Oxidation of Metals最新文献

The effect of water vapor content on the oxidation behavior of In625 at 900 °C in synthetic air was reported. The higher the water vapor content, the greater the oxidation and volatilization rates were. Increasing the water vapor content led to an increase in the proportion of spinel and rutile-type oxides in the oxide scale compared to chromia, and the proportion of Al-rich oxides within the alloy. A k_p-k_v mass variation model was used to quantify the experimental results, and Fluent Ansys® CFD simulations of the gas phase were used to predict volatilization rates. CFD simulations were used to calculate local gas velocity, temperature and composition along with local volatilization rates at each point on the sample surface. It was possible to explain not only the variations in volatilization between upstream and downstream samples, but also the increased volatilization at sample corners. For longer durations, it was shown experimentally that the rate of volatilization decreases. This was explained by the enrichment of the oxide scale with spinel and rutile-type oxides.

This paper highlights that hot corrosion at 750 °C can develop on the surface of AM1 nickel-based single-crystal superalloy (SX) turbine blades, whether in the As-Cast (AC) or Fully Heat-Treated (FHT) states even in the absence of SO₃ (g) flow. It was found that the 1 and 3 mg/cm² Na₂SO₄ deposits induce sulphidation, oxidation and basic flux at such low temperature like in Type I hot corrosion. Sulphidation is mainly located in the γˈ-depleted zone irrespective the substrate (AC and FHT). The metallurgical segregations in the AC superalloy extend the incubation period in contrast to what is observed upon pure oxidation. The increase in salt content showed a reduction in hot corrosive attack by forming a barrier layer.

The microstructural evolution of T91 steel by pre-steam oxidation during liquid lead-bismuth eutectic (LBE) dissolution corrosion was investigated. A bi-layered pre-oxide film with Fe-rich outer layer and Cr-rich inner layer was formed on T91 steel, which was similar to the oxide film of T91 after oxidation corrosion in LBE. The pre-oxide film effectively protects the matrix from LBE corrosion at 620 °C. However, the composition and microstructure of the pre-oxide film changed dramatically. Unlike the original duplex structure, the pre-oxide film exposed to LBE undergoes a process of reduction of the outer layer and oxidative growth of the inner layer and changes into five layers, the loose and easily peeling outer Fe layer, the dense and intact inner layers successively consisting of Fe–Cr spinel layer, a transition layer of matrix and Cr-rich oxide, continuous Cr-rich oxide layer with tetragonal distorted spinel structure and amorphous SiO₂ layer. The evolution mechanism of the pre-oxide film during LBE dissolution corrosion is discussed.

The focus of this study was to compare the isothermal oxidation behavior of IN 939 nickel-based superalloys produced by selective laser melting and casting. Oxidation experiments were performed on both heat-treated and non-heat-treated, as cast and additively manufactured samples, to reveal the role of heat treatment and manufacturing methods on oxidation behavior. As cast samples underwent a two-step aging at 1080 and 843 °C, while a one-step aging was carried out for additively manufactured samples at 845 °C. The microstructure of the as cast IN 939 exhibited a dendritic structure with gamma prime precipitates. Following the heat treatment, primary and secondary gamma prime precipitates were formed. Additively manufactured IN 939 exhibited clearly visible melt pools and no trace of gamma prime precipitates. After heat treatment the melt pools disappeared, and gamma prime precipitates formed. Oxidation experiments were performed at 800, 900 and 1000 °C. All samples exhibited similar weight gain characteristics and obeyed a parabolic rate law. Spallation did not occur at 800 and 900 °C, whereas at 1000 °C all samples experienced spallation. The activation energies of all samples, calculated for three temperatures (800, 900, and 1000 °C), were similar, ranging between 260.99 and 287.51 kJ/mole. XRD and EDS analyses indicated that the oxide scale formed on all IN 939 samples was mainly Cr₂O₃ and TiO₂ in rutile form. The internal oxidation and nitridation zones were investigated using SEM and image analysis. The results showed that at 1000 °C, internal oxidation and nitridation extended deeper into the bulk material for additively manufactured samples due to the finer and columnar grains along the building direction which contained extensive amounts of precipitates compared to cast microstructure.

Durability is still a critical factor that limits solid oxide cell (SOC) technology industrialization. In order to maintain a good level of performance for the overall targeted lifetime of about 40 kh, the oxidation of the interconnects made of ferritic stainless steel and Cr volatilization from this material to the cell electrodes have to be restricted. CeCo-based coatings were applied by PVD HiPIMS on AISI441 alloy. Their ability to reduce the thickness of the poorly conductive formed oxide and improve Cr retention was studied at sample scale by measurements of weight gain and Cr content by ICP-OES after 5000 h of exposure in ambient air at 700 and 800 °C. In the testing conditions, post-test characterization by SEM/EDX showed that oxide scale thickness was reduced when coatings were applied compared to bare AISI441 steel. Moreover, the strong oxide scale spallation observed at 800 °C with bare AISI441 steel was avoided. Cr volatilization was also strongly decreased. Post-test SEM/EDX and ToF–SIMS characterization of a short stack integrating coatings on the air side in some repeat units (RU) confirmed the limited Cr diffusion in the strontium doped lanthanum manganite (LSM) contact layer when the coating is present after 5200 h of solid oxide electrolysis cell operation (SOEC).

Carbon migration and subsurface transformation, among the corrosion processes of P92 steel in high-temperature CO₂ were investigated using in situ monitoring technology. During monitoring reaction intermediate CO was used to distinguish between the oxidation and carbonization reactions. The CO generation rate on P92 steel at 550 °C and 600 °C reached the highest value after 80 min and 100 min, respectively. Honeycomb pores in Fe₃O₄ oxide scale were observed as the main channels for CO₂ and CO gas diffusion and carbon deposition. The deposited carbon gradually diffused into the matrix in an ionic state and transformed into carbides.

When operating at very high temperatures (starting from 1200 °C), thermal barrier coatings (TBCs) start interacting with oxide particles such as CMAS (CaO-MgO-Al₂O₃-SiO₂), found in sand or volcanic ashes. Namely, CMAS can infiltrate the TBC and tamper the thermal and mechanical properties of said TBC, leading to its deterioration. This study aimed to understand the interaction between yttria partially stabilized zirconia (YSZ) TBCs and CMAS particles under a thermal gradient. The TBC was made through an EB-PVD process. The experimental study was conducted with a laser rig. TBC samples were heated up to 1200 °C and exposed to a cylinder-shaped CAS (CaO-Al₂O₃-SiO₂) deposit for different durations. The study was conducted in presence of a through-thickness thermal gradient of up to 150 °C in the sample. It was observed that the infiltration is a rather quick phenomenon; while, the dissolution of the TBC and the precipitation of the crystalline phases worked on a longer timeline. Both phenomena can then be considered uncoupled under these test conditions and modeled as such. A heat transfer model was implemented as to better understand the different phenomena happening. The model was fitted to experimental data through a test-calculation dialog.

As ammonia does not emit greenhouse gases when burned, it is considered a fuel instead of fossil fuels. However, there are few reports on the corrosion behavior of materials when ammonia is used as fuel. Therefore, this study measured the hydrogen produced by the reaction between ammonia and iron using a hydrogen sensor containing a proton conductor. As a result, the amount of hydrogen produced by the decomposition of ammonia was small at low temperatures but increased at high temperatures. Also, when the ammonia content was low, oxidation of iron occurred preferentially. This way, the relationship between the amount of hydrogen generated and corrosion behavior could be clarified.

This work introduces a new high-throughput method to characterize the oxidation behavior of chemically graded Ni-based alloys in order to feed databases destined to numerical metallurgy approaches. A Ni–wCr–3Al (w ∈ [0, 30]) chemically graded material was obtained from two homogeneous samples by a diffusion couple method at 1300 °C for 100 h. The composition range was selected in order to observe the three types of oxidation behavior identified in the reference work of Giggins and Pettit (Giggins and Pettit in Journal of The Electrochemical Society 118:1782, 1971). The excellent agreement between simulated and experimental diffusion profiles validated the experimental method used to manufacture the chemically graded material (CGM). The CGM was then oxidized at 1200 °C in air. Surface and cross-section characterization was conducted by SEM/EDS and Raman spectroscopy to identify the oxides formed on the CGM. To accelerate the Raman characterization treatment, a method linking principal component analysis and K-means unsupervised clustering algorithm was developed. It allowed for the identification of the oxide type without peak indexation issues and is well suited for CGM. These results show that results similar to well-recognized reference experiments (Giggins and Pettit in Journal of The Electrochemical Society 118:1782, 1971) can be achieved using only one CGM.