Cr-enriched Oxide Research Articles

Effect of Mo content on the corrosion resistance of (CoCrFeNi)1−xMox thin films in sulfuric acid

(CoCrFeNi)1−xMox thin films with various Mo content (0–10 at.%) were grown by magnetron sputtering on a stainless steel substrate. The films with 0–2 at.% presented two crystal structures: one FCC phase and one sigma phase, while films with higher Mo content only had the FCC structure. All films have a (111) texture and follow the topography of the substrate. The corrosion resistance of the films was evaluated in 0.05 M H2SO4 at room temperature and at 80 °C. It was observed that the corrosion current densities considerably decreased for Mo > 2 at%, and that the current densities were higher at the elevated temperature. Scanning Kelvin Probe Force Microscopy showed a large potential difference between the main FCC phase and sigma phase for the Mo0–2 films. This would suggest that preferential dissolution of the FCC phase occurs over the sigma and reduces the corrosion resistance. Such preferential dissolution does not occur for the higher Mo content films with only the FCC phase. The high corrosion resistance was also attributed to the inhibition of Fe and Cr dissolution by Mo and the stabilisation of the Cr enriched oxide by incorporating Mo oxides into the passive film, identified by X-ray photoelectron spectroscopy. The low corrosion current densities (below 1 µA/cm2) make these thin films possible candidates for protective coatings of bipolar plates in PEM fuel cells.

The environmental degradation behavior of FeNiMnCr high entropy alloy in high temperature hydrogenated water

Co-free high entropy alloy (HEA) 28%Fe-27%Ni-27%Mn-18%Cr shows good irradiation resistance and could be a potential candidate for structural material in nuclear power industry. In this work, the environmental degradation behavior of this HEA was investigated in high temperature hydrogenated water. This HEA shows extensive intergranular cracking after constant extension rate tensile test, indicating that it is very susceptible to stress corrosion cracking (SCC) initiation. The microstructure analysis indicates that Mn and Cr diffuse outwards along the grain boundary during oxidation, inducing grain boundary migration. The Cr flux promotes the formation of Cr-enriched oxide layer above the grain boundary which prevents intergranular oxidation when the sample is unstressed. However, Mn is unstable in the oxide and tends to dissolve into the solution, making the oxide porous. The porous oxide fractures readily when stressed, inducing SCC initiation.

Nanoscale early oxidation mechanisms of model FeCrNi austenitic stainless steel surfaces at room temperature

Scanning tunneling microscopy was applied to investigate the topographical and structural alterations at small spatial and temporal scales of a model 304 austenitic stainless steel surface resulting from ultra-low exposures to gaseous oxygen at room temperature. The 2D nucleation and growth mechanisms were observed until surface saturation by a fully covering and nanogranular Cr(III)-enriched oxide film. Local supply of metallic Fe and Cr for oxide nucleation and growth is concluded to be at the origin of heterogeneity developed in the oxide film at the nanoscale. This work provides nanoscale understanding basis for composition and morphology tuning of Cr-enriched oxide film for improved local stability of passivity.

The passivity of low-temperature carburized austenitic stainless steel AISI-316L in a simulated boiling-water-reactor environment

The passive film on low-temperature-carburized AISI-316L austenitic stainless steel was studied in an oxygenated, simulated boiling-water-reactor environment with normal water chemistry. The microstructure was studied using scanning electron microscopy, grazing-incidence X-ray diffractometry, Auger electron spectroscopy, X-ray energy-dispersive spectroscopy, and transmission electron microscopy. For non-surface engineered AISI-316L, three distinct layers were observed after exposure: an outer layer of larger nickel-oxide enriched particles with the spinel structure, an intermediate layer of small Cr-oxide-enriched hematite particles, and a compact inner layer of Cr-enriched oxide also having the spinel structure. Exposure of low-temperature-carburized AISI-316L, in contrast, resulted in only two distinct layers. These layers did not include a compact Cr-rich inner layer. Instead, the innermost layer consisted of an oxide with the spinel structure that was a mixture of Fe, Cr and Ni oxide. The outer layer on the low-temperature-carburized specimen consisted of large loosely stacked particles with the spinel structure of mixed composition. In addition to passive-film characterization, the corrosion rate was studied by monitoring mass loss. After 500 h of immersion, the mass changes measured were relatively low for all specimens indicating good adhesion and low oxidation rates.

Effect of electropolishing on corrosion of Alloy 600 in high temperature water

Effect of change in surface composition and microstructure by electropolishing on corrosion of Alloy 600 in high temperature water was investigated. The investigation was conducted by comparing the corrosion of electropolished surface (EPS) with that of the 40nm-colloidal silica slurry polished surface (CPS), which has few composition change as well as few surface residual strain. The result revealed while EPS had a thicker oxide scale at the early stage of corrosion, it showed a lower corrosion rate when the exposure time was >25h. This is attributed to formation of a Cr-enriched oxide film at the primary surface by electropolishing.

Direct observation of thin membrane passive film over the growing pit on sputtered nanocrystalline austenitic stainless steel film

Abstract Potentiostatic polarization with a temperature scan was applied to initiate formation of a single pit on a sputtered nanocrystalline austenitic stainless steel film and the thin membrane passive film over the growing pit was directly observed for the first time in this study. The results showed that the passive film consisted of two different layers, both of which exhibited an amorphous structure. The outer layer comprised of a Fe-enriched oxide, whereas the inner layer was a Cr-enriched oxide. Compared with conventional coarse-grained stainless steel, the passive film of sputtered nanocrystalline austenitic stainless steel possessed higher elastic modulus and hardness.

Electrochemical analysis on the growth of oxide formed on stainless steels in molten carbonate at 650 °C

Abstract The oxide growth on stainless steel (SS) 310S and 316L, used as a cathode current collector material of molten carbonate fuel cell (MCFC), were examined in the mixture of 62 mol% Li2CO3–38 mol% K2CO3 at 650 °C by measuring the change in corrosion potential and potentiodynamic response of the alloys and also in terms of impedance analysis on the alloy|oxide layer|electrolyte system. The corrosion potential of SS 316L was in an active region for 12 h-immersion, whereas that of SS 310S drastically increased after 6 h-immersion due to an active to passive transition. The corrosion rate of the two SSs decreased with immersion due to the growth of protective oxide. However, the decrease in the corrosion rate of SS 310S is much greater than that of SS 316L. The oxide formed on the two SSs was found to be duplex layer, composed of inner Cr enriched oxide and outer Fe enriched oxide. However, the inner Cr enriched layer of 310S is more clearly separated from the outer Fe enriched layer than that of SS 316L due primarily to the higher Cr content in SS 310S. The drastic increase in the corrosion potential of SS 310S after 6 h-immersion is closely associated with the growth of the inner Cr enriched oxide layer. The corrosion resistance of SS depends dominantly on the resistance of the inner Cr enriched oxide that is determined form the impedance analysis on the alloy|oxide layer|electrolyte system. The higher corrosion resistance of SS 310S compared with SS 316L results from the more protective inner Cr enriched oxide layer, as confirmed by its higher resistance associated with the higher Cr content in SS 310S.

Influence of surface modifications on pitting corrosion behavior of nickel-base alloy 718. Part 2: Effect of aging treatment

A two-step aging treatment is applied to nickel-base alloy 718 that has been previously surface-treated by mill finishing (MF) and machine hammer peening (MHP). As a result, a Cr-enriched oxide layer is formed along with a nano-precipitates layer that consists of high precipitate fractions of γ′/γ″ on the top surface. Surface hardness increases after aging and the compressive residual stress is almost relaxed. The synergistic effects of MHP and aging on pitting corrosion behavior are studied. The results show that the corrosion resistance of the MHP specimens decreases after aging, although the corrosion resistance is still higher than the MF condition.

ORIGIN OF PLATINUM-GROUP MINERALS IN MAFIC AND ULTRAMAFIC ROCKS: FROM DISPERSED ELEMENTS TO NUGGETS

Based on the studies of paragenetic assemblages of platinum-group minerals (PGM) from placers and chromitite ores associated with zonal-ring dunite intrusions in the Aldan Shield, on the Siberian platform, a novel model for the PGM formation is offered, which implies separation of a Cr-rich oxide liquid with significant concentration of the platinum-group elements (PGE) from a picritic magma. The presence of PGM alloys in sulfide-poor chromitite ores from layered intrusions and zonal gabbro–dunite massifs indicates that PGE mineralization is genetically related not just to sulfide formation but to chromite formation as well. A hypothetical (MgO + FeO + CaO) – SiO2 – (Cr2O3 + Fe2O3 + Al2O3) triangular diagram shows that fractional crystallization of olivine results in the enrichment of the residual melt in Cr, PGE, a range of metals, and easily fusible and volatile elements, and in magma separation into a light silicate-rich liquid and a dense PGE–Cr-rich oxide liquid. In fluid-saturated magma, a Cr-enriched oxide melt completely separates from a silicate melt, its crystallization temperature is lower, and the solubility of PGE is higher. It is likely that the presence of PGE in the Cr-rich oxide liquid takes the form of sybotactic domains, clusters surrounded by ligands of easily fusible and volatile elements, which prevents the PGE from removal via an untimely crystallization from the melt as micronuggets. Crystallization of a PGE-, Cr-rich oxide melt can be traced in the schematic chromite – Os – Pt ternary diagram proposed, which shows a wide field of immiscibility existing between chromite and the PGE. Owing to the low solubility of PGE in chromite, crystallization of the Cr-rich melt results in accumulation of PGE in the interstitial liquid. With IPGE prevailing over PPGE, the crystallization trend does not reach the immiscibility field, ending in synchronous crystallization of chromite and small grains of Pt–Ru–Ir–Os alloys. In case of Pt dominance, the crystallization trend reaches the immiscibility field, Pt gradually accumulates in the residual melt, with final crystallization occurring at a triple eutectic point, which creates favorable conditions for the formation of large Pt nuggets from tens of grams to a few kilograms in weight, which are characteristic only of PGE–chromite deposits of the Uralian–Alaskan and Aldanian types.

Reactive gas atomization processing for Fe-based ODS alloys

Gas atomization reaction synthesis was employed as a simplified method for processing oxide dispersion forming precursor Fe-based powders (e.g., Fe–Cr–Y–Hf). During this process a reactive atomization gas (i.e., Ar–O2) was used to oxidize the powder surfaces during primary break-up and rapid solidification of the molten alloy. This resulted in envelopment of the powders by an ultra-thin (t<50nm) metastable Cr-enriched oxide shell that was used as a vehicle to transport oxygen into the consolidated microstructure. Subsequent elevated temperature heat treatment promoted thermodynamically driven oxygen exchange reactions between trapped films of Cr-enriched oxide and internal (Y, Hf)-enriched intermetallic precipitates, resulting in highly stable nano-metric mixed oxide dispersoids (i.e., Y–Hf–O) that were identified with X-ray diffraction. Transmission electron microscopy and atom probe tomography results also revealed that the size and distribution of the dispersoids were found to depend strongly on the original rapidly solidified microstructure. To exploit this, several oxide dispersion strengthened microstructures were engineered from different powder particle size ranges, illustrating microstructural control as a function of particle solidification rate. Additionally, preliminary thermal–mechanical processing was used to develop a fine scale dislocation substructure for ultimate strengthening of the alloy.

Grain boundary microstructure and fatigue crack growth in Allvac 718Plus superalloy

The correlation between grain boundary microstructure and fatigue crack growth with hold-times was investigated for two conditions of the superalloy Allvac 718Plus; a Standard condition with the recommended distribution of grain boundary phases and a Clean condition with virtually no grain boundary phases. Fatigue testing was performed at 704°C using 10Hz cyclic load with intermittent hold-times of 100s at maximum tensile load. Microstructural characterization and fractography were conducted using scanning- and transmission electron microscopy techniques. Auger electron- and X-ray photoelectron spectroscopy techniques were used for oxide analyses on fracture surfaces. It was found that in the Standard condition crack growth is mostly transgranular for 10Hz loading and intergranular for hold-times, while for the Clean condition crack growth is intergranular in both load modes. The lower hold-time crack growth rates in the Standard condition are attributed to grain boundary δ-phase precipitates. No effect of δ-phase was observed for 10Hz cyclic loading crack growth rates. Two different types of oxides and oxide colours were found on the fracture surfaces in the Standard condition and could be correlated to the different loading modes. For cyclic loading a bright thin Cr-enriched oxide was dominate and for hold-times a dark and slightly thicker Nb-enriched oxide was dominant These oxide types could be related to the oxidation of δ-phase and the matrix respectively. The influence of δ-phase precipitates on crack propagation is discussed.

ESCA investigation of the oxide layers on some Cr containing alloys

Cr is an important constituent of many technically important alloys, such as stainless steels and those used for metal implants. The following characteristics of such alloys are revealed by ESCA measurements: (1) Heating the sample in air below 400°C leads to oxide layers with high contents of Fe or Co oxides. For temperatures higher than 400°C the Cr becomes mobile enough to migrate to the surface and to build up Cr-enriched oxide layers. (2) In H 2O and HNO 3 a Cr-rich oxide layer is always observed. (3) The chemical shift for Fe oxide in stainless steels is smaller than for pure Fe or Fe-Si alloys; i.e. there is a larger amount of Fe(II) oxide. (4) The intensity ratio Fe metal : Cr metal or CO metal : Cr metal after exposure of the sample to H 2O or HNO 3 is lower than for oxidation in air, i.e. Cr is enriched not only in the oxide layer but in the metal underneath. (5) If the alloy also contains Mo, the intensity ratio Cr oxide : Mo(VI) oxide is about 10 : 1 for exposure to H 2O, as well as to diluted and concentrated HNO 3. In H 2O and diluted HNO 3, Mo(IV) is found also. The ESCA results were obtained without any ion bombardment because the sampling depth is large enough to permit analysis of oxide layers below contamination and even of the metal below a thin oxide layer. This non-destructive method is of great advantage because in ion bombardment of the sample Cr is preferentially sputtered.