Growth Of Chromium Oxide Research Articles

Effects of imposed stresses on high temperature corrosion behaviour of T91

T91 steel specimens were tested at a high temperature with and without Na2SO4-NaCl molten salt under different stresses to investigate the effect of imposed stress on the corrosion behaviour. The imposed tensile stress facilitated chromium-oxide growth near the substrate and strengthened the corrosion resistance. The mechanism of the stress-salt-gas coupling effect on the high temperature corrosion of alloy T91 was discussed. The “optimal stress” of 20 MPa provided T91 with the best corrosion resistance.

Growth, structure, and electronic properties of nonpolar thin films on a polar substrate:Cr2O3on ZnO (0001) and ZnO (0001¯)

The growth and geometric and electronic structures of ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ layers on the polar ZnO surfaces were characterized to determine how polar substrates can influence the properties of nonpolar films. X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy (UPS), high resolution transmission electron microscopy (HRTEM), reflection high energy electron diffraction, low energy electron diffraction, x-ray diffraction (XRD), and x-ray reflectivity (XRR) were employed to characterize the growth mode, film quality, and interfacial electronic properties. Chromium oxide growth on ZnO $(000\overline{1})$ and $(0001)$ followed the same trends: two-dimensional growth with initial disorder followed by the formation of epitaxial ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3} (0001)$. Despite the initial disorder, HRTEM and XRD/XRR measurements on thicker films revealed an abrupt interface with the ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ lattice extending all the way to the interface. This indicates that above a critical thickness of 10--15 $\mathrm{Cr}\ensuremath{-}{\mathrm{O}}_{3}\ensuremath{-}\mathrm{Cr}$ repeat units, the entire film reorganizes into an ordered structure. It is postulated that the oxygen remained ordered throughout the growth but that the chromium initially filled interstices randomly in the oxygen sublattice, which allowed the film to eventually grow with a well-defined epitaxial relationship with the substrate. The polar interfaces showed a small band offset that decayed with increasing film thickness, suggesting that the compensating charges at the interface may partially migrate to the film surface. No evidence of formal changes in the Cr oxidation state at the interfaces was seen. On the other hand, statistical analyses of UPS valence band spectra revealed an enhanced density of states near the valence band edge for ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ on ZnO $(0001)$, consistent with stabilization of the positive interface by filling surface electronic states. In contrast, no significant valence band differences were observed between bulk ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ and thin ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ layers on ZnO $(000\overline{1})$, suggesting a different charge compensation mechanism on the negative surface. The potential impact of these findings on the surface properties of chromium oxide thin films is discussed.

Preparation and Characterization of Protective LSM Coatings Produced by Atmospheric Plasma Spraying

Lanthanum strontium manganite oxides with perovskite structure are widely employed materials for protective coatings on chromium-contained metallic interconnectors in the intermediated temperature solid oxide fuel cells (ITSOFCs). The application of protective coatings is used to decrease the growth of chromium oxide and the evaporation of chromium trioxide and chromium hydroxide from the surfaces of metallic interconnectors. In this study, La0.8Sr0.2MnO3-δ (LSM) protective coatings are produced by the promising atmospheric plasma spraying (APS) technique on the substrates of Crofer 22 H, Crofer 22 APU and SS441 ferritic steels with or without pre-oxidation treatment. The substrates with pre-oxidation treatment were heated to 800°C and dwelled for 12 hrs in air before APS coating process. The cross-sectional micrographs show that the LSM coatings produced by APS technique are quiet dense without penetrating cracks. The XRD results identify that the LSM coatings produced by APS under 50 kW torch power reveal desired perovskite structure without any X-ray detectable second phase. After 600 hrs ageing in air at 800°C, the initial and final ASR values of the coated Crofer 22 APU sample with pre-oxidation treatment are 1.350 and 1.694 mΩcm2, respectively. The measured ASR increasing rate is only about 0.573 μΩcm2/hr. Thus, LSM coating prepared by APS technique can dramatically decrease the growth of chromium oxide to protect the metallic interconnector and the generation of gaseous Cr-contained species to avoid cathode poisoning at the operating temperatures of ITSOFCs.

Influence of Hydrogen and Water Vapour on the Kinetics of Chromium Oxide Growth at High Temperature

In support of the selection of structural materials for heat exchangers in helium-cooled high temperature reactors, the oxidation behaviour of the Ni-base chromia-former alloy 230 was investigated at 850 °C in diluted helium atmosphere with a low water vapour content. In such a media, the equivalent partial pressure of oxygen (imposed by the $$ P_{{{\text{H}}_{2} {\text{O}}}} $$ / $$ P_{{{\text{H}}_{2}}} $$ ratio) is very low ( $$ P_{{{\text{O}}_{ 2} }}^{\text{eq}} $$ around 10−16 Pa). The equivalent partial pressure of oxygen has no straight influence on the parabolic rate constant (k p); on the other hand, $$ P_{{{\text{H}}_{2} }} $$ and $$ P_{{{\text{H}}_{2} {\text{O}}}} $$ demonstrate a complex influence on k p. Photoelectrochemistry analyses revealed that this oxide could simultaneously contain two types of cationic defects. Specific oxidation tests with D2O showed that the oxide scale also contains hydrogen. A mechanist model is proposed in order to describe the scale growth using both cationic defects. Those theoretical results show, at least qualitatively, how $$ P_{{{\text{H}}_{2} }} $$ and $$ P_{{{\text{H}}_{2} {\text{O}}}} $$ may concurrently influence the oxidation rate.

Advanced TEM characterization of stress corrosion cracking of Alloy 600 in pressurized water reactor primary water environment

Advanced transmission electron microscopy techniques were carried out in order to investigate stress corrosion cracking in Alloy 600 U-bend samples exposed in simulated PWR primary water at 330 °C. Using high-resolution imaging and fine-probe chemical analysis methods, ultrafine size oxides present inside cracks and intergranular attacks were nanoscale characterized. Results revealed predominance of Cr 2O 3 oxide and Ni-rich metal zones at the majority of encountered crack tip areas and at leading edge of intergranular attacks. However, NiO-structure oxide was predominant far from crack tip zones and within cracks propagating along twin boundaries and inside grains. These observations permit to suggest a mechanism for intergranular stress corrosion cracking of Alloy 600 in PWR primary water. Indeed, the results suggest that stress corrosion cracking is depending on chromium oxide growth in the grain boundary. Oxide growth seems to be dependent on oxygen diffusion in porous oxide and chromium diffusion in strained alloy and in grain boundary beyond crack tip. Strain could promote transport kinetic and oxide formation by increasing defaults rate like dislocations.

Chromium-oxide Growth on Fe–Cr–Ni Alloy Studied with Grazing-emission X-ray Fluorescence

Using grazing-emission X-ray fluorescence (GEXRF), isothermal oxidation of the alloys 55Fe–25Cr–20Ni and 55Fe–25Cr–20Ni(+0.3Y) (wt.%) were studied as a function of oxidation time at 750 °C in O2. In addition, the effect of thermal cycling was studied. Using GEXRF, oxide thickness, the Cr-depletion zone in the substrate, and Fe and Ni concentrations in the oxide were monitored as a function of oxidation time. Scanning-electron microscopy was used to independently measure the Cr-depletion zone. Raman spectroscopy was used to measure the concentration of Fe2O3 appearing in the oxides in early oxidation (less than 2 h). Both GEXRF and Raman measurements show that the thermally-grown chromium oxide purifies with extended oxidation; initially abundant Fe2O3 became undetectable after 2 h of oxidation. However, the total Fe concentration was still ∼3% after 2 h but systematically decreased with further oxidation. Thermal cycling had no effect on these results.

Electrical and Thermal Properties of Spinels

The properties of spinel oxides were studied in an effort to identify a potential solid oxide fuel cell (SOFC) interconnect coating which can hinder chromium oxide growth and evaporation as well provide acceptable electrical conductivity. Transition metal spinels based on aluminum, chromium, manganese and iron were examined. The electrical conductivities were measured in air from 500 to 800°C by the standard four-probe DC method. Additionally, the thermal expansion coefficients of some samples were measured in air from room temperature to 1000°C. The stoichiometric aluminates, chromites and manganites have thermal expansion coefficients that lie consistently in the 7-9 ppm/K range. Thermal expansion coefficients of the femtes are in the 11 -13 ppm/K range, and their conductivities are generally 0.1-10 S/cm. Conductivities of the aluminates are all less than 10 -2 S/cm. Among the potential candidates for SOFC interconnect coatings, Co 2 MnO 4 has a conductivity of 55 S/cm at 800°C and a thermal expansion coefficient of 9.7 ppm/K and CuFe 2 O 4 has a conductivity of 9.1 S/cm at 800°C and a thermal expansion coefficient of 11.2 ppm/K. The ternary spinel oxides of CuMn 2-x Cr x O 4 (x = 0.2, 0.6, 0.8 and 1,0) and Co 2 Mn 0.5 Cr 0.5 O 4 are also acceptable in terms of their electrical conductivities and thermal expansion coefficients.

Surface morphology study on chromium oxide growth on Cr films by Nd-YAG laser oxidation process

Grain sized (60–100 nm) Cr 2O 3 thin films were prepared on Cr thin film surfaces by Nd-YAG laser photothermal oxidation process. Surface morphology study showed crack-free short plateau-like oxide films formed. Increase of dislocation density after pulsed laser irradiation was found. Thin film external surfaces, grain boundaries and dislocations are main paths of laser surface oxidation. Pinning and sealing of grain boundary was the reason that deeper oxidation did not produce. Grain growth and agglomeration of Cr sub-layer yielded tensile stress on the surface Cr 2O 3 thin film. It was the reason that short plateau-like surface morphology formed and cracks appeared sometimes. In oxygen annealing at 700 °C, grain boundaries were considered not to be pinned at the surface, mixture diffusion was main mechanism in growth of oxide. Compression stress development in whole film led to extrusion of grains that was the reason that multiple appearances such as pyramid-like and nutshell-like morphology formed.

An analysis of intergranular segregation of sulphur in a low alloy steel by stress-induced diffusion process

T91 steel specimens were tested at a high temperature with and without Na2SO4-NaCl molten salt under different stresses to investigate the effect of imposed stress on the corrosion behaviour. The imposed tensile stress facilitated chromium-oxide growth near the substrate and strengthened the corrosion resistance. The mechanism of the stress-salt-gas coupling effect on the high temperature corrosion of alloy T91 was discussed. The “optimal stress” of 20 MPa provided T91 with the best corrosion resistance.