Oxidation of Metals最新文献

Short-time oxidation exposures of the Ni-based TROPEA single crystal superalloy was implemented to determine the nature and quantities of transient oxides in the 680–1000 °C temperature range. Experiments were carried out in situ in the SEM with reduced air pressure (150 Pa, ({P}_{{O}_{2}}) ~ 31.5 Pa) compared to atmospheric conditions (10⁵ Pa, ({P}_{{O}_{2}}) ~ 2.1 10⁴ Pa). TEM characterization after oxidation showed the complexity of the oxidation products developed. Aluminum underwent internal oxidation between 680 and 1000 °C. During the limited duration of oxidation, the TROPEA alloy only formed a continuous alumina layer at 1000 °C. At 680 and 850 °C, the low diffusion rate and small amount of Al in the Ni-based single crystal led to the formation of a significant amount of transient oxides such as (Ni,Co)O, compared to the desired chromia or alumina protective oxides. The lower the temperature, the smaller the size of the internal Al₂O₃ precipitates and the larger the transient oxide amount, which would lower the resistance of TROPEA to Type II hot corrosion. In contrast after a transient period shorter than 22 h, during which multiple transient oxide developed, the oxidation resistance would be ensured at 1000 °C by the formation of a continuous Al₂O₃ scale.

The microstructure and oxidation resistance at 900 and 1000 °C of additively manufactured (AM) by directed energy deposition (DED) and conventionally manufactured (CM) Inconel 625 alloys were studied. The microstructure of the AM samples was cellular, with Nb and Mo segregations located in the dendritic and interdendritic regions. At 900 °C, the oxidation rate was similar for both materials, but was clearly higher for the AM material at 1000 °C, being related to the segregation and porosity present in the microstructure of the AM samples. Decrease in porosity by DED changing parameters allowed better oxidation resistance, but still considerably inferior than CM samples at 1000 °C. After oxidation, a layer of Cr2O3 was identified under all conditions, providing high resistance to oxidation. Internal oxidation of alumina was also observed in the CM and AM samples. The delta phase Ni3(Nb, Mo) was observed for the CM and AM alloys at the grain boundaries (900 °C) and at the metal/oxide interface for both temperatures as a result of chromium depletion. Finally, the oxide layer formed was compact and dense, and some voids were formed in the subsurface region of the samples produced by AM.

Alloys of Ni–25Cr–(2Mn–1Si) under mixed deposits of ash + (0, 10, 50 and 90) wt% sulphate were exposed to an Ar–60CO₂–20H₂O gas at 650 and 750 °C for up to 300 h, forming both protective chromia and regions of Ni-rich oxide. The presence of ash + sulphate mixtures improved Ni–25Cr alloy protection, increasing surface coverage by thin, protective chromia compared with the deposit-free condition. Increasing sulphate proportions in these mixtures led to an accelerated chromia scale growth and reduced internal oxidation zone (IOZ). These beneficial effects were more significant at 750 °C, where surface coverage by the protective scale was increased, and a chromia band was formed beneath nonprotective regions at the IOZ-substrate interface. Alloy additions of Mn and Si generally slowed the growth of outer NiO and IOZ but did not lead to exclusive chromia scale formation.

The influence of oxidizing atmosphere on the degradation mechanisms of the Ni-based superalloy Rene 65 was studied in this work. Oxidation was carried out in synthetic air, argon and water vapor (Ar + 18 vol.%H₂O) between 700 and 900 °C, with samples built following additive manufacturing and forging processing routes. The results showed that the processing route and hence, the derived microstructure did not significantly affect the oxidation behavior. In contrast, the oxidizing atmosphere markedly modified the oxidation kinetics, the growth of the oxide layers and the overall oxidation mechanisms. The resulting thin oxide scales were made of NiCr₂O₄ and Cr₂O₃ at 700 °C, while at 800 and 900 °C the oxide layer was composed of an external Cr₂O₃ layer on top of an internal α-Al₂O₃ layer resulting from the lower partial pressure of oxygen underneath the chromia layer. The presence of nitrogen in the synthetic air favored the internal formation of TiN, while the absence of nitrogen in argon revealed the doping effect of Ti on the formation of the Cr₂O₃ layer. The effect of water vapor in Ar was not significant as the oxidation behavior was close to that observed under argon.

This study investigates the effect of the Alloy 625 manufacturing process on the high-cycle fatigue (HCF) performance of oxidized samples. Conventional manufacturing processes (wrought and casting) and additive manufacturing (AM) processes (laser powder bed fusion and direct energy deposition) were studied. Results of Alloy 625 isothermal oxidation at 950 °C in air revealed that AM samples showed faster oxidation kinetics and enhanced intergranular oxidation (IGO) with associated voids; the latter two were attributed partially to the alloy's greater amount of interstitial oxygen compared to conventional manufacturing processes. The HCF results showed that oxidized AM samples have a shorter life than oxidized wrought counterparts, where the earlier crack initiation in the oxidized AM samples is attributed to greater oxidation-induced subsurface degradation. This subsurface degradation includes the enhanced IGO and associated voids.

Molten chloride salts represent a very corrosive medium due to the amount of impurities they contain and that essentially comes from moisture. In this work, an industrial nickel-based alumina-forming alloy was preoxidized and corroded for 500 h in the NaCl–MgCl₂ eutectic. Electrochemistry and SEM analyses were used to prepare and analyse the corrosion test. Both the nickel-rich matrix and the alumina scale formed during preoxidation seemed to remain stable during the corrosion test contrary to some of the chromium carbides initially present in the columnar microstructure of the alloy. The use of X-ray tomography coupled with SEM observation revealed a preferential dissolution of the chromium carbides connected to the alloy/salt interface. X-ray tomography reveals a chromium carbides network enabling a deep molten salt infiltration within the alloy due to their preferential dissolution. Molten salt infiltration in the dissolved carbides network then leads to the oxidation of aluminium present in the alloy into a mixed MgAl₂O₄ spinel. An oxoacido-basic reaction between the alumina scale formed at the alloy surface during preoxidation and MgO dissolved in the salt is also discussed. This work shows that nickel-based alumina-forming alloy present a realistic interest and that the microstructure of the alloy should be optimized in further work to enhance corrosion resistance.

This study has investigated the effect of NaCl and different gaseous environments on the stress corrosion cracking susceptibility of CMSX-4 at 550 °C. The presence of SO_x leads to the rapid dissociation of NaCl into Na₂SO₄ and the release Cl₂ and HCl, which then trigger an active oxidation mechanism and stress corrosion cracking. The incubation time for crack initiation at 690 MPa and in the presence of a sulphur containing environment is 10 min. A working hypothesis is that stress corrosion cracking occurs due to the hydrogen released at the oxide/alloy interface when metal chlorides are formed; however, this hypothesis needs to be further explored.

Fundamental understanding of surface oxidation dynamics is critical for rational corrosion protection and advanced manufacturing of nanostructured oxides. In situ environmental TEM (ETEM) provides high spatial (nano- to atomic- scale) and temporal (< 0.1 s) resolution to investigate the early-stage oxidation/corrosion dynamics of metals and alloys. Thin samples with facets are widely used to enable cross-sectional observation of the oxidation dynamics in ETEM. However, how different facet orientations oxidize under the same conditions, and how these facets change the oxidation process, has not been investigated before. Using in situ ETEM, we systematically compare the oxidation dynamics of Cu(001) thin films, with faceted holes exposing {100} and {110} facets at temperatures ranging from 250–600 °C under 0.03 Pa O₂. Oxidation preference is observed to change, from Cu(110) facets at lower temperatures to Cu(100) facets at ~ 500 °C. Oxide growth mechanisms change from outward growth on Cu₂O surfaces at low temperatures, to inward growth on Cu-Cu₂O interfaces at high temperatures. At high temperatures (500–600 °C), a rod-like Cu₂O morphology is observed, with side facets of ~ {024} and top facets of {100} on Cu(100). This differs from the square-shaped Cu₂O exposing {110} facets formed on Cu(001) surfaces. Rod-like oxides exhibit directional growth along their lengths with linear growth rates, regardless of rod length and width. This suggests that O from Cu(001) surfaces, rather than Cu(100) facets, serves as an O source for oxide growth. These results show a direct comparison of oxidation at different orientations with temperature, underscoring the temperature dependence of oxidation preference. Our results also suggest future in situ ETEM experiments viewing oxidation corrosion cross-sectionally should be cautious when oxide size is comparable with sample thickness, as the oxidizing mechanism may change due to sample thickness.

Multicracking tests are carried out in an SEM on nickel specimens preoxidized at high temperature. These tests are monitored by acoustic emission. By combining the analysis of the acoustic emission signals with SEM observations of the specimens, it is possible to find the signatures of the two active crack propagation modes. In mode I (propagation perpendicular to the metal–oxide interface), the acoustic emission signals have high amplitudes and short durations, whereas for propagation in mode II (along the metal–oxide interface), the AE signals have low amplitudes and long durations.

Additions of titanium nitrides (TiN) can reduce cracking sensitivity of FeCrAl alloys manufactured by laser powder bed fusion through grain refinement. However, the oxidation behavior of TiN-added FeCrAl alloys is not reported up to date. In the present work, high-temperature oxidation of additively manufactured (AM) FeCrAl alloys with Ti additions from 0.5 to 1.1 w% has been studied in air at 1250 °C during 1000 h. The AM Ti-added FeCrAl alloys have shown a higher oxidation rate than their cast reference alloy. The degradation kinetics during high-temperature exposure of the model AM alloy are described and discussed with respect to the microstructural examination. The Ti addition is shown to affect the spallation kinetics. The formation of TiN precipitates at the metal/oxide interface and their growth within the alumina scale during the exposure at 1250 °C were revealed for the first time in FeCrAl material.