Formation Of Protective Alumina Scale Research Articles

Effect of Nb and Cu Additions on High Temperature Oxidation Behavior and Mechanical Properties of Al2O3-Forming Austenitic Alloys

The addition of Cu to austenitic steels was found to be effective for decreasing the critical Al content and promoting protective alumina scale formation from our recent studies. It was also reported that Nb addition to the alloy promotes the formation of alumina. These two elements are also known to improve the mechanical properties of steels by forming fine precipitates of Cu and intermetallics such as Laves and γ” phases, thus combined addition of these elements is expected to improve both the oxidation and mechanical properties of steels, however, there is no systematic study on the effects of the combined addition of these elements on the oxidation resistance and mechanical properties of austenitic heat-resistant alloys. The aim of this study was to investigate the effects of Nb and Cu on the oxidation behavior and mechanical properties of austenitic heat-resistant alloys containing Nb and Cu.Fe-25Ni-17Cr-(5~8)Al-(0, 0.5, 1)Nb-(0, 3, 5)Cu (in at.%) alloys were prepared by Ar-arc melting pure constituent metals. The ingots were cold-rolled by 90% and heat-treated at 1200ºC for 1 h, followed by aging at 800 ºC or 600 ºC for up to 500 h. Oxidation samples were cut from the cold-rolled plates and polished down to a 3 µm diamond paste. Oxidation tests were performed at 800°C in a box furnace in air for up to 100 h. The alloys were also subjected to a Vickers hardness test at room temperature and tensile tests at room temperature and 600ºC. The tensile test was performed in Ar at a strain rate of 0.005mm/s. Samples were analyzed by SEM, STEM, TEM, EPMA, GD-OES and XRD.Fig. 1 shows the oxidation kinetics and room temperature hardness of the alloys with/without different Nb and Cu contents. The oxidation kinetics of the alloys with/without different Nb and Cu contents shown in Fig. 1(a) and (b) reveals that the oxidation mass gain of the alloys without Cu decreased significantly by the addition of Nb and it further decreased with increasing Nb content. The oxidation mass gain of the 1Nb doped alloys tended to increase slightly with increasing Cu content. From the STEM-EDS analysis, it was confirmed that the protective alumina scale was formed on the Cu and/or Nb added alloys. An α-alumina was confirmed to develop on the alloys with Nb addition at about 600 ºC during heating, which is much lower temperature than that reported for formation of an α-alumina, suggesting that Nb addition promotes α-alumina formation. Vickers hardness of all alloys at room temperature increased with aging time, however, the maximum hardness was higher for the alloys aged at 600 ºC than 800 ºC. The hardness of the 5Cu alloy was higher than 0Cu alloy, which might be attributed to a formation of very fine Cu precipitates confirmed by TEM observation. Figure 1

Cathodic Protection Using Aluminum Metal in Chloride Molten Salts as Thermal Energy Storage Material in Concentrating Solar Power Plants

The new generation of concentrated solar power (CSP) plants to be developed presents a great challenge related to the increase in maximum operating temperature since molten salt CSP technologies require alternative salt chemistries such as chloride. The cathodic protection strategy involves the addition of a sacrificial metal to prevent corrosion of the alloy tested as container material in a CSP plant. In this paper, aluminum (Al) metal was analyzed as a corrosion inhibitor in OCT and HR224 alloys, obtaining corrosion rates of 4.37 and 0.27 mm/y, respectively. It has been confirmed that the use of Al metal can reduce the anodic current which is directly related to the corrosion rate. The formation of protective alumina scales (Al2O3) was assessed by scanning electron microscopy (SEM) and X-ray diffraction (XRD), confirming the corrosion model results from electrochemical impedance spectroscopy monitoring tests.

(Invited) Advanced Techniques for the Study of Elementary Mechanisms of Oxide Formation and Growth on Co-Al-W Model Alloys at High Temperatures

γ′-strengthened Co-base superalloys, based on the Co-Al-W system and its modifications, show high potential for use at high temperatures, with promising mechanical properties. A challenge remaining is the high-temperature oxidation behavior, as oxidation resistance above 800 °C is relatively poor as compared for instance to corresponding Ni-based superalloys. This is due to hindered formation of continuous, protective Al2O3 oxide layer. The presentation will discuss elementary mechanisms in the early stages of oxidation of Co-Al-W model alloys. A special focus of our studies is to clarify the role of the γ/γ'-microstructure in the formation and growth of oxide scales. In situ oxidation experiments in SEM, ex situ high-resolution TEM, and further surface analytical techniques were employed to determine critical factors and mechanisms of high-temperature oxidation of this alloy system. The results demonstrate the key role of the ratio of elemental fluxes (diffusion of oxygen and aluminum) on the formation of protective alumina scales.

Surface preparation effect on oxidation kinetics of Ni-base superalloy

In the present study an oxidation behavior of commercially available Ni-base superalloy IN 713C with different surface finishing, namely polished up to 1 μm diamond paste (resulting in Ra = 0.023) and ground by SiC paper with 80 grit (Ra = 1.23), were investigated at 950°C in air. It was found that ground sample developed very thin, protective α-Al2O3 scale. Moreover, locally formation of θ-Al2O3 in the forms of spikes was observed. For the sample with polished surface formation of outer chromia scale accompanied with internal oxidation of aluminum was observed. Moreover, for specimen which does not form continuous alumina scale or sub-scale, the GD-OES depth profiles showed an enrichment of boron at the oxide scale/alloy interface. It was proposed that the formation of protective alumina scale on ground samples is caused by introducing the defects into the near-surface region of the material, which are an easy diffusion paths for the elements forming a protective oxide scales.

Development of 1100 °C Capable Alumina-Forming Austenitic Alloys

Recently developed alumina-forming austenitic (AFA) alloys based on ~12–32 weight % (wt%) Ni offer an attractive combination of oxidation resistance and creep resistance at relatively low alloy cost. However, they exhibit a transition to internal oxidation and nitridation of Al above ~750–950 °C depending on composition and exposure environment. In order to identify AFA compositions capable of higher-temperature operation for applications such as ethylene cracking, the oxidation behavior of a series of developmental, as-cast nominal Fe–(25–45)Ni–(10–25)Cr–(4–5)Al–1Si–0.15Hf–0.07Y–0.01B wt% base alloys with and without Nb, Ti, and C additions was evaluated at 1100 °C in air with 10% water vapor. Protective alumina scale formation was observed at levels of 35Ni, 25Cr, and 4Al with additions of Nb and C, indicating promise for 1100°C capable cast AFA alloys.

Long-Term Oxidation of Candidate Cast Iron and Stainless Steel Exhaust System Alloys from 650 to 800 °C in Air with Water Vapor

The oxidation behavior of candidate cast irons and cast stainless steels for diesel exhaust systems was studied for 5,000 h at 650–800 °C in air with 10 % H2O. At 650 °C, Ni-resist D5S exhibited moderately better oxidation resistance than did the SiMo cast iron. However, the D5S suffered from oxide scale spallation at 700 °C, whereas the oxide scales formed on SiMo cast iron remained relatively adherent from 700 to 800 °C. The oxidation of the cast chromia-forming austenitics trended with the level of Cr and Ni additions, with small mass losses consistent with Cr oxy-hydroxide volatilization for the higher 25Cr/20–35Ni HK and HP type alloys, and transition to rapid Fe-base oxide formation and scale spallation in the lower 19Cr/12Ni CF8C plus alloy. In contrast, small positive mass changes consistent with protective alumina scale formation were observed for the cast AFA alloy under all conditions studied. Implications of these findings for exhaust system components are discussed.

Increasing the Upper Temperature Oxidation Limit of Alumina Forming Austenitic Stainless Steels in Air with Water Vapor

A family of alumina-forming austenitic (AFA) stainless steels is under development for use in aggressive oxidizing conditions from ~600–900 °C. These alloys exhibit promising mechanical properties but oxidation resistance in air with water vapor environments is currently limited to ~800 °C due to a transition from external protective alumina scale formation to internal oxidation of aluminum with increasing temperature. The oxidation behavior of a series of AFA alloys was systematically studied as a function of Cr, Si, Al, C, and B additions in an effort to provide a basis to increase the upper-temperature oxidation limit. Oxidation exposures were conducted in air with 10% water vapor environments from 800–1000 °C, with post oxidation characterization of the 900 °C exposed samples by electron probe microanalysis (EPMA), scanning and transmission electron microscopy, and photo-stimulated luminescence spectroscopy (PSLS). Increased levels of Al, C, and B additions were found to increase the upper-temperature oxidation limit in air with water vapor to between 950 and 1000 °C. These findings are discussed in terms of alloy microstructure and possible gettering of hydrogen from water vapor at second phase carbide and boride precipitates.

Microstructural Characterization of Silicon Added Titanium Aluminide

Titanium aluminides intermetallic compounds have received great attention during the past decade, since they have the potential, in aircraft and automotive engines, to replace the high density Ni-base superalloys However, these intermetallics possess poor oxidation properties at high temperatures. Previous studies showed that protective alumina scale formation on γ-TiAl can be obtained by small additions (around 2 at.%) of Ag. In the present study, a number of cast Ti–Al–Si alloys were investigated in relation to transient oxide formation in air at 1300°C. After various oxidation times the oxide composition, microstructure and morphology were studied by combining a number of analysis techniques. The TiAl–Si alloys appear to form Al Ti and Si oxides. However, the formation of silicon oxide at the interface of base metal and scale slows down the oxidation rate significantly.

Alloying effects on creep and oxidation resistance of austenitic stainless steel alloys employing intermetallic precipitates

Microstructure evolution during creep testing at 750 °C and 100 MPa of Fe–20Cr–30Ni–2Nb (at.%) alloys with and without 0.4 Si, 0.2 Zr or 5.0 Al additions has been studied, in order to explore the viability of Fe-rich austenitic stainless alloys strengthened by intermetallic phases. Fine Fe 2Nb Laves phase dispersions with the size of less than 1 μm within the γ-Fe matrix were obtained in the base and Si-modified alloys after aging at 800 °C. The addition of Si helped to refine and stabilize the size of particles, resulting in finer and denser Fe 2Nb dispersion than that in the base alloy. The alloys with only solution heat-treatment exhibited superior creep resistance to the aged ones, which is due to dynamic precipitation of the Fe 2Nb Laves phase during creep testing with a size of 300–400 nm, resulting in more effective pinning of dislocations. The base alloy also showed the meta-stable γ″-Ni 3Nb phase with a size of 50 nm during the transient state of the creep testing. The Zr-modified alloy achieved significant improvement of creep properties. This is hypothesized to be due to the stabilization of δ-Ni 3Nb phase relative to Fe 2Nb, resulting in the formation of multiple fine dispersions of stable intermetallic compounds of δ and Fe 2Nb within the γ-Fe matrix. A small amount of a {Ni, Zr and Nb}-enriched unidentified phase was also observed. The addition of Al significantly improved the oxidation resistance because of the formation of protective alumina scales on the surface. The alloy also showed superior creep resistance to that of the base alloy due to the formation of a dense dispersion of spherical Ni 3Al, typically 30 nm in diameter. Collectively, these findings provide the alloy design basis for creep and oxidation-resistant austenitic stainless steel alloys via intermetallic precipitates.

Effects of minor alloy additions and oxidation temperature on protective alumina scale formation in creep-resistant austenitic stainless steels

Niobium was found to be beneficial for alumina scale formation in newly developed creep-resistant austenitic stainless steels. Additions of both Ti and V degraded alumina scale formation, whereas alloys with individual additions of Ti or V were able to form protective alumina scales. The ability to form external alumina scales was lost in the lower Al- and Nb-containing alloys between 800 and 900 °C. Compositions with the potential to form alumina at 900 °C were identified.

Effect of Combined Yttrium and Zirconium Additions on Protective Alumina Scale Formation on High Purity FeCrAl Alloys during Oxidation in the Temperature Range of 1200 to 1300°C

Effect of Combined Yttrium and Zirconium Additions on Protective Alumina Scale Formation on High Purity FeCrAl Alloys during Oxidation in the Temperature Range of 1200 to 1300°C

Transient oxidation of alumina forming Ti–Al–Ag-based alloys and coatings studied by SEM, AFM, XPS and LRS

γ-TiAl based intermetallics possess poor oxidation properties at temperatures above approximately 700°C. Previous studies showed that protective alumina scale formation on γ-TiAl can be obtained by small additions (around 2 at.%) of Ag. Recently, this type of materials has therefore been proposed as oxidation resistant coatings for high strength TiAl alloys. In the present study, a number of cast Ti–Al–Ag alloys and magnetron sputtered Ti–Al–Ag coatings were investigated in relation to transient oxide formation in air at 800°C. After various oxidation times the oxide composition, microstructure and morphology were studied by combining a number of analysis techniques, such as SEM, ESCA, AFM and LIOS-RS. The γ-TiAl–Ag alloys and coatings appear to form an α-Al2O3 oxide scale from the beginning of the oxidation process, in spite of the relatively low oxidation temperature of 800°C. The formation of metastable alumina oxides seems to be related to the presence of Ag-rich precipitates in the alloy matrix.

The role of chromium in protective alumina scale formation during the oxidation of ternary TiAlCr alloys in air

Ternary TiAlCr alloys show promise for practical application as coating materials due to their excellent oxidation stability in air at elevated temperatures. The main feature of these alloys is the formation of a stable alumina layer at the surface during oxidation. Some alumina forming alloys were studied i.e. two ternary TiAlCr alloys, Ti 0.25Al 0.67Cr 0.08 (τ-phase) and Ti 0.28Al 0.44Cr 0.28 (Laves phase-based alloy), and compared with a binary Ti 0.25lAl 0.75 alloy and some ‘model’ single phase alloys. An intensive study of the surface characteristics using scanning and transmission electron microscopy, scanning Auger spectroscopy and secondary ion mass spectroscopy was performed. It was shown that to achieve an oxidation resistance greater than for the binary alloy, the components in a ternary TiAlCr alloy must collaborate synergistically during oxidation. Cr alloying of titanium aluminides leads to the formation of a fine-grained, protective alumina layer with little or no rutile formation. The multi-phase component Laves and τ phase alloys have superior oxidation resistance to single component alloys. Possible explanations of this phenomenon are discussed.

Fundamental considerations for the development of oxidation-resistant alloys and coatings based on γ-TiAl

Alloys based on γ-TiAl are promising high-temperature materials that may replace conventional heat-resistant steels and superalloys in applications where high strength in combination with low density is required. However,an important hindrance to the use of γ-TiAl alloys at high temperatures is their relatively poor oxidation resistance and sensitivity against environmentally induced embrittle-ment. This material degradation is related to the poor protective properries of the mixed TiO 2 /Al 2 O 3 suface scales which form on the surface during high-temperature exposure. Recently, it was shown that protective alumina scale formation on γ-TiAl can be obtained by small additions of Ag. This effect was found to be related to the formation of Z phase in the subscale depletion layer at the expense of α 2 -Ti 3 Al. It was found that the beneficial effect of Ag can be suppressed if the alloys contain additional α 2 -stabilizing elements, such as Nb, as is the case for most (semi)commercial, high-strength alloys. Therefore, recent efforts have concentrated on developing Ag-containing γ-TiAl alloys as oxidation-resistant coatings for high-strength titanium aluminides. Preliminary results using magnetron sputtering have shown that, due to the similarities in chemical and physical properties of the coating and base material, the Ag-containing material offers promising potential to be qualified as a coating material for reducing the oxidation-induced degradation of titanium aluminides.

Transient oxidation of alumina forming Ti–Al–Ag-based alloys and coatings studied by SEM, AFM, XPS and LRS

γ-TiAl based intermetallics possess poor oxidation properties at temperatures above approximately 700°C. Previous studies showed that protective alumina scale formation on γ-TiAl can be obtained by small additions (around 2 at.%) of Ag. Recently, this type of materials has therefore been proposed as oxidation resistant coatings for high strength TiAl alloys. In the present study, a number of cast Ti–Al–Ag alloys and magnetron sputtered Ti–Al–Ag coatings were investigated in relation to transient oxide formation in air at 800°C. After various oxidation times the oxide composition, microstructure and morphology were studied by combining a number of analysis techniques, such as SEM, ESCA, AFM and LIOS-RS. The γ-TiAl–Ag alloys and coatings appear to form an α-Al2O3 oxide scale from the beginning of the oxidation process, in spite of the relatively low oxidation temperature of 800°C. The formation of metastable alumina oxides seems to be related to the presence of Ag-rich precipitates in the alloy matrix.

Comparison of the High-Temperature Oxidation of Uncoated and CVD-FBR Aluminized AISI-304 Stainless Steel

Aluminide coatings were obtained by means of the CVD–FBR technique at 525°C for 1.5 hr under a reactive-gas mixture composed of 10 vol.% H2+1 vol.% HCl, the rest being Ar as the fluidizing inert gas. Subsequent heat treatment at temperatures to 900°C was conducted to enhance interdiffusion of the components. As a result, substoichiometric Ni–Al phases were found to form. Uncoated and coated plus heat-treated specimens were then subjected to 950°C oxidation for up to about 200 hr under isothermal conditions. The coated plus heat-treated specimens had much lower oxidation rates than the uncoated ones because of the formation of protective alumina scales. Loss of protective behavior occurred only by spalling of the alumina scales upon cooling from the oxidation temperature. The higher oxidation rates of uncoated specimens have been attributed to nodule formation and minor subsequent spalling of the oxide scale.

The role of Cr in promoting protective alumina scale formation by γ-based TiAlCr alloys— II. Oxidation behavior in air

The oxidation behavior of single-phase γ, single-phase Laves, and two-phase γ + Laves TiAlCr alloys was investigated at 1000°C in air. The addition of 4–5 at % Cr to the γ phase resulted in protective alumina scale formation in dry air, but titania-based nodule formation in humid room air. Two-phase γ + Laves alloys exhibited protective alumina scale formation in both dry air and humid room air. The difference in oxidation behavior between γ and γ + Laves alloys was interpreted in terms of compositional changes at the alloy/scale interface during oxidation.

The role of Cr in promoting protective alumina scale formation by γ-based TiAlCr alloys—I. Compatibility with alumina and oxidation behavior in oxygen

The substitution of greater than 8–10 at.% Cr for Ti in TiAl reduces the level of Al needed for protective alumina scale formation during oxidation, the “Cr effect”. To elucidate the mechanism of the Cr effect, the interaction of binary TiAl and ternary TiAlCr alloys with alumina was examined at 1000°C by hot-pressed diffusion couples. The oxidation behavior of the same alloys was also examined at 1000°C in dry oxygen. The dissolution of alumina in an alloy/alumina diffusion couple correlated with an inability of the alloy to form a protective alumina scale on exposure in dry oxygen because of internal attack. Both of these effects were associated with high oxygen permeability in the alloy. It was postulated that the Cr effect results from the Ti(Cr,Al) 2 Laves phase, which has a low oxygen permeability and is capable of alumina scale formation despite its relatively low Al content of 37–42 at.% Al.

97/00268 Suppression of scale formation during primary hydrogenation of Victorian brown coal. (1). Effect of thermal treatment in solvent on scale form

The oxidation behavior of single-phase γ, single-phase Laves, and two-phase γ + Laves TiAlCr alloys was investigated at 1000°C in air. The addition of 4–5 at % Cr to the γ phase resulted in protective alumina scale formation in dry air, but titania-based nodule formation in humid room air. Two-phase γ + Laves alloys exhibited protective alumina scale formation in both dry air and humid room air. The difference in oxidation behavior between γ and γ + Laves alloys was interpreted in terms of compositional changes at the alloy/scale interface during oxidation.

97/00269 Suppression of scale formation during primary hydrogenation of Victorian brown coal. (2). Effects of hydrothermal treatment of Victorian brown

The oxidation behavior of single-phase γ, single-phase Laves, and two-phase γ + Laves TiAlCr alloys was investigated at 1000°C in air. The addition of 4–5 at % Cr to the γ phase resulted in protective alumina scale formation in dry air, but titania-based nodule formation in humid room air. Two-phase γ + Laves alloys exhibited protective alumina scale formation in both dry air and humid room air. The difference in oxidation behavior between γ and γ + Laves alloys was interpreted in terms of compositional changes at the alloy/scale interface during oxidation.