Steam Corrosion Research Articles

Microstructure understanding of high Cr-Ni austenitic steel corrosion in high-temperature steam

The microstructure and microchemistry of the oxide scales formed on Fe-21Cr-32Ni and Fe-17Cr-9Ni steels after exposure to deaerated high-temperature high-pressure steam at 600 °C for 1500 h have been analysed and compared by several advanced characterization techniques. By comparing the oxide scales formed at different-stages of exposure, it is shown that Fe-21Cr-32Ni steel was internally oxidized at the early-stage, and then an external oxide scale was developed together with an inner chromia band under the internal oxidation zone. In comparison, Fe-17Cr-9Ni steel was internally oxidized together with an external Fe-rich oxide scale during the entire experimental period. The thicknesses of the internal oxidation zone of Fe-21Cr-32Ni and Fe-17Cr-9Ni steels were ∼7 and ∼70 µm, respectively. Further characterisation revealed that the internal oxidation zone contained (Cr, Fe, (Ni))3O4 and nanoscale nickel networks, together with numerous nano-pores. The effects of these structures on mass transfer and reaction product formation were discussed, in connection with the alloy composition and the formation of the chromia layer.

High-temperature corrosion of HfSiO4 environmental barrier coatings exposed to water vapor/oxygen atmosphere and molten calcium magnesium aluminosilicate

The hot corrosion behavior of plasma-sprayed HfSiO4 coatings exposed to molten calcium magnesium aluminosilicate (CMAS) and steam was investigated. No reactive crystallization was observed in the CMAS-attacked HfSiO4 coating which consist of tetragonal HfSiO4 and monoclinic HfO2. The penetration did not bring obvious volume expansion thanks to the buffering effect of the pores/microcracks. The bilayer environmental barrier coating (EBC) systems using HfSiO4 as top coat and Si as bond coat have the great resistance against steam corrosion. The coating degradation at high temperatures was associated with TGO volatilization and phase transformation as well as thermomechanical incompatibility.

Structural and Phase Evolution in U3si2 During Steam Corrosion

Structural and Phase Evolution in U3si2 During Steam Corrosion

Microstructural and mechanical evolution of SiCf/SiC composites in wet oxygen atmosphere above 1000 °C

As thermal-structural materials used in gas turbine hot sections, continuous silicon carbide fiber reinforced silicon carbide matrix (SiCf/SiC) composites serve under high temperature gas steam corrosion. Therefore the long-term durability of the composites is strongly dependent on the microstructural and mechanical stability under the corrosion. Here, the microstructural and mechanical evolution of SiCf/SiC were investigated in a simulated steam environment (22 vol% H2O + 78 vol% O2, 1100–1300 °C). The results indicate that the temperature plays a major role in the corrosion of the composites. As the temperature rises up to 1200 °C, the fiber/matrix interphase (boron nitride) is completely dissipated due to oxidation. Meanwhile, a dense SiO2 scale forms on the composite surface and cristobalite phase tends to precipitate with the increase of temperature. The dissipation of interphase can lead to significant mechanical degradation on the composites. Furthermore, the degradation mechanism was also proposed in terms of stress distribution and fracture morphology. This study can offer new insight for understanding the wet-oxygen corrosion of SiCf/SiC and thus constructing SiCf/SiC with high stability in gas steam.

Corrosion behaviour of alumina-forming heat resistant alloy with Ti in high temperature steam

The corrosion behaviour of an alumina-forming heat resistant alloy with high Ti (Fe-35Ni-16Cr-4.5Al-3Ti, wt%) was evaluated in high temperature steam. The alloy developed interfacial α-Al2O3 layer during the pre-oxidation treatment in air at 750 °C. However, it formed external Cr2O3 and internal γ-Al2O3 oxides on the austenite matrix during the steam corrosion at 750 °C for 1000 h. This behaviour is attributed to the increased oxygen permeability into the austenite matrix from the synergistic effect of steam and Ti. Meanwhile, the pre-oxidized alloy with interfacial α-Al2O3 layer showed improved corrosion protection with reduced weight gain in steam environment.

Diamond Coating Reduces Nuclear Fuel Rod Corrosion at Accidental Temperatures: The Role of Surface Electrochemistry and Semiconductivity.

If we want to decrease the probability of accidents in nuclear reactors, we must control the surface corrosion of the fuel rods. In this work we used a diamond coating containing <60% diamond and >40% sp2 “soft” carbon phase to protect Zr alloy fuel rods (ZIRLO®) against corrosion in steam at temperatures from 850 °C to 1000 °C. A diamond coating was grown in a pulse microwave plasma chemical vapor deposition apparatus and made a strong barrier against hydrogen uptake into ZIRLO® (ZIRLO) under all tested conditions. The coating also reduced ZIRLO corrosion in hot steam at 850 °C (for 60 min) and at 900 °C (for 30 min). However, the protective ability of the diamond coating decreased after 20 min in 1000 °C hot steam. The main goal of this work was to explain how diamond and sp2 “soft” carbon affect the ZIRLO fuel rod surface electrochemistry and semi conductivity and how these parameters influence the hot steam ZIRLO corrosion process. To achieve this goal, theoretical and experimental methods (scanning electron microscopy, Raman spectroscopy, electrochemical impedance spectroscopy, carrier gas hot extraction, oxidation kinetics, ab initio calculations) were applied. Deep understanding of ZIRLO surface processes and states enable us to reduce accidental temperature corrosion in nuclear reactors.

Study of early P91 dual corrosion in steam and simulated combustion gases from a gas-fired boiler

Abstract P91 ferritic steel pipes face dual environments during boilers operation: steam-side and fire-side. This P91 steel assessment differs from the dual studies performed to simulate coal-fired boilers -oxyfuel/steam atmospheres- since the fuel source is replaced by natural gas. This research work includes designing a device to reproduce dual corrosion studies at 650 °C and testing times up to 200 h. One coupon face was exposed to combustion gases while the other to steam. As a main result, the duplex's inner layer allowed to state that combustion gases overcome the steam oxidation rate by a factor of 1.6. Besides, we supplied physical-chemistry information about the surface and bulk of oxide layers by atomic force microscopy, scanning electron microscopy, x-ray photoelectron spectroscopy, and x-ray diffraction analysis. Thus, our experiments aimed to obtain data about the P91 early degradation under the simultaneous 72.73N2/8.30CO2/3.37O2/15.60H2O %mol and steam influence. We last for a future work the isolated evaluation of both environments to determine their role on the corrosion rate obtained in the current study.

Cr-doped U3Si2 composite fuels under steam corrosion

Dense Cr-doped U3Si2 composite fuels were manufactured by spark plasma sintering, and the effects of Cr addition on mechanical properties and oxidation resistance were investigated. Dynamic oxidation testing by thermogravimetric analysis revealed significantly improved oxidation resistance of U3Si2 with minimal doping of 3 wt% Cr. The onset oxidation temperature increased to above 550 °C in air and ∼520 °C in steam conditions for the 5 wt% and 10 wt% Cr-doped composites. Steam corrosion testing under 360 °C for 24 hours indicated well-maintained pellet integrity without pulverization for the 10 wt% Cr-doped U3Si2 pellet which only showed minor surface oxidation. The first promising results open up the possibility of designing and manufacturing metal additive-doped U3Si2 composite fuels with significantly-improved corrosion resistance as a potential candidate for accident tolerant fuels.

Al–Si @ Al2O3 @ mullite microcapsules for thermal energy storage: Preparation and thermal properties

Encapsulation is an effective method to prevent leakage of molten phase change materials (PCMs). However, it is difficult for microcapsules to acquire high latent heat capacity as well as good thermal cycling performance. In the present study, double shell microcapsules, using aluminum silicon alloy as the core, Al2O3 as the inner shell, and mullite as the outer shell, were prepared for heat storage by steam corrosion followed by silica sol immersion and high-temperature calcination. A cross-section of microcapsule showed that the total thickness of the double shell was approximately 1.5 μm. The latent heat storage and latent heat release of the microcapsules were 367.1 J g−1 and 298.4 J g−1 and still remained 90.4% and 80.0% after 3000 thermal cycles, respectively. The latent heat storage and release rate were 72.0 J g−1·min−1 and 20.3 J g−1·min−1, respectively. With an increase of temperature from 40 to 950 °C, the calculated total heat absorption of the microcapsules reached as high as 1850.3 J g−1, 22.7% greater than that without phase change. This double shell microcapsule has a high potential to be used in thermal energy storage systems.

Water Vapor Corrosion Behavior of Yb2SiO5 Environmental Barrier Coatings Prepared by Plasma Spray-Physical Vapor Deposition

Tri-layer Si/mullite/Yb2SiO5 environmental barrier coating (EBC) was prepared on the SiCf/SiC ceramic matrix composite (CMC) by plasma spray-physical vapor deposition (PS-PVD). The EBC samples were carried out with water vapor corrosion at 1300 °C for 200 h. After steam corrosion, Yb2SiO5 layer forms a gradient porous structure. This is mainly due to the inclusion of SiO2-rich layer which is precipitated from the gasification inside the coating and existing a small amount of Yb2O3 separately. During the corrosion process, water vapor infiltrates into the coating and reacts with the SiO2 and Yb2O3 to generate volatile substances. This forms a porous structure to make the coating brittle, resulting in mud cracks finally. In addition, the results show that the Yb2SiO5 can react with the water vapor at the coating surface, forming an Yb2Si2O7 top layer.

Evaluating Deposit Formation of Iron-Corrosion Products in High- and Low-Pressure Evaporative Circuits When Starting a Combined-Cycle Plant

One of the main causes of equipment damage at TPPs is corrosion of structural materials and deposit formation on the heat-transfer surfaces of the steam-water tract. This process occurs during stationary operation of the equipment and during start-ups and shutdowns of the power unit. In this paper, we assess the deposit formation of corrosion products in the evaporative circuits during the start-up of a combined cycle plant with the subsequent prediction of the number of starts after which it is necessary to carry out a chemical washing of the boiler-utilizer. The changes in the product concentration of iron corrosion in water and steam when starting a 450-MW power unit from a cold state are shown. The probability for the deposit formation of iron-corrosion products is estimated based on the experimental data obtained at start-up, with the subsequent calculation of the material balance for iron-corrosion products for low- and high-pressure circuits. A comparative analysis of the probability for deposit formation of iron-corrosion products in the high- and low-pressure circuits during the start-up period and at nominal operating parameters is performed. As a result of studies, it was found that the iron concentration in the feed and boiler water and saturated steam of the high- and low-pressure circuit significantly exceeds the normalized values by the time when the boiler-utilizer reaches its operating parameters. The specific amount of iron-corrosion products that can be deposited on heat-transfer surfaces in one start for the low- and high-pressure circulation circuit is determined. To reduce the probability for deposits of corrosion products during start-up, it is proposed to use feed water with a lower iron concentration or to increase the purge of the boiler-utilizer. Recommendations on the shutdown of the boiler-utilizer for washing when the maximal specific deposit amount of 200 g/m2 is reached taking into account the number of permissible starts for the low-pressure circuit are given.

Zr alloy protection against high-temperature oxidation: Coating by a double-layered structure with active and passive functional properties

In this work, a new concept of metal surface protection against degradation caused by high-temperature oxidation in water environment is presented. We were the first to create a double-layered coating consisting of an active and passive part to protect Zr alloy surface against high-temperature oxidation in a hot water environment. We investigated the hot steam corrosion of ZIRLO fuel cladding coated with a double layer consisting of 500 nm nanocrystalline diamond (NCD) as the bottom layer and 2 μm chromium-aluminum-silicon nitride (CrAlSiN) as the upper layer. Coated and uncoated ZIRLO samples were exposed for 4 days at 400 °C in an autoclave and for 60 min at 1000 °C (nuclear reactor accident temperature) in a hot steam furnace. We have shown that the NCD coating protects the Zr alloy surface against oxidation in an active way: carbon from NCD layer enters the Zr alloy surface and, by changing the physical and chemical properties of the Zr cladding tube surface, limits the Zr oxidation process. In contrast, the passive CrAlSiN coating prevents the Zr cladding tube surface from coming into physical contact with the hot steam. The advantages of the double layer were demonstrated, particularly in terms of hot (accident-temperature) oxidation kinetics: in the initial stage, CrAlSiN layer with low number of defects acts as an impermeable barrier. But after a longer time (more than 20 min) the protection by more cracked CrAlSiN decreases. At the same time, the carbon from NCD strongly penetrates the Zr cladding surface and worsen conditions for Zr oxidation. For the double-layer coating, the underlying NCD layer mitigates thermal expansion, reducing cracks and defects in upper layer CrAlSiN.

Current-driving dissolution of nanoscale brittle precipitates produced by spinodal decomposition in FeCrAl alloys

Due to its excellent resistance to high temperature oxidation and steam corrosion, FeCrAl alloys have received much attention recently as a highly potential candidate cladding material for light water reactors. However, under the effect of thermal aging and neutron irradiation applied in reactor, the nanoscale brittle precipitates will occur in the ferritic FeCrAl alloys after spinodal decomposition due to the presence of Cr, which greatly increase the hardness and reduce the toughness of the material, thus seriously affecting the operation of the nuclear reactor. In this work, the alloy after heat aging was treated by pulsed electric current at room temperature. The micro-hardness and electrical properties of the aged alloy are greatly restored. In addition, compared with the heat treatment at the same temperature, the dissolution time of precipitates in aged alloys into the matrix is reduced by 10 times by electropulsing treatment with higher parameters. In comparison to the ex-situ conventional annealing treatment, this in-situ pulsed treatment can be used as a novel efficient and convenient method to repair the performance degradation of aged materials. Based on the thermodynamic and kinetics, electric pulses reduces the thermodynamic barrier for precipitation dissolution and increases the atomic diffusion rate.

Preparation and formation mechanism of Al-Si/Al2O3 core-shell structured particles fabricated via steam corrosion

Abstract In this study, Al-Si/Al2O3 core-shell structured particles were fabricated via pressurized steam corrosion for 1 h followed by heating for 3 h at 1100 °C. After steam corrosion, a layer composed of disordered crystals covered the surfaces of the Al-Si alloy particles. After heating, Al-Si/Al2O3 core-shell structured particles with complete shells were prepared. The thickness of the shell was approximately 2 μm, and it enclosed the Al-Si alloy core. The shell exhibited excellent thermal stability because, even at 1100 °C, the mass gain ratio of the encapsulated particle was less than 0.5%. Scalloped patterns of alumina were formed by the oxidation of Al, which was inlaid through and upon the alumina shell. The shell formation mechanism suggested that the α-Al2O3 shell resulted from the combination of the decomposition of surface Al(OH)3 crystals and the oxidation of Al from the core.

Hydrothermal Corrosion of Coatings on Silicon Carbide in Boiling Water Reactor Conditions

SiC/SiC ceramic matrix composites are an attractive material for use as accident tolerant fuel cladding. However, despite its resistance to corrosion in high-temperature steam, SiC is known to diss...

Corrosion of the bonding at FeCrAl/Zr alloy interfaces in steam

The interfacial bonding at protective coatings and Zr alloy substrates could have a significant effect on the corrosion behaviors of the coatings. In this work, FeCrAl-Zr couples were fabricated at 900 °C by spark plasma sintering (SPS), and the interfacial bonding of the couples was modified through the addition of Si and/or thermal diffusion at 1100 °C–1300 °C. Uphill diffusion of silicon occurred during SPS. The corrosion resistance of the couples in 400 °C/10.3 MPa steam, and the effects of the interfacial bonding characteristics were studied. Due to the presence of ZrSi2 in the interfacial layer, the interfacial bonding of the FeCrAl/Si-Zr couple had a better corrosion resistance than that of the FeCrAl-Zr couple, withstanding 14 days of corrosion in steam compared to 7 days for the FeCrAl-Zr couple. The results also suggested that bimetallic effect could accelerate the oxidation of Zr alloy at the interface of the couples, and galvanic corrosion significantly contributed to the serious corrosion of the interface of the diffusion bonded couples with thermal diffusion. The influence of oxide growth stress around interface on the bonding lifetime of the couples was also discussed.

Evaluation of steam corrosion and water quenching behavior of zirconium-silicide coated LWR fuel claddings

This study investigates steam corrosion of bulk ZrSi2, pure Si, and zirconium-silicide coatings as well as water quenching behavior of ZrSi2 coatings to evaluate its feasibility as a potential accident-tolerant fuel cladding coating material in light water nuclear reactor. The ZrSi2 coating and Zr2Si-ZrSi2 coating were deposited on Zircaloy-4 flats, SiC flats, and cylindrical Zircaloy-4 rodlets using magnetron sputter deposition. Bulk ZrSi2 and pure Si samples showed weight loss after the corrosion test in pure steam at 400 °C and 10.3 MPa for 72 h. Silicon depletion on the ZrSi2 surface during the steam test was related to the surface recession observed in the silicon samples. ZrSi2 coating (∼3.9 μm) pre-oxidized in 700 °C air prevented substrate oxidation but thin porous ZrO2 formed on the coating. The only condition which achieved complete silicon immobilization in the oxide scale in aqueous environments was the formation of ZrSiO4 via ZrSi2 coating oxidation in 1400 °C air. In addition, ZrSi2 coatings were beneficial in enhancing quenching heat transfer - the minimum film boiling temperature increased by 6–8% in the three different environmental conditions tested. During repeated thermal cycles (water quenching from 700 °C to 85 °C for 20 s) performed as a part of quench tests, no spallation and cracking was observed and the coating prevented oxidation of the underlying Zircaloy-4 substrate.

Thermal behavior and mechanical properties of Y2SiO5 environmental barrier coatings after isothermal heat treatment

Y2SiO5 coatings are deposited by a flame-spray technique as protection layer on SiC substrates to prevent oxidation and steam corrosion. In this research, Y2SiO5 coatings were isothermally heat treated at different temperatures and different exposure times in a laboratory environment. The thermal behaviors such as phase transformation, microstructural change and thermally grown oxide (TGO) formation have been examined by XRD, SEM, and EDS analysis. Different modes of TGO growth behavior were found at different temperatures. In addition, the mechanical properties were evaluated by a Martens hardness tester. The results show that the change of microstructure and composition is not too critical, but higher temperatures and longer heating times do lead to the formation of Y2SiO5 crystalline phases and a β-Y2O3 phase. Thus, the isothermal heat treatment improves the hardness and elastic modulus of Y2SiO5 coatings.

Effect of Ethanolamine, Ammonia, Acetic Acid, and Formic Acid on Two-Phase Flow Accelerated Corrosion in Steam–Water Cycles

Effects of ethanolamine, ammonia, and acetic and formic acid on two-phase flow-accelerated corrosion were investigated in an experimental loop simulating the conditions found in a water-steam cycle. Results indicate that the effects of acetic acid and ethanolamine on the corrosion rate neutralize each other. The effect of acetic acid on the corrosion rate was most pronounced at the highest tested steam quality. A model simulation for liquid film pH at 90% steam quality suggests that at very high steam qualities the protection ethanolamine provides increases, while the protection provided by ammonia goes down. A linear relation between calculated liquid film pH and the measured corrosion rate was found for 24% steam quality within a pH range from 5.6 to 6.3. Formic acid was thermally unstable at the tested temperatures and had no effect on the corrosion rate.

Ultra-high temperature steam corrosion of complex silicates for nuclear applications: A computational study

Stability of materials under extreme conditions is an important issue for safety of nuclear reactors. Presently, silicon carbide (SiC) is being studied as a cladding material candidate for fuel rods in boiling-water and pressurized water-cooled reactors (BWRs and PWRs) that would substitute or modify traditional zircaloy materials. The rate of corrosion of the SiC ceramics in hot vapor environment (up to 2200°C) simulating emergency conditions of light water reactor (LWR) depends on many environmental factors such as pressure, temperature, viscosity, and surface quality. Using the paralinear oxidation theory developed for ceramics in the combustion reactor environment, we estimated the corrosion rate of SiC ceramics under the conditions representing a significant power excursion in a LWR. It was established that a significant time – at least 100h – is required for a typical SiC braiding to significantly degrade even in the most aggressive vapor environment (with temperatures up to 2200°C) which is possible in a LWR at emergency condition. This provides evidence in favor of using the SiC coatings/braidings for additional protection of nuclear reactor rods against off-normal material degradation during power excursions or LOCA incidents. Additionally, we discuss possibilities of using other silica based ceramics in order to find materials with even higher corrosion resistance than SiC. In particular, we found that zircon (ZrSiO4) is also a very promising material for nuclear applications. Thermodynamic and first-principles atomic-scale calculations provide evidence of zircon thermodynamic stability in aggressive environments at least up to 1535°C.