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.     

The oxidation of Fe-Cr alloys containing an oxide dispersion or reactive metal additions

The oxidation of an iron-16% chromium alloy containing a dispersion of yttria particles and of iron-16 to 18% chromium alloys containing small additions of yttrium or zirconium has been studied at 1100 and 1200°C in 100 Torr oxygen. The yttria-containing alloy exhibited the excellent oxidation resistance usually associated with oxide-dispersion-containing alloys, having a thin, adherent, virtually iron-free scale which resisted the breakaway rapid oxidation behavior commonly found in Fe-Cr alloys in this composition range. Of the alloying additions intended to form a fine oxide dispersion during oxidation, only zirconium affected the oxidation behavior in a beneficial way, the scale on the yttrium-containing alloy being possibly less protective than that on the equivalent binary alloy.Supported by Battelle, Columbus Laboratories, Columbus, Ohio.     

Temperature and water vapour effects on the cyclic oxidation behaviour of Fe-Cr alloys

Binary Fe-Cr alloys were subjected to cyclic oxidation at 600, 700 and 950 °C in flowing gases of Ar-20O₂ and Ar-20O₂-5H₂O (vol.%). The minimum chromium concentration required to achieve protective scale growth decreased as temperature increased from 600 to 700 °C. This change is attributed to faster chromium diffusion at higher temperature. Conversely, this minimum chromium level increased when the temperature was raised from 700 to 950 °C. This is attributed to faster scale growth, leading to its rapid mechanical failure, along with formation of slow-diffusing austenite. Water vapour accelerated scaling, leading to a need for higher chromium concentrations to resist breakaway oxidation.     

The role of nickel in the high-temperature oxidation of Fe-Cr-Ni alloys in oxygen

The oxidation of several largely austenitic Fe-Cr-Ni alloys in 1 atm oxygen at 800–1200°C has been studied thermogravimetrically, metallographically, and in detail by electron probe micro analysis. Fe-Cr-Ni alloys of this type are protected by Cr₂O₃-healed scale, which thickens slower than on the corresponding binary Fe-Cr and Ni-Cr alloys, presumably because nickel and iron ions dope the Cr₂O₃ more effectively together than singly and/or because the alloy composition and ability to absorb cation vacancies are such as to produce a smaller vacancy activity gradient or level in the scale, or voids within it. The scale adhesion, as on Ni-Cr alloys, is generally good after long times, at least partly due to the convoluted alloy-oxide interface, in some cases to large intergranular Cr₂O₃-rich stringers, and possibly to the general specimen mechanical properties. Nonprotective stratified scale development is relatively unusual and often produces nickel-rich, alloy-particle-containing nodules, as on Fe-Ni alloys. Careful selection of ternary and more complex alloys with appropriate alloy interdiffusion coefficients and oxygen solubilities and diffusivities should permit development of materials with the best compromise between ease of Cr₂O₃ establishment, avoidance of breakaway, and readiness of scale healing.     

The Effect of Water Vapor on Selective Oxidation of Fe–Cr Alloys

Binary Fe–Cr alloys containing 10 and 20 mass% Cr were studied with respect to isothermal oxidation behavior at 900 and 1,050 °C in Ar–20%O₂, Ar–7%H₂O and Ar–4%H₂−7%H₂O. Thermogravimetric analyses in combination with analytical studies using SEM/EDX and Raman Spectroscopy revealed, that in atmospheres in which water vapor is the source of oxygen, Cr exhibits a higher tendency to become internally oxidized than in the Ar–O₂ gas. Contrary to previous studies which showed the presence of water vapor to affect transport processes in the scale, the present results thus reveal that the presence of water vapor also affects the transport processes in the alloy. This mechanism is an “easy” explanation of the frequently observed effect that Fe–Cr alloys with intermediate Cr contents (e.g. 10–20%, depending on temperature) exhibit protective chromia-rich scale formation in dry gases but breakaway type Fe-rich oxides in wet gases, provided the oxygen partial pressure is sufficiently high for Fe to become oxidized.     

The High temperature oxidation of iron-chromium alloys containing 0.1 wt. % cerium or cerium oxide

The effects of 0.1 wt% Ce in both metal and oxide form on the high temperature oxidation of Fe-Cr (Cr = 10, 12, 14, 16, 18, 20) have been investigated at 1000°C in a 0.1315 bar O₂/He mixture at a total pressure of 1 bar. The presence of Ce and CeO₂ markedly affects the oxidation characteristics of the tested alloys, causing rapid initial coverage with protective oxide, decreased overall rates of oxidation, modifications in scale morphologies and enhanced scale adhesion. There is a possibility of a “trade-off” between the amount of Cr in the Fe-Cr materials and the presence of Ce of CeO₂, since such additions have been shown to decrease the amount of Cr necessary to form a protective Cr₂O₃ scale on the alloy surface. The mechanisms by which the Ce or CeO₂ improve the oxidation behaviour of Fe-Cr alloys are discussed.     

The mechanisms of breakaway oxidation of three mild steels in high-pressure CO2 at 500°C

The mechanisms of breakaway oxidation of a rimming steel, a low-alloy steel (1 Cr-0.5 Mo), and a free-cutting steel (En1A) have been studied in high-pressure CO₂ at 500°C. Average compressive stresses up to 170–280 ton in^–2. in the scale have been derived from foil elongation and creep data. Carbon contents of up to 6 and 15% of the weight gain have been found in protective and breakaway scale and are larger than previously reported.¹ Most of the carbon is deposited near the scale-metal interface, showing that CO₂ diffuses through the scale. Oxidation in CO₂-tritiated water mixtures gives a maximum tritium content in the metal at 250–500 h, which declines thereafter. Treatment with some sulfur compounds before oxidation, or the presence of sulfur in the metal, reduces the rates of protective and breakaway oxidation in wet CO₂ and carbon transfer to the metal, but not to the scale. It is proposed that breakaway is initiated by an effect of hydrogen such as the accumulation of hydrogenous gases at the scale-metal interface under pressures sufficient to rupture the inner scale. Carbon deposition may assist initiation, and is probably the main factor in propagating breakaway oxidation.     

The effect of water vapor on the oxidation behavior of Ni-Pt-Al coatings and alloys

Turbines fired with hydrogen or syngas from coal gasification will have significantly higher water vapor contents in the combustion gas than natural gas fired turbines. The effect of increased water vapor on alumina-forming coatings and model alloys was investigated at 1100 °C in furnace cyclic testing. Increasing the water vapor content from 10% to 50 vol.% increased the amount of scale spallation on undoped alumina-forming alloys. Compared to dry O₂, increased spallation was observed for β and γ/γ' phase coatings on the substrates of alloys 142 and N5. In all cases, the addition of water vapor appeared to reduce the formation of alumina whiskers and ridges at the scale-gas interface, but did not significantly change the alumina growth rate. The addition of water vapor may have a detrimental effect on the selective oxidation of Al in γ/γ' alloys and coatings.     

Effects of water vapour on isothermal oxidation of chromia-forming alloys in Ar/O2 and Ar/H2 atmospheres

Fe9Cr, Fe17Cr and Fe25Cr alloys were subjected to isothermal oxidation in Ar/O₂ and Ar/H₂ atmosphere at 700 °C as high temperature corrosion for 48 h. Oxidation weight change measurement showed increasing Cr content reduced the oxidation rate. The oxidized Cr alloys were analysed using SEM, TEM and XRD. The addition of water vapour accelerates the onset of breakaway oxidation kinetics for Fe9Cr. The presence of water vapour promotes internal oxidation of Cr within Fe9Cr. For Fe17Cr and Fe25Cr, the water vapour effect is not significant due to the large Cr reservoir due to continue growth of Cr₂O₃.     

Oxidation Behavior of ODS Fe–Cr Alloys

Four experimental oxide dispersion strengthened (ODS)Fe-(13–14 at. %)Cr ferritic alloys were exposed for up to 10,000 hr at 700–1100 °C in air and in air with 10vol.% water vapor. Their performance has been compared to other commercial ODS and stainless steel alloys. At 700–800°C, the reaction rates in air were very low for all of the ODS Fe–Cr alloys compared to stainless steels. At 900°C, a Y₂O₃ dispersion showed a distinct benefit in improving oxidation resistance compared to an Al₂O₃ dispersion or no addition in the stainless steels. However, for the Fe-13 %Cr alloy, breakaway oxidation occurred after 7,000 hr at 900°C in air. Exposures in 10 % water vapor at 800 and 900°C and in air at 1000 and 1100°C showed increased attack for this class of alloys. Because of the relatively low Cr reservoirs in these alloys, their maximum operating temperature in air will be below 900°C.     

The Oxidation of Ferritic Stainless Steels in Simulated Solid-Oxide Fuel-Cell Atmospheres

The cyclic oxidation of a variety of chromia-forming ferritic stainless steels has been studied in the temperature range 700–900°C in atmospheres relevant to solid-oxide fuel-cell operation. The most detrimental environment at 800°C and 900°C was found to be air with 10% water vapor. This resulted in excessive oxide spallation or rapid scale growth. Impurities in the alloys, particularly Al and Si, were found to have a significant effect on the oxidation behavior. Oxide growth was slow at 700°C but the higher-Cr-content alloys were observed to form sigma-phase at this temperature. The sigma phase formation was accelerated by higher silicon contents, and remarkably, by the presence of water vapor in the exposure environment. Alloys containing Mn were observed to form an outer layer of MnCr₂O₄ over the chromia scale. The potential for this overlayer to suppress reactive evaporation of the chromia scales has been analyzed.     

Chloridation-oxidation of Fe-Cr and Ni-Cr alloys at 800°C

The chloridation-oxidation behavior of Fe-Cr (0–25 wt. %Cr) and Ni-Cr (0–20 wt.%Cr) alloys was studied at 800°C in three different H₂-HCl-H₂O() environments. In a low-HCI and low-H₂O() environment, where Cr₂O₃ is thermodynamically stable, the corrosion resistance of the Fe-Cr alloys increased with increasing Cr content in the alloys. In a high-HCl and high-H₂O() environment, where FeCr₂O₄ is stable and CrCl₂ is metastable, the corrosion resistance of the Fe-Cr alloys depended similarly on the Cr content. Low-Cr-Fe-Cr alloys exhibited large weight losses, while Fe-Cr alloys with higher than 19 wt. %Cr showed good corrosion resistance. In an environment of high-HCl in the absence of H₂O(), the evaporative corrosion rate was fast and limited by gas phase diffusion, and independent of the Cr content in the Fe-Cr alloys. Ni and Ni-Cr alloys generally showed good corrosion resistance in the environments of high H₂O() because of the low NiCl₂ vapor pressure and formation of a protective Cr₂O₃ scale. However, in the environment of high HCl in the absence of H₂O(), selective formation and evaporation of CrCl₂ occur, which results in Cr depletion and networks of voids for even a high-Cr Ni-Cr alloy.     

The protection of 9% chromium steels using vapour-deposited silica coatings

The corrosion behaviour of un-oxidized Fe-Cr binary alloys, in atmospheres typical of the advanced gas-cooled reaction (A.G.R.) can be greatly improved by a vapour-deposited silica coat. Such a treatment would be nearly always applied to steels already partly corroded. The coatings could be damaged during reactor operation. The behaviour of samples simulating these more realistic conditions has been investigated. Silica coatings have been deposited on clean Fe?8.6 Cr binary alloy, and then scratched to simulate damage. They have been deposited on binary alloy which was already extensively corroded, and on a 9%Cr steel typical of that used in A.G.R. power station boilers. The time taken for the samples to reach their linear ‘breakaway’ oxidation rate was increased by a factor of ca. 10 in all cases. In the case of the damaged sample, the improvement was due to geometrical factors. Analysis of its behaviour has given some insight into the mechanism of breakaway corrosion. If the corroded samples were already in the breakaway condition, silica coatings did not alter their oxidation rate. To be effective the coatings had to be applied before breakaway oxidation commenced.     

Factors governing breakaway oxidation of FeCrAl‐based alloys

At temperatures above around 1100 °C the life time of FeCrAl based alloy components can be limited by oxidation. Growth and spalling of the protective alumina scale leads after long exposure times to a depletion of aluminium in the alloys, eventually resulting in breakaway oxidation. This life time limit can be predicted using a recently developed model, taking into account scale growth rate (characterized by the parameters k and n), initial alloy Al content (C_o), critical Al content for protective alumina formation (C_B), oxide adherence and component geometry. Based on the evaluation of long term oxidation data for a number of commercial and model FeCrAl alloys it is shown that the life time can substantially be increased by decreasing the oxide growth rate and/or increasing C_o, whereby application of the latter factor is in most practical cases limited due to restrictions imparted by the alloys' mechanical properties. For typical commercial ODS materials C_B is around 1 wt‐%, however, this value is strongly affected by the exact alloy composition, especially Cr‐content. C_B seems to be higher for dispersion strengthened alloys than for conventional wrought materials. The adherence of the oxide scale not only depends on type and exact amount of reactive element (oxide) addition but also on other common minor alloying additions, such as Ti. Indications were found, that oxide adherence is also affected by the mechanical strength of a material and/or component.     

High-temperature sulfur corrosion of iron-chromium alloys

The kinetics and the mechanism of sulfurization of Fe-Cr alloys containing from 0.35 to 74 at. % Cr were studied in the temperature range 700–1000°C. The sulfurization was conducted in sulfur vapor at atmospheric pressure. The reaction rate was determined by the continuous gravimetric method. The scale composition was studied by X-ray and electron microprobe methods. The contribution of the reactants to the process of material transport through the scale was examined with both the marker method and the autoradiographic method using the radioactive isotope³⁵S. It has been found that, irrespective of alloy composition and temperature, the sulfurization follows a parabolic rate law. On the dilute alloys (up to about 2 at. % Cr) the scales formed are monophase consisting of Fe_1–xS only. In the range 2–40 at. % Cr the scale is a heterophase mixture of Fe_1–xS and the mixed spinel FeFe_2–xCr_xS₄. On the chromium-rich alloys the scale is monophase and is built of a solid solution of FeS in Cr₂S₃. The scale growth on the alloys under examination proceeds without the participation of the inward diffusion of sulfur. The mechanism of the scale formation has been proposed.     

Two calculations concerning the critical aluminium content in Fe20Cr5Al alloys

Alloys for high temperature applications rely on a protective oxide layer formed by selective oxidation. In Fe20Cr5Al type alloys the useful lifetime is governed by the depletion of aluminium by oxidation down to a critical aluminium content. Following spallation of the oxide scale the bare metal is exposed to air and catastrophic failure due to breakaway oxidation occurs if the aluminium content is too low. In this paper a model for the calculation of the critical aluminium content is presented. Based on Wagner's ideas on selective oxidation in ternary alloys, the critical aluminium content is calculated as that corresponding to the transition between external and internal oxidation. The resulting aluminium contents are temperature dependent and cover the range of values 0 to 3.5 wt.% reported in the literature. In the second part of the paper aluminium depletion profiles in Fe20Cr5Al plates have been calculated assuming a parabolic growth law for the oxidation process. The solution of the differential equation is an extension of the one dimensional solution presented by Whittle and coworkers for chromium depletion in FeCr sheet material, to three dimensions. Because of the increased consumption of aluminium at the corners of the sample, the critical aluminium content is first reached there. Based on the calculation, the reduction in lifetime, due to premature breakaway oxidation at the corners can be estimated.     

Effect of NaCl vapor on the oxidation of Ni-Cr alloys

Ni-Cr alloys are known for their resistance to high temperature oxidation. The kinetics of scale formation and the nature of the scale in these alloys are affected by NaCl liquid or vapor. There have been a few investigations dealing with the influence of NaCl on long-time exposure. But the nature of reaction at short times can provide information on the initiation of such attack. In this investigation, Ni-Cr alloys with Cr varying from 0 to 25 wt% were exposed to NaCl vapor at 850°C for a few minutes. The surface chemistry of these alloys along with the unattached ones was analyzed by Auger electron spectroscopy. The nature of scale and the distribution of chlorine was found to vary with the Cr content in the alloys, which has a direct bearing on the rate of oxidation of these alloys in NaCl vapor.     

Effect of water vapour on cyclic oxidation of Fe–Cr alloys

Model Fe–Cr alloys containing 9, 17 or 25 wt% Cr were subjected to repeated 1 h cycles of exposure at 700 °C to flowing gas mixtures of Ar‐20O₂, Ar‐20O₂‐5H₂O and Ar‐5O₂‐20H₂O (all in volume %) for up to 400 cycles. The kinetics and morphological development of these reactions were compared with those found during isothermal exposure to the same gases. Under isothermal conditions, all alloys developed thin protective chromium‐rich scales in dry oxygen. Addition of 5% H₂O induced breakaway for Fe‐9Cr within 48 h, but had little effect on higher chromium alloys. Isothermal chromia scale growth on Fe‐17Cr and Fe‐25Cr was accelerated by the addition of 20% H₂O, but breakaway did not result. Under cyclic conditions in dry oxygen, Fe‐9Cr quickly entered breakaway, oxidising according to fast, linear kinetics, but the higher chromium alloys exhibited protective behaviour. When 5% H₂O was added to the oxygen, the 17% Cr alloy also underwent fast breakaway oxidation, but Fe‐25Cr continued to be protected by a chromia scale. In the 20% H₂O gas, all alloys failed under cyclic conditions, producing thick, iron‐rich oxide scales. The synergistic effects of water vapour and temperature cycling are discussed in terms of alloy chromium depletion and the effects of H₂O(g) on oxide transport properties.     

The oxidation behavior of Fe-Cr alloys containing HFO2-dispersed phase

The present investigation examines the high-temperature oxidation behavior of Fe-Cr ferritic alloys containing 1.0% Hf which has been convened into an oxide dispersion. The oxide dispersions were produced by an internal oxidation treatment using a 50-50 Cr-Cr₂O₃ powder mixture in a sealed quartz capsule at 1100°C: the samples were not in direct contact with the powders. The effect of the dispersed oxide was spectacular under isothermal oxidation conditions, but it had almost no effect during thermal cycling conditions. Unlike the TiO₂-containing Fe-Cr ferritic alloys, virtually no particle coarsening was observed. The absence of the coarse particles caused poor thermal cycling behavior, whereas in TiO₂-containing ferritic alloys, coarse particles acted as oxide pegs giving a keying-on effect.     

High temperature oxidation of Fe-Al and Fe-Cr-Al alloys: The role of Cr as a chemically active element

Good high-temperature corrosion resistance of Fe-Al alloys in oxidizing environments is due to the α-Al₂O₃ film which is formed on the surface provided temperature is above 900 °C and the Al-content of the alloy exceeds the critical value. Ab initio calculations combined with experiments on Fe-13Al, Fe-18Al, Fe-23Al and Fe-10Cr-10Al alloys show that the beneficial effect of Cr on the oxidation resistance is significantly related to bulk effects. The comparison of experimental and calculated results indicates a clear correlation between the Fe-Cr chemical potential difference and the formation of the protective oxide scales.