Oxidation of molybdenum-tungsten-chromium-silicon alloys

The oxidation kinetics of (50–60) wt.% Mo-(35–47) wt.% Cr-(2–5) wt.% Si and (30–40) wt.% Mo-(30–40) wt.% W-(27–37) wt.% Cr-(0–3) wt.% Si alloys were studied between 1000 and 1200°C in a pure oxygen atmosphere. The oxidation of Mo-W-Cr-Si alloys resembled that of Mo-Cr-Si alloys but was much more oxidation resistant. In general, oxidation resistance increased with increasing Cr and Si content. Alloys with good oxidation behavior had a thin outer Cr₂O₃ layer and an internal oxidation zone (in both Mo and Mo-W alloys). Alloys displaying poor oxidation behavior had a porous Cr₂O₃ layer (in Mo alloys) or layers of oxides of W and Cr (in Mo-W alloys). Although the alloy systems were not truly oxidation resistant, definite improvement in oxidation resistance was achieved.     

Oxidation behavior of Mn and Mo alloyed Fe-16Ni-(5-8)Cr-3.2Si-1.0Al

Oxidation tests were conducted on a master alloy, Fe-16Ni-(5–8)Cr-3Si-lA1, to which (0–4) wt/o pct Mn and/or Mo were added. Tests were conducted at temperatures ranging from 1073–1273 K for times up to 1000 hr. Additions of Mn resulted in formation of a dual oxide structure and decreased oxidation protection. Addition of Mo significantly improved oxidation protection by formation of an intermetallic Fe(Mo)Si precipitate that eventually formed a protective SiO₂ oxide sublayer. Increasing the Cr concentration in an alloy containing both Mn and Mo resulted in a slight increase in weight gain. To first order, the apparent oxidation activation energy for all the alloys was nearly constant, 121 kJ/mol, suggesting that the same mechanism controlled the oxidation for all compositions. The oxidation protection was related to the alloy components and concentration.     

Oxidation behavior at 300–1000 °C of a (Mo,W)Si2-based composite containing boride

The oxidation behavior of a (Mo,W)Si₂ composite with boride addition was examined at 300–1000 °C for 24 h in dry O₂. The oxidation kinetics was studied using a thermobalance, and the oxide scales were analyzed using a combination of electron microscopy (SEM/EDX, FIB, BIB) and XRD. Accelerated oxidation was found to occur between 500 °C and 675 °C, with a peak mass gain at 625 °C. The rapid oxidation is attributed to the vaporization of molybdenum oxide that leaves a porous and poorly protective silica layer behind. At higher temperature (700–1000 °C) a protective scale forms, consisting of a dense SiO₂/B₂O₃ glass.     

Cyclic oxidation behaviour of two-phase Ni---Cr---Al alloys at 1100°C

The cyclic oxidation behaviour in static air of four Ni---Cr---Al alloys of different chromium contents was studied at 1100°C. All of the alloys were two-phase, consisting of nickel-rich β-NiAl and α-Cr. The volume fractions of the two phases were different in each of the alloys, although their compositions were kept relatively constant. All of the alloys exhibited relatively large weight gains and formed an external scale containing Al₂O₃. The large weight gains were attributed to extensive internal Al₂O₃ formation which was in turn associated with scale spallation. The extent of internal attack was greatest for alloys containing up to 30 at.% Cr. In these alloys, a destabilization of the original α-Cr + β-NiAl equilibrium to γ-Ni + β-NiAl occurred as a result of the selective oxidation of aluminium. For an alloy containing 40 at. % Cr, the selective oxidation of aluminium resulted in a destabilization of the α-Cr + β-NiAl equilibrium to α-Cr + γ-Ni. Such a destabilized structure was more resistant to internal Al₂O₃ formation.     

The third-element effect in the oxidation of Ni–xCr–7Al (x = 0, 5, 10, 15 at.%) alloys in 1 atm O2 at 900–1000 °C

The oxidation in 1 atm of pure oxygen of Ni–Cr–Al alloys with a constant aluminum content of 7 at.% and containing 5, 10 and 15 at.% Cr was studied at 900 and 1000 °C and compared to the behavior of the corresponding binary Ni–Al alloy (Ni–7Al). A dense external scale of NiO overlying a zone of internal oxide precipitates formed on Ni–7Al and Ni–5Cr–7Al at both temperatures. Conversely, an external Al₂O₃ layer formed on Ni–10Cr–7Al at both temperatures and on Ni–15Cr–7Al at 900 °C, while the scales grown initially on Ni–15Cr–7Al at 1000 °C were more complex, but eventually developed an innermost protective alumina layer. Thus, the addition of sufficient chromium levels to Ni–7Al produced a classical third-element effect, inducing the transition between internal and external oxidation of aluminum. This effect is interpreted on the basis of an extension to ternary alloys of a criterion first proposed by Wagner for the transition between internal and external oxidation of the most reactive component in binary alloys.     

Effect of Ti content and nitrogen on the high-temperature oxidation behavior of (Mo,Ti)5Si3

The binary intermetallic compounds Mo₅Si₃ (T1) and Ti₅Si₃ are prone to rapid oxidation below 1000 °C. Recent investigations on (Mo,Ti)₅Si₃, however, revealed that macro-alloying with 40 at.% Ti can result in a very good oxidation resistance in a wide temperature range (750–1300 °C) due to the formation of a duplex layer composed of a silica matrix with dispersed titania. Additionally, Ti decreases density making (Mo,Ti)₅Si₃ a promising key constituent of quaternary Mo-Si-B-Ti alloys considered for ultrahigh temperature structural applications. The aim of this study is to obtain an in-depth understanding of the influence of different Ti concentrations as well as of nitrogen on the oxidation behavior of (Mo,Ti)₅Si₃ at intermediate and elevated temperatures. The microstructure and oxidation mechanisms were analyzed using various experimental techniques. The experimental results were supported by ab initio and thermodynamic calculations.     

Sulfidation behavior of Fe-27Mn-(0-17.3)Mo(a/o) alloys

Iron-base alloys containing ca. 27 a/o (atomic percent) manganese and up to 17.3 a/o molybdenum were sulfidized in H₂/H₂S gases of 4 Pa sulfur partial pressure at temperatures of 700–1000° C. Three-layered scales developed on all the molybdenum-containing alloys, and an internal sulfidation zone was observed in most cases. The overall scaling process and individual layer growth all followed parabolic kinetics. The outer and intermediate layers comprised Fe(Mn)S and Mn(Fe)S, respectively. Sulfidation rates varied with the morphology and constitution of the inner layer. The reaction product Fe_xMo₆S_8–z, which was restricted to the inner layer, is permeable to sulfur, iron and manganese, but not molybdenum.For high-molybdenum levels, the overall scaling rate decreased, as a result of the slow diffusion of iron in Fe_xMo₆S_8–z. For low-molybdenum levels, this beneficial effect is small and outweighed by the formation of an inner two-phase layer.     

High-temperature oxidation of Fe-Cr-Al-Si alloys extruded into honeycomb structures

The oxidation behavior of Fe–20Cr–5Al–(0.5–5)Si and Fe–(12–20)Cr–(5–7)Al–(1–2)Si alloys extruded into honeycomb structures has been investigated at 1150°C in air for up to 500 hr. The oxidation weight gains decrease with increasing Si and Cr contents in the 5-Al alloys. Si additions are more efficient than Cr additions to reduce the weight gain. Increasing Si content in the 5-Al alloys suppresses the formation of an iron-chromium complex oxide, forming mullite and vitreous silica in the scale, although the location is not clearly indicated. The 5-Si alloy shows anisotropy in elongation of the honeycomb specimen during oxidation in the Fe–20Cr–5Al–xSi alloys, whereas alloying with Si and Cr does not improve the oxidation resistance of the 7-Al alloys significantly. These results are explained by Wagner's theory of a secondary getter. However, we point out additionally that the difference between Si and Cr in the Pilling-Bedworth ratio and the solubility of their oxides in the Al₂O₃ scale may contribute to the significant effect of Si additions. Finally, this paper demonstrates that the selected Fe–Cr–Al–Si honeycombs having walls 200 m thick show excellent oxidation resistance over 500 hr at 1150°C in air. The time to catastrophic oxidation is roughly proportional to the wall thickness in extruded honeycombs.     

Oxidation behaviour of Mo–Si–B–(Al,Ce) ultrafine-eutectic dendrite composites in the temperature range of 500–700 °C

A comparative study has been carried out to investigate the effects of Al and/or Ce additions on microstructure of Mo–Si–B alloys and their isothermal oxidation behaviour at 500 and 700 °C in laboratory air for 24 h. Microstructure of arc melted Mo–Si–B–(Al, Ce) alloys consists of bcc α-Mo dendrites embedded in ultrafine lamellar Mo₃Si and Mo₅SiB₂ eutectic matrix. Isothermal oxidation kinetics of ultrafine structured Mo–Si–B alloy at 500 °C has been found to show hardly any mass change during 24 h exposure. Addition of Al to Mo–Si–B alloy refines the microstructure, decreases the net mass loss at 700 °C by ～43%, whereas Ce does not bring about any significant change. The enhanced oxidation resistance of Mo–Si–B–Al alloy is due to the formation of Al–O rich inter-layer at the alloy/oxide interface along with the formation of a protective and dense Al₂(MoO₄)₃ outer layer, which reduces the sublimation of MoO₃ at 700 °C. Various transient/complex oxides formed on the alloys during their high temperature exposure have been examined to determine the oxidation mechanisms.     

High temperature oxidation of Mo–Si–B alloys: Effect of low and very low oxygen partial pressures

The oxidation mechanism of a Mo–Si–B alloy in two different oxygen partial pressure ranges was investigated between 820 and 1200 °C. Oxygen partial pressures between 10^?19 and 10^?12 bar were applied in order to suppress Mo oxide formation. Weight gain kinetics were determined resulting from simultaneous external and internal oxidation. Silica scale formation was found to lead to a droplet shape because of the high evaporation rates of B₂O₃ and limited wetting of the silica. In the oxygen partial pressure range 10^?6–10^?4 bar Mo–Si–B alloys suffer from severe degradation due to continuous formation of volatile MoO₃. Catastrophic oxidation was observed as a consequence of the formation of a highly porous and non-protective silica scale.     

Air Oxidation of Ni–Ti Alloys at 650–850°C

The oxidation of pure Ni and three Ni–Ti alloys containing 5, 10, and 15 wt.% Ti over the temperature range 650–850°C in air was studied to examine the effect of titanium on the oxidation resistance of pure nickel. Ni–5Ti is a single-phase solid solution, while the other two alloys consisted of nickel solid solution (-Ni) and TiNi₃. The oxidation of Ni–Ti alloys at 650°C follows an approximately parabolic rate law and produces a decrease in the oxidation rate of pure Ni by forming an almost pure TiO₂ scale. At higher temperatures, Ni–Ti alloys also follow an approximately parabolic oxidation, and their oxidation rates are close to or faster than those of pure Ni. Duplex scales containing NiO, NiTiO₃ and TiO₂ formed. Some internal oxides of titanium formed, especially at 850°C. In addition, the two-phase structure of Ni–10Ti and Ni–15Ti was transformed into a single-phase structure beneath the scales.     

Effects of Si, W and W–Mo on isothermal oxidation behaviors of Nb/Nb5Si3 in situ composites at high temperature

The effects of Si, W and W–Mo on the isothermal oxidation behaviors of Nb/Nb₅Si₃ in situ composites in static air at 1000 and 1200 °C for 20–100 h were investigated on as-cast materials. The results show that the oxidation kinetics of each alloy was not changed whether at 1000 or 1200 °C, and the oxidation mechanism were not changed. The oxidation resistance of Nb/Nb₅Si₃ in situ composites was sensitive to Si content, and the oxidation rate of Nb-10Si alloy was more than twice as many that of Nb–20Si alloy. By alloying of W, the oxidation resistance of Nb–20Si–10W alloy was improved significantly, because the WO₃ scale can provide the adherence for the creaked Nb₂O₅ scale and reduce the diffusion of oxygen through the scale. Comparing to alloying with W, the poor oxidation resistance of Nb–20Si–10W–10Mo alloy was attributed to the evaporation of MoO₃ and highly porous scale.     

High-temperature oxidation of directionally solidified Ni-Cr-Nb-AI (γ/γ′-δ) eutectic alloys

The oxidation of Ni-23.1Nb-4.4Al and Ni-19.7Nb-6 Cr-2.5Al alloys in air at temperatures in the range 870–1100°C has been studied for times up to 168 hr, in the as-cast, slowly cooled, and directionally solidified forms. The oxidation rate decreases with increasing temperature for the ternary alloy, and this appears to be due to the increasing tendency to establish a continuous Al₂O₃ layer at the metal surface, although at no temperature in this range is a complete layer established. At the lowest temperature the -Ni₃Nb lamellae are preferentially oxidized, with fingers of oxide extending into the metal, but at 900°C and above a continuous single-phase 8-free layer is established at the metal surface very early in the oxidation. The oxidation rate of the quaternary alloy increases with increasing temperature. At the lower temperatures a continuous Al₂O₃ layer is established at the metal surface, but at the highest temperature the aluminum oxidizes internally and a continuous layer is not established, internal oxidation penetrating down the lamellae. It appears that niobium, like chromium, is able to promote the formation of external Al₂O₃ layers; if this fact is accepted, the beneficial role of chromium in these alloys is difficult to explain.     

Oxidation Resistance of Ti5Si3 and Ti5Si3Zx at 1000°C (Z = C,N, or O)

The oxidation behavior of Ti₅Si_3+y (y=0 or 0.2) and Ti₅Si₃Z_x (Z=C, N or O, x=0.25 or 0.5) was studied at 1000°C in air or argon–oxygen mixtures for up to 500 h. Ti₅Si₃ has poor oxidation resistance in air because of the formation of an oxide scale rich in rutile and subscale formation of TiN, TiSi, TiSi₂ and Si. In contrast, Ti₅Si_3.2 has excellent oxidation resistance because of the formation of a silica scale. Samples with interstitial oxygen or nitrogen show only slight improvements in the early stages of oxidation, compared to Ti₅Si₃, which is in stark contrast to previous research. However, samples with interstitial carbon displayed excellent oxidation resistance at 1000°C, consistent with previous research.     

Oxidation of molybdenum-chromium-palladium alloys

Isothermal and cyclic oxidation studies have been carried out on (80–90) wt.% Mo-(8–17) wt.% Cr-(0.5–3) wt.% Pd alloys in atmospheres of both air and pure oxygen at temperatures between 1000 and 1250°C. The Pd additions decreased the oxidation rate with the most pronounced effect being observed for an alloy of 80 wt.% Mo-17 wt.% Cr-3 wt.% Pd. Palladium played a major role in providing the necessary oxidation protection by accelerating the formation of Cr₂O₃ layers at low Cr concentrations. Contrary to the behavior of most metals, an increase in oxidation resistance with increase in temperature was observed. Although the alloy systems were not truly oxidation resistant, definite improvement in oxidation resistance was achieved. The oxidation mechanism of Mo-Cr-Pd alloys is described.     

Effects of aging at 475 °C on corrosion properties of tungsten-containing duplex stainless steels

Effects of aging at 475 °C on the corrosion and mechanical properties of Fe–25Cr–7Ni–0.25N–xMo–yW (x=0–3, y=0–6) duplex stainless steels were investigated by an anodic polarization test in HCl solution, a modified double-loop electrochemical potentiodynamic reactivation (DL-EPR) test, and an impact test. Corrosion resistance of the alloys was degraded with aging at 475 °C due to the depletion of Cr around α^′ precipitates where numerous micropits were formed during the anodic polarization test. Especially for over-aged alloys, a second anodic current loop appeared in the passive region during the anodic polarization in 1 M HCl solution. The peak value of the second anodic current loop as well as the ratio of the maximum current in reactivation loop to that in anodic loop (i_r/i_a) determined from the modified DL-EPR test were found to be an effective measure of the precipitation of α^′-phase during the aging. However, the degradation in corrosion resistance and impact toughness of the alloys during the aging was retarded with an increase in the W content of the designed DSS, suggesting that W in duplex stainless steels delays the precipitation rate of α^′-phase due to a slower diffusion rate of W compared with that of Mo in ferrite. Influences of aging on the galvanic corrosion behaviors between austenite and ferrite phases were discussed by atomic force microscopy observation.     

Early oxidation behaviors of Mg–Y alloys at high temperatures

The early oxidation behaviors of Mg–Y alloys (Y = 0.82, 1.09, 4.31 and 25.00 wt.%) oxidized in pure O₂ have been investigated at high temperatures. The results showed that the oxidation behaviors of the Mg–Y alloys (Y = 4.31 and 25.00 wt.%) obeyed a parabolic law, while that of the Mg–Y (Y = 0.82 and 1.09 wt.%) exhibited both parabolic and linear kinetics depending on the oxidation temperature. Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses indicated that an oxide film with a single structure composed of MgO and Y₂O₃ had formed. Moreover, the higher the oxidation temperature was, the thicker the oxide film was. Finally, the corresponding oxidation mechanism has been discussed, and the improved oxidation resistance of the Mg–Y alloys can be due to the formation of a continuous Mg-dissolving Y₂O₃ protective film.     

Oxidation of γ-Ni−Al−Si alloys at 1073 K

The formation and development of oxides in Ni–4Al and Ni–4Al–xSi (at.%, x=1, 3, 5) alloys at 5–9×10^–6 and 1 atm oxygen pressure at 1073 K have been studied. The oxidation rate increased with an increase of silicon content in the alloy at the early stage of oxidation, but decreased after longer time exposure due to formation of an intermediate layer composed of NiO and spinel (NiAl₂O₄ and Ni₂SiO₄) between the top NiO layer and the internal-oxidation zone. This intermediate layer became a barrier for releasing stress, generated by the volume expansion associated with oxidation of solute atoms, resulting in high dislocation density and severe distortion in the internal-oxidation zone for the Ni–Al–Si alloys. In Ni–4Al alloy where no complete intermediate-layer formation occurred, stress was easily released by an enhanced vacancy gradient, and therefore an enhanced vacancy-injection rate into the alloy, resulting in a higher oxidation rate than the situation where a sample was oxidized at an oxygen pressure associated with the dissociation of NiO.     

Effect of H3BO3, BCl3, and BF3 pretreatments on oxidation of molten Al-Mg alloys in air

The effect of treating Al-Mg alloys with boron compounds on subsequent oxidation rates of the molten metal in air was determined by gravimetry. Untreated Al alloys containing 4.0–4.5% Mg gained over 10% weight, corresponding to complete consumption of Mg in spinel formation, in 75–100 hr in air at 750°C. Samples adequately treated before melting by dusting H₃BO₃ on the surface, by dipping in aqueous H₃BO₃ and drying, or by passing BCl₃ (but not BF₃) over the surface for 1 min gained less than 0.1% weight in up to 190 hr at 750°C. In all cases, B₂O₃ incorporated into the MgO surface film apparently provided this oxidation inhibition.