Thin Oxide Scale Research Articles

New insight on the formation of uneven oxide scales on AFA steel in supercritical CO2: Roles of recrystallization degree on the high temperature corrosion resistance

Uneven oxide scale is a common phenomenon in high-temperature corrosion, but the formation mechanism has not been fully uncovered. A new alumina-forming austenitic (AFA) stainless steel with superior corrosion resistance in 650°C/15 MPa SC-CO2, has been systematically investigated. It is demonstrated that the uneven oxide was formed by the significant impact of the recrystallization behavior of AFA steel on its microstructural evolution and subsequent corrosion behavior. Compared with the thin oxide scale, the thick oxide scale was caused by the relatively higher density of dislocations and grain boundaries of matrix resulting from delayed recrystallization, which facilitated the rapid outward diffusion of ions.

Comparison of diffusion coatings resistance on ferritic and austenitic-stainless steels at 650 °C in steam oxidation

Iron-aluminide coatings were deposited on T24, P92 and Incoloy 800HT (IN-800HT) by pack-cementation and by a water-based slurry. According to the differences in heat treatments between the two processes, Al-rich phases, i.e. Fe4Al13 and Fe2Al5 were obtained by pack cementation and only B2-(Fe, Ni)Al by slurry. Vertical or horizontal cracks were present on certain coatings, the former being due to the coefficient of thermal expansion (CTE) mismatch with the substrate while the latter were related to the brittleness of the Al-rich phases. The oxidation behaviour of uncoated and coated substrates was studied at 650 °C up to 1000 h under a low steam flow rate (1.07 × 10−5 m/s). The aluminide coatings were very protective on the low Cr substrates (i.e. T24 and P92) because of the formation of a very thin oxide scale compared to the non-protective thick layers of Fe3O4 and Fe2O3 that grew on both uncoated substrates. In contrast, none of the coatings significantly improved the oxidation resistance of the IN-800HT steel despite the growth of a thin oxide layer made of (Fe, Cr)2O3 and (Fe, Cr)3O4 at 650 °C.

Dual-function yttrium-silicate-modified SiC ceramics with prominent antioxidant performance

The oxidation of dense yttrium-silicate-modified and unmodified SiC-based ceramic (YPS-SiC and RMI-SiC) at 1200, 1300, and 1400 °C was studied. Oxide scale formed on the surface protects internal ceramics from further oxidation. Y2Si2O7 promoted the rapid crystallisation of cristobalite and no large catastrophic bubbles were observed. The high-crystallinity oxide scale of YPS-SiC slowed down the diffusion of the oxidant. The results showed the better oxidation resistance of the YPS–SiC ceramic with a thinner oxide scale than RMI-SiC. Based on oxidation thermodynamics and kinetics analysis, oxidation models were established to reveal the oxidation resistance mechanism of yttrium-silicate-modified SiC-based ceramics.

Investigation on the failure mechanistic evolution of fretting corrosion of Alloy 690 with increasing cycles in high-temperature water

In practical applications, Alloy 690 tubes are not only susceptible to fretting wear, but also subjected to corrosion by high temperature and high pressure water. In this work, the fretting corrosion behavior of Alloy 690 tube was investigated in high temperature high pressure water, and the coupling effects of corrosion and mechanical wear on the microstructure evolution of Alloy 690 tube with increasing testing time have been revealed. Results show that fretting wear accelerates the material oxidation by material deformation, cracking and grain refinement. Moreover, the structure and the composition of the oxide layer formed on the fretting worn surface are completely different from those formed on the generally corroded area: a dual layer oxide scale with the loose and porous outer oxide layer composed of Fe2O3 and Fe3O4 and the dense inner oxide film composed of Cr2O3 and Fe-rich spinel oxides is formed on the worn surface, while an extremely thin single layer oxide scale composed of Cr2O3 is formed on the generally corroded area. As fretting cycles increase, the influence of mechanical wear on material oxidation and structural evolution becomes more pronounced. However, friction-induced defects (e.g., cracks and voids) destroy the integrity of the oxide scale and aggravate material damage, which result in larger wear volume and higher wear depth of the Alloy 690 tube.

Surface oxidation behavior of spark plasma sintered AlCrCuFeMnWx high entropy alloys at an elevated isothermal temperature

The high-temperature surface oxidation behavior of high-entropy alloys (HEAs) with compositions of AlCrCuFeMnWx (x = 0, 0.05, 0.1, and 0.5) was studied at 800 °C as an isothermal temperature for 50 h. The high entropy alloys (HEAs) were synthesized by mechanical alloying (MA) followed by spark plasma sintering (SPS). This study focused on the effect of oxidation resistance properties with different elevated temperatures on the high entropy alloy over the surfaces. The XRD and Raman analysis confirmed the formation of various oxides like Al2O3, Cr2O3, Fe2O3, and CrO2 mainly. The HEAs follow the single-stage parabolic law at 800 °C. To achieve better resistance against the heat, rather than adding more amount of refractory elements. Consequently, AlCrCuFeMnWx HEAs showed excellent oxidation resistance at elevated temperatures in a specific amount of tungsten. Different values of mass gain, loss, and complex oxidation kinetics were observed for the varying W-containing HEAs. These alloys formed inhomogeneous oxide scales possessing different regions with porous oxide layers and some areas revealed thin oxide scales due to the formation of discontinuous Mn, Cr, and Al-rich scales evidence found through the SEM images. It is noted that complicated oxides formed considerably more frequently on the W0 alloy than on other alloys, indicating that the addition of tungsten in the presence of Cr may inhibit the growth of certain types of multiphasic oxides and prevent spallation.

Effects of Cr additions on Corrosion and Erosion-Corrosion Behaviors in the Weld Heat-Affected Zone of Hadfield Steel in Brine Environments

The corrosion and erosion-corrosion behaviors in the weld heat-affected zone (HAZ) of Hadfield steels with varying Cr contents (1, 2, and 3 wt%) were examined. Various experimental methods, including electrochemical polarization, impedance, and weight loss measurements, were utilized. Two types of isothermal heat treatments were conducted in a box furnace to simulate the intercritical HAZ, known to be the most vulnerable region in terms of mechanical properties and environmental stabilities, and large-scale samples for the erosion-corrosion experiment were fabricated. The results showed that increasing the Cr content improved the resistance to corrosion and erosion-corrosion, but there was an inflection point where adding more Cr had the opposite effect. Up to 2 wt%, a higher resistance was exhibited owing to the formation of a thin and protective oxide scale enriched with Cr that adhered to the steel surface. On the other hand, adding 3 wt% of Cr resulted in decreased resistance. This was due to the formation of coarse M<sub>7</sub>C<sub>3</sub> (M: Cr) precipitated along the grain boundary, which caused the development of a thick and unstable oxide scale that detached locally. Based on these findings, it is essential to optimize the Cr content to ensure a high resistance to corrosion and erosion-corrosion in the HAZ of Hadfield steel.

Oxidation Behavior of Atmospheric Plasma Sprayed NiAl Coating at High Pressure Oxygen Atmosphere

The high-temperature and high-pressure oxygen-rich combustion gases often cause the high temperature gas conduit in rocket engine to suffer severe oxidation, resulting in a reduced service life. The present study deposited a dense NiAl coating on GH4202 superalloy using atmospheric plasma spraying and investigated its oxidation behavior in a high temperature and high-pressure oxygen atmosphere. The results showed that the NiAl coating presented excellent bonding with the superalloy substrate throughout tests. The uncoated GH4202 substrate was severely oxidized during high pressure oxygen atmosphere. It was found that only a continuous very thin Al2O3 oxide scale evolved on the NiAl coating surface and no oxidation was observed at the interface between the NiAl coating and GH4202 substrate. Moreover, no significant inner oxidation occurred to NiAl coating. Therefore, the present APS NiAl coating can provide effective protection of superalloy from oxidation at high pressure oxygen combustion gas atmosphere.

The surface grinding-induced oxide scale exfoliation of an austenitic alloy in supercritical CO2

This work studied the effect of surface grinding on the corrosion behaviour of an austenitic stainless steel (SS) in the supercritical CO2. The results show that the polished surface had a typical oxide structure including an outer and an inner oxide layer while the ground surface had a much thinner oxide scale with Cr2O3, SiO2, and Mn2O3 after 3000 h exposure. The C-enrichment lamellar was observed in the oxide scale which was attributed to the exfoliation of the oxide scale and further penetrated into the matrix as Cr-rich carbides in the specimen with a ground surface.

Effect of crystal orientation of AlN coatings on its high temperature oxidation in wet air

Aluminum nitride is discussed for different high temperature applications, due to its unique properties. Lately it also received attention as a coating for different substrate materials. Within the present study oriented AlN-coatings with two different crystal orientations were produced via reactive magnetron sputtering and a high temperature CVD process on SiC-based substrates. The oxidation resistance of these coatings in comparison to bulk AlN was investigated in wet synthetic air at 1200 °C. Both, the bulk AlN and (101¯0)-oriented AlN-coating formed a porous Al2O3-scale, which was not protective and resulted in high oxidation rates. In contrast, the (0001)-oriented coatings were able to form a thin oxide scale and remained protective for the total exposure time of up to 200 h at 1200 °C. The underlying mechanism of the oxidation of AlN in wet air was discussed, considering the strong influence of the crystal orientation.

Corrosion Behavior of Alumina-Forming Austenitic Steel in Supercritical Carbon Dioxide Conditions: Effects of Nb Content and Temperature.

The corrosion behavior of alumina-forming austenitic (AFA) stainless steels with different Nb additions in a supercritical carbon dioxide environment at 500 °C, 600 °C, and 20 MPa was investigated. The steels with low Nb content were found to have a novel structure with a double oxide as an outer Cr2O3 oxide film and an inner Al2O3 oxide layer with discontinuous Fe-rich spinels on the outer surface and a transition layer consisting of Cr spinels and γ'-Ni3Al phases randomly distributed under the oxide layer. Oxidation resistance was improved by accelerating diffusion through refined grain boundaries after the addition of 0.6 wt.% Nb. However, the corrosion resistance decreased significantly at higher Nb content due to the formation of continuous thick outer Fe-rich nodules on the surface and an internal oxide zone, and Fe2(Mo, Nb) laves phases were also detected, which prevented the outward diffusion of Al ions and promoted the formation of cracks within the oxide layer, resulting in unfavorable effects on oxidation. After exposure at 500 °C, fewer spinels and thinner oxide scales were found. The specific mechanism was discussed.

High temperature oxidation of additively manufactured Rene 65

This paper investigates the initial oxidation stages of additive manufactured Rene 65 Ni-based superalloy between 700 and 900 °C in air and argon for 48 h, with different building directions and heat treatments. The results show that the only factor that impact the oxidation behavior is the oxidizing atmosphere. The resulting thin oxide scales are made of NiCr2O4 and Cr2O3 at 700 °C while at 800 and 900 °C the oxide layer is composed of an external Cr2O3 layer on top of an internal layer of α-Al2O3 due to the lower partial pressure of oxygen underneath the chromia layer. Data availabilityThe raw/processed data required to reproduce these findings are available upon request from the corresponding authors.

A protective SiAlN coating on topographic surface of laser powder bed fusion manufactured Ti alloy

Protective SiAlN coating with Mo interlayer has been directly deposited onto as-built topographic surface of laser-based powder bed fusion manufactured Ti6Al4V alloy where high surface roughness, even with irregular geometrical features and native oxide scale on it. The topographic surface of Ti6Al4V is well protected by SiAlN/Mo coatings without spallation, cracks or even oxidation after 50 h of exposure at 800 °C for 10 cycles in air. The conformal nature of sputtered SiAlN/Mo coatings enables topographic surface to be well covered, and Mo diffuses through thin native oxide scale on topographic surface into Ti alloy, thereby forming strong chemical bonding between coatings and Ti substrates.

Substrate controlled degradation rate of protective SiAlN coating via mitigating interfacial reaction

The degradation rates of protective SiAlN coatings (0.9 µm thick) on pure Ti, Ti6Al4V, and Timetal 834 alloys in air at 800 °C were investigated and compared. The SiAlN coating on pure Ti was depleted and then formed a thin oxide scale after exposure of 50 h, whereas a 0.6 µm thick remnant SiAlN coating without any signs of oxidation was observed on Timetal 834. Interfacial reactions between SiAlN coatings and different substrates caused depletion of SiAlN, thereby determining the degradation rates of SiAlN coatings. The non-reactive alloying elements Sn and Al in Timetal 834 alloy could retard such interfacial reactions, thereby impeding coating degradation.

Effect of solid oxidation products on the ablation mechanisms of ZrC and HfC based coatings above 2000°C

Solid and liquid oxidation products have been considered to be the key issues to influence the ablation resistance of UHTCs coatings. However, the effect of solid oxidation products on the ablation performance is not clear. In the present work, four kinds of UHTCs coatings (ZrC, ZrC-30vol.%SiC, HfC, HfC-30vol.%SiC) were selected to study the effect of different solid oxides on the ablation behaviors of UHTCs coatings. The results showed that the HfC-based coatings had thinner oxide scale and SiC-depleted layer, which shown better ablation resistance than the ZrC-based coatings. Based on the characterization of oxygen vacancy and density functional theory (DFT) calculation, it is thought that the solid oxidation products of HfO2 possesses higher defect formation energy compared with ZrO2, which can help to restrain the diffusion of oxygen and contribute to the better ablation resistance.

Effect of buffer layer on oxidation and corrosion resistance of CrN coatings on Zr alloy prepared by FCVAD technology

To improve the oxidation resistance of fuel cladding in case of nuclear accidents, CrN coatings with different metal buffer layers (Cr, Ti, Al) were deposited on Zr 702 alloy via filtered cathode vacuum arc deposition (FCVAD) technology. The phase composition, microstructure, mechanical properties, oxidation resistance and corrosion resistance of the coatings were systematically investigated. CrN coating with Cr buffer layer showed excellent oxidation resistance at 800–1000 °C in air. Cr buffer layer effectively retarded the diffusion of oxygen and the decomposition of CrN by generating a Cr2O3 layer. CrN coating with Cr buffer layer had the lowest mass gain and the thinner oxide scale after oxidation at 800–1000 °C compare with that of Ti and Al buffer layer. The electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization test revealed that Cr buffer layer effectively improved the corrosion resistance of CrN coating by generating Cr2O3 passive film. In addition, Cr/CrN coating showed excellent mechanical properties. Cr buffer layer made more contribution to the comprehensive properties of CrN coatings than Ti and Al, which illustrated utilization potentiality for the protection of fuel cladding.

Effectiveness of Ni-based and Fe-based cladding alloys in delaying hydrogen generation for small modular reactors with increased accident tolerance

This study investigates the high temperature oxidation behaviour of a Ni–20Cr-1.2Si (wt.%) alloy in steam from 1200 °C to 1350 °C by Thermogravimetric Analysis (TGA), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS) and X-ray Diffraction (XRD). The results demonstrate that exposed Ni-based alloy developed a thin oxide scale, consisted mainly of Cr2O3. The oxidation kinetics obtained from the experimental results was applied to evaluate the hydrogen generation considering a simplified reactor core model with different cladding alloys following an unmitigated Loss-Of-Coolant Accident (LOCA) scenario in a hypothetical Small Modular Reactor (SMR). Overall, experimental data and simulations results show that both Fe-based and Ni-based alloys may enhance cladding survivability, delaying its melting, as well as reducing hydrogen generation under accident conditions compared to Zr-based alloys. However, a substantial neutron absorption occurs when Ni-based alloys are used as cladding for current uranium-dioxide fuel systems, even when compared to Fe-based alloys.

Improving oxidation resistance of porous FeAl-based intermetallics with high boron/yttrium alloying

The coalloying with high contents of chromium (Cr), boron (B) and yttrium (Y) for porous B2-structured FeAl intermetallics fabricated through reactive synthesis was conducted. The oxidation behaviors of porous FeAl-based materials were investigated by evaluating the pore-structure evolution, oxidation kinetics and oxide-scale configuration. The results show that with the coalloying of high contents of Cr, B and Y, the oxidation mass gains of porous FeAl materials at 600−800 °C are significantly reduced. The combination of B enriched on the surface of oxide scales and Y located at the scale−metal interface promotes the formation of thin protective nodular α-Al2O3 oxide scales. It is indicated that introducing relatively high contents of reactive elements such as B and Y can benefit the selective growth of α-Al2O3 scales at relatively low temperatures without pre-treatment.

On the role of mechanical deformation in the environmental degradation of 310S stainless steels in supercritical carbon dioxide

The corrosion behavior of 310 S stainless steel with and without mechanical deformation (surface grinding and cold work) has been investigated. The experimental results show that mechanical deformation enhances corrosion resistance of materials. Multilayer oxide scale is observed on the material without mechanical deformation, which owns a loose structure and contributes to the severe carburization of the material. Thin and protective oxide scale (Cr/Mn-rich and Si-rich) is formed on the material after mechanical deformation, which hinders the internal carburization. Mechanical deformation reduces grain size and makes grain boundary diffusion become predominant, which improves the corrosion resistance of materials.

Influence of Si on the oxidation behavior of TM-Si-B2±z coatings (TM = Ti, Cr, Hf, Ta, W)

The concept of Si alloyed transition metal (TM) diborides – well explored for bulk ceramics – is studied for five different physical vapor deposited TM-Si-B2±z (TM = Ti, Cr, Hf, Ta, W) coatings, focusing on the oxidation behavior up to 1200 °C. In their as deposited state, all coatings exhibit single phased AlB2 prototype structures, whereby the addition of Si results in dense, refined morphologies with no additional phases visible in the X-ray diffractograms. With already low amounts of Si, the slope of the mass increase during dynamic oxidation flattens, especially for Ti-Si-B2±z, Cr-Si-B2±z, and Hf-Si-B2±z. Above distinct Si contents, the formation of a steady state region exhibiting no further mass increase is promoted (starting at around 1000 to 1100 °C). Best results are obtained for Hf0.21Si0.18B0.61 and Cr0.26Si0.16B0.58 (both around 2.4 μm thick in the as deposited sate), revealing drastically retarded oxidation kinetics forming 400 nm thin oxide scales after 3 h at 1200 °C in ambient air (significantly lower compared to bulk ceramics). This highly protective oxidation mechanism is attributed to the formation of an amorphous Si rich oxide scale. The Si content needed to form these oxide scales largely differs between the TM-Si-B2±z coatings investigated, also diversifying the prevalent oxidation mechanism, especially for Cr-Si-B2±z.

Synthetic Biofuels by Molten‐Salt Catalytic Conversion: Corrosion of Structural Materials in Ternary Molten Chlorides

The molten‐salt catalytic conversion of biomass into synthetic fuels is one of the promising renewable energy topics of the 21st century. Mixtures of Zn–Na–K//Cl have gained attention, acting as catalytically active heat transfer fluids in the conversion process, due to their low costs and principally low melting points which effectively lower the required operating temperature in the catalytic reactor. Herein, the corrosion behavior of three candidate steels, SS316L, SS321, and Alloy 800, in the pure molten salt, in a salt–biomass mix, and in a salt–biomass mix containing a corrosion inhibitor (zinc oxide (ZnO)) is comprehensively investigated. The herein described study demonstrates that biomass additions increase the penetration depth of the salt, and the corrosion mechanism leads to partial spalling of the oxide scale. This is attributed to the presence of oxygen and moisture from the biomass leading to the formation of reactive HCl gas. The addition of ZnO reduced the corrosivity of the molten salt–biomass mix, and all alloys obtained a protective, adherent, and thin oxide scale leading to the conclusion that ZnO indeed is an effective corrosion inhibitor for the presented salt system. These findings constitute a major advance in the implementation of molten salt‐catalytic reactors for renewable synthetic fuel production.