Steel In Supercritical Water Research Articles

Corrosion behavior of 304 stainless steel in supercritical water

Supercritical water (SCW) has strong reactivity and the ability to fuse non-polar substances, but it requires higher corrosion resistance of structural materials. To select reliable candidate materials for a supercritical water system, this study investigated the corrosion behavior of 304 stainless steel (304 SS) in supercritical water (798 K/24 MPa). After soaking 304 SS in supercritical water for 200 h, the oxidation kinetics, surface morphology, and element diffusion were studied by weight measurement, scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersion spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The surface of 304 SS shows an isolated and discontinuous oxidation deposition. It is determined that the oxide layer is a single layer, mainly composed of Fe-Cr-rich spinel, with a small amount of magnetite attached to the surface of the oxide layer. Besides, the oxidation mechanism was also studied since some pores were found on the surface due to pitting.

Corrosion behavior of HR3C austenitic stainless steel in supercritical water near the critical point

AbstractThe corrosion behavior of HR3C stainless steel was investigated in supercritical water near the critical point. Results show that HR3C steel displays an unusual weight change behavior. Traditional weight gain behavior is detected in the initial corrosion stage because of the formation of spinel oxide scale and crater‐like Nb‐rich oxides. The detaching of crater‐like Nb‐rich oxides results in weight loss behavior during the middle corrosion stage. After 1200 h exposure, the diffusion‐controlled growth of outer Fe2O3 particles leads to the reoccurring of weight gain behavior. The formation of crater‐like Nb‐rich oxides is closely associated with the primary Z‐phase precipitates.

Oxidation behavior of the T91 ferritic/martensitic steel and 304 austenitic stainless steel in supercritical water

PurposeThis study aims to report the oxidation behaviors of the T91 ferritic/martensitic steel (T91 steel) and 304 austenitic stainless steel (304 steel) in supercritical water (SCW) at 600°C.Design/methodology/approachThe microstructure, elemental distribution and phase structure of the oxidation layers derived from the corrosion of the T91 steel and 304 steel were analyzed by scanning electron microscopy, Oxford Instrument X-ray spectroscopy, electron scattered diffraction and transmission electron microscopy.FindingsThe oxidation layers on the T91 steel and 304 steel have duplex structure. The two steels all suffer internal oxidation, and the phase of the internal oxidation layers are indexed as Fe-Cr spinel, although their morphologies are different. The formation of a continuous Cr-rich layer is not detected because of the relatively low Cr content of the steels, which is attributed to the corrosion property.Originality/valueThe accelerated corrosion and corrosion mechanism of the T91 steel and 304 steel with low Cr occurring in SCW at 600°C was clarified.

A high-resolution study of the different surface state effects on the corrosion behaviors of a ferritic steel and an austenitic steel in supercritical water

The corrosion behaviors of two iron-based steels in 600 ºC deaerated supercritical water (SCW) were investigated to study the effect of surface finishing. For the austenitic stainless steel (Fe-19Cr-9Ni), grinding significantly enhanced the oxidation resistance by favoring the formation of a thin and continuous chromia layer. This is because that grinding induced ultra-fined grain layer with a high dislocation density promotes the outward Cr diffusion, thus favors the transition from internal to external oxidation. For the ferritic steel (Fe-9Cr), the oxidation rate remained at a high level regardless of the surface condition mainly because the Cr diffusivity is already high enough but the Cr content is too low to promote the internal-external oxidation transition.

Chemical-Reaction-Induced deformation of Body-Centered cubic iron in supercritical water leading to high risk of cleavage Fracture: A reactive Molecular dynamics study

To improve the reliability of BCC iron-based steels in supercritical water, it is crucial to understand the atomic-scale deterioration mechanisms under stress coupled with chemical reactions at the iron/water interface. Molecular dynamics simulations with the reactive force field were employed to examine the atomic-scale deterioration mechanisms of BCC-iron and the role of chemical reactions with supercritical water. The simulation results revealed a lower yield stress and strain for BCC-iron in supercritical water than in vacuum. Yielding of the BCC-iron in both the vacuum and supercritical water occurred through the generation of partial dislocations at the surface. The deterioration of iron in supercritical water was found to originate from chemical reactions between the iron surface and water molecules; specifically, the formation of Fe–OH bonds on the iron surface due to the dissociative adsorption of water induces displacement of the surface iron atoms, accelerating the generation of partial dislocations.

Revealing the oxidation mechanism of 310S stainless steel in supercritical water via high-resolution characterization

This work studied the surface oxide scale of 310S stainless steel exposed to 650 °C supercritical water for 1000 h. The oxide scale showed a complex structure with an outer oxide layer, an inner oxide layer, and a much thinner Cr2O3 layer. The outer oxide layer consisted of Fe2O3, NiFe2O4 and NiO. Fe-Cr-Ni spinel was identified in the inner oxide layer due to internal oxidation. Transition from internal to external oxidation occurred at the metal-oxide interface, resulting in the formation of a protective Cr2O3 layer. The formation mechanisms of each oxide layer were discussed.

The effect of dissolved oxygen on the oxidation of an austenitic steel in supercritical water

ABSTRACT The oxidation experiments of austenitic steel TP347HFG were carried out at 620°C under 25 MPa in supercritical water with the dissolved oxygen content of 0–2000 ppb. The oxidation rate of TP347HFG increased with the rise in dissolved oxygen content. A double-layered oxide scale composed of a Fe-rich outer layer and a Cr-rich inner layer developed on the TP347HFG steel surface. Haematite was detected in supercritical water with 2000 ppb dissolved oxygen while absent in deaerated supercritical water. The content of dissolved oxygen has a great impact on the formation and distribution of Cr2O3. A thicker and continuous Cr2O3 were observed in the grain boundary near the interface of oxide and the substrate in deaerated supercritical water. The Cr evaporation may be responsible for the oxidation resistance of TP347HFG steel in supercritical water with different dissolved oxygen content. The influence mechanisms of dissolved oxygen on oxidation rate and oxides composition were discussed in this paper.

Domain knowledge-guided interpretive machine learning: formula discovery for the oxidation behavior of ferritic-martensitic steels in supercritical water

A general formula with high generalization and accurate prediction power is highly desirable for science, technology and engineering. In addition to human beings, artificial intelligence algorithms show great promise for the discovery of formulas. In this study, we propose a domain knowledge-guided interpretive machine learning strategy and demonstrate it by studying the oxidation behavior of ferritic-martensitic steels in supercritical water. The oxidation Cr equivalent is, for the first time, proposed in the present work to represent all contributions of alloying elements to oxidation, derived by our domain knowledge and interpretive machine learning algorithms. An open-source tree classifier for linear regression algorithm is also, for the first time, developed to materialize the formula with collected data. This algorithm effectively captures the linear correlation between compositions, testing environments and oxidation behaviors from the data. The sure independence screening and sparsifying operator algorithm finally assembles the information derived from the tree classifier for linear regression algorithm, resulting in a general formula. The general formula with the determined parameters has the power to predict, quantitatively and accurately, the oxidation behavior of ferritic-martensitic steels with multiple alloying elements exposed to various supercritical water environments, thereby providing guidance for the design of anti-oxidation steels and hence promoting the development of power plants with improved safety. The present work demonstrates the power of domain knowledge-guided interpretive machine learning with respect to the data-driven discovery of physics-informed formulas and the acceleration of materials informatics development.

Mechanistic understanding for the formation of Cr-rich haematite layer on the T12 ferritic steel in supercritical water

ABSTRACT The oxides formed on T12 ferritic steel surface after exposure to 560 °C/25 MPa supercritical water with dissolved oxygen contents of 300 ppb was characterised by various techniques. It was found that the weight gain and thickness of the oxide scale increase with the increasing exposure time and the oxidation kinetics of T12 followed with parabolic law. The Cr content at the outermost layer increases while the Cr content at the inner/outer layer interface decreases with the increasing of exposure time. The Cr-rich haematite layer oxide was observed on the specimen surface when the test duration extended to 1000 h. The mechanism for the formation of the Cr-rich haematite layer was proposed based on the Cr can diffuse outward through the magnetite layer by short-circuits of the oxide layer and reach the magnetite/haematite interface.

Analysis of oxide scale thickness and pores position of HCM12A steel in supercritical water

Abstract More attention has been paid to the exfoliation of oxide scale on high-temperature heating surface of utility boiler. The oxidation mechanism of HCM12A steel in supercritical water is proposed and the growth of oxide film is simulated. The duplex scale contains an outer magnetite layer and an inner Cr-rich spinel layer. According to the data of Backhaus and Töpfer, the diffusion coefficient values of iron in magnetite layer are discussed and the function of R V, R I {R}_{\text{I}} for oxygen activity can be used for calculation of iron diffusion coefficients in Cr-rich spinel layer. Based on Wagner’s oxidation theory, the oxidation rate constants of HCM12A are calculated at 500 and 600°C in supercritical water, compared with experimental data of the relevant literatures. The oxygen activities at the interfaces of alloy/Cr-rich spinel oxide and magnetite/supercritical water are estimated. The simulation results of weight gain are matched with the test data. The iron diffusion mechanisms inside the magnetite layer and the Cr-rich spinel layer are analyzed. The iron diffusion coefficient at the interface of Cr-rich spinel/magnetite is discontinuous, while the oxygen activity is continuous in the whole double layer. The thickness of oxide scale on inner tube walls of the final superheater coils (T91) of a 600 MW supercritical boiler is calculated by using the calculation method provided by the paper. The modeling results, the measured data, and the calculation results by the method are compared. Accurate calculation of the thickness of the inner and outer oxide scales can provide a necessary basis for predicting the stress and exfoliation of oxide scales.

Effects of tensile stress on oxidation behavior of a Ferrite-martensite steel in supercritical water

ABSTRACT The oxidation behaviour of ferritic-martensitic steel P92 with different tensile stress conditions has been investigated in supercritical water at 600°C for up to 1000 h. The results indicated that the oxidation rate increased with an increase in tensile stress. The thickness of the oxide scale with atensile stress of 30 MPa is about 1.22 times greater to that at without stress, while the ratio of the thickness of oxide scale to that observed at 60 MPa is 1.73 times greater to that at 0 MPa. The thickness ratio of the outer and inner layer increased with the increasing tensile stress. Crack develops in all cases and is affected by the tensile loading. Furthermore, the thickness of the diffusion layer increased with an increase in tensile stress. The related mechanism has been discussed in this paper.

Oxidation behavior analysis of a ferritic ODS steel in supercritical Water

Abstract The corrosion behavior of 14Cr ODS steel exposed to the supercritical water (SCW) at 600 °C under 25 MPa with the dissolved oxygen content of 300 ppb was investigated. The weight gain measurement and corrosion layers analysis were used to determine the corrosion behavior and corrosion mechanism of the 14Cr ODS steel. The results indicate that the 14Cr ODS steel exposed in this work possesses better corrosion resistance than the other ODS steel with a similar composition. Both uniform corrosion regions and nodular corrosion regions can be seen on the exposed specimens, which is due to the bimodal microstructure of the ODS steel. The corrosion products have a multi-layer structure, including the outer oxide layer, the inner oxide layer, and the transition layer. The oxide in the inner layer is FeCr2O4. The oxides in the outer layer change from the initial Fe3O4 to Fe2O3 with the increase of exposure time. The possible corrosion mechanism is analyzed as well, combining with the microstructure characters of the 14Cr ODS steel.

The characterization of oxide scales formed on ferritic-martensitic steel in supercritical water with dissolved oxygen

The oxidation behaviour of ferritic-martensitic steel in supercritical water has been studied at 600−700 °C and 25 MPa. Exposure tests as functions of temperature, time and distance from the leading edge of the specimen provide evidence that hydrogen influence the surface morphology and oxide phase. Hematite is apt to develop on the top of the magnetite at a lower temperature, prolonged time, and smaller distance from the leading edge of the specimen. By the consideration of convective mass transfer, diffusive mass transport and oxidation rate, a model has been proposed to calculate the partial pressure of hydrogen.

A Diffusion–Reaction Continuum Damage Model for the Oxidation Behaviors of High-Cr Steels in Supercritical Water

In this paper, a diffusion–reaction continuum damage model is proposed to study the oxidation behavior of high-Cr steels in supercritical water. The model is formulated based on a three-dimensional (3-D) setting. By considering the diffusion and reaction effects of free oxygen in a domain of solid material, a partial differential equation named as the diffusion–reaction equation is derived, which contains an oxidation damage variable representing the fraction of the oxidized metal at a local material point. The evolution equation of the oxidation damage variable can also be derived, which together with the diffusion–reaction equation formulates the governing system of the model. To show the efficiency of the model, a 1-D problem is first studied. By taking P92 steel as an example, the values of the parameters in the governing system are determined. Based on the numerical solutions to the governing system, some key features of the oxidation behavior of high-Cr steel sample are analyzed. The influences of the parameters on the oxidation process have also been discussed. The model is further implemented into the ANSYS UserMatTh subroutine. Then, some typical examples of FEM simulations are introduced. It can be seen that our current model is suitable to simulate the global oxidation behaviors of high-Cr steel specimens with different geometrical shapes and subject to different boundary conditions. Therefore, the model would be helpful for the design and residual lifetime evaluation of high-Cr steel components in practical applications.

Understanding the surface oxide evolution of T91 ferritic-martensitic steel in supercritical water through advanced characterization

The surface oxide scale formed on T91 steel after exposure to deaerated supercritical water (SCW) at 600 °C for 100–1500 h are characterized in detail to reveal the oxide evolution over time. A triplex structure with outer, inner and internal oxide layers has been confirmed. The thickness of each oxide layer increases with the exposure time following the power law kinetics. The different oxide phases in each oxide layer are the results of the decreasing partial pressure of oxygen inwards the depth of oxide scale. The inner and internal oxide layers are not uniform in Fe and Cr contents. The internal oxide layer consists of Cr-rich oxide precipitates surrounded by metal matrix in the grain interior, which could be explained by the internal oxidation mechanism. After a further long-time exposure, a continuous thin Cr-rich spinel oxide layer is formed at the internal oxide layer-metal matrix interface, which is the result of transition from internal to external oxidation when the outward diffusion of Cr exceeds the inward diffusion of oxygen. The inner oxide layer consists of Cr-rich spinel oxide precipitates surrounded by Fe-rich spinel oxide. The formation of the inner oxide layer is the result of further oxidation of the internal oxide layer where the metal matrix in the grain interior is oxidized into Fe-rich spinel.

Microstructural investigation of the nodular corrosion of 304NG stainless steel in supercritical water

304NG stainless steel was exposed to deaerated 600 °C supercritical water for 300 h. The thin Cr-rich oxide film and corrosion nodules were characterized in detail by cutting edge techniques. The results show that the formation of MnCr2O4 is an important factor that deteriorates the protectiveness of the Cr-rich thin oxide film. Reaction zones ahead of the inner oxide are observed. The reaction zones are composed of oxide particles and remaining metal, indicating the occurrence of internal oxidation. The internal oxidation controls the further corrosion after the breakdown of Cr-rich thin oxide film, and subsequently the generation of corrosion nodules.

Formation mechanism of the outer layer of duplex scales on stainless steels in oxygenated supercritical water

The dependence on water density of the formation and growth of the outer scale layer was revealed by using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) to analyze the effects of additive phosphates on morphology and composition of oxide scales formed on 316 stainless steel in supercritical water (SCW) with an initial dissolved oxygen of 100 mg·L−1 at 400 °C and 600 °C. In 400 °C SCW with a high density (exceeding 100 kg·m−3) the outer layer thickens via a precipitation process, hence the promotion of pre-existent phosphates in cationic precipitation results in the presence of smaller product particles and phosphorus on the scale surface. In 600 °C SCW with a low density, the formation of the outer scale layer follows a solid-state growth mechanism.

Effects of Si, Mn on the corrosion behavior of ferritic–martensitic steels in supercritical water (SCW) environments

Ferritic–martensitic (F/M) steels with various amounts of alloying elements (Cr: 12 wt. %, Si: 0.6–2.2 wt. %, Mn: 0.6–1.8 wt. %) were exposed to supercritical water (SCW) environments at 500 °C and 25 MPa for up to 1000 h. Gravimetric, Scanning Electron Microscope, Transmission Electron Microscopy (TEM), Energy-dispersive X-ray Spectroscopy (EDX) and X-ray Diffraction (XRD) analyses were conducted to characterize the corrosion behavior of F/M steels in supercritical water. The results showed that the contents of the alloying elements (Si, Mn) affected both the oxidation kinetics and the oxidation scales formed on F/M steels. The F/M steel (Si: 0.6 wt. %, Mn: 1.8 wt.%) showed the highest corrosion rate in SCW with an intermittent oxide scale formed on the surface. Increasing the Si content in F/M steel improved the oxidation resistance and promoted the formation of a uniform oxide scale. The mechanism associated with the corrosion of these F/M steels under SCW conditions is discussed.

Point defect model for the corrosion of steels in supercritical water: Part I, film growth kinetics

A Point Defect Model has been developed to describe theoretically the corrosion of metals and alloys in supercritical aqueous systems. The model, SCW_PDM, accounts for the kinetics of growth of the barrier layer and the total scale thickness and for the oft-reported growth of the barrier layer with the barrier/outer layer interface remaining at the location of the original metal surface. The barrier layer grows into the metal via the production of oxygen vacancies at the metal/barrier layer interface and their annihilation at the barrier layer/outer layer interface. The proposed atomic-level rate laws for the scale growth and oxidation weight gains, not only can act as deterministic equations for extracting values of some fundamental parameters in SCW_PDM, but also can be employed to describe the oxidation kinetics of steels directly and successfully.

Corrosion behavior of 14Cr ODS steel in supercritical water: The influence of substituting Y2O3 with Y2Ti2O7 nanoparticles

14Cr-4.5Al ODS steels were prepared by adding Y2Ti2O7 and Y2O3 nanoparticles respectively. Bilayer-structure corrosion scales were formed while the Y2Ti2O7-ODS steel showed a lower mass gain of 108.1 mg/dm2 in SCW for 1500 h. Compared with Y2O3, Y2Ti2O7 with finer size and higher number density exhibited more effective hindrance impacts on the diffusion of ions. Furthermore, adding Y2Ti2O7 in the ODS steel can provide more zero valent Al to form alumina film through inhibiting the formation of coarse Y-Al-O particles. Our work demonstrates that substituting Y2O3 with Y2Ti2O7 is an effective route to improve the corrosion resistance of Al-adding ODS steels.