The chemistry of the scale/metal interface of non-adherent oxides has been investigated and related to its microstructure. Al2O3 or Cr2O3 scales were formed at elevated temperatures in oxygen on Fe-18wt%Cr-5wt% Al and Ni-25wt%Cr alloys. Newly exposed interfaces were made by scratching the oxide with a diamond stylus located in the vacuum chamber of a scanning Auger microprobe. The scratching operation often caused the scales to fracture and exfoliate at the scale/metal interface. On both the Al2O3- and the Cr2O3-forming alloys, sulfur was found to be present everywhere on the metal surface. This sulfur coverage was no more than 3 monolayers thick. On areas where the scale had lost contact with the metal at temperature, the sulfur was embedded in a thin oxide layer formed during cooling of the specimen.

The lifetimes due to chemical failure during high-temperature oxidation of Co and Ni-rich MCrAlY type coating alloys were investigated. They were found to depend critically on the oxidation rate and Al diffusivity in alloy. Re and Al additions increased the life by providing higher 3-phase contents and by lowering the Al diffusivity and oxidation rates. The effects of Re were more pronounced in the Co-rich than the Ni-rich alloys. The observed difference was a result of different alloy phases, with a microstructure of gamma, beta, and sigma in the Co-rich alloys and gamma, beta, and alpha in the Ni-rich alloys. (C) 2015 Elsevier Ltd. All rights reserved.

In the present study, two types of subscale Cr and Re accumulation found at scale/alloy interface of two batches of Co32Ni21Cr10Al3.5ReY alloys are described. During oxidation, one alloy developed a supersaturated Cr-Re rich layer and another formed Cr-Re precipitates beneath the oxide in the subscale region. Diffusional and thermodynamic modelling showed the accumulation is a result of decreasing Cr and Re chemical potentials with Al depletion. The observed difference in accumulation forms is caused by the different phases in two alloys, with the former being gamma, beta, alpha, sigma and gamma, beta and the latter having only gamma, beta and sigma. (C) 2014 Elsevier Ltd. All rights reserved.

Microstructural changes following oxidation at high temperatures of a complex coating or alloy can significantly influence its properties. This paper investigates possible solutions that can accurately model alloy microstructures that result from such a process in a cobalt-rich CoNiCrAlReY bond coat alloy system. Four different alloys with varying aluminum content were studied after heat treatments at 1000 °C or 1150 °C. SEM, EPMA and XRD were used to characterize the alloy microstructure, and hardness testing was performed to determine the role of microstructure on mechanical properties. Experimental microstructures were modeled using two databases of Thermo-Calc, namely TTNI8 and SSOL5. Effects of Al content on phase changes and materials properties were quantitatively investigated. Data showed that a 1 wt. % increase of Al in the alloy caused an 8% increase in the beta phase, and a Cr and Re rich sigma phase precipitated preferentially at high Al concentrations. In addition, alloy hardness was found to increase by 4 and 81%, respectively, with a 10% increase in the beta and sigma phases. Modeled results using the TTNI8 database showed excellent agreement with experimental observations, but the SSOL5 database produced erroneous information.

Early-stage tensile stress evolution in a-Al2O3 scales during oxidation of FeCrAlY at 1000, 1050, 1100, and 1200?degrees C was monitored in situ by use of synchrotron radiation. Tensile stress development as a function of oxidation temperature indicated a dynamic interplay between stress generation and relaxation. An analysis of the time dependence of the data indicated that the observed relaxation of the initial tensile stress in the oxide scales at 1100 and 1200?degrees C is dominated by creep in the a-Al2O3. A thin layer of a (Fe,Cr,Al) oxide was observed at the oxide-gas interface, consistent with a mechanism whereby the conversion of (Fe,Cr,Al)2O3 to a-Al2O3 produces an initial tensile stress in the alumina scale.

Alloys designed for high-temperature applications need to form a protective oxide scale to withstand further degradation by oxidation. The importance of the interfacial chemistry for the adhesion of the protective scale was demonstrated in experiments with β-NiAl, iron-aluminides, NiCrAlYs and Ni-based superalloys. Interfaces between alloys both with and without thermal barrier coatings and their thermally grown alumina scales were studied as well as interfaces between alloys and deposited alumina coatings. It was found, that in all cases improved oxidation performance and scale adhesion could be achieved by doping the alloys with reactive elements. The improvement was shown to be due to interfacial segregation of the reactive element. Segregation of sulfur to internal interfaces on the other hand caused scale spallation.

The electronic properties of chromia scales grown between 800°C and 900°C on chromium metal and chromia-forming ferritic stainless steels were determined using room temperature PhotoElectroChemistry (PEC) experiments and the relative importance of the n- and p-character of the scales could be assessed. According to the thermodynamic previsions of defects structures, the external part of all the scales grown in oxygen exhibits band gap energy around 3.5 eV, with a marked p-type character on chromium and a possibly n-type behaviour on stainless steels. On the contrary, the internal part of the scales is always n-type, with predominant interstitial chromium defects. A major change appears when chromium or stainless steels are oxidised in water vapour-argon mixtures, where the absence of a p‑type semiconductor in the scales could be evidenced. Hydrogen defects are thought to be responsible of this particular behaviour which leads to a strong reduction of residual stresses due to increased high temperature relaxation. Moreover, the inversion of the growth direction resulting from high mobility of the OH defects makes the chromia scales grown in water vapour more adherent than when grown in oxygen.

Raman spectroscopy has in recent years been used by several research groups to study the stresses that develop in thermally grown oxide layers, particularly Cr2O3, at elevated temperatures between 700 and 900 °C. This paper presents an overview of the Raman technique and describes quantitatively the factors that, in addition to stress, affect the Raman peak shifts, including temperature, non-stoichiometry, impurities, stresses and instrumental calibration. It also summarizes and reviews published work in this area and in doing so, discusses the pros and cons of this technique for oxidation stress measurements.

The “reactive element effect”, modified from its earlier representation of the “rare earth effect”, is a well known term within the oxidation community. It describes several beneficial outcomes on the oxidation behavior of alumina and chromia forming alloys. Any element can be considered “reactive” if it is more oxygen active than the scale forming element, namely that of Al or Cr. However, the relative effectiveness of each element can be quite different. Numerous scientific studies have been carried out on this topic since its discovery more than 70 years ago to gain understanding of the manifestations of and reasons for these effects. This paper gives an overview that summarizes current understandings on this effect and points to issues that warrant further studies.

The oxidation behavior of a Co32Ni21Cr8Al0.6Y (wt%) alloy with and without the addition of 3.5 wt% rhenium, 2 wt% aluminum or a combination of the two was investigated at 1000 °C. Results showed that increasing the Al content from 8 to 10 wt% led to an increase of the alloy β-phase, but did not affect the oxidation behavior. Re addition induced (Cr,Re,Y)-rich phase to precipitate in the alloy, accelerated the θ- to α-alumina transformation, reduced the oxidation rate and enhanced the rate of alloy Al diffusion. Adding both Al and Re further improved the oxidation behavior by promoting the development of the external alumina scale and suppressing the formation of Ni, Co containing spinel. This alloy also showed the largest reduction of oxidation rate and emerged to be the most beneficial. A continuous Cr–Re rich layer was observed at the oxide/alloy interface of the Re, Al containing alloy after longer oxidation times, but this layer is not expected to affect the continued growth of the alumina scale.

The microstructures and oxidation behaviour of the modified Tribaloy T-800 alloys by additions of yttrium and yttrium plus aluminium have been studied. At the presence of yttrium alone, the oxidation rate decreased, and the selective oxidation of chromium was promoted, which was related to the refinement of alloy phase size. The addition of yttrium plus aluminium further reduced the oxidation rate. The selective oxidation of chromium and aluminium were both promoted significantly. The benefits were especially pronounced at 1000 degrees C, with the formation of protective alumina external layer and no internal oxides, which may be detrimental to the alloy mechanical property. (C) 2010 Elsevier Ltd. All rights reserved.

Heat-exchanger tubes in fluidized bed combustors (FBCs) often suffer material loss due to combined effect of corrosion and erosion. Some field studies have indicated a possible chlorine effect, where coals containing higher chlorine contents cause higher wastage rates. The effect, however, is neither understood nor certain due to inconclusive field and laboratory data. The purpose of this paper is to report the results of experiments using a wastage simulator to evaluate the effect of chlorine on in-bed tube wastage. The rig was designed to simulate dense particle impacts on tube bottoms with well-controlled parameters, and has been proven to closely reflect situations found in operating bubbling FBCs. In this study, HCl gas was chosen to be the chlorine source and was introduced into the fluidized bed at 40~50 μg/g concentrations. Tests were performed at temperatures ranging from ambient to 400°C using 1018 low carbon steel rods in a bed of commercial SiO2 sand that had an average size of 800 μm. The wear profile after each test was measured using profilometer. Results showed a significant increase in material wastage rates in the presence of HCl, and that the rate was greater with higher HCl concentrations. The magnitude of the effect observed appears to be of the same order as that reported in practice.

The segregation of sulfur and other elements at the interface between thermally grown alumina and a few coatings have been reviewed and compared with studies made at oxide/metal interfaces formed on model alloys. The coatings studied were NiPtAl on CMSX-4 or AM1 with two different bulk sulfur contents, and NiCoCrAlY on PWA 1484. The segregation behavior at the oxide/PWA1484 interface was also reported. Auger electron microscopy was used to study the chemistry at the oxide/coating interface after portions of the oxide were removed in ultra high vacuum (UHV) by scratches made on the oxidized sample surface. The extent of oxide spallation in relation to the scratch width was utilized to evaluate the interfacial strength, which was then correlated with the interface impurity level. Results showed strong relationship between sulfur segregation and the composition of the alloy substrates. In addition to substrate sulfur content, the degree of sulfur segregation was most significantly increased by Cr co-segregation or decreased by Y doping of the coating. Pt and Hf could stop segregation only when present together. P was found as a significant segregand in one case where sulfur segregation was prevented by Y. These behaviors are discussed in terms of various thermochemical interactions in the bulk and at the interface.

Thermally grown oxide scales are often under compressive residual stresses, especially upon cooling, which can lead to spontaneous spallation and a loss of their protectiveness. This study uses nickel oxide formed on different purity of nickel as an example to investigate the spallation process. Samples oxidized after different times between 800 and 1100 °C were observed during cooling. The oxide scale buckled and spalled after reaching a critical thickness that depended on oxidation temperatures, substrate thickness, and metal purity. The observed buckling was only a secondary process that followed scale delamination under local tensile stresses at sample edges or corners. When the delamination eventually extended over a large enough area on the face of the specimen, the scale above it buckled, driven by the residual compressive stress in the oxide. Growth of the buckles took place by crack extension along regions of high pore densities in the oxide scale. The development of pores in the oxide layer, which depended strongly on substrate impurity levels, was found to be the most important factor controlling failures of the NiO scales. Sulfur segregated on pore surfaces, whereas TEM studies showed no S at NiO/Ni interfaces.

The strains in alumina thin films growing on high-temperature alloys at 1,000−1,100°C and during cooling have been successfully measured in-situ using a novel x-ray technique, exploiting synchrotron radiation at the Advanced Photon Source at Argonne National Laboratory. This paper summarizes results obtained from model alloys, with or without the presence of a reactive element, such as Zr, Hf, and Y, to show the importance of the dynamic nature of the stress evolution process and the effects of alloy composition on the generation and relaxation of these stresses.

The interfacial chemistry that developed as a result Al2O3-scale growth on gamma-Iei + gamma'-Ni3Al alloys at 1150 A degrees C was studied using scanning Auger microscopy after the oxide layer was scratched to spall under ultra-high vacuum. The extent of scale spallation was used to evaluate semi-quantitatively the interfacial strength. The alloys investigated were primarily gamma' in structure, containing 22 at.% Al plus further additions of Pt, Cr and/or Hf. In the case of the binary gamma + gamma' alloy, it was found that a sub-monolayer of sulfur segregated at the alloy/scale interface. Platinum reduced and hafnium eliminated sulfur segregation, but chromium enhanced it through Cr-S co-segregation, even on Pt- and Hf-containing alloys. Platinum also segregated slightly at the alloy/scale interface. The interface strength was a strong function of the sulfur content. Beyond the effect of eliminating S segregation, Pt and Hf both showed additional beneficial effects on alumina scale adhesion.

The interfacial chemistry that developed as a result Al2O3-scale growth on γ-Νi + γ′-Ni3Al alloys at 1150 °C was studied using scanning Auger microscopy after the oxide layer was scratched to spall under ultra-high vacuum. The extent of scale spallation was used to evaluate semi-quantitatively the interfacial strength. The alloys investigated were primarily γ′ in structure, containing 22 at.% Al plus further additions of Pt, Cr and/or Hf. In the case of the binary γ + γ′ alloy, it was found that a sub-monolayer of sulfur segregated at the alloy/scale interface. Platinum reduced and hafnium eliminated sulfur segregation, but chromium enhanced it through Cr–S co-segregation, even on Pt- and Hf-containing alloys. Platinum also segregated slightly at the alloy/scale interface. The interface strength was a strong function of the sulfur content. Beyond the effect of eliminating S segregation, Pt and Hf both showed additional beneficial effects on alumina scale adhesion.

In the context of solid oxide fuel cell (SOFC) applications, the adhesion strength and failure location of chromia scale that developed on 430 stainless steel after various surface modifications prior to oxidations between 600 and 800 °C were evaluated. Results demonstrated that the tensile strength and nature of adhesion of the oxide/alloy interface on 430 stainless steel can be compromised by polishing, but can be improved by reducing surface impurities, increasing surface roughness and applying a coating that contains a reactive element, such as Y-nitrate. Optimally, a combination of firing in a reducing atmosphere and applying a thin yttrium nitrate coating was found to be especially effective. These findings identify surface modification techniques that improve scale adhesion for Cr2O3-forming metallic interconnects whether independently or beneath a protective coating.