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.     

High temperature scaling of Ni-xSi-10 at.% Al alloys in 1 atm of pure O2

The oxidation of Ni-xSi-10Al alloys (with x = 0, 2, 4 and 6 at.%), has been studied at 900 and 1000 °C in 1 atm of pure O₂ to examine the effect of different silicon additions on the behavior of ternary Ni-Si-10Al alloys. The kinetic curves of Ni-10Al are approximately parabolic at both 900 and 1000 °C. Conversely, the kinetics of the ternary alloys at both temperatures correspond generally to a rate decrease faster than predicted by the parabolic rate law, except for the oxidation of Ni-6Si-10Al at 1000 °C, which exhibits a single nearly-parabolic stage. Oxidation of the binary alloy formed at both temperatures an internal oxidation zone beneath a layer of NiO. Oxidation of Ni-2Si-10Al at both temperatures and of the other two alloys at 900 °C formed initially a zone of internal oxidation of Al + Si. However, a layer of alumina forming at the front of internal oxidation after some time blocked the internal oxidation and produced a gradual conversion of the metal matrix of this region into NiO, with a simultaneous decrease of the oxidation rate. Conversely, the oxidation of Ni-4Si-10Al and Ni-6Si-10Al at 1000 °C did not produce an internal oxidation, but formed an alumina layer directly on the alloy surface after an initial stage when also Ni was oxidized. Therefore, silicon exerts the third-element effect by reducing the critical Al content needed for the transition from its internal to its external oxidation with respect to the corresponding Ni-Al alloy. This result is interpreted by means of an extension to ternary alloys of Wagner’s criterion for the same transition in binary alloys based on the attainment of a critical volume fraction of internal oxide.     

The development of the wustite-fayalite scale on an iron-1.5 wt.% silicon alloy at 1000°C

An investigation has been carried out on the morphological development and structure of the wustite-fayalite scale formed on a ferritic Fe-1.5 wt.% Si alloy exposed to carbon dioxide-carbon monoxide atmospheres at 1000°C. The amorphous silica film formed on the metallographically polished specimens crystallized to -cristobalite at the reaction temperature. Wustite and fayalite developed within nodules which grew laterally to cover the alloy surface. Concurrent with growth of the nodules externally, oxygen diffusion into the underlying alloy led to precipitation of silica as -tridymite. This internal oxidation zone was sufficiently depleted in silicon for its transformation to an austenitic phase. A fully developed scale was composed of an external wustite layer and an inner wustite-fayalite conglomerate layer, interspersed with discontinuous fayalite bands.This work forms part of a research project sponsored by the American Iron and Steel Institute and the National Research Council of Canada.     

Role of Na2CO3 on an AISI 330 austenitic stainless steels oxidation at 900°C

This study shows the influence of sodium carbonate coatings on the austenitic AISI 330 (Fe–35Ni–19Cr–1.3Si) oxidized during 48 hr at 900°C. The N₂‐5 vol% H₂ gaseous environment was used to simulate industrial heat treatment conditions. Silica scale formation is promoted by low oxygen‐containing gaseous environments and the high alloy silicon content. On this alloy, an amorphous silica scale is formed after the blank material oxidation. It indicates that silicon is free to diffuse in the alloy and forms a silica scale at the internal interface. On Na₂CO₃‐coated specimens, no silica scale is formed. Then, sodium combines with silicon to form amorphous glass particles. A comparison has been performed with results obtained on a AISI 330Cb niobium containing alloy in the same oxidizing conditions. It is then concluded that sodium carbonate coatings could only favor silica formation on niobium containing alloy due to a reaction between sodium and niobium.     

Cyclic and isothermal oxidation behavior on some Ni-Cr alloys

Additions of 3 wt.% Mn and 3 wt.% Si were made to Ni-20Cr. These alloys, along with Ni-20Cr and Ni-40Cr were oxidized for 100 1-hr cycles at 1100°C and 50 1-hr cycles at 1200° C. Oxidation behavior was judged by sample weight and thickness change, metallography, x-ray diffraction, and electron microprobe analysis. These tests showed that Ni-40Cr and Ni-20Cr-3Si were about the same and were the most oxidation-resistant alloys. Ni-20Cr-3Mn was not as oxidation resistant, especially at 1200° C. Ni-20Cr was far less oxidation resistant than any of the other alloys. The Ni-40Cr and Ni-20Cr-3Si relied on a protective layer of Cr₂O₃ for their oxidation resistance. A SiO₂ layer was noted beneath the Cr₂O₃ layer on the Ni-20Cr-3Si, but had apparently only a second-order effect. The source of improved protection of the Ni-20Cr-3Mn was apparently the formation of a relatively adherent MnCr₂O₄ layer at the metal-oxide interface.     

The oxidation of three Ni-6Si-xAl alloys in 1 atm O2 at 1000 °C

The oxidation of three ternary Ni-6Si-xAl alloys containing 6, 10 and 15 at.% Al and of the corresponding binary Ni-Al alloys has been studied at 1000 °C under 1 atm O₂ to examine the effect of different Al additions on the behavior of ternary Ni-Al-Si alloys containing 6 at.% Si. Of the three binary Ni-Al alloys only Ni-15Al was able to form external alumina scales. Conversely, all the three ternary alloys formed an innermost layer of alumina directly in contact with the alloy following very similar and approximately parabolic kinetics after a short faster initial stage due to transient formation of NiO. Thus, the presence of silicon is very effective to reduce the critical Al content needed to form exclusive alumina scales with respect to binary Ni-Al alloys. The third-element effect due to silicon is interpreted on the basis of an extension of Wagner’s criterion for the transition from the internal to the external oxidation of the most reactive component in binary alloys.     

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.     

The influence of silicon on the high-temperature oxidation of nickel

An investigation of the oxidation of nickel-silicon alloys has been carried out in order to ascertain the mode of development of partially or fully protective SiO₂ layers. The addition of 1% Si has little effect on the oxidation rate of nickel at 1000°C but is sufficient for partial-healing layers of amorphous SiO₂ to be established. These layers are incorporated into the inner part of the duplex NiO scale but do not react with the oxide to form a double oxide. Increasing the silicon concentration to 4% or 7% facilitates the development of apparently continuous amorphous SiO₂ layers at the base of the NiO scale, resulting in reduced rates of oxidation. However, these layers develop imperfections, possibly microcracks resulting from oxide growth stresses, and are unable to prevent some continued transport of Ni²⁺ ions into the NiO scale and oxygen into the alloy, particularly for Ni-4% Si. Although the formation of SiO₂-healing layers can reduce the rate of oxidation of nickel, they provide planes of weakness that result in considerable damage under the differential thermal contraction stresses during cooling. In particular, severe scale spalling occurs for Ni-4% Si and Ni-7% Si as failure occurs coherently within the SiO₂ layer.     

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.     

High-temperature oxidation of Fe-16.7% Mn-2.1% Ni-6.6% Si alloy in SO2

The oxidation of an austenitic Fe-16.7% Mn-2.1% Ni-6.6% Si (by weight) alloy in SO₂ in the temperature range 600–900°C is described. The corrosion products formed on this alloy in this environment below 800°C consist only of oxides, rather than a mixture of oxides and sulfides as is observed for unalloyed Fe or Mn. The kinetics of oxidation of the alloy in SO₂ in this temperature range are similar to those in O₂. It is proposed that these characteristics result from the presence of a thin silicate layer near the scale-metal interface that alters the gradient of oxygen potential within the scale.     

Modification of the oxidation behavior of high-purity austenitic Fe-14Cr-14Ni by the addition of silicon

The oxidation behavior of Fe-14Cr-14Ni (wt.%) and of the same alloy with additions of 1 and 4% silicon was studied in air over the range of 900-1100° C. The presence of silicon completely changed the nature of the oxide scale formed during oxidation. The base alloy (no silicon) formed a thick outer scale of all three iron oxides and an internally oxidized zone of (Fe,Cr,Ni) spinels. The alloy containing 4% silicon formed an outer layer of Cr₂O₃ and an inner layer of either (or possibly both) SiO₂ and Fe₂SiO₄. The formation of the iron oxides was completely suppressed. The oxidation rate of the 4% silicon alloy was about 200 times less than that of the base alloy, whereas the 1% silicon alloy exhibited a rate intermediate to the other two alloys. The actual ratio of the oxidation rates may be less than 200 due to possible weight losses by the oxidation of Cr₂O₃ to the gaseous phase CrO₃. The lower oxidation rate of the 4% silicon alloy was attributed to the suppression of iron-oxide formation and the presence of Cr₂O₃, which is a much more protective scale.     

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.     

Accelerated oxidation,internal oxidation,intergranular oxidation,and pesting of intermetallic compounds

The compounds MoSi₂, NiAl, and NbAl₃ all form protective oxide films, particularly at high temperatures where the diffusion of Si or Al is more rapid and, for the case of MoSi₂, the transient oxides evaporate. However, at low temperatures, all three can undergo accelerated oxidation. The mechanisms of degradation are unique to the particular compound although there are some similarities. The accelerated oxidation of MoSi₂ occurs at temperatures below 600°C by the rapid growth of Mo oxides which prevent development of a continuous silica film. Internal or intergranular oxidation does not occur. If the specimen contains cracks or pores, the rapid oxidation in these defects leads to fracture of the specimen or pesting. The accelerated oxidation of NiAl occurs at temperatures below 1000°C at reduced oxygen partial pressures as the result of internal oxidation and rapid intergranular oxidation. The intergranular oxidation does not lead to pesting. Special circumstances are required for the accelerated oxidation of NiAl as it does not appear to occur in flowing gases unless sulfur is present. The accelerated oxidation of NbAl₃ also occurs at temperatures less than 1000°C and at reduced oxygen partial pressures and takes the form of intergranular oxidation of Al. The intergranular oxidation results in pesting of NbAl₃. The phenomena of accelerated oxidation, internal oxidation, intergranular oxidation, and pesting have not been investigated in detail for most other intermetallic compounds but one or more of these phenomena seems to afflict most aluminides and silicides.     

High-temperature oxidation behavior of a wrought Ni-Cr-W-Mn-Si-La alloy

An investigation was carried out to study the kinetics and products of oxidation of a wrought Ni–Cr–W–Mn–Si–La alloy at temperatures in the range of 950 to 1150°C. Oxidation kinetics were evaluated from measurements of weight change, metal loss, and internal penetration. Analytical electron microscopy, scanning electron microscopy, electron probe microanalysis, and X-ray diffraction were used to characterize the scale microstructure. Initially, La was observed to segregate within a surface layer of about 5 m thick, which promoted selective oxidation of Cr and Mn. Oxidation kinetics were found to follow a parabolic-rate law with an activation energy of about 232kJ/mol. During steady-state oxidation, the scale consisted of an inner adherent layer of -Cr₂O₃ modified by the presence of La and Si, and shielded by an outer layer of MnCr₂O₄. Most of the La was segregated to grain boundaries of the -Cr₂O₃ scale, however, Si was homogeneously distributed. It was concluded that the characteristic oxidation resistance of the alloy was related to the synergistic effects of Ni and Cr and to the effective minor additions of La, Si, and Mn; however, the useful life of the scale was limited by rupture and surface depletion in Cr, leading to accelerated internal oxidation.     

The effect of Si additions on the high temperature oxidation of a ternary Ni-10Cr-4Al alloy in 1 atm O2 at 1100 °C

The oxidation of four Ni-10Cr-ySi-4Al alloys was studied at 1100 °C to examine the effects of Si additions (from 2 to 6 at.%) on the behavior of the alloy Ni-10Cr-4Al. Addition of 2 at.% Si prevented completely nickel oxidation, but could not form alumina scales. Larger Si additions produced alumina only over part of the alloy surface (about 20% with 4 at.% Si and 30% with 6 at.% Si), but could not prevent completely the internal oxidation of Al. The results are interpreted by extending to quaternary alloys the mechanism of the third-element effect already proposed for ternary alloys.     

Influence of atmosphere on high‐temperature oxidation of Fe‐Cr‐Si model alloy

The aim of this study is to show the influence of the gaseous environment on the ferritic Fe‐Cr‐Si model alloy oxidation during 70 hrs, at 900 and 950°C. Two different atmospheres have been used, air or nitrogen containing 5 vol% hydrogen (N₂–5%H₂). After air oxidation, a nonadherent chromia scale formed. In the N₂–5%H₂ gaseous environment, it clearly appears that silicon segregation near the internal scale‐metal interface is favoured. In this low oxygen‐containing gas, adherent chromia, and silica scales have been formed. Silica subscale associated to an adherent chromia scale obtained in low‐oxygen conditions are a good protection barrier against carburisation.     

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.     

The effect of yttrium and thorium on the oxidation behavior of Ni-Cr-Al alloys

The effect of quaternary additions of 0.5% Y and 0.5 and 1.0% Th to a base alloy of Ni-10Cr-5Al on the oxidation behavior and mechanism was studied during oxidation in air over the range of 1000–1200°C. The presence of yttrium decreased the oxidation kinetics slightly, whereas the addition of thorium caused a slight increase. Oxide scale adherence was markedly improved by the addition of the quaternary elements. Although a number of oxides formed on yttrium-containing alloys, quantitative x-ray diffraction clearly showed that the rate-controlling step was the diffusion of oxygen through short-circuit paths in a thin layer of alumina that formed parabolically with time. Mixed oxides containing both aluminum and yttrium formed by the reaction of Y₂O₃ to form YAlOP₃ initially, and Y₃Al₅O₁₂ (YAG) after longer times. Although the scale adherence of the yttrium-containing alloy was considerably better than the base alloys, spalling did occur that was attributed to the formation of the voluminous YAG particles that grew in a mushroom-like manner, lifting the protective scale off the substrate locally. The YAG particles formed primarily at grain boundaries in the substrate in which the yttrium originally existed as YNi₉. This intermetallic compound reacted to form Y₂O₃, liberating metallic nickel that subsequently reacted to form NiO or NiAl₂O₄ spinel or both. The Y₂O₃ reacted with aluminum to ultimately form the YAG mushrooms. Thorium did not form any mixed oxides; the only oxide involving thorium was ThO₂, which existed as small particles at the oxide-metal interface. A highly beneficial effect of the thoria particles in reducing film spalling was observed. Scale spalling in the base alloy was attributed to void formation at the oxide-metal interface, the voids forming by condensation of excess vacancies from the Kirkendall effect associated with fast back-diffusion, of nickel into the substrate as aluminum was preferentially oxidized and diffused slowly outward. The mechanism of improved scale adherence in the quaternary alloys was the elimination of voids by annihilation of the Kirkendall vacancies at vacancy sinks introduced by the noncoherent interfaces between yttrium and thorium-containing intermetallics or oxides or both.This work is based on a portion of the dissertation of Arun Kumar in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Engineering, University of California, Los Angeles.Supported by NASA-Ames under grant No. NGR 05-007-352.     

Inherent oxidation protection of Fe-5Cr-15Ni-2Si-4.5Mo

The oxidation mechanism of Fe-5Cr-15Ni-2Si-4.5Mo alloy was investigated in order to determine the role of Si and Mo in providing oxidation resistance. It was determined that the oxidation protection in the temperature range 750–950°C resulted from formation of a continuous oxide sublayer of SiO ₂ (or possibly Fe ₂ SiO ₄).Molybdenum formed an intermetallic Fe ₂ Mo _1–x Si _x that eventually diffused out into the grain boundaries and formed a protective barrier to the oxidation process. The mechanism behind the improved oxidation is the formation of a SiO ₂ layer at the metal-oxide interface that retards the outward diffusion of Fe. It was also established that the oxidation mechanism was controlled by an activation energy equal to that of Fe ³⁺ ions diffusing through SiO ₂.     

Influence of nitridation and temperature cycling on oxidation of Ti-0.96 wt % Si alloy

The oxidation kinetics of prenitrided, and subsequently oxidized in either air or oxygen, Ti-0-96 wt % Si alloy have been investigated in the temperature range of 900–1100°C. The activation energies for various gas-alloy reactions are reported. Comparative kinetics data are given for oxidation of the alloy in oxygen or air without prenitridation. The oxidation resistance of nitrogen pretreated alloy is about the same as its resistance to oxidation in air. Temperature cycling during oxidation does not exert a beneficial effect on the oxidation characteristics of the alloy. X-ray and electron microprobe analyses reveal the presence of TiO₂, Ti₂O, -TiN, and -TiN in the scale and the concentration profiles of Ti, Si, O, and N in the alloy.