The Effect of Water Vapor on the Transition from Internal to External Oxidation of Austenitic Steels at 1,073 K

The effect of water vapor on the transition from internal to external oxidation of austenitic alloys has been conducted at 1,073 K under the equilibrium oxygen partial pressure for the coexistence of Fe and FeO. Critical Cr concentrations in the Fe–Cr–30Ni (at.%) austenitic alloys were determined to be 30 at.% in dry atmosphere and 37 at.% in humid atmosphere. Thus, water vapor significantly affected the transition from internal to external oxidation of austenitic alloys. Two oxides of Cr₂O₃ and FeCr₂O₄ precipitated in the Fe–5Cr–30Ni (at.%) alloy and solid state reaction for the formation of FeCr₂O₄ may be influenced by water vapor. Oxygen permeability, which was estimated by considering the effective stoichiometric ratio, was also enhanced by water vapor.     

Magnesium-assisted formation of metal carbides and nitrides from metal oxides

A series of carbides (TiC, V₂C, Mo₂C) were synthesized by the corresponding metal oxides (TiO₂, V₂O₅, MoO₃), CaC₂ and magnesium as starting materials in a stainless steel autoclave at 600 °C. Through similar processes, transition metal nitrides (TiN, VN and CrN) could also be produced by employing the corresponding metal oxides (TiO₂, V₂O₅, Cr₂O₃), NaNH₂, and magnesium as starting materials at 550 °C. The FE-SEM image showed that the TiC sample was mainly consisted of flower-like microstructures. TEM image showed the other carbides (V2C, Mo2C) and the obtained nitride (TiN, VN, CrN) were consisted of nanoparticles. The possible synthesis mechanism of TiC had been described.     

Volatilization kinetics of chromium oxide,manganese oxide,and manganese chromium spinel at high temperatures in environments containing water vapor

Performance degradation of solid oxide fuel cells due to chromium volatilization is a well‐investigated issue in the literature. Therefore, retention coatings were developed to distinctly reduce the chromium volatilization. One approach was by alloying with manganese to ferritic steels to form manganese chromium spinel which is reported to decrease chromium volatilization by 61–75%. In the present paper, the volatilization rates of pure manganese chromium spinel ceramics were examined as well as those of the two oxides forming this spinel—pure chromium oxide and pure manganese oxide—in synthetic air containing 10% water vapor (high p(O₂)) and argon/hydrogen containing 10% water vapor (low p(O₂)) at 850°C, 950°C, and 1,050°C. Chromium oxide showed higher volatilization rates in high p(O₂), whereas manganese oxide demonstrated higher volatilization rates in low p(O₂). Contradictory to the literature, manganese chromium spinel displayed the highest volatilization rates in both atmospheres and nonlinear kinetics behavior. This deviation from linear behavior can be attributed to diffusion‐controlled volatilization in high p(O₂).     

Untersuchungen über die Korrosion einiger Metalle und Metalloxyde in Natriumhydroxydschmelzen

Research on the corrosion of some metals and metal oxides in sodium hydroxide melts The investigations were aimed at contributing to the clarification of reactions encountered in the salt baths specially used for the descaling of high-alloyed chrome and chrome-nickel steels, particularly the Hooker bath. The investigations were carried out on the metals silver, gold, platinum, nickel and iron and on the metal oxides FeO, Fe₃O₄, Fe₂O₃ and Cr₂O₃ in NaoH melts at temperatures around 500°sC. Special attention was paid to the influence of the gas atmosphere on the corrosion rate. With the exception of silver, the corrosion of all the metals and oxides investigated is considerably more intense under oxygen than under nitrogen, and an admixture of water vapour has a strong corrosion inhibiting effect, especially under oxygen. Silver was found to be the only metal to show a higher corrosion rate in the presence of water vapour than under dry gases. The corrosion products were examined chemically, metallographically, and by means of X-ray and electron diffraction. The low corrosion rate under nitrogen shows that the reaction between metals or metal oxides and the NaOH melt takes place rather slowly, if at all. The accelerating effect of oxygen and the inhibiting effect of water vapour indicate that, in most cases, it is the sodium peroxide content of the melt which governs the corrosion rate.     

Oxidation behavior of the single crystal Ni-based superalloy at 900 °C in air and water vapor

The oxidation behavior of a single crystal Ni-based superalloy TMS-82+ was studied in cyclic air and water vapor (air plus 15% H₂O) at 900 °C for 200 h. Oxidation kinetics was evaluated by mass gain measurements and the oxide scale was analysed by XRD, SEM and EDS, including the quantitative elemental concentration profiles by EPMA. Water vapor accelerated the growth rate of the oxides of NiO, Cr₂O₃ and spinel of NiCr₂O₄ due to the incursion of H, resulting in a higher mass gain and a thicker oxide scale with larger oxide grains of (Ni,Co)O.     

Nucleation and growth of selective oxide particles on ferritic steel

In the continuous annealing process, steel sheets are annealed at 800 °C in an atmosphere of nitrogen and hydrogen (5 vol.%) containing low partial water pressure (20-50 Pa). Under these conditions, the most oxidizable alloying elements in the steel segregate towards the surface where they form oxide particles. The nucleation and growth of those oxides were examined. Oxide nucleation mainly occurs between 650 and 750 °C. During their growth, the oxides take the form of a spherical cap and are composed of MnO, Mn₂SiO₄ (or MnSiO₃), MnAl₂O₄, SiO₂, Al₂O₃ and B₂O₃. Particle nucleation and growth are favored on grain boundaries.     

New environmental barrier coating system on carbon-fiber reinforced silicon carbide composites

Carbon-fiber-reinforced silicon carbide composites (C/SiC) are promising materials for high-temperature, light weight structural components. However, a protective coating and environmental barrier coating (EBC) are necessary to prevent the oxidation of the carbon and the reaction of the formed silica scale with water vapor. Current EBC systems use multiple layers, each serving unique requirements. However, any mismatch in the coefficients of thermal expansion (CTE) creates internal stresses and might lead to crack formation. In this case, oxygen and water vapor penetrate through the EBC, reducing the lifetime of the component. Mullite (Al₆Si₂O₁₃) is used in many known EBC systems on silicon-based ceramics either as an EBC itself or as a bondcoat. Due to its low CTE and its sufficient thermal cycling behavior, mullite was chosen in this investigation as a first layer. As mullite suffers loss of SiO₂ when exposed to water vapor at high temperatures, an additional protective top coat is needed to complete the EBC system. Different oxides were evaluated to serve as top coat, especially high-temperature oxides with low coefficients of thermal expansion (LCTE). An EBC containing mullite as bondcoat and the LCTE oxide La₂Hf₂O₇ as a top coat is proposed. Both layers were applied via atmospheric plasma spraying. In this paper, results of the influence of processing conditions on the microstructure of single mullite and LCTE oxide layers as well as mullite/LCTE oxide systems are presented. Special emphasis was directed toward the crystallinity of the mullite layer and, in the top layer, toward low porosity and reduced crack density.     

The cyclic oxidation behavior of the single crystal TMS‐82+ superalloy in humidified air

The cyclic oxidation behavior of a single crystal Ni‐based superalloy TMS‐82+ was studied at 800 and 900 °C for 200 h in water vapor (air plus 15% H₂O). Regardless of the exposure temperature, time‐dependence of the growth rate of the scale for the superalloy was fitted by a subparabolic relationship. The oxidation rate was enhanced with increase in exposure temperature, which was evidenced by a higher mass gain and thicker scale. The oxides on the specimen at 800 °C consisted of (Ni,Co)O, CrTaO₄, AlTaO₄, Cr₂O₃, and θ‐Al₂O₃, whereas for the specimen exposed at 900 °C, spinels of NiCr₂O₄ and (Ni,Co)Al₂O₄ as well as α‐Al₂O₃ were observed. An innermost dense α‐Al₂O₃ layer was responsible for a stable growth rate of the scale after the initial rapid oxidation.     

One component metal oxide sintering additive for β-SiC based on thermodynamic calculation and experimental observations

This paper examines a range of metal oxides, including those containing relatively safe elements under neutron irradiation, such as Cr, Fe, Ta, Ti, V and W, as well as widely used oxides, Al₂O₃, MgO and Y₂O₃, as a sintering additive for β-SiC theoretically and experimentally. After selecting the most probable SiC oxidation reaction at 1973–2123 K, the condition where the metal oxide additive does not decompose SiC was calculated based on the standard Gibbs formation free energies. Thermodynamic calculations revealed that Al₂O₃, MgO and Y₂O₃ could be an effective sintering additive without decomposing SiC under hot pressing conditions, which was demonstrated experimentally. On the other hand, no one component metal oxide that contains a safe element for nuclear reactor applications was found to be an effective sintering additive due to the formation of metal carbides and/or silicides. Overall, the simulation based on thermodynamic calculations was found to be quite useful for selecting effective metal oxide additives.     

The Transport Properties and Defect Structure of the Oxide (Fe,Cr)2O3 formed on Fe?Cr alloys

The protective properties of a scale forming on a metal are intimately relat-ed to the way in which matter is transported through the oxide. This report re-views the oxidation behaviour the bi-nary iron-chromium alloys which form the basis of many technologically important materials and attempts to relate it to the defect structure and transport properties of (Fe, Cr)₂O₃ oxides. Existing information on this system is mainly restricted to the end members, Cr₂O₃ and Fe₂O₃. The former is a cation deficient, p-type oxide and cation transport dominates. Fe₂O₃ is n-type, probably containing iron interstitials and oxygen vacancies, with comparable transport rates for both species, but the defect structure and properties of both oxides have been only partly characterized. New measurements by thermogravimetric, electrical and high temperature deformation techniques of these properties are briefly described. Electrical measurements support the hypothesis that certain solid solutions of Cr₂O₃-Fe₂O₃have lower defect concentrations and ionic transport rates than the pure oxides. There is evidence that scales of about the required composition form on the binary alloy containing about 20% Cr and are responsible for its low oxidation rate. Possible extensions of the theory to the design of highly stoichiometric scales for the protection of tornary and more complex alloys are briefly mentioned.     

Quantitative analysis of iron oxides using Fourier transform infrared spectrophotometry

In this study, a systematic approach based on the application of Fourier transform infrared spectrophotometry (FTIR) was taken, in order to quantitatively analyze the corrosion products formed in the secondary cycle of pressurized water reactors (PWR). Binary mixtures of iron oxides were prepared with known compositions containing pure commercial magnetite (Fe₃O₄), maghemite (γ-Fe₂O₃), and hematite (α-Fe₂O₃) for calibration purposes. Calcium oxide (lime) was added to all samples as a standard reference in obtaining the calibration curves. Using regression analysis, relationships were developed for intensity versus concentration for absorption bands corresponding to each of the phases in their corresponding FTIR spectrum. Correlation coefficients, R², of 0.82, 0.87, and 0.86 were obtained for maghemite-magnetite, hematite-magnetite, and hematite-maghemite systems, respectively. The calibration curves generated were used to quantify phases in multi-component unknown field samples that were obtained from different components (moisture separators, condensers, and high- and low- pressure heaters) of the two units (units 1 and 2) of the secondary cycle of the Comanche Peak PWR.     

Water Vapor Effects on the Oxidation Behavior of Fe–Cr and Ni–Cr Alloys in Atmospheres Relevant to Oxy-fuel Combustion

The oxidation behavior of a number of Fe–Cr- and Ni–Cr-based alloys was studied in atmospheres relevant to oxyfuel combustion at 650?°C. Oxidation was greatly enhanced in ferritic model alloys exposed in low p(O₂) CO₂?+?30%H₂O and Ar?+?30%H₂O gases. Rapidly growing iron oxides appear to be porous and gas permeable. Transition from non-protective to protective oxidation occurs on alloys with higher Cr contents between 13.5 and 22?wt% in H₂O. Excess oxygen, usually found in the actual oxyfuel combustion environments, disrupts the selective oxidation of Fe–Cr alloys by accelerating vaporization of early-formed Cr₂O₃ in combination with accelerated chromia growth induced by the H₂O. Rapid Cr consumption leads to the nucleation and rapid growth of iron oxides. On the contrary, Ni–Cr alloys are less affected by the presence of H₂O and excess O₂. The difference between Fe–Cr and Ni–Cr alloys is not clear but is postulated to involve less acceleration of chromia growth by water vapor for the latter group of alloys.     

High-Temperature Corrosion of Iron–Chromium Alloys in Oxidizing–Chloridizing Conditions

An investigation has been carried out into the effects of 0.1 to 1.0% HCl onthe oxidation of Fe–28%Cr and Fe–28%Cr–1%Y inargon–20%O₂ at 600 and 700°C. At the higher temperature,the additions of HCl to the gas caused considerable increases in corrosionof the binary alloy, with the rates of metal loss actually being greaterthan those of iron in the 0.5 and 1% HCl-containing environments. Thick andmultilayered scales were observed; these were oxides, particularlyFeCr₂O₄ and Fe₂O₃, that developedfollowing formation and vapor phase transport of chlorine-containing speciesfrom the metal surface. The main metal-loss processes were evaporation ofFeCl₂, CrCl₂, and CrO₂Cl₂, withthe first two of these reacting with oxygen to form solid oxides in thescale, while the third was lost mainly to the environment. The addition of1% Y to the alloy resulted in a marked improvement in corrosion resistanceat 700°C, because of the reactive element facilitating rapidestablishment of a protective Cr₂O₃-rich layer andpromoting the formation of condensed chlorides rather than the more volatileCrO₂Cl₂ phase. At 600°C, additions of HCl toargon–20%O₂ caused formation of some localized condensedchlorides on both alloys, but the corrosion rates were relatively low,because of protection by a Cr₂O₃-rich oxide scale.     

Influence of Pre-oxidation on the Corrosion and Mechanical Strength of Fe–25Cr and Fe–25Cr–20Ni Alloys at 973 K

The influence of pre-oxidation on the corrosion and mechanical strength of Fe–25Cr and Fe–25Cr–20Ni alloys was investigated in N₂–0.1SO₂ at 973 K with and without mechanical loadings. About 0.1μm-thick Cr₂O₃ scales formed on the Fe–25Cr alloy by pre-oxidation in Ar. However, spinel oxides of (Fe,Cr)₃O₄ remained on the Cr₂O₃ and voids formed at the oxide/metal interface on Fe–25Cr–20Ni after pre-oxidation in Ar. The preformed oxides are very effective in preventing corrosion of the alloy surfaces. The preformed oxides are also beneficial to increase the strength of the alloys in corrosive environments. The effects of pre-oxidation on Fe–25Cr are stronger than those of Fe–25Cr–20Ni due to the different characteristics of the preformed oxides.     

Stability of protective oxide films in waste incineration environment—solubility measurement of oxides in molten chlorides

Solubility measurements of several oxides in molten NaCl-KCl and NaCl-KCl-Na₂SO₄-K₂SO₄ were conducted in three different levels of basicity. The dissolution behavior of the oxides showed almost the same tendency as that shown by the dissolution behavior of the oxides in molten Na₂SO₄ in literature. In a waste incineration environment, a protective Cr₂O₃ film easily dissolves in molten chlorides as CrO₄²⁻ because pO²⁻ of the molten chlorides tends to have a small value due to the effect of water vapor contained in the combustion gas. From the result of the solubility measurement, the addition of molybdenum and/or silicon was expected to improve the corrosion resistance of alloys. Laboratory corrosion tests confirmed this expectation. However, the scale analysis suggested that the effect of molybdenum could not be explained completely by only the mechanism derived from the result of the solubility measurement.     

Investigations of plasma polymerized SiOx barrier films for polymer food packaging

Nano-scale SiO_x layers were deposited on polyethylene terephthalate (PET) foils by plasma enhanced chemical vapor deposition (PECVD) in an electron cyclotron resonance (ECR) plasma process in order to enhance their barrier properties towards water vapor. Oxygen (O₂) and hexamethyldisilazane (HMDSN) served as reactive gas and precursor, respectively. The effect of layer thickness and O₂:HMDSN (x:1) gas mixture ratio on the water vapor transmission rate was systematically investigated. Measurements by infrared spectroscopy and scanning electron microscopy for the characterization of the chemical composition and of the surface structure of the SiO_x layers, respectively, showed that both chemical composition and surface structure of the layers have a noticeable effect on their barrier properties. For low O₂ content in the O₂:HMDSN gas mixture ratio, organic layers were deposited. When increasing the O₂ content, the growing number of inorganic compounds in the SiO_x layers found by the infrared spectroscopy gave rise to a decrease in the water vapor transmission rate. A reduction of the water vapor permeation by more than a factor of 2 in comparison with the uncoated PET foil was achieved by the best performing SiO_x layer. Further increase of the O₂ content led to the onset of a columnar-like layer growth which showed to be causative for the water vapor permeation rising again.Finally, the barrier properties towards water vapor of 100 nm thick SiO_x films deposited from different O₂:HMDSN gas mixtures were contrasted with their corresponding barrier properties towards O₂. The minimum water vapor and O₂ permeation results were found for the SiO_x films plasma deposited from almost identical O₂:HMDSN gas mixture ratios in the range of 25 ≤ x ≤ 30.     

Formation and breakdown of a protective layer of chromium sesquioxide on L-605 alloy at 1100° C

The formation and breakdown of a protective layer of Cr₂O₃ on L-605 during oxidation at 1100° C was investigated. The effects of surface deformation, pressure, and water vapor on the breakdown time were evaluated. It was found that increasing surface deformation, increased the time to breakdown. Decreasing pressure below 1 atm to 0.13 N/m² increased time of breakdown as did decreasing water content from 25,000 to 2.5 ppm. By metallographic and microprobe examinations of samples during breakdown a model was deduced. Surface deformation promotes Cr₂O₃ formation, while increasing pressure and moisture increases the volatility of Cr₂O₃. Thus, the Cr₂O₃ grows for a time determined by these three factors. At the end of this time the growth stresses cause the oxide to crack, exposing a chromium-depleted metal surface to the oxidizing gas. The resultant rapid oxidation of this surface lifts the remaining Cr₂O₃, exposing more depleted metal. When the depletion zone is consumed and a very thick oxide has formed, the rate of oxidation slows and no further disruption is noted.     

Self-Repairing Metal Oxides

The oxidation of Cu, Zr, and alloys forming chromia, alumina, and zirconia was studied in a closed reaction chamber in O₂ gas near 20 mbar. Information on the position of oxide growth has been gained from the ¹⁸O/SIMS technique. Rates of O₂ dissociation on metal oxides, Au, and Pt have been evaluated from measurements in labeled O₂. The experimental results indicate that hydrogen in the metal substrates induces increased metal-ion transport in internal oxide surfaces during oxidation, which leads to increased oxide growth at the oxide–gas interface. Experiments also show that oxides of rare-earth metals (REM) and Pt catalyze the dissociation of O₂. An increased rate of O₂ dissociation can lead to increased transport of oxygen ions in the oxides and increased oxide growth at the substrate–oxide interface. A balanced transport of metal and oxygen ions in metal oxides that leads to oxide growth at both the metal–oxide and at the oxide–gas interface is found to be favorable for the formation of protective oxides with good adherence to the metal substrate. Depending on the original proporation of metal–to–oxygen ion transport in the oxide, an addition of hydrogen will increase or decrease the oxidation kinetics. In analogy, an addition of REM will increase or decrease the oxidation kinetics, depending on the original proportion of metal-to-oxygen ion transport.     

Indication of Chromium Oxide Hydroxide Evaporation During Oxidation of 304L at 873 K in the Presence of 10% Water Vapor

The oxidation of type 304L stainless steel wasinvestigated at 873 K in the presence of O₂and O₂ + 10% H₂O. Oxidation timevaried between 1 and 672 hr. The oxidized samples wereinvestigated by a number of surface-analytical techniques includinggrazing-angle XRD, SEM/EDX, auger spectroscopy, SIMS andXPS. Oxidation in dry oxygen results in the formation acorundum-type oxide (Me₂O₃) withadditional formation of spinel oxides after prolonged exposure. Theoxide layer contained mainly chromium, with smalleramounts of Fe and Mn. Oxidation in the presence of watervapor results in an oxide that contains more Fe and less Cr, the outer part of the oxide beingdepleted in Cr. In the presence of water vapor, a massloss is detected after prolonged exposure. We show thatthe mass loss is caused by chromium evaporation. The volatile species is suggested to beCrO₂(OH)₂.     

Zur Oxidschicht von Aluminium

The oxide layer of aluminium The oxide layer formed by the atmospheric oxidation of aluminium contains rather different oxides, namely AlO, Al₂O₂ and Al₂O₃. The presence of these suboxides and for their mixed crystals is responsible for the adhesion of the oxide layers to the metal because the suboxides are still able by their residual valancy electrons to react with a metal. It is consequently advisable to carry out chemical treatments in such a way as to yield suboxides, too.