Steam Corrosion Research Articles

Enhanced steam thermal cycle resistance of Yb2Si2O7 environmental barrier coatings via Al modification

SiC-based ceramic matrix composites are susceptible to performance losses in steam environments, necessitating the use of environmental barrier coatings for their protection. The high Si activity of Yb2Si2O7 material may induce the emergence of Si(OH)4 in steam environments, resulting in the development of a pore structure within Yb2Si2O7. This pore structure weakens the service life of the coating, highlighting the importance of enhancing the steam corrosion resistance of environmental barrier coatings as a key factor in improving their overall longevity. In the present study, we employed Al modification technique to improve the steam corrosion resistance of Yb2Si2O7 coatings. The results indicated that Al-modified Yb2Si2O7 coatings exhibited superior steam thermal cycle corrosion resistance compared to the non-modified Yb2Si2O7 coatings after testing performed at 1523 K for 1000 h (10 h per cycle). This improvement was attributed to the presence of Yb3Al5O12 and Al2O3 composite phases within the Al-modified Yb2Si2O7 coatings, which exhibited low steam activity and effective steam corrosion resistance. Moreover, Al-modified Yb2Si2O7 coatings obtained the dual excellent performance of improving the steam corrosion resistance of Yb2Si2O7 coating and not weakening the thermal cycle ability, providing a guarantee for the longer life of Yb2Si2O7 coating.

Effect of pulse laser nitriding on the superheated steam corrosion behavior of Zr alloy

Zirconium (Zr) alloy fuel cladding is exposed to harsh environments such as high temperature and high pressure for a long time. Surface modification technology can effectively improve its high-temperature oxidation resistance. In this work, pulse laser nitriding technology was used to perform laser nitriding treatment on Zr alloys at different laser energies. The effect of different energy levels of laser nitriding treatments on the superheated steam corrosion resistance of Zr alloys was studied. First-principles was used to reveal the mechanism of how the diffusion layer improves the performance of Zr alloys in resisting superheated steam corrosion after nitriding treatment. Results indicate that the surface of Zr alloys treated with laser nitriding exhibits a molten feature. When the laser energy is 100 mJ, N mainly exists in the form of a diffusion layer in the Zr alloy substrate. As the laser energy rises to 200 mJ and above, the ZrN phase is generated. After 1000 h of superheated steam corrosion, the corrosion weight gain of the 100 mJ sample was reduced by approximately 50 % compared to the Zr alloy. The corrosion weight gain of the 400 mJ sample increased by approximately 12 %. The presence of the ZrN phase accelerates the oxidation degradation rate of Zr alloy. When N atoms exist as a diffusion layer, the transition from tetragonal zirconia to monoclinic zirconia can be suppressed. First-principles calculations showed that when O and N atoms coexist in the Zr alloy substrate, their binding performance with surface O is significantly decreases, thus further improving the 100 mJ sample's resistance to superheated steam corrosion.

Investigation of Yb2SiO5/BSAS/Si coating on Cf/SiC composites: processing and steam corrosion behavior

Environmental Barrier Coatings (EBCs) are used to protect silicon carbide-based composites in the high-temperature steam combustion environment of the latest generation of high-efficiency gas turbine aero-engines. The Yb2SiO5/BSAS/Si coatings with gradient coefficient of thermal expansion were prepared using atmospheric plasma spraying (APS) method on the carbon fiber reinforced silicon carbide ceramic matrix composites (Cf/SiC) composites surface. The coating preparation process parameters were optimized by evaluating the influence of spraying power on the bonding strength, deposition efficiency, and porosity. The corrosion behaviors and mechanism of the optimized Yb2SiO5/BSAS/Si coating was investigated at 1200 ℃ by thermal cycle test in steam conditions. The results indicated that the decomposition of Yb2SiO5 and the conversion of SiO2 to Si(OH)4 were the main corrosion reactions of Yb2SiO5 top coating. Noticeably, the SiO2 component acted as an obvious sign of the reaction front diffusing, forming the clear SiO2-gradient layers. The gradient Yb2SiO5/BSAS/Si coating remained intact after thermal cycles for 500 h at 1200 ℃ in steam conditions which effectively enhanced the resistance of Cf/SiC composites to high temperature steam corrosion.

High-performance Cr2AlC MAX phase coatings for ATF application: Interface design and oxidation mechanism

Surface-modified Zr-based alloy (ZIRLO) claddings with advanced ceramic coatings are increasingly required for accident-tolerant fuel (ATF) systems in light-water reactors. Cr2AlC MAX phase coatings are promising for this purpose owing to their remarkable properties combining radiation/oxidation/corrosion resistance. However they are suffering from weak interface compatibility to ZIRLO substrate and poor structural densities for long-term services. Herein, we fabricated densely high-purity Cr2AlC MAX phase coatings with uniquely designed Cr/CrCx interfacial layers. The oxidation behavior of the coatings was focused under steam environments at 1000–1200 °C. Results showed that Cr2AlC coatings exhibited an oxidation mass gain of 8.9 mg/cm2 and an oxide thickness of 680 nm after oxidation at 1200 °C for 30 min, which were about 10% and 0.5% of ZIRLO substrate, respectively. Based on microstructural evolutions, the embedded interfacial layers significantly suppressed the rapid diffusion of Al in Cr2AlC coatings to the substrate and the premature delamination of oxidized coatings. Particularly, the formed oxides were identified as dense yet pure α-Al2O3, which endowed the protection against further oxidation and excellent resistance to high-temperature steam corrosion.

Mg2Al4Si5O18 as a novel bond coat material for advanced thermal/environmental barrier coatings on SiCf/SiC composites

This study proposes cordierite (Mg2Al4Si5O18, MAS) as a novel bond coat material for thermal/environmental barrier coatings (T/EBC), deposited using atmospheric plasma spray on SiC fiber reinforced SiC matrix composites (SiCf/SiC-CMCs) coupons, forming a Mg2Al4Si5O18/Yb2Si2O7/Yb2SiO5/LaMgAl11O19 coating system. The phase composition and thermal properties of the as-sprayed MAS coating were investigated, revealing cordierite as the primary phase in the as-sprayed coating with minor amounts of spinel and quartz phases which disappeared after heat treatment. Additionally, the as-sprayed MAS coating exhibited a low thermal expansion coefficient (3.19 × 10−6 K−1). The thermal cycling behavior and steam corrosion performance of the T/EBC coating system with cordierite as the bond coat were studied. Detailed analysis revealed that during thermal cycling tests, the coating failure was primarily related to liquid-phase sintering caused by eutectic reactions. In steam corrosion tests, the coating delamination was linked to the formation and growth of cracks and corrosion voids due to the residual stress release.

Effect of grain refinement on high temperature steam oxidation of an FeCrAl alloy

FeCrAl alloys are promising candidates to replace Zr alloys as fuel cladding materials in nuclear light-water reactors. Grain refinement has been indicated to improve irradiation resistance. To enhance corrosion resistance as well, the effects of grain refinement on steam corrosion behavior were investigated in this work. Samples of Kanthal D alloy (Fe-21Cr-5Al) with two different grain sizes (coarse-grained and ultrafine-grained) were exposed to steam at 1200 °C for 2 hrs. Results indicate improved steam corrosion resistance in ultrafine-grained Kanthal D with formation of a thinner protective Al oxide layer and the presence of a thin underlying Cr oxide layer.

Stability assessment of alumina and SiC based refractories in a high temperature steam environment as potential thermal energy storage materials

Solar energy can be utilized not only for electricity generation but also for synthetic fuel production, making it a versatile option in the transition to a low-carbon and sustainable energy system. However, to enable economically viable solar fuel production, the development of efficient thermal energy storage (TES) materials is crucial. Ceramic materials have been identified as a potential solution for high-temperature TES applications. In this study, two commercially available refractories, alumina coated recrystallized silicon carbide and alumina-based refractory were investigated for their stability under long-term steam corrosion tests to evaluate their suitability as TES materials for a potential TES unit. X-ray diffraction (XRD) revealed that recrystallized silicon carbide is composed of two different SiC polytypes (moissanite 6H and moissanite 4H) while alumina-based refractory contained corundum and mullite. Upon being exposed to steam, both materials exhibited distinct characteristics. SiC experienced significant mass gain, a subtle lightning of its surface color and erosion of a porous alumina coating on the SiC. Amorphous silica phases were detected as an indication of oxidation of SiC from X-ray diffractrograms. These remarkable changes clearly indicate that SiC is unsuitable for use in a potential thermal energy storage (TES) unit. On the other hand, there was no significant visual changes in the alumina-based refractory and the weight change was only marginal after corrosion tests. However, XRD indicated mullite completely disappeared after steam corrosion tests possibly due to its decomposition into Al2O3 and Si(OH)4.

Characterization of surface coarse-grained structure and improved corrosion resistance for Zr-4 alloy at high temperature

A surface coarse-grained structure (SCGS) layer is introduced in Zr-4 alloy after surface mechanical rolling treatment and then vacuum annealed at 800 ℃. The microstructures and textures of SCGS is characterized by Electron Backscattered Diffraction, and grains orientation are analyzed in detail by Inverse Pole Figure and Pole Figure. The corrosion tests of the Zr-4 alloy with SCGS layer and as-received were carried out at 360 ℃/18.7 MPa in water and in steam at 700–800 ℃, respectively. The coarse grain in SCGS layer is 10 times larger than that of as-received samples and has <101¯0> preferred orientation in cross section of cylindrical samples. The weight gain of the SCGS specimens is lower than that of the as-received specimens at high temperatures. As for corrosion in steam at 700 ℃ for 1 hour, the weight gain of SCGS sample is 40% less than that of the as-received sample. The high temperature oxidation process is dominated by the diffusion of O2− along the grain boundaries. The grain boundary density of the SCGS layer is six times less than that of the as-received specimens, which reduces the grain boundary diffusion channels for oxygen ion, and improves corrosion resistance of Zr-4 alloy at high temperature.

Crack Inhibition and Performance Modification of NiCoCr-Based Superalloy with Y2O3 Nanoparticles by Laser Metal Deposition.

A new precipitation strengthening NiCoCr-based superalloy with favorable mechanical performance and corrosion resistance was designed for ultra-supercritical power generation equipment. The degradation of mechanical properties and steam corrosion at high temperatures put forward higher requirements for alternative alloy materials; however, when the superalloy is processed to form complex shaped components through advanced additive manufacturing techniques such as laser metal deposition (LMD), hot cracks are prone to appear. This study proposed that microcracks in LMD alloys could be alleviated with powder decorated by Y2O3 nanoparticles. The results show that adding 0.5 wt.% Y2O3 can refine grains significantly. The increase in grain boundaries makes the residual thermal stress more uniform to reduces the risk of hot cracking. In addition, the addition of Y2O3 nanoparticles enhanced the ultimate tensile strength of the superalloy at room temperature by 18.3% compared to original superalloy. The corrosion resistance was also improved with 0.5 wt.% Y2O3, which was attributed to the reduction of defects and the addition of inert nanoparticles.

The effect of porosity, mixed molecular/Knudsen diffusion, and a surface barrier layer on steam corrosion of Yb2Si2O7

Yb2Si2O7 is a promising candidate environmental barrier coating (EBCs) for ceramic aero-engine components. The effect of microstructure, especially porosity, on steam corrosion of Yb2Si2O7 was studied for sintered compacts of different porosities at 1350 °C. A porous layer of Yb2SiO5 formed on the surface, with the level of porosity determining corrosion rate independent of grain size. Mixed molecular/Knudsen diffusion significantly reduced effective gaseous diffusion through the pores. A thin deposit of dense Yb3Al5O12 on the surface offered protection from corrosion suggesting that microstructural control of pore formation and the inclusion of Al2O3 in Yb2Si2O7 coatings can have significant beneficial effects steam corrosion resistance of EBCs.

Accelerated steam corrosion behavior of Fe-ion irradiated RAFM steel: The role of displacement damage

In this study we investigated the accelerated corrosion behavior of Fe-ion irradiated RAFM steel (SIMP) in high-temperature steam. The thickness, elementary composition, surface morphology, crystal structure and microstructure of the formed oxide films were studied. Consistent with the unirradiated sample, the oxide films of the irradiated samples consisted of an outer layer of Fe3O4, a porous inner layer of Fe-Cr spinel, and an internal oxide zone composed of (Cr, Si)2O3. The thickness of the oxide films increased with the increasing displacement damage. Compared with austenitic steels, the increase of corrosion rate of SIMP steel by displacement damage is rather moderate.

Effect of post-annealing on thermal shock and steam corrosion of LaMgAl11O19/Yb2Si2O7 thermal/environmental barrier coatings

LaMgAl11O19/Yb2Si2O7 (LMA/YbDS) thermal/environmental barrier coatings (T/EBCs) on SiCf/SiC composites were annealed at 1200 °C for 5 h in air or Ar atmosphere. The effect of post-annealing in different atmospheres on the microstructure, thermal shock and steam corrosion of the LMA/YbDS T/EBCs was investigated. Results indicated heat treatment in air and argon eliminated the amorphous in the coatings, avoiding the aging stress associated with the crystallization. And the argon-annealed layers (LMA and YbDS) exhibited the less elastic moduli than the air-annealed ones, resulting in the lower thermal mismatch stress in the argon-annealed T/EBCs upon subsequent thermal cycling. Thus, the argon-annealed T/EBCs exhibited an improved thermal cycling lifetime than the as-sprayed and the air-annealed ones. In addition, the unbroadened vertical cracks in the argon-annealed LMA-TBC layer limited the reactant (water-vapor) access to the silica-TGO, leading to the greater resistance of the argon-annealed T/EBCs against steam corrosion than the other two systems.

Al-modified environmental barrier coatings for protection against water vapor corrosion

High-temperature water vapor corrosion is a great challenge for long lifetime of Si/mullite/Yb2SiO5 Environmental Barrier Coatings. In this work, the Yb3Al5O12 modified layer was in-situ synthesized on the surface of Yb2SiO5 layer to improve the steam resistance. The reaction mechanism of the Al-modified layer was explored. The steam corrosion experimental results indicated that the defects in Yb2SiO5 layer healed, and the oxidation rate of Si-bonded coating was slowed down under the protection of modified layer. Additionally, the calculation results of first-principle molecular dynamics confirmed that Yb3Al5O12 has excellent steam corrosion resistance.

Polycrystalline diamond and magnetron sputtered chromium as a double coating for accident-tolerant nuclear fuel tubes

Controlling the hot steam corrosion of nuclear fuel tubes is crucial for power plant efficiency and safety. We demonstrate an effective method of protecting ZIRLO ® (ZIRLO) fuel tubes against corrosion at accident temperatures in a water-cooled nuclear reactor environment. The tubes surfaces were coated by double layers consisting of the polycrystalline diamond (PCD) and magnetron-sputtered Cr. Coated and bare ZIRLO tubes were subjected to hot steam tests for 30 min at 900 °C followed by 40 min at 1000 °C. Surprisingly, Cr and PCD double layers ability to protect ZIRLO fuel tubes against hot steam corrosion strongly depends on the order of the layers in coatings. ZIRLO coated with Cr as the bottom layer and PCD as the top layer exhibited the best ability to slow down the oxidation process. This double layer coating reduced ZIRLO tube corrosion by 47% compared to unprotected ZIRLO. On the contrary, the double layer coating with PCD as bottom and Cr as the top layer reduced the ZIRLO tube corrosion by 20–25% compared to unprotected ZIRLO. To explain the specific roles of Cr and PCD coatings experimental and theoretical procedures were performed. Our results pointed to two basic parameters, significantly affecting the corrosion of coated ZIRLO tubes: 1. the amount of Cr carbides in the surface part of the coated ZIRLO fuel tubes and 2. adhesion of the coatings to the ZIRLO substrate.

Effect of Annealing Temperatures on Phase Stability, Mechanical Properties, and High-Temperature Steam Corrosion Resistance of (FeNi)67Cr15Mn10Al5Ti3 Alloy

The effect of different annealing temperatures on the phase stability and mechanical properties of (FeNi)67Cr15Mn10Al5Ti3 high-entropy alloys (HEAs) was studied. The phase stability was analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron backscattering diffraction (EBSD). The mechanical properties of the alloy were characterized by hardness and tensile tests. Furthermore, the heat-resistant corrosion properties of the (FeNi)67Cr15Mn10Al5Ti3 alloy after annealing at 800 °C was tested under high-temperature steam. The results indicated that HEAs exposed to different annealing temperatures always exhibited the face-centered cubic (FCC) phase. With rising annealing temperature, the dendrite structure of the alloys in the as-cast condition gradually disappeared, with recrystallization and precipitation of larger grains. The tensile strength of the alloy first increased and then decreased with the rising annealing temperature, the hardness and yield strength of the alloy decreased slightly, and the tensile elongation varied greatly. These findings can be used as a basis for improving the phase stability and mechanical properties of a Cr-Fe-Ni-Mn-HEA system with unequal atomic ratios. The heat and corrosion resistance of the alloy at 360 °C and 400 °C was better than that of Zr-4 alloy.

Influence of alumina addition on steam corrosion behaviour of ytterbium disilicates for environmental barrier coating applications

The sintering and steam corrosion behaviour of Yb2Si2O7 with and without the addition of Al2O3 has been studied on compacted specimens. While, the addition of Al2O3 to Yb2Si2O7 enhanced the sintering of Yb2Si2O7 significantly, it also promoted the formation of a thin (80–100 nm), dense and protective Yb3Al5O12 layer at the Yb2Si2O7 surface when exposed to steam. TEM analysis confirmed that the Yb3Al5O12 layer is formed in steam environment via surface diffusion of aluminium from residual Al2O3 present on the surface of Al2O3/Yb2Si2O7 composite. These findings demonstrate the benefit of Al2O3 addition to protect environmental barrier coatings from steam corrosion.

Oxidation kinetics of SPS-densified U3Si2 fuels—Microstructure impact

U3Si2 is a potential candidate for accident tolerant fuels because of its high uranium density and excellent thermal conductivity in comparison to UO2. However, U3Si2 suffers from oxidation, steam corrosion, and subsequent disintegration/pulverization. The detailed investigation of kinetics that incorporates fundamental treatment of oxidation of U3Si2 is scarcely reported, and the oxidation mechanisms have not been fully elucidated. In this paper, the oxidation behavior of microcrystalline (mc-) and nanocrystalline (nc-) U3Si2 have been systematically investigated using a thermogravimetric analysis (TGA) apparatus through a series of isothermal and non-isothermal kinetic studies. The isothermal kinetic study with a model-fitting approach indicates oxidation activation energy of 85 kJ/mol for dense mc-U3Si2 and 96.4 kJ/mol for nc-U3Si2 pellets, while the isoconversional approach leads to an activation energy in the range of 70–85 kJ/mol for mc-U3Si2 and 75–86 kJ/mol for nc-U3Si2 with three most common model-free methods, including Kissinger–Akahira–Sunose, Flynn–Wall–Ozawa, and Friedman methods. The derivation of oxidation activation energies using both isothermal and isoconversional methods highlights the approach to evaluate the oxidation resistance of nuclear materials using TGA quantitatively and makes it possible to compare among various nuclear fuels.

Structural and phase evolution in U3Si2 during steam corrosion

U3Si2 nuclear fuel is corroded in deuterated steam with in situ neutron diffraction. Density functional theory is coupled with rigorous thermodynamic description of the hydride including gas/solid entropy contributions. H absorbs in the 2b interstitial site of U3Si2Hx and moves to 8j for x ≥ 0.5. Hydriding forces lattice expansion and change in a/c ratio linked to site preference. Rietveld refinement tracks the corrosion reactions at 350–500 °C and preference for the 8j site. Above 375 °C, formation of UO2, U3Si5 and USi3 take place in the grain boundaries and bulk. Hydriding occurs in bulk and precedes other reactions.

Effect of dwell time on water–oxygen corrosion behavior and mechanism of Si-18at%Y eutectic alloy at 1300 ℃

In this study, Si-18at%Y alloys were corroded in water–oxygen atmosphere at 1300 ℃ for 90 h to investigate their corrosion behavior. With the increase in the corrosion time, gravity segregation occurred in the alloy. Moreover, SiO2 was consumed by steam corrosion, leading to the gradual increase in the surface roughness of alloy with time. The molten alloy inside flowed out from the cracks on the surface; therefore, the weight gain rate of alloys first increased and then decreased over time. Owing to the oxygen concentration gradient, temperature gradient, and aluminum cations in corrosion layers, gradient structures were formed in which different structures of yttrium silicate coexisted.

Effect of HfO2 framework on steam oxidation behavior of HfO2 doped Si coating at high temperatures

HfO2 doped Si is designed as bond coat material in thermal/environmental barrier coatings (TEBCs). In this work, the HfO2-Si composite coatings with different HfO2 contents were prepared by atmospheric plasma spraying (APS). The steam oxidation behavior of the coatings was comparatively studied at 1300 °C and 1400 °C. Volatilization of Si occurred during spraying, leading to the deviation of coating compositions. The sprayed coatings contained different HfO2 structures. During steam oxidation, HfSiO4 phase was formed at the SiO2/HfO2 interface by solid-state reaction between them. The HfSiO4 or HfO2/HfSiO4 mixture particles worked to deflect or pin micro-cracks, thus improving the resistance of the coating to cracking. At 1300 °C, a protective oxide scale was formed on the traditional Si coating or the HfO2-Si coating with isolated HfO2 particles. However, the HfO2-Si coating with inter-connected HfO2 framework revealed poor oxidation-resistance. At 1400 °C, accelerated oxidation degradation, steam corrosion volatilization, interface reaction and sintering occurred. The HfO2 framework structure played a dominating role in determining the steam oxidation resistance of the HfO2-Si coating, and the connected HfO2 framework and TGO network provided a rapid diffusion path for oxidants (H2O, O2− and OH−) and deteriorated the oxidation resistance.