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 Ni-Cr-1ThO2 alloys at 1000 and 1200°C

The oxidation behavior of Ni-13.5-33.7Cr-1ThO₂ alloys in flowing oxygen at 150 Torr was investigated in the temperature range 1000–1200°C. Gravimetric measurements of the oxidation kinetics have been combined with microstructural studies of the reacted samples in order to evaluate the reaction mechanisms. The oxide products formed on the alloys were a function of Cr content, sample surface preparation, reaction time, and temperature. The presence of ThO₂ appears to produce two effects during alloy oxidation. First, enhanced Cr diffusion to the alloy surface results in rapid formation of a Cr₂O₃ subscale beneath NiO on Ni-13.5Cr-1ThO₂ and selective oxidation of Cr for Ni-22.6Cr-1ThO₂. Second, the mechanism of formation of Cr₂O₃ is apparently different from that for simple Ni-Cr alloys, resulting in about an order of magnitude reduction in the Cr₂O₃ growth rate. The oxidationvaporization of Cr₂O₃ to CrO₃ becomes rate controlling for the higher Cr alloys after only a few hours of exposure at 1200°C.     

The oxidation of Ni-20 wt. % Cr-2ThO2

The oxidation of TD NiCr (Ni-20 wt. % Cr-2ThO₂) has been investigated between 900 and 1200°C, and the oxidation behavior is compared with Ni-30 wt. % Cr and Co-35 wt. % Cr alloys. All alloys develop Cr₂O₃ scales but the weight changes obtained for the NiCr and CoCr alloys show an increase with time whereas above 1000° C the TD NiC shows a progressive loss in weight from the evaporation of CrO₃ from the scale, and the reaction products appear to be formed mainly at the alloy-scale interface. However, no mechanism for its formation has been established.     

Relation between impurities and oxide-scale growth mechanisms on Ni-34Cr and Ni-20Cr alloys. I. Influence of C,Mn, and Si

The oxidation behavior of Ni-Cr alloys (34 and 20 wt.% Cr) was investigated between 850 and 1200°C in oxygen for a maximum duration of about 70 hr. The oxide-growth mechanism is a diffusion process controlled by either outward diffusion of chromium in Cr₂O₃ (Ni-34Cr alloy) or by an increase in grain size (Ni-20Cr alloy). In the case of the Ni-34Cr alloy, low values of chromium diffusion were found for the growth of Cr₂O₃ by taking into account the general equation of Wagner. The influence of impurities (Si, C, Mn, Ni) diffusing from the underlying alloy is analyzed because of their doping effect in the outer oxide scales.     

Effect of internal oxidation pretreatments and Si contamination on oxide-scale growth and spalling

Internal oxidation pretreatments carried out in quartz capsule with a Rhines pack were found to have a profound effect on the subsequent oxidation behavior of alloys. Specimens of Co-15 wt.% Cr, Co-25 wt.% Cr, Ni-25 wt.% Cr, and Ni-25 wt.% Cr-1 wt.% Al were tested at 1100°C after pre-oxidation treatments. Even without the development of internal oxide particles, pretreated binary CoCr and NiCr alloys oxidized with significantly lower rates. Selective oxidation of chromium was observed on the non-Cr₂O₃-forming Co-base alloys, whereas on the Cr₂O₃-forming Ni-base alloys, elimination of base-metal oxide, reduction in the Cr₂O₃ growth rate, and better scale adhesion were found. These effects were more apparent with pre-oxidation temperatures greater than 1000°C and with longer pretreatment times. Contaimination of Si from the quartz is believed to be the cause.     

A TEM study of the oxide scale development in Ni-Cr-Al alloys

In the present study the isothermal oxidation behaviours of Ni-10Cr-5Al, Ni-20Cr-5Al and Ni-30Cr-5Al alloys were investigated. The alloys were oxidised in air for 50 h at 1000 °C. Analytical transmission electron microscopy was used to characterize the morphology, structure and composition of the oxide scale. The oxide formed adjacent to the alloy was α-Al₂O₃ such that the higher was the Cr content of the alloy the easier was its formation. The Ni-30Cr-5Al alloy formed a complete layer of α-Al₂O₃ in the initial stages of oxidation through ‘oxygen gettering’ by Cr. A decrease in scale thickness and an increase in scale adherence were observed with an increase in Cr content from 10 to 30 wt.%.     

Effect of yttrium on the sulfidation behavior of Ni-Cr-Al alloys at 700°C

The sulfidation of Ni-10Cr-5Al, Ni-20Cr-5Al, and Ni-50Cr-5Al, and of the same alloys containing 1% Y, was studied in 0.1 atm sulfur vapor at 700°C. The sulfidation process followed linear kinetics for all the alloys except Ni-50Cr-5Al-1Y, and possibly Ni-50Cr-5Al, which followed the parabolic law. The reaction rates decreased with increasing chromium content in alloys without yttrium, and the addition of yttrium reduced the rates by at least a factor of two for the alloys containing 10 and 20% Cr and by an order of magnitude for Ni-50Cr-5Al. Alloys containing 10 and 20% Cr (with and without yttrium) formed duplex scales consisting of an outer layer of NiS_1.03 and an inner lamellar layer of a very fine mixture of Cr₂S₃ and A1₂O₃ in a matrix of NiS_1.03. The two alloys containing 50% Cr formed only a compact layer of Cr₂S₃, which was brittle and spalled during cooling. The lamellae in the duplex scales were parallel to the specimen surface and bent around corners. The lamellae were thicker than those on Ni-Al binary alloys. The lamellae were also thicker in scales on the 20% Cr alloy than on the 10% Cr alloy. The presence of yttrium refined the lamellae and increased the lamellae density near the scale/metal interface in the 10% alloy, but in the 20% Cr alloy the lammellae were thicker and more closely spaced. Platinum markers were found in the inner portion of the exterior NiS_1.03 layer close to the lamellar zone. A counter-current diffusion mechanism is proposed involving outward cation diffusion and inward sulfur diffusion, although diffusion was not rate controlling for alloys containing 10 and 20% Cr. Auger analysis of scales formed on Ni-50Cr-1Y showed an even distribution of yttrium throughout the layer of Cr₂S₃, suggesting that some yttrium dissolved in the sulfide. The reduced sulfidation rate of samples containing yttrium is explained by the possible dissolution of yttrium as a donor. The presence of Y⁴⁺ would then decrease the concentration of interstitial chromium ions in the N-type layer of Cr₂S₃, which would decrease the reaction rate.     

High temperature corrosion of Ni-20Cr and Ni-15Cr-8Fe alloys in sulfur dioxide

Two nickel-base alloys, Ni-20Cr and Ni-15Cr-8Fe, in the form of wire specimens, have been exposed to 100 mbar of sulfur dioxide between 550 and 850°C. For Ni-20Cr, an outer Cr₂O₃ layer formed only at the beginning of the reaction, but very quickly Ni₃S₂ grew preferentially at the exterior by outward diffusion of nickel. The reaction rate is regulated by an external interfacial process. A barrier effect was noted near 645°C associated with the formation of NiCr₂O₄; a new acceleration takes place above 680°C. The external growth of Ni₃S₂ is attributed to the low radius of curvature of the samples. For Ni-15Cr-8Fe, the reaction mechanism is rather similar, except that no barrier effect occurred. A protective Cr₂O₃ layer formed above 800°C in both cases.     

Effect of alloy grain size and silicon content on the oxidation of austenitic Fe-Cr-Ni-Mn-Si alloys in a SO2-O2 gas mixture

Austenitic Fe-18Cr-20Ni-1.5Mn alloys containing 0, 0.6, and 1.5 wt.% Si were produced both by conventional and rapid solidification processing. The cyclic oxidation resistance of these alloys was studied at 900°C in a SO ₂-O ₂ gas mixture to elucidate the role of alloy microstructure and Si content on oxidation properties in bioxidant atmospheres. All the large-grained, conventionally processed alloys exhibited breakaway oxidation during cyclic oxidation due to their poor rehealing characteristics. The rapidly solidified, fine-grained alloys that contained less than 1.5 wt.% Si exhibited very protective oxidation behavior. There was considerable evidence of sulfur penetration through the protective chromia scale. The rapidly solidified alloys that contained 1.5 wt.% Si underwent repeated scale spallation that led to breakaway oxidation behavior. The scale spallation was attributed to the formation of an extensive silica sublayer in the presence of sulfur in the atmosphere.     

Effect of alloy grain size and silicon content on the oxidation of austenitic Fe-Cr-Ni-Mn-Si alloys in pure O2

Austenitic Fe-18Cr-20Ni-1.5Mn alloys containing 0, 0.6, and 1.5 wt.% Si were produced both by conventional and rapid solidification processing. The isothermal and cyclic oxidation resistance of the alloys were studied at 900°C in pure O₂ to elucidate the role of alloy microstructure and Si content on oxidation properties. The conventionally-processed, large-grained alloy that contained no silicon formed Fe-rich nodules during oxidation. The nodule formation was effectively eliminated by either reducing the alloy grain size by rapid solidification or by adding Si to the alloy. The lowest weight gains were achieved when a continuous silica layer formed between the alloy and the external chromia scale. The formation of the continuous silica layer required a ombination of fine alloy grain size and high Si content. The presence of S in the alloy was found to be detrimental to oxide scale adherence when the silica layer was continuous.     

Oxidation of Fe-8Cr-14Ni-Al-Si-Mn from 700 to 1000°C

This paper reports an investigation into reducing the Cr concentration in commercial-grade stainless steels while maintaining oxidation protection at elevated temperatures. Aluminum and Si were added as partial substitute alloy elements to enhance the reduced operation protection resulting from Cr concentration reduced by approximately 50 pct of that found in stainless steels. The goal of this study was to determine the oxidation mechanism of such an Fe, Al-Si alloy: Fe-8Cr-14Ni-1Al-3.5Si-1Mn. During the initial oxidation period the protection resulted from a thin film of Al₂O₃ over an Fe and Cr spinel. Long-term oxidation protection resulted from the gradual formation of a Cr sesquioxide (Cr₂O₂) inner oxide layer. Eventually an outer oxide layer formed that was a mixed composition spinel of Cr and Mn (MnO · Cr₂O₃). The Al₂O₃, which was part of the original protective layer flaked off early in the oxide testing, and the aluminum oxide that formed later appeared as an internal oxide precipitate.     

The effect of preoxidation on the sulfidation of Ni-20Cr (2–5)Al Alloys

Ni-20Cr alloys with 2, 3.5, and 5 wt.% Al have been preoxidized up to 100 hr at 1000°C in dry H₂, in H₂/23% H₂O and in air and subsequently exposed to an H₂/5% H₂S atmosphere at 750° C. During the preoxidation treatment different types of oxide scales were formed which affect the sulfidation protection in different ways. Optimum results were obtained for alloys with 3.5 and 5 wt.% Al after 20 hr exposure to dry H₂ at 1000°C. A thin Al₂O₃ scale is formed which decreases the sulfur attack by more than one order of magnitude. Preoxidation conditions for Ni-20Cr-2Al alloys in H₂ and for Ni-20Cr-2Al and Ni-20Cr-3.5Al in H₂/H₂O were observed to be less effective. No improvement was found for preoxidation in air or for Ni-20Cr-5Al alloys preoxidized in H₂/H₂O.     

The effect of silicon and manganese on the oxidation mechanism of Ni-20 Cr

Additions of 3% silicon or manganese to Ni-20 Cr reduced the oxidation rate, whereas additions of 1% had little effect. Three percent silicon alloys formed an inner scale of SiO₂, and 3% manganese alloys formed an inner spinel layer of essentially pure MnCr₂O₄. The experimentally determined solid-state growth rate of NiCr₂O₄ was about 1000 times slower than the growth rate for Cr₂O₃. It has been established that the protective layer on Ni-20 Cr (Nichrome alloys) is the spinel and not Cr₂O₃ as previously postulated. The mechanism for scale growth is discussed for Ni-20 Cr alloys.This work was performed at Stanford Research Institute, Menlo Park, Calif. and was supported by the National Aeronautics amd Space Administration, Contract NAS 3-11165.     

The oxidation of Fe-19.6Cr-15.1Mn stainless steel

The oxidation behavior in air of Fe-19.6Cr-15.1Mn was studied from 700 to 1000°C. Pseudoparabolic kinetics were followed, giving an activation energy of 80 kcal/mole. The scale structure varied with temperature, although spinel formation occurred at all temperatures. At both 700 and 800°C, a thin outer layer of -Mn₂O₃ formed. The inner layer at 700°C was (Fe,Cr,Mn)₃O₄, but at 800°C there was an intermediate layer of Fe₂O₃ and an inner layer of Cr₂O₃ + (Fe, Cr,Mn)₃O₄. Oxidation at 900°C produced an outer layer of Fe₃O₄ and an inner layer of Cr₂O₃+(Fe,Cr,Mn)₃O₄. Oxidation at 1000°C caused some internal oxidation of chromium. In addition, a thin layer of Cr₂O₃ formed in some regions with an intermediate layer of Fe₃O₄ and an outer layer of (Fe,Mn)₃O₄. A comparison of rates for Fe₃O₄ formation during oxidation of FeO as well as for the oxidation of various stainless steels, which form spinels, gave good agreement and strongly suggests that spinel growth was rate controlling. The oxidation rate of this alloy (high-Cr) was compared with that of an alloy previously studied, Fe-9.5Cr-17.8Mn (low-Cr) and was less by about a factor of 12 at 1000°C and by about a factor of 100 at 800°C. The marked differences can be ascribed to the destabilization of wustite by the higher chromium alloy. No wustite formation occurred in the high-Cr alloy, whereas, extensive wustite formed in the low-Cr alloy. Scale structures are explained by the use of calculated stability diagrams. The mechanism of oxidation is discussed and compared with that of the low-Cr alloy.     

Sulfidation behavior of Ni-Cr-Mo alloys at 700°C

The effect of molybdenum additions 5, 10, 15, and 20 wt. %, on the sulfidation behavior of Ni-20Cr, and the effect of chromium additions, 5, 10, 15, and 20 wt.%, on the sulfidation of Ni-20Mo were studied in pure sulfur vapor at 700°C. In general, the alloys followed a linear or near-linear rate law, the sulfidation rate of Ni-20Mo being slightly less than that of Ni-20Cr. The alloys having the lowest ternary addition, e.g., Ni-Cr-5Mo and Ni-20Mo-5Cr. exhibited the most rapid reaction rates. The highest alloying additions of 20 wt.% had no appreciable benefit on reaction rates. Scale structures were complex but generally consisted of several layers. The outer layer was always NiS_1.03, although both binaries formed Ni₃S₂ within the NiS_1.03. An inner layer of Cr₃S₄ existed in which there was considerable dissolved molybdenum. A thin, intermediate layer of Cr₂S₃ generally formed between the Cr₃S₄ and the outer nickel sulfide. An innermost layer of MoS₂ formed on all alloys containing more than 10 wt. % Mo, and a second phase of Mo₂S₃ formed within the MoS₂ on Ni-20Mo. Although the scales changed with alloy composition, no significant changes in reaction rate were observed. Notable differences in both scale structure and reaction kinetics between this study and previous studies were apparent. The differences and possible reaction mechanisms are discussed.     

Effects of mode-I stresses on the oxidation and failure mechanisms of Ni-20Cr and Ni-15Cr-8Fe alloys in sulfur dioxide

Two alloys, Ni-20Cr and Ni-15Cr-8Fe, as wire specimens, were exposed to sulfur dioxide between 325 and 800°C, with applied external stresses (mode I). Their mechanical properties have been investigated, and the variation of their radius has been precised by conductivity measurements. For the Ni-20Cr alloy, below 550°C, the failure process combines cracking and corrosion: Cr₂S₃ crystals formed at the tip of the cracks facilitate their propagation. Above 550°C, no barrier effect is observed, and intergranular corrosion takes place. For the Ni-15Cr-8Fe alloy, mode-I stresses bolster intergranular corrosion.     

Resistance of Ni-Cr-Al alloys to cyclic oxidation at 1100 and 1200°C

A series of Ni-rich alloys in the Ni-Cr-Al system were cyclically oxidized in still air for 500 1 -hr heating cycles at 1100°C and 200 1 -hr heating cycles at 1200° C. The specific sample weight-change data for each sample were then used to determine both a scaling constant k₁ and a spalling constant k₂ for each alloy, using the regression equation w/A=k ₁ ^1/2 t^1/2 – k₂t±.These in turn were combined to form an oxidation attack parameter K_a,where K_a= (k ₁ ^1/2 + 10 k₂).Log K_a was then fitted to a fourth-order regression equation as a function of the Cr and Al content at the two test temperatures. The derived estimating equations for log K_a were presented graphically as iso-attack contour lines on ternary phase diagrams at each temperature. At 1100°C compositions estimated to have the best cyclic oxidation resistance were Ni-45 at. % Al and Ni-30 at. % Cr-20 at. % Al, while at 1200°C compositions estimated to have the best cyclic oxidation resistance were Ni-45 at. % Al and Ni-35 at. % Cr-15 at. % Al. In general, good cyclic oxidation resistance is associated with Al₂O₃ and/or NiAl₂O₄ formation. The analysis also indicated that alloys prepared by zirconia crucible melting, compared to other types of melting, had tramp Zr pickup, which significantly improved the cyclic oxidation resistance. The nature of the improvement in oxidation due to tramp Zr pickup, however, is not yet understood.     

Effects of cerium and manganese on corrosion of Fe–Cr and Fe–Cr–Ni alloys in Ar–20CO2 gas at 818 °C

Model alloys Fe–9Cr, Fe–20Cr and Fe–20Cr–20Ni (wt.%) with Ce (0.05%, 0.1%) or Mn (1%, 2%) were exposed to Ar–20CO₂ gas at 818 °C. Scales on Fe–9Cr alloys consisted of FeO and FeCr₂O₄, Fe–20Cr–(Ce) alloys formed only Cr₂O₃, and Fe–20Cr–(Mn) alloys formed Cr₂O₃ and MnCr₂O₄. All Fe–20Cr–20Ni alloys formed Fe₃O₄, FeCr₂O₄ and FeNi₃. Cerium additions had little effects, but additions of 2% Mn significantly improved oxidation resistance of Fe–20Cr and Fe–20Cr–20Ni alloys. Most alloys also carburized. All alloys developed protective chromium-rich oxide scales in air. Different behavior in the two gases is attributed to faster Cr₂O₃ scaling rates induced by CO₂.     

Oxide scale adhesion and impurity segregation at the scale/metal interface

The chemistry at scale/metal interfaces was studied using scanning Auger microscopy after removal of the scale in ultra-high vacuum using an in situ scratching technique. Al₂O₃ and Cr₂O₃ scales formed between 900°C and 1100°C on Fe-18 wt.% Cr-5 wt.% Al and on Ni-25 wt.% Cr alloys, respectively, were investigated. The adhesion of these scales was determined qualitatively by way of micro-indentation and scratching on the surface oxide. All of the alumina scales fractured to the same degree to expose the metal surface, regardless of the oxidation temperature. The chromia-forming alloy on the other hand, developed more adherent scales at lower oxidation temperatures. About 20 at.% sulfur was found at the metal surface in all cases, and its presence was not only detected on interfacial voids, but also on areas where the scale was in contact with the alloy at temperature. Results from this study clearly demonstrated that sulfur as an alloying impurity does segregate to the scale/alloy interface. However, for alumina scales and chromia scales, the effect of this segregation on oxide adhesion is noticeably different.     

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.