedi, Sabbah Ataya

    The effects of oxidation and alumina addition on the physical and mechanical properties of Ti/Al2O3 composites were studied. The variation in alumina addition used in this study was 0, 10, 20 and 30 wt%. The mixture of Ti and Al2O3 was prepared by semi-powder metallurgy method and then pressed and followed by sintering in air atmosphere at 1000 °C for 2 h. The present results show that the density of the sintered Ti/Al2O3 decreased with increasing alumina amount and oxidation. XRD and EDX analysis indicates that the sample free of alumina produced the oxides in the form of TiO2 on the surface of the composite. With alumina addition, the AlTiO2 oxide appears besides TiO2. This occurrence confirms that the oxidation of Ti increases with increasing the amounts of alumina. The intermetallic phase Ti3Al has appeared in the Ti/Al2O3 composites, which might be due to reduction in alumina by Ti. The oxidation of Ti/Al2O3 composites decreases the hardness and compressive yield strength and hardness values. The decrease in mechanical properties becomes more obvious with increasing the alumina amount which enhanced the formation of oxidation scales after sintering.

    更新日期:2019-11-04
  • Effect of Nb Addition on Oxidation Mechanisms of High Cr Ferritic Steel in Ar–H 2 –H 2 O
    Oxid. Met. (IF 1.805) Pub Date : 2019-08-05
    A. Vayyala, I. Povstugar, T. Galiullin, D. Naumenko, W. J. Quadakkers, H. Hattendorf, J. Mayer

    High chromium ferritic steels are being used as construction materials for interconnects in solid oxide electrolysis cells (SOEC). Addition of niobium in the range of a few tenths of a percent is suitable for increasing the high-temperature creep strength of this type of ferritic steel. In the present work, the high-temperature isothermal oxidation behavior of a niobium containing ferritic steel at 800 °C was investigated in Ar–4%H2–4%H2O gas simulating the service environment in an SOEC (cathode side) and compared with that of a Nb-free counterpart alloy. Gravimetric data were correlated with the results from microstructural analyses using, among others, scanning and transmission electron microscopy as well as glow discharge optical emission spectroscopy. Atom probe tomography was used for obtaining atomic-scale insight into the segregation processes in external oxides and their interfaces. The oxidation rate was substantially higher for the Nb-containing than for the Nb-free alloy. Both alloys formed double-layered oxide scales consisting of inner chromia and outer MnCr2O4 spinel. Additionally, a thin layer of rutile-type Nb(Ti,Cr)O2 oxide of 200–300 nm thickness was observed at the scale–alloy interface in the Nb-containing steel. Nb addition to the alloy led to its segregation at chromia grain boundaries which affected the diffusion of Cr and other solute species such as Ti, Mn and Si.

    更新日期:2019-11-04
  • Effect of Surface Finish on High-Temperature Oxidation of Steels in CO 2 , Supercritical CO 2 , and Air
    Oxid. Met. (IF 1.805) Pub Date : 2019-10-23
    Richard P. Oleksak, Gordon R. Holcomb, Casey S. Carney, Lucas Teeter, Ömer N. Doğan

    Current and future power systems require steels resistant to high-temperature oxidation in CO2-rich environments. The introduction of structural defects by various surface treatments can profoundly affect the oxidation/corrosion behavior of steels in many environments. This effect is largely unexplored for steels exposed to high-temperature CO2, which is the focus of this work. We prepared Grade 22, Grade 91, 347H, and 310S steels with three different surface finishes, ranging from little to substantial surface damage, and exposed the steels to 1 bar CO2, 200 bar supercritical CO2, and laboratory air at 550 °C for up to 1500 h. Surface finish had little impact on the oxidation behavior of low-Cr (2 wt%) Grade 22 and high-Cr (25 wt%) 310S steels. In contrast, intermediate-Cr steels Grade 91 (8 wt%) and 347H (17 wt%) generally showed improved oxidation and carburization resistance with increasing extent of surface damage, which was attributed to the delay or prevention of the onset of Fe-rich oxide nodule growth. Comparison between exposure environments suggests that this effect is more complex for CO2 compared to air and that it is additionally affected by CO2 pressure. The results suggest that surface treatments should be considered as one approach to achieve improved corrosion resistance in high-temperature CO2, particularly for steels containing Cr levels near the transition that is required to form and maintain a protective Cr-rich oxide scale.

    更新日期:2019-11-04
  • Effect of Minor Alloying Elements on the Oxidation Behavior of Ni-Base Alloys in a High-Temperature Steam Environment
    Oxid. Met. (IF 1.805) Pub Date : 2019-10-22
    Sung Hwan Kim, Chaewon Kim, Ji-Hwan Cha, Changheui Jang, Young Soo Yoo

    Model alloys based on Alloy 617 were fabricated with modified minor alloying elements for improvement in oxidation resistance in high-temperature steam environments. Model alloys were grouped as one with Ti and Al and the other without. Mn content was varied from 0 to 2 wt% for the alloys within each group. The alloys were subjected to oxidation testing in steam at 900 °C for up to 1000 h. Alloys with Ti and Al showed high weight gains due to fast oxide growth on top of grain boundaries and internal oxidation. Mn addition caused formation of Mn-rich spinels and contributed somewhat to weight gain. On the other hand, for the alloys without Ti and Al, Mn content greater than 0.5 wt% prevented oxide spallation. Tensile properties were not significantly affected by the minor alloying element modifications, and one of the model alloys exhibited comparable creep rupture life to Alloy 617.

    更新日期:2019-11-04
  • Sulphidation of Low-Alloyed Steels Used in Power Industry
    Oxid. Met. (IF 1.805) Pub Date : 2019-07-27
    T. Dudziak, K. Jura, P. Dudek, L. Boroń, J. Rutkowska

    This paper reports the results obtained from sulphidation tests on a low-alloyed steels 15HM (T/P12), 16M (T/P1), 18K and 10HM2 (T/P22) in the temperature range 450–550 °C for 100 h. Tests were conducted using a gas mixture of 1%H2S 99% Ar at 1 bar. The results indicate that the low-alloyed steels underwent a high degree of corrosion degradation due to the formation of an Fe1−XS scale. The scale thicknesses and chemical compositions of the scales formed on the exposed samples were analysed by means of standard techniques, including scanning electron microscopy coupled with energy X-ray dispersive spectroscopy. In addition, a phase diagram for the Fe–S system was calculated using FACTSAGE software to aid interpretation of the results. Based on the study conducted, relevant defect equations were proposed.

    更新日期:2019-11-04
  • Nodular Corrosion of Zr–0.85Sn–0.16Nb–0.37Fe–0.18Cr Alloy in 500 °C Steam Caused by High-temperature Processing
    Oxid. Met. (IF 1.805) Pub Date : 2019-10-10
    Tianguo Wei, Xun Dai, Boquan Chen, Junsong Zhang, Chongsheng Long

    Zr–0.85Sn–0.16Nb–0.37Fe–0.18Cr alloy samples prepared by low-temperature process (LTP) and high-temperature process (HTP) show different corrosion behaviors in 500 °C steam. The former exhibits uniform corrosion, while the latter exhibits nodular corrosion casually. The occurrence of nodular corrosion on HTP samples is attributed to the formation of local alloying elements depletion region in the Zr matrix. During hot rolling at 800 °C, transformation of zirconium from α-phase to β-phase happens locally. Alloying elements migrate to β-phase from its neighboring α-phase. The local β-phase dissolves in the subsequent cold rolling and final annealing, leaving the microstructure with agglomeration of precipitate particles and neighboring alloying elements depletion regions in Zr matrix. And the undesirable microstructure becomes the causation of nodular corrosion.

    更新日期:2019-11-04
  • Influence of W Content on the Oxidation Behaviour of Ternary $$\gamma ^{\prime }$$ γ ′ -Strengthened Co-Based Model Alloys Between 800 and $$900\,^{\circ }{\hbox {C}}$$ 900 ∘ C
    Oxid. Met. (IF 1.805) Pub Date : 2019-08-10
    Martin Weiser, Sannakaisa Virtanen

    The oxidation behaviour of ternary Co–Al–W alloys with W contents between 7 and 10 at.\(\%\) was investigated in detail. Continuous thermogravimetry was conducted between 800 and \(900\,^{\circ }{\hbox {C}}\) to compare the impact of \(\gamma ^{\prime }\) volume fraction on the overall mass gain during the comparably long transient stages of oxide-scale growth. Resulting cross-sections were assessed using electron microscope-based analysis methods. To understand prevailing oxidation mechanisms, the development and composition of multilayered oxide scales were the main focus of the present study. Elemental distribution of three individual scale sections was investigated with electron probe microanalysis. No striking impact of the W level in the alloy was found. However, the development of diffusion-limiting barrier layers was demonstrated to change with varying W contents. Continuous alumina layers were only observed after 100-h exposure at \(800\,^{\circ }{\hbox {C}}\). For samples with 9 at.\(\%\) W, the grain structure within the outer Co-oxide layer was characterized with electron backscattered diffraction after the maximum duration of exposure. Individual growth kinetics for three distinguished layers within the oxide scales were used to compare elemental transport depending on the W content in the alloy and the exposure temperature.

    更新日期:2019-11-04
  • The Effect of Air Fraction in Steam on the Embrittlement of Zry-4 Fuel Cladding Oxidized at 1273–1573 K
    Oxid. Met. (IF 1.805) Pub Date : 2019-10-24
    Martin Negyesi, Masaki Amaya

    This paper deals with the effect of air fraction in steam on the embrittlement of Zry-4 nuclear fuel cladding tubes exposed under steam–air atmospheres (air fractions of 10–100%) in the temperature range of 1273–1573 K. Ring compression tests were carried out in order to evaluate the embrittlement of fuel cladding. Furthermore, the microhardness of prior β-phase was measured and fractured surfaces were observed under scanning electron microscopy. The degree of the embrittlement is discussed against the results of metallographic and hydrogen analyses. The microstructure and the hydrogen pickup were substantially affected by nitride formation. Accelerated oxidation kinetics enhanced shrinking of the prior β-region. The enhanced hydrogen absorption resulted in the increased microhardness of prior β-phase. The degree of the fuel cladding embrittlement, expressed by the plastic strain at failure and the maximum load, correlated well with the microhardness and the thickness of prior β-phase.

    更新日期:2019-11-04
  • Effect of Iron-Aluminide Coating on the Fracture Mechanism of Ferritic–Martensitic Steel in Coal-Fired Boilers Environment
    Oxid. Met. (IF 1.805) Pub Date : 2019-10-22
    Huan-Chang Liang, Chaur-Jeng Wang

    Due to the coal combustion that generates halides, steel components can confront hot corrosion during applications at high temperature. The hot-dipping aluminum (HDA) was operated on the 9Cr–Mo steel (grade 91) to form the iron aluminide layer. A hot corrosion-loading test of aluminized grade 91 (HDA-91) was carried out by covering a salt mixture of NaCl/Na2SO4 under static load ranging from 75 to 100 MPa at 600 °C and 700 °C, respectively. The failure mechanism was assessed after various elongations using scanning electron microscopy and optical microscopy. The results showed that HDA-91 presented higher hot corrosion resistance than the uncoated grade 91. The aluminide layer formed a higher ductility oxide and prevented the substrate form grain-boundary oxidation at high temperatures, resulting in durability. The results also revealed a significant improvement in reduction of area during the hot corrosion-loading test for grade 91 that underwent HDA treatment.

    更新日期:2019-11-04
  • Effects of Hf, Y, and Zr on Alumina Scale Growth on NiAlCr and NiAlPt Alloys
    Oxid. Met. (IF 1.805) Pub Date : 2019-07-23
    DongEung Kim, Shun-Li Shang, Zhuoqun Li, Brian Gleeson, Zi-Kui Liu

    The effects of Hf, Y, and Zr additions on the growth of a thermally grown alumina scale formed on NiAlCr and NiAlPt alloys were investigated. Isothermal and thermal cycling oxidation experiments were carried out at 1150 °C, and cross sections of the oxidized samples were characterized using scanning electron microscopy. It was observed that single doping as well as co-doping of Hf, Y, and Zr reduces the rate of alumina scale growth on NiAlCr and NiAlPt alloys, with Hf showing a more significant effect than Y or Zr. The following possible contributing factors to these observations were assessed: (1) the ionic size of the minor alloying elements and (2) the bond strength between the doping elements and oxygen in the oxide grain boundaries in terms of formation and melting enthalpies of oxides. It was concluded that the bond strength between the doping elements and oxygen within the oxide grain boundaries plays an important role in retarding the alumina scale growth.

    更新日期:2019-11-04
  • Effective Reduction of Chromium-oxy-hydroxide Evaporation from Ni-Base Alloy 690
    Oxid. Met. (IF 1.805) Pub Date : 2019-08-29
    T. Sand, C. Geers, Y. Cao, J. E. Svensson, L. G. Johansson

    The corrosion of the Ni-base alloy 690 (60Ni, 30Cr and 10Fe) in humidified air was studied at 500–800 °C, and the rate of CrO2(OH)2 volatilization was measured quantitatively as a function of exposure time using a denuder technique. Different gas velocities were employed in exposures with a maximum duration of 200 h. Corrosion morphology was investigated by SEM/EDX using BIB-milled cross sections. The rate of chromium volatilization increased with increasing temperature and gas velocity. The rate of volatilization decreased with exposure time. Two oxide scale morphologies were observed, depending on temperature and gas velocity. In the 500–700 °C range, the scale consisted of chromia-rich corundum-type oxide, while exposures with high gas velocities at 800 °C produced an entirely different type of scale that included a Ni-rich and Cr-poor cap layer. The latter scale morphology is suggested to result from extensive chromium depletion of the alloy substrate which triggers a new mode of oxidation involving formation of NiCr spinel oxide. Continued volatilization of CrO2(OH)2 causes the NiCr spinel to decompose into a Ni-rich oxide that forms a cap layer on the scale surface. This cap layer is very efficient in decreasing the rate of chromium volatilization, allowing the chromium levels in the substrate to recuperate. We show that volatilization of chromium (VI) from the alloy can be mitigated by an oxidation pre-treatment that allows the Ni-rich cap layer to form.

    更新日期:2019-11-04
  • Reactive Element Effect Applied by Alloying and SiHfBCN Coating on the Oxidation of Pure Chromium
    Oxid. Met. (IF 1.805) Pub Date : 2019-07-23
    Anke S. Ulrich, Timo Kaiser, Emanuel Ionescu, Ralf Riedel, Mathias C. Galetz

    To reduce scale spallation, scale volatilization, and nitrogen embrittlement in Cr-based systems, reactive elements such as Y, Zr, Hf, La, and Ce were introduced as oxide dispersions into pure chromium. In addition, Hf coating systems were investigated. One was a Hf sputter layer with varying thickness, and the other one a Hf-containing precursor ceramic, i.e., SiHfBCN, which may be considered as suitable material for environmental barrier coating applications. Oxidation tests at \(1050\,^{\circ }\hbox {C}\) in synthetic air for 50 h were carried out using thermogravimetric analysis. The samples were analyzed via X-ray diffraction, optical microscope, electron microprobe analysis, and scanning electron microscope. All reactive elements led to a decrease in total mass gain after oxidation compared to pure Cr, with Y and Zr showing the strongest effect. Improvements in oxide attachment, oxide growth rate, volatilization rate as well as nitridation resistance were observed. Concerning these experiments, Y showed the most promising results. Concerning Hf, coating systems, especially SiHfBCN, showed a higher effect on improving the oxidation resistance. The reason for this outcome might be that not only Hf is active in the precursor ceramic layer. A more complex oxide layer has formed, which consisted of not only Cr2O3 but also of Hf and Si oxides. This layer prevents the material from any nitridation under the selected oxidation conditions.

    更新日期:2019-11-04
  • Thermodynamic Phase Diagrams of Ternary Alloys Exposed to a Single Oxidant
    Oxid. Met. (IF 1.805) Pub Date : 2019-07-06
    Jun Shen, Xiaohui Guo, Yan Niu

    A simple example of an isothermal thermodynamic phase diagram for a quaternary system composed of a ternary A–B–C alloy reacting with a single oxidant (O) has been calculated assuming that oxides are insoluble and that they do not interact with each other. The 2D thermodynamic diagrams of the ternary metal–oxygen systems composed of the three types of binary alloys plus the oxidant are reviewed first, and then the 2D sections of the 3D phase diagram along planes of constant values of one of the variables involved are explored in detail. Finally, the structure of the 3D diagram is presented by means of appropriate figures and is discussed by examining in detail the connections between the critical curves, surfaces and volumes in 3D space with the corresponding critical features, including points, curves and surfaces, of the 2D sections. In addition, the possible application of thermodynamic diagrams to predict the nature of the oxidation products is briefly discussed in combination with the concept of oxidation maps.

    更新日期:2019-11-04
  • Oxidation Behaviour of Steel During hot Rolling by Using TiO 2 -Containing Water-Based Nanolubricant
    Oxid. Met. (IF 1.805) Pub Date : 2019-07-13
    Hui Wu, Chengyang Jiang, Jianqiang Zhang, Shuiquan Huang, Lianzhou Wang, Sihai Jiao, Han Huang, Zhengyi Jiang

    The formation and performance of oxide scale on a low-alloy steel were investigated during hot rolling at 850 and 950 °C under various lubrication conditions, including benchmarks (dry condition and water) and water-based nanolubricants containing various concentrations of nano-TiO2 from 1.0 to 8.0 wt%. The results showed that the addition of nano-TiO2 particles in the lubricant significantly reduced the thickness of oxide scale and surface oxide roughness. The reduction reached the maximum when the concentration of TiO2 was 4.0 wt%. Detailed oxide phase characterisation and oxide component fraction determination revealed that hot rolling destroyed the conventional multi-layer oxide scale and promoted magnetite and haematite formation because of easy access of oxygen from the deformed structure. The effect of TiO2 was explained by the decrease in the rolling force, which led to a higher fraction of dense retaining wustite and therefore reduced the extent of further oxidation. Increasing temperature did not change the trend of lubrication effect but raised the rate of steel oxidation in general.

    更新日期:2019-11-04
  • Oxide Dispersion Strengthened Bond Coats with Higher Alumina Content: Oxidation Resistance and Influence on Thermal Barrier Coating Lifetime
    Oxid. Met. (IF 1.805) Pub Date : 2019-07-29
    C. Vorkötter, S. P. Hagen, G. Pintsuk, D. E. Mack, S. Virtanen, O. Guillon, R. Vaßen

    The oxidation resistance of the bond coat in thermal barrier coating systems has significant influence on thermal cycling performance of the protective coating. In this study, the influence of varying the alumina content of plasma-sprayed oxide dispersion strengthened bond coats with CoNiCrAlY matrix material on the oxidation resistance was analysed by thermogravimetric analysis, SEM and TEM. Yttrium ions at the alumina scale grain boundaries and the grain size in the scale appear as major factors influencing oxidation properties. The ODS material with 2, 10 and 30 wt% alumina content was applied in TBC systems as an additional thin bond coat. The thermal cycling performance of those advanced TBC systems, in burner rig tests, was evaluated with respect to the ODS material properties. Thermal cycling behaviour is in good correlation with the isothermal oxidation resistance. All results indicate that TBC systems with 10 wt% alumina content in the ODS bond coat have a superior thermal cycling performance, as compared to ODS bond coats with lower or higher alumina content.

    更新日期:2019-11-04
  • The Oxidation Behavior of Ni–15Cr–5Al– x Si ( x = 0, 1, 3, 5 wt%) Alloys in Air at 1100 °C
    Oxid. Met. (IF 1.805) Pub Date : 2019-07-06
    Erpeng Wang, DuanJun Sun, Haifei Liu, Mingyang Lu, Wen Guo, Bingxin Zheng, Xiuhai Zhang

    In order to explore effect of silicon on the oxidation resistance of Ni-based superalloys, the cyclic oxidation behavior of Ni–15Cr–5Al–xSi (x = 0, 1, 3, 5 wt%) at 1100 °C was studied. The results show that the addition of Si promotes the nucleation of metastable and stable alumina, which facilitates selective Al oxidation to enable a rapid formation of continuous metastable and stable alumina scales, which in turn greatly enhances the oxidation resistance of alloys. The addition of alloyed Si can also enhance the adhesion between matrix and oxide scale. It was determined that the optimal Si content for scale adherence was 3 wt%. However, the adhesion between oxide scale and matrix decreased with higher Si content. One reason could be that the volume shrinkage caused by transformation from metastable alumina to stable alumina. The other is an increase in the thermal stresses generated in the scale with higher silicon contents.

    更新日期:2019-11-04
  • Behaviour of Metallic Materials in Simulated Service Environments of CO 2 /H 2 O Co-electrolysis Systems for Power-to-X Application
    Oxid. Met. (IF 1.805) Pub Date : 2019-07-25
    J. Zurek, N. Margaritis, D. Naumenko, N. H. Menzler, W. J. Quadakkers

    In the present study, the ferritic steel Crofer 22 H as potentially suitable interconnect material for SOEC stacks as well as joints between the steel and Ni- and CuNi contact materials was investigated with respect to the behaviour in simulated service environments of an SOEC system for CO2/H2O co-electrolysis. Exposures up to 1000 h at temperatures between 600 and 800 °C were carried out in CO2/H2O- and CO/H2-rich gases, thus simulating conditions at the stack inlet and outlet, respectively. It was found that the steel formed protective surface oxide scales consisting of chromia and/or Cr/Mn spinel in all studied test conditions. No indication of carbon transfer from the gas atmosphere into the steel was found even in the high carbon activity CO/H2-rich gas simulating stack outlet conditions. However, in the latter gas substantial carbon transfer from the gas to the steel via the Ni- or CuNi-wires resulted in the formation of a carburized zone with substantial M23C6 and/or M7C3 precipitate formation. This effect was more pronounced for the joints of the steel with the Ni-wire than with the CuNi-wire. In the gas simulating the service environment at the stack inlet, only minor carbon transfer was found in case of the Ni/steel joint at 600 °C but not at 800 °C. In case of the CuNi-wires, partial loss of contact between wire and interconnect steel and formation of Kirkendall voids as a consequence of interdiffusion between wire and steel were observed. The experimental results are discussed using thermodynamic considerations involving gas equilibria and stability of possible external and/or internal formation of oxide and carbide phases.

    更新日期:2019-11-04
  • Oxidation Behavior of γ′-Ni 3 Al-Based Ni–20Al–5Cr Alloys With and Without Reactive Elements Under Different Heating Conditions
    Oxid. Met. (IF 1.805) Pub Date : 2019-06-29
    Wei-Ting Chen, Brian Gleeson, Arthur Heuer

    High-temperature oxidation of a model γ′-Ni3Al-based Ni–20Al–5Cr alloy and versions of this alloy doped with Y, Hf, or Si, or some combination of these, was investigated. Oxide scale evolution was characterized using SEM, TEM, and TOF–SIMS. The resulting scale microstructures depended quite sensitively on the heating rate to the oxidation temperature of 1100 °C. However, whatever the heating rate, the dopant additions improved the oxidation resistance of the base alloy. The reactive elements Y and Hf, if segregated to the surface during preheating under conditions where appreciable oxidation was not possible, suppressed transient oxidation and facilitated the exclusive growth of the thermodynamically stable α-Al2O3 scale.

    更新日期:2019-11-04
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