Short Term Oxidation Behavior of Si-Free Low-Cr Alloy Sputtered Coating

A sputtered coating of a low-Cr alloy without Si was deposited on the cast alloy with the same composition. The short term (100 h) oxidation behavior of the sputtered coating and the cast alloy was evaluated in air at 800 °C. The results indicated that the sputtered coating exhibited a higher oxidation resistance than the cast alloy. It was found that the mass gain of the cast alloy increased continuously with oxidation time and was higher than that of the sputtered coating, which demonstrated only a slight increase in mass gain with oxidation time after 5 h thermal exposure. During the initial thermal exposure of 0.5 h, the oxide scale formed on the cast alloy consisted of Fe₂O₃ and (Fe,Co,Cr)₃O₄ spinel with a small amount of Cr. However, (Fe,Co,Cr)₃O₄ spinel and Fe₂O₃ were thermally grown on the sputtered coating. After oxidation for 100 h, the oxide scale formed on the cast alloy consisted of Co₃O₄ and (Fe,Co)₃O₄ with internal oxide of Cr, while a double-layer oxide consisting of an outer (Fe,Co,Cr)₃O₄ spinel layer and an inner Cr₂O₃ layer was developed on the sputtered coating.     

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.     

Oxidation of Fe-3 wt.% Cr alloy

The oxidation behavior and the oxide microstructure on Fe-3 wt. % Cr alloy were investigated at 800°C in dry air at atmospheric pressure. Two distinct oxidation rate laws were observed: initial parabolic oxidation was followed by nonparabolic growth. The change in the oxidation kinetics was caused by microchemical and microstructural developments in the oxide scale. Several layers developed in the oxide scale, consisting of an innermost layer of (Fe,Cr)₃O₄ spinel, an intermediate layer of (Fe,Cr)₂O₃ sesquioxide, and two outer layers of Fe₂O₃ hematite, each with different morphologies. Wustite (Fe_1–xO) and distorted cubic oxide (-(Fe,Cr)₂O₃) were observed during the iniital parabolic oxidation only.     

Air Oxidation of FeCoNi-Base Equi-Molar Alloys at 800–1000°C

The oxidation behavior of FeCoNi, FeCoNiCr, and FeCoNiCrCu equi-molar alloys was studied over the temperature range 800–1000 °C in dry air. The ternary and quaternary alloys were single-phase, while the quinary alloy was two-phase. In general, the oxidation kinetics of the ternary and quinary alloys followed the two-stage parabolic rate law, with rate constants generally increasing with temperature. Conversely, three-stage parabolic kinetics were observed for the quaternary alloy at T 900°C. The additions of Cr and Cu enhanced the oxidation resistance to a certain extent. The scales formed on all the alloys were triplex and strongly dependent on the alloy composition. In particular, on the ternary alloy, they consist of an outer-layer of CoO, an intermediate layer of Fe₃O₄, and an inner-layer of CoNiO₂ and Fe₃O₄. Internal oxidation with formation of FeO precipitates was also observed for this alloy, which had a thickness increasing with temperature. The scales formed on the quaternary alloy consisted of an outer layer of Fe₃O₄ and CoCr₂O₄, an intermediate layer of FeCr₂O₄ and NiCr₂O₄, and an inner layer of Cr₂O₃. Finally, the scales formed on the quinary alloy are all heterophasic, consisting of an outer layer of CuO, an intermediate-layer of CuO and Fe₃O₄, and an inner-layer of Fe₃O₄, FeCr₂O₄, and CuCrO₂. The formation of Cr₂O₃ on the quaternary alloy and possibly that of CuCrO₂ on the quinary alloy was responsible for the reduction of the oxidation rates as compared to the ternary alloy.     

The oxidation behavior of an Fe61B15Zr8Mo7Co5Y2Cr2 bulk metallic glass at 550–700 °C

The oxidation behavior of an Fe₆₁B₁₅Zr₈Mo₇Co₅Y₂Cr₂ bulk metallic glass (Fe7-BMG) and its crystalline counterpart (XFe7) was studied over the temperature range of 550–700 °C in dry air. In general, the results showed that the oxidation kinetics of the Fe7-BMG and XFe7 alloys followed a parabolic-rate law, with their oxidation rates increasing with temperature. It was found that the oxidation rate constants (k_p values) of the Fe7-BMG are slightly higher than those of XFe7, indicating that the amorphous alloy underwent a fast oxidation reaction. The scale formed on the Fe7-BMG at 550 °C for 48 h was thin and heterophasic mixture of iron oxides (Fe₂O₃/Fe₃O₄) and tetragonal-ZrO₂ (t-ZrO₂). At higher temperatures (T ≥ 600 °C), duplex scales formed on the glassy alloy consisted of an outer layer of iron oxides (Fe₂O₃/Fe₃O₄) and minor amounts of B₂O₃, and of a heterophasic inner layer of mostly t-ZrO₂ intermixed with non-oxidized Fe₃B, FeCo, and ZrB₁₂. The formation of the crystalline intermetallics indicated the occurrence of the substrate crystallization at the temperature range of interest.     

Distribution and characterization of high temperature air corrosion products on iron-chromium alloys by Raman microscopy

Raman microscopy has been used to study the nature and distribution of corrosion products formed on iron and iron-chromium alloys in air at high temperatures. Fe and Fe-Cr alloys containing 2, 5, 14, and 18% Cr were oxidized at 400, 600, and 850°C for 2 hr, in addition samples of each alloy were oxidized for 24 hr at 400°C to obtain thicker scales at this temperature. The corroded samples showed varying distributions of the oxides Fe₂O₃, Fe₃O₄, Cr₂O₃, and FeCr₂O₄. Fe₂O₃ and Fe₃O₄ were formed exclusively on the pure iron and the 2 and 5% chromium alloys at all temperatures and on the 14% chromium alloy at 400°C. The 14 and 18% Cr alloys formed scales containing Cr₂O₃ and FeCr₂O₄ at the higher temperatures (600 and 850°C). Examples of small regions of Fe₂O₃ being formed within Cr₂O₃-FeCr₂O₄ scales are suggested as possible indications of breakaway corrosion initiation sites.     

Continuous and cyclic oxidation of T91 ferritic steel under steam

The oxidation behaviour of T91 ferritic steel in steam has been studied under isothermal and non-isothermal conditions within a temperature range between 575 and 700 °C. Isothermal treatments resulted in parabolic oxidation kinetics. Three clearly defined oxide layers constituted the oxide scales. The innermost layer was a (Fe,Cr)₃O₄. The intermediate layer was porous magnetite (Fe₃O₄) followed by a compact thinner layer of hematite (Fe₂O₃). Under non-isothermal conditions the oxide scales were irregular and evidently cracked. An increase of the oxidation temperature produces an acceleration of the oxidation process, causing an increase of the oxide scale thickness that depends on the temperature increase and the exposure time.     

A Cr2O3-deposited Sm(Co0.68Fe0.22Cu0.08Zr0.02)7.5 magnet with increased oxidation resistance at 700 °C

A Sm(Co_0.68Fe_0.22Cu_0.08Zr_0.02)_7.5 alloy was arc-ion-plated with a thin Cr₂O₃ film. It completely prevented the external oxidation and sufficiently suppressed the internal oxidation of the alloy in air at 700 °C for 20 h, causing the alloy to form only a very shallow layer where the Sm oxidation occurred. The mechanism for the effect of the Cr₂O₃ film on the oxidation of the alloy was proposed based on phase characterization of the oxidized layer.     

The Oxidation of Fe-2 and 5 at.% Y Alloys at 600-800°C in Air

The oxidation of Fe-Y alloys containing 2 and 5at.% Y and pure iron has been studied at 600-800°Cin air. The oxidation of pure iron follows the parabolicrate law at all temperatures. The oxidation of Fe-Y alloys at 600°C approximatelyfollows the parabolic rate law, but not at 700 and800°C, where the oxidation goes through severalstages with quite different rates. The oxide scales on Fe-2Y and Fe-5Y at 700 and 800°C arecomposed of external pure Fe oxides containingFe₂O₃,Fe₃O₄, and FeO, with FeO being themain oxide and an inner mixture of FeO andYFeO₃. The scales on Fe-2Y and Fe-5Y at 600°C consist ofFe₂O₃,Fe₃O₄, andY₂O₃, with a minor amount of FeO.Significant internal oxidation in both Fe-Y alloysoccurred at all temperatures. The Y-containing oxidesfollow the distribution of the original intermetalliccompound phase in the alloys. The effects of Y on theoxidation of pure Fe are discussed.     

Oxidation Behavior of the CrMnFeCoNi High-Entropy Alloy

Oxidation of the Cr₂₀Mn₂₀Fe₂₀Co₂₀Ni₂₀ (at%) high-entropy alloy (HEA) was investigated at 500–900 °C in laboratory air. At 600 °C the oxide was mainly Mn₂O₃ with a thin inner Cr₂O₃ layer; at 700 and 800 °C it was mainly Mn₂O₃ with some Cr enrichment; at 900 °C it was Mn₃O₄. The oxidation rate was initially linear but became parabolic at longer times with an activation energy of 130 kJ/mol, comparable to that of Mn diffusion in Mn oxides but much lower than that for sluggish diffusion of Mn in the HEA. The diffusion of Mn through the oxide is considered to be the rate-limiting process.     

The oxidation of Iron-Chromium-Manganese alloys at 900°C

The oxidation of nine ternary iron-chromium-manganese alloys was studied at 900°C in an oxygen partial pressure of 26.7 kPa. The manganese concentration was set at 2, 6, and 10 wt. %, and chromium at 5, 12, and 20 wt. %. The scales formed on the low-chromium alloys consisted of (Mn,Fe)₂O₃, -Fe₂O₃, and Fe₃O₄. These alloys all exhibited internal oxidation and scale detachment upon cooling. The scales formed on the higher-chromium alloys were complicated by nodule formation. Initially, these scales had an outer layer of MnCr₂O₄ with Cr₂O₃ underneath, adjacent to the alloy. With the passage of time, however, nodules formed, and the overall reaction rate increased. This tendency was more marked at higher manganese contents. Although these alloys contained a high chromium content, the product chromia scale usually contained manganese. It was concluded that the presence of manganese in iron-chromium alloys had an adverse effect on the oxidation resistance over a wide range of chromium levels.     

Characterisation of oxide films formed on Co–29Cr–6Mo alloy used in die-casting moulds for aluminium

Oxide films formed at 700 °C on Co–29Cr–6Mo alloy were characterised extensively to improve the corrosion resistance of the alloy to liquid Al, enabling its use in Al die-casting moulds. Film of duplex layer consisting of an outer CoO-rich layer and an inner Cr₂O₃-rich layer was observed in samples subjected to oxidation for 4 h. With an increase in duration of oxidation, CoO was gradually replaced by Cr₂O₃, resulting in a single-layered oxide film dominantly composed of Cr₂O₃. The oxide film evolved with duration of oxidation treatment indicating the possibility of optimising films for Al die-casting moulds.     

Oxidation of an Fe-12% Ni alloy in oxygen at 700–1000°C

The oxidation of an Fe-12% Ni alloy has been studied at 700–1000°C using thermogravimetric, metallographic, and electron probe microanalysis techniques. At all temperatures parabolic kinetics were observed and the activation energy for the process was 48±4 kcal mole^–1. At 700°C Fe₃O₄ and Fe₂O₃ were present in the external scale and scaling was accompanied by a progressive Ni enrichment of the underlying alloy. When the Ni-enriched zone contained 50–60% Ni, this metal entered the spinel phase, eventually leading to the formation of Ni_xFe_3–xO₄ where x had a value of 0.24 close to the alloy and <0.01 close to the Fe₂O₃ phase boundary. At higher temperatures (900–1000°C) Ni entered the spinel phase very early in the oxidation process. There was a buildup in Ni concentration in the Ni_xFe_3–xO₄ phase to x values of 0.4 and at 900°C only this corresponded to a transition to a lower parabolic rate of oxidation. The internal oxide phase was identified as Ni_0.7Fe_2.3O₄. The mechanism of oxidation of the alloy is discussed in the light of present knowledge concerning the Fe-Ni-O system.     

High temperature corrosion of pure Fe, Cr and Fe-Cr binary alloys in O2 containing trace KCl vapour at 750 °C

Pure Fe, Cr and Fe-Cr binary alloys were corroded in O₂ containing 298 ppm KCl vapour at 750 °C. The corrosion kinetics were determined, and the microstructure and the composition of oxide scales were examined. During corrosion process, KCl vapour reacted with the formed oxide scales and generated Cl₂ gas. As Cl₂ gas introduced the active oxidation, a multilayer oxide scales consisted of an outmost Fe₂O₃ layer and an inner Cr₂O₃ layer formed on the Fe-Cr alloys with lower Cr concentration. In the case of Fe-60Cr or Fe-80Cr alloys, monolayer Cr₂O₃ formed as the healing oxidation process. However, multilayer Cr₂O₃ formed on pure Cr.     

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.     

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.     

Air oxidation of an Fe72B22Y6 bulk amorphous alloy at 600–700 °C

The oxidation behavior of the Fe₇₂B₂₂Y₆ bulk metallic glass and its crystalline counterpart was studied over the temperature range of 600–700 °C in dry air. The results showed that the oxidation kinetics of both glassy and crystalline alloys in general follow a parabolic rate law although a two-stage kinetics was noted at 700 °C for the glassy alloy. The oxidation rates of the two alloys increased with increasing temperature, and the parabolic rate constants of the glassy alloy are much lower than those of the crystalline counterpart. The scales formed on the glassy alloy consisted mainly of boron oxide (B₂O₃) and minor amounts of iron oxides (Fe₃O₄/FeO). Conversely, duplex scales formed on the crystalline counterpart were composed of an outer layer of Fe₂O₃ and an inner layer of Fe₃O₄–YBO₃ mixture. The formation of B₂O₃ is responsible for the reduced oxidation rates of the glassy alloy as compared to those of crystalline counterpart.     

High-Temperature Oxidation Behavior of Sputtered IN 738 Nanocrystalline Coating

A sputtered nanocrystalline coating of IN 738 alloy was obtained by means of magnetron sputtering. The isothermal oxidation behavior at 800, 900, and 1000°C and the cyclic oxidation behavior at 950°C of the coating were studied in comparison with IN 738 cast alloy. The results indicated that a double external oxide scale was formed on the nanocrystalline coating at 900, 950, and 1000°C without internal oxide and nitride. The scale consisted in an outer mixture of Cr₂O₃, TiO₂, and NiCr₂O₄ and an inner, continuous Al₂O₃ layer, which offered good adhesive and protectiveness. However, at 800°C a continuous Al₂O₃ scale could not be formed during oxidation of nanocrystalline coating and aluminum was still oxidized internally.     

Oxidation of Ni-base alloys in atmospheres with widely varying oxygen partial pressures

The oxidation behavior of the Ni-base alloys IN 617, IN 713 LC, Ni20Cr, and Ni20Cr+Si has been investigated in the temperature range from 850°C to 1000°C in air and at low-oxygen partial pressure p(O₂) (10^–19 to 10^–16 bar). With the exception of alloy IN 713 LC, the materials show no influence of p(O₂) on the oxidation mechanisms and the kinetics. This result can be explained by the formation of a dense Cr₂O₃ layer, the growth rate of which is controlled by the Cr ion interstitial concentration in Cr₂O₃ at the phase boundary oxide/alloy and the mobility of Cr ions in Cr₂O₃. For the alloy IN 713 LC which develops a dense Al₂O₃ layer in air, a modified transition mechanism at low p(O₂) leads to the formation of Cr₂O₃ at the surface and a strong internal oxidation of Al.     

Improving the high-temperature oxidation resistance of a β-γ TiAl alloy by a Cr2AlC coating

A Cr₂AlC coating was deposited on a β-γ TiAl alloy. Isothermal oxidation tests at 700 °C and 800 °C, and thermocyclic oxidation at 800 °C were performed in air. The results indicated that serious oxidation occurred on the bare alloy. Thick non-protective oxide scales consisting of mixed TiO₂ + α-Al₂O₃ layers formed on the alloy surface. The coated specimens exhibited much better oxidation behaviour by forming an Al-rich oxide scale on the coating surface during the initial stages of oxidation. This scale acts as diffusion barrier by effectively blocking the ingress of oxygen, and effectively protects the coated alloys from further oxidation.