Preformed Oxide Scales Research Articles

Investigation on the hot corrosion behavior of a single crystal Ni-based superalloy in molten Na2SO4+K2SO4 salts improved by different preoxidation treatments

This study investigated the hot corrosion behavior of N5 in molten sulfate at 850 ℃ after different peroxidation treatments. The preoxidation was conducted at 900, 1000 and 1100 ℃ for 20 and 100 h, respectively. The preformed oxide scale featured a multilayered structure and varying degrees of Ta oxides. Among all, the samples preoxidized at 900 ℃ exhibited the highest corrosion resistance. But a porous structure with a large number of internal oxides and sulfides was observed when preoxidized at 1000 and 1100 ℃. The alloy without preoxidation directly reacted with the molten salt to form a porous structure with internal oxides and sulfides.

Rearrangement of Oxide Scale on Ni-11Fe-10Cu-6Al Pre-Oxidized at 950 °C during Anodic Polarization in Molten Carbonate

The effect of anodic polarization in molten Na2CO3-K2CO3 at 750 °C was investigated on the structure of oxide scale formed by pre-oxidation of Ni-11Fe-10Cu-6Al alloy at 950 °C in air. The pre-formed oxide scale evolves and rearranges under anodic polarization related to melt corrosion and non-uniformly distributed electric field. Both of pre-oxidized and as-rearranged electrodes can serve as inert anodes with oxygen evolution. Anodic polarization exhibits a negative rearrangement-destructivity effect for the pre-formed oxide scale with corrosion protection of the rearranged oxide scale decreasing. The structure rearrangement of pre-formed oxide scale is also discussed during anodic polarization in the melt.

Decarburization of 60Si2MnA in Atmospheres Containing Different Levels of Oxygen, Water Vapour and Carbon Dioxide at 700–1000 °C

The decarburization behaviour of 60Si2MnA in atmospheres containing 0–21% O2, < 20 ppm–17%H2O, and with or without 8%CO2, at 700–1000 °C, was investigated. The new findings of the current study were: (a) severe decarburization was associated with the formation of wustite (FeO) scale on the steel surface, (b) the carbon activity at the steel–FeO interface was most likely determined by the reaction equilibrium between FeO and dissolved carbon in steel, (c) when a ferrite layer was able to form, the decarburization tendency was determined by the relative carbon permeability (defined as the product of carbon concentration difference at the two interfaces of the ferrite layer and carbon diffusivity) through the ferrite layer, and therefore, (d) the decarburization tendency at 800 °C was greater than those at 700 and 900 °C as the relative carbon permeability at 800 °C was the greatest. If FeO was absent when heating in dry O2-containing gases, however, possibly as a result of the formation of a SiO2 layer at the steel surface, decarburization was very much alleviated or avoided. At 1000 °C, the decarburization tendency was alleviated even when FeO was able to form because formation of a ferrite layer was not possible and carbon diffusivity in austenite was much lower than that in ferrite. A preformed oxide scale was effective in providing decarburization protection only when the steel was exposed to dry O2-containing atmospheres.

Oxidation of 60Si2MnA Steel in Atmospheres Containing Different Levels of Oxygen, Water Vapour and Carbon Dioxide at 700–1000 °C

The oxidation behaviour of the spring steel 60Si2MnA in atmospheres containing 0–21% (volume per cent) O2, < 20 ppm (part per million) to 17%H2O, with some containing 8%CO2, at 700–1000 °C was investigated. The oxide scale thicknesses formed in both 17%H2O–N2 and dry O2-containing atmospheres were less than 6 μm after 20 min of oxidation, significantly smaller than those formed in atmospheres containing both oxygen and water vapour, and the scale structures developed in the three different scenarios were also very different. The scale formed in 17%H2O–N2 contained wustite only, the scale formed in dry O2-containing atmospheres comprised primarily hematite and some magnetite, and that in O2–H2O mixtures developed a multi-layered structure, generally with an innermost Fe2SiO4 + FeO layer, followed by FeO/Fe3O4/Fe2O3 layers towards the scale surface. A preformed oxide scale and the presence of 8%CO2 in the atmosphere had little effects on further steel oxidation. The mechanisms of forming different scale structures are discussed.

The Influence of Oxide-Scale Microstructure on KCl(s)-Induced Corrosion of Low-Alloyed Steel at 400\xa0\xb0C

The high-temperature corrosion of low-alloyed steels and stainless steels in the presence of KCl(s) has been studied extensively in the last decades by several authors. The effect of KCl(s) on the initial corrosion attack has retained extra focus. However, the mechanisms behind the long-term behavior, e.g., when an oxide scale has already formed, in the presence of KCl(s) are still unclear. The aim of this study was to investigate the effect of the microstructure of a pre-formed oxide scale on low-alloyed steel (Fe–2.25Cr–1Mo) when exposed to small amounts of KCl(s). The pre-oxidation exposures were performed at different temperatures and durations in order to create oxide scales with different microstructures but with similar thicknesses. After detailed characterization, the pre-oxidized samples were exposed to 5%O2 + 20%H2O + 75%N2 (+KCl(s)) at 400 °C for 24, 48, and 168 h and analyzed with scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and focused ion beam. The microstructural investigation indicated that Cl-induced corrosion is a combination of oxide thickness and microstructure, and the breakaway mechanism in the presence of KCl(s) is diffusion-controlled as porosity changes prior to breakaway oxidation were observed.

Rearrangement of oxide scale on Ni-11Fe-10Cu alloy under anodic polarization in molten Na2CO3-K2CO3

The effect of anodic polarization on a pre-formed oxide scale of Ni-11Fe-10Cu alloy was investigated in Na2CO3-K2CO3 melt at 750 °C. The pre-formed oxide scale rearranged under anodic polarization and enabled the alloy as an inert anode for oxygen evolution. The anodic polarization has a positive effect on repairing defects in the original oxide scale, i.e., repair-enhancement effect, which can enhance the protectivity of an alloy substrate and eliminate further corrosion and oxidation. This study demonstrates an effective way to prepare a protective oxide scale, and reveals the rearrangement of pre-formed oxide scale under anodic polarization.

Effects of pre-formed nanostructured surface layer on oxidation behaviour of 9Cr2WVTa steel in air and liquid Pb–Bi eutectic alloy

A nanostructured surface layer was pre-formed on a 9Cr2WVTa reduced activation ferritic/martensitic steel sample by means of surface mechanical rolling treatment (SMRT). In comparison with the initial sample, oxidation experiments at 600°C in air showed that the oxidation resistance of the SMRT sample is markedly enhanced, due to the formation of a more continuous, homogenous, and compact oxide scale enriched with Mn. Furthermore, oxidation experiments at 550°C in oxygen-saturated liquid Pb–Bi eutectic alloy showed that the oxidation resistance is improved on the SMRT sample after pre-oxidation in air, due to depressed diffusivities in the pre-formed oxide scale.

Effect of Sulfur Partial Pressures on Oxidation Behavior of Fe–Ni–Cr Alloys

Fe–Ni–Cr alloys containing different contents of Si with and without pre-formed oxide scale at the surface were tested in oxidation environments at 1,050 °C with varied sulfur partial pressures. The oxide-scale growth on Fe–Ni–Cr alloys was accelerated by increasing sulfur partial pressures in the oxidizing-carburizing environments. This accelerated oxidation was characterized by the formation of plate-shaped MnCr2O4 spinel crystallites and the nodular clusters at the site of scale spallation. Pre-oxidized Fe–Ni–Cr alloys generally did not suffer from sulfur attack because of excellent protection of pre-formed oxide scale. Scale spallation and sulfur attack were found only on high-Si alloy subjected to the maximum sulfur potential, which was attributed to accelerated oxidation and selective oxidation and sulfidation at the sites where oxide scale spallation had occurred. For bare alloys in absence of pre-formed oxide layers, scale spallation was found to occur at lower level of sulfur potential on low-Si alloy than on high-Si alloy. A higher content of Si is necessary for the formation of protective silica sub-layer, which is believed to be the main cause of the difference in scale spallation observed.

Hot corrosion of a novel (Ni,Co)O/(Ni,Co)Fe 2O 4 composite coating thermally converted from an electrodeposited Ni–Co–Fe 2O 3 composite coating

Ni–Co–Fe 2O 3 composite coatings were successfully developed by sediment co-deposition. In order to improve their hot corrosion resistance, a pre-oxidation treatment was conducted at 1000 °C for 6 h. The corrosion behaviour of the oxidised composite coating was investigated at 960 °C in an atmosphere consisting of a mixture of Na 3AlF 6–AlF 3–CaF molten salts and air. They exhibited good hot corrosion resistance due to not only the pre-formed oxide scale with (Ni,Co)O and (Ni,Co)Fe 2O 4 phases after pre-oxidation, but also the formation of (Ni,Co,Fe)Al 2O 4 phases in the outer layer and a well-distributed NiFe 2O 4-enriched phase along the grain boundaries in the subscale area during the corrosion process.

Anodic behaviour of oxidised Ni–Fe alloys in cryolite–alumina melts

Nickel-iron alloys have been identified as promising inert anode candidates for the Hall–Héroult process. In this study, binary Ni–Fe alloys of various compositions were subjected to short-term galvanostatic electrolysis in a cryolite–alumina bath at 960 °C. Prior to electrolysis, the anodes were oxidised at 800 °C for 48 h, forming a protective scale. Fe 2O 3, Ni x Fe 3− x O 4 and Ni x Fe 1 − x O were identified as the major scale components using a combination of X-ray diffraction (XRD) analysis and energy dispersive X-ray spectroscopy (EDX). Anodes having Ni content of 50–65 wt% performed adequately during short-term electrolysis, operating at a steady potential of 3–3.5 V vs. AlF 3/Al. Overall, it was found that the pre-formed oxide scale was effective in reducing anode wear and fluoridation. In the absence of a pre-formed scale, anodes were shown to undergo appreciable internal corrosion and/or passivation due to metal fluoride formation. Analysis of the anodes following electrolysis was performed using XRD and electron microprobe analysis (EPMA).

Effects of CeO2 applied to preformed oxide scales on subsequent oxidation of Fe–20Cr at 1000°C

In order to explore the relationships between rare earth elements existing in oxides and growth rate and adhesion of oxide scales, CeO2 thin films were prepared on Fe–20Cr alloys after pre-oxidation at 1000°C; subsequent isothermal and cyclic oxidation was carried out, and the oxide adhesion was determined by using the tensile pull test. The results demonstrated that similar to the effects of CeO2 on the as polished Fe–20Cr alloy, the application of CeO2 to the preformed oxides could decrease the subsequent oxidation rate, improve the cyclic oxidation resistance of the alloy and raise the adhesion strength of the oxide scales. These beneficial effects of the applied CeO2 decreased with increasing pre-oxidation time. The fact that virtually none of CeO2 applied on the preoxidised preformed layer reached the metal/oxide interface suggests that the 'sulphur trapping effect', through which CeO2 may act, is minimal in the present system.

Beneficial Effects of Ce Implantation into Preformed Cr2O3Scales on the Subsequent Oxidation of Ni–20Cr Alloy

The influence of Ce implantation into preformed Cr2O3 scales with a dose of 1 × 1017 ions/cm2 on the subsequent oxidation behavior of Ni–20Cr alloy at 1050°C in air has been investigated. The pre-oxidation was carried out at 1050°C in air for 0.5 and 1 hr respectively Cr2O3 and NiCr2O4 formed on Ni–20Cr alloy. The oxidation rate was decreased remarkably due to Ce implantation regardless of whether it was implanted into the alloy or into the pre-formed oxide scales, and the beneficial effect decreased with increasing pre-oxidation time, the alloy implanted directly with Ce had the lowest oxidation rate constant. During cyclic oxidation (350 cycles) Ce implantation played a similar benefical effect on the oxide-spallation resistance for blank and pretreated alloys. The result indicates that Ce incorporated into the oxide scale affected the diffusion of the reaction species and also the spallation resistance of the oxide scales. The change of the oxidation process is attributed to the segregation of Ce at the oxide grain boundaries

The effect of pre-oxidation and the influence of deformation on the corrosion behaviour of two heat resistant steels in a sulphur-oxygen-carbon bearing environment

The influence of pre-oxidation on the corrosion resistance of the austenitic steels AC 66 (Fe-32Ni-27Cr-0.07Ce) and Alloy 800H (Fe-32Ni-20Cr) was studied in a sulphur-oxygen-carbon bearing atmosphere at 700°C. For AC 66 the corrosion resistance was significantly improved by preoxidation, whereas this effect was less marked for Alloy 800H. This can be explained by a much better adherence of the preformed oxide scale for AC 66. The corrosion resistance was shown to decrease by superimposed deformation which leads to deeply penetrating intergranular corrosion paths.

Computer simulation of the effect of pre-oxidation in erosion-corrosion environments

The combined effects of erosion and corrosion can lead to a number of material wastage scenarios in industrial environments. In some cases, for example at high temperatures in fluidized bed environments, the wear may be corrosion dominated where chipping of an oxide scale is the predominant process, while in more aggressive environments such as gas turbine conditions, formation of a protective scale may never occur. Thus, pre-oxidation to form a protective scale or coating of a substrate may be effective in reducing material loss in corrosion-dominated environments since particle energies may be low enough to prevent significant deformation of the underlying surfaces. A computer model has been developed to simulate the erosion of a preformed scale or a coating on a metal substrate. This model uses computer graphics to demonstrate the sequential stages of erosion-corrosion. By incorporating established erosion and corrosion algorithms to calculate the wastage rate, the surface topography and extent of wear can be shown after exposure to erosion-corrosion conditions of varying severity. Thus, one can extrapolate from this technique whether erosion of oxide or metal is likely to be dominant wastage mechanism. This model was developed to describe the erosion-corrosion of a preformed oxide scale on a metal and comparisons are made between the erosion results for pre-oxidized surfaces and the graphics after computer simulation of erosion-corrosion. However, since the factors which determine adhesion of a preformed scale may be similar in many respects to those of a coating, the model may equally describe the erosion-corrosion behaviour of a coated metal. Hence, the paper shows the stages of the model development and compares the experimental results with those of the model.

Sulfidation behavior of Fe-25Cr-20Ni-1.5Al2O3 in simulated coal combustion/gasification environments

The effect of minor addition of α-Al2O3 dispersoids on the sulfidation behavior of Fe-25Cr-20Ni was investigated over a range of pO2, 1.13×10−20 to 1.18×10 ****−22 atm. at constant pS2=1.22×10−8 atm. Fe-25Cr-20Ni and Fe-25Cr-20Ni 1.5 Al2O3 with and without preformed oxide scales were exposed to bioxidant gas mixtures H2/H2O/H2S/Ar at 700° C. Both isothermal and cyclic exposures were included. Scales were characterized by a combination of several surface analytical tools. A remarkable improvement in sulfidation resistance is observed in Fe-25Cr-20Ni-1.5Al2O3 under the conditions investigated here. This is attributed to the ability of the alloy to form and maintain a predominantly Cr2O3 scale with reduced Fe-diffusion and content. Possible scientific reasons for such improvement are discussed. The base alloy, Fe-25Cr-20Ni, fails to develop and retain such a Cr2O3 scale and undergoes sulfidation within a few minutes of exposure. The scale breakdown process by sulfidation is explained qualitatively. Experimental evidence suggests that sulfur in the environment enhances Fe-diffusion and content in the scale.

Oxidation and sulfidation behavior of Fe-20Cr-16Ni-4Al-1Y2O3 oxide-dispersion-strengthened alloy

High-temperature oxidation and sulfidation studies were conducted on an oxide-dispersion-strengthened alloy of composition Fe-20Cr-16Ni-4Al-1Y2O3. The oxidation studies were conducted in air and low-PO2 environments over a temperature range of 650–1200°C. Results are also reported on the sulfidation resistance of preformed oxide scales and the influence of reoxidation of preformed sulfide scales. Detailed microstructural results and x-ray diffraction analysis data are presented to substantiate the corrosion behavior of the alloy over the wide range of conditions anticipated in fossil-energy systems. Data are also presented on the corrosion behavior of the alloy in oxygen/sulfur mixed gas atmospheres, and the results are used to compare the corrosion behavior of the present alloy with other chromia- and alumina-forming alloys.

Penetration of sulfur through preformed protective oxide scales

Thermodynamic assessment of sulfur penetration through otherwise protective scales such as Cr2O3, Al2O3 has been carried out for Co-Cr- and Co-Cr-Al-base alloys. Limiting conditions for sulfide formation following gas molecular transport and solution-diffusion transport have been established and the results partially confirmed by experiments carried out on Co-10Cr, Co-25Cr, and Co-10Cr-5Al alloys in sulfurous atmospheres. The results show that molecular transport of sulfurous gas species through the growing oxide scale definitely occurs. It was not possible to confirm or disprove the solutiondiffusion mechanism.

Mechanism of sulphur transport through preformed oxide scales

AbstractThe possibilities for transport of sulfur through preformed oxide scales by both solution‐diffusion and molecular gas (24) permeation mechanisms are examined thermodynamically to establish the limiting conditions under which each is viable.The results are tested, using nickel and cobalt specimens, and it is concluded that, although both mechanisms may operate in parallel, the permeation of gas molecules is the more dangerous since it can operate over wider ranges of gas atmosphere composition. The permeation of SO2 molecules throu through oxide scales growing on cobalt is clearly demonstrated.