Thermodynamic probability analysis of the effects of Rb on the corrosion susceptibility of Cr-containing steels for nuclear materials canisters

Rubidium (Rb) generated from the β-decay of Kr-85 has been theorized to be corrosive toward steel, specifically in the storage of Kr-85 nuclear waste streams. In the present study, the phase equilibria of RbxCryOz oxides with Rb in dry oxygen and water are investigated to understand a possible pathway to unusual deterioration of the corrosion resistance of canister steels in the presence of Rb. It was found that, in dry oxygen environments, the accumulation of Rb (more than 0.01 mol) can completely consume the Cr in 1 mol of AISI 4130 steel by forming -Rb2CrO4 and Rb3CrO4 and prevent the formation of protective Cr2O3 scale. In aqueous environments, RbxCryOz oxides are metastable species. In order to investigate their role, the probability of forming various oxides is invoked in order to avoid the all-or-nothing approach to oxide formation typical of E-pH diagram, which only predicts the most stable species dissolved, ionized or solid ionized. Thus, the probability of forming RbxCryOz was considered and reported herein. It was found RbxCryOz can possess a larger than 7% probability of forming over Cr2O3 in Rb rich case and 15% in Cr rich case, indicating that it is expected to find small amount of RbxCryOz in the thermodynamically formed reaction products. Even though Cr2O3 is more stable than RbxCryOz oxides, the protective Cr2O3 scale is likely to have some vulnerability to Rb, leading to one possible route for the decline in the corrosion resistance of steel canisters in aqueous environments. Therefore, from a thermodynamic perspective, the current study supports the hypothesis that Rb can thermodynamically react with Cr in steels and can lead to formation of RbxCryOz at certain potentials and pH levels, showing the Rb influence of steel corrosion cannot be discounted. The paper considers experimental mixed potential and pH levels observed and relationship to thermodynamic probability. From this relative corrosion resistance can be assessed in a preliminary way in aqueous environments.

Multiscale Characterization of An Oxide Scale Formed on the Creep-Resistant ATI 718Plus Superalloy during High-Temperature Oxidation

The ATI 718Plus® is a creep-resistant nickel-based superalloy exhibiting high strength and excellent oxidation resistance in high temperatures. The present study is focused on multiscale 2D and 3D characterization (morphological and chemical) of the scale and the layer beneath formed on the ATI 718Plus superalloy during oxidation at 850 °C up to 4000 h in dry and wet air. The oxidized samples were characterized using various microscopic methods (SEM, TEM and STEM), energy-dispersive X-ray spectroscopy and electron diffraction. The 3D visualization of the microstructural features was achieved by means of FIB-SEM tomography. When oxidized in dry air, the ATI 718Plus develops a protective, dense Cr2O3 scale with a dual-layered structure. The outer Cr2O3 layer is composed of coarser grains with a columnar shape, while the inner one features fine, equiaxed grains. The Cr2O3 scale formed in wet air is single-layered and features very fine grains. The article discusses the difference between the structure, chemistry and three-dimensional phase distribution of the oxide scales and near-surface areas developed in the two environments. Electron microscopy/spectroscopy findings combined with the three-dimensional reconstruction of the microstructure provide original insight into the role of the oxidation environment on the structure of the ATI 718Plus at the nanoscale.

Phase, Composition and Structure Changes of CoCrNi-Based Concentrated Alloys Resulting from High Temperature Oxidation

In this work, CoCrNi, FeCoCrNi and CoCrFeMnNi concentrated alloys with a Y-Ti oxide particle dispersion were prepared by mechanical alloying and Spark Plasma Sintering. The alloy consists of an FCC Ni-based matrix with a Y-Ti oxide dispersion and additional phases of Cr23C6 and Cr2O3. The effect of Fe, Mn, and Y-Ti oxide particles on the formation of oxide scales and the composition of the adjacent CoCrNi and FeCoCrNi alloys was studied. It was found that alloys without Mn in their composition form a protective Cr2O3 scale. The Cr23C6 particles provide an alternative mechanism for balancing the chromium loss during the oxidation. Y and Ti from the oxide particles participate in the formation of the protective oxide scales. Fe promotes Y and especially Ti diffusion through the Cr2O3 scale, resulting in the formation of Ti-depleted regions in the alloy. The findings will serve for the further development of these new materials.

Air and Wet Air Oxidation of 9Cr-3Co-3W Creep Resistant Ferritic Steel at 650 °C

The new experimental creep resistant ferritic steel of the 9Cr-3Co-3W type was oxidised at 650 °C in air and wet air. The oxidation kinetics was measured by intermittent weight measurement. The scales formed were analysed using techniques of XRD, SEM and EDS. The results showed that the oxidation rate was more than a magnitude faster in wet air than in air. The oxidation kinetics in air obeyed the parabolic rate law of oxidation only in a limited oxidation period of up to 1726 h whereas it did not follow any power rate law of oxidation in wet air. The steel cannot form a protective Cr2O3 scale either in air or in wet air at 650 °C. Instead, the scale formed in air consisted of an outer (Fe0.6Cr0.4)2O3 layer and an inner Cr-rich (Fe,Cr)2O3 layer containing Cr2O3 particles, but in wet air it consisted of an outer Fe3O4 layer and an inner (Fe,Cr)3O4 layer.

Effect of RE on Cyclic Oxidation Behavior of Ferrite Stainless Steel

A primary study on the resistance to high-temperature-oxidation of 430 ferrite stainless steel and its oxidation process before and after the addition of RE elements was reported. Results show that the oxidation resistance of 430 ferrite stainless steel after adding RE is indeed great. The value of oxidation rate of Sample 1 (without adding RE) is 2.87 times higher than Sample 2, respectively at 1423K after oxidizeing for totally 144h. And the dense and adherent Cr2O3 scale and FeCr2O4 scale are formed and played the protection role to the 430 ferrite stainless steel. In the internal oxidation layer, the pinning effect of silicon dioxide is strengthened by RE.

High-Temperature Oxidation Behaviour of Co-Re-Cr-Based Alloys: Limitations and Ways to Improve

Ni-based alloys are the most widely used alloy system in high-temperature applications. However, the use of Ni-based alloys is limited to temperatures below 1100°C. The experimental Co-Re-Cr-based alloys are promising for high-temperature applications for service temperatures beyond 1200°C. A complete miscibility in the Co-Re system allows to steadily elevate the melting point of the system with the rhenium content. In addition, rhenium takes the role as solid solution strengthening element. In the case of Co-based alloys, the oxidation resistance at high temperature is mainly based on the formation of a protective Cr2O3 scale. The purpose of the present investigations is to gain an insight into the oxidation mechanisms of the model Co-Re-Cr alloys and to find ways to improve oxidation resistance of this class of materials. Earlier investigations of the authors showed a rather poor oxidation resistance during exposure to laboratory air. Oxidation at 1000°C in air yielded an oxide scale that consists of a Co-oxide outer layer on a thick and porous Co-Cr oxide and a semicontinuous and therefore non-protective Cr-oxide film on the base metal substrate. As a consequence of the lacking protectiveness of the oxide layer the vaporization of rhenium oxide takes place and hence leads to a rapid loss of Re. The aim of recent investigations is to study the effect of Si on the high-temperature oxidation behaviour of Co-Re-Cr alloys by means of kinetic and microstructural examinations. It was found that Si stabilizes the Cr2O3 scale, enhancing the oxidation resistance significantly. Hence, the synergetic effect of chromium with silicon could be considered as an encouraging perspective to improve the oxidation resistance of Co-Re-Cr alloys. Apart from that, other concepts to enhance the oxidation resistance of this class of materials are discussed, such as the formation of a borosilicate layer or protective Al2O3 scale on the substrate surface.

Ternary and Quaternary Interdiffusion in γ (fcc) Fe-Ni-Cr-X (X = Si, Ge) Alloys at 900°C

Interdiffusion in Fe-Ni-Cr (fcc γ phase) alloys with small additions of Si and Ge at 900°C was studied using solid-to-solid diffusion couples. Alloy rods of Fe-24 at.%Ni, Fe-24 at.%Ni- 22at.%Cr, Fe-24 at.%Ni-22at.%Cr-4at.%Si and Fe-24 at.%Ni-22at.%Cr-1.7at.%Ge were cast using arc-melt, and homogenized at 900°C for 168 hours. Sectioned alloy disks from the rods were polished, and diffusion couples were assembled with in Invar steel jig, encapsulated in Argon after several hydrogen flushes, and annealed atz 900°C for 168 hours. Polished cross-sections of the diffusion couples were characterized to determine experimental concentration profiles using electron probe microanalysis with pure elemental standards. Interdiffusion fluxes of individual components were calculated directly from the experimental concentration profiles, and the moments of interdiffusion flux profiles were examined to determine the average ternary and quaternary interdiffusion coefficients. Effects of alloying additions on the interdiffusional behavior of Fe-Ni- Cr-X alloys at 900°C are presented with due consideration for the formation of protective Cr2O3 scale.