Stainless 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.

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.

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.

Effect of exposure temperature on the corrosion behaviors of TP347H austenitic stainless steel in supercritical water

Corrosion behaviors of TP347H austenitic stainless steel were investigated in supercritical water at different temperatures. Results show that weight gains follow roughly parabolic kinetics except after 1800 h exposure at 650 °C. Some outer isolated Fe2O3 particles are interspersed on the inner nano-sized spinel scale at 380 °C. At 550 °C, the oxide layer with outer magnetite and inner spinel gradually increases and the (Fe,Cr)2O3 layer decreases with exposure time increasing. At 650 °C, the thick magnetite layer is firstly formed and then spalled associating with a chromia layer generated at the spinel/matrix interface, resulting in significant weight loss.

Effect of salt deposit on corrosion behavior of Ni-based alloys and stainless steels in supercritical water

The under-deposit corrosion behavior of Ni-based alloys 625, 825 and Fe-based alloy 800 in supercritical water (SCW) at 400 °C and 500 °C was investigated. Newly designed experimental apparatus was used to ensure that the surface of alloy specimens was covered by NaCl or KCl deposit layers during the corrosion test in SCW environment. Based on the cross-sectional observation and surface chemical analyses, the oxide film on the samples after exposure contains an outer layer containing Fe-Ni-Cr spinel and Fe3O4, and an inner layer of Cr2O3. NiCl2 was also found on the alloy surface. At 400 °C, hydrolysis of the metallic ions occurred, forming HCl with chloride ions. At 500 °C, the oxides react with the solid-phase salt deposit via the formation and transformation of the HCl vapor. The salt deposit layer acted as a barrier for the diffusion of SCW and HCl vapor at 500 °C.

Corrosion mechanisms of 316L stainless steel in supercritical water: The significant effect of work hardening induced by surface finishes

The oxidation of 316 L stainless steel in hydrogenated supercritical water at 600 °C is strongly dependent on the effects of work hardening induced by surface finishes. The oxide scale formed under these conditions is always double-layered with an external layer of Fe-rich oxides. However, when a hardening threshold is reached, a switch in oxidation mechanisms leads to a considerable thinning of the oxide scale. This thinning results from the formation of a Cr-rich internal oxide layer that acts as a diffusion barrier against ionic species responsible for its growth but also against Fe cations implied in the external layer growth.

Stress corrosion cracking behavior of 310S stainless steel in supercritical water at different temperature

The stress corrosion cracking (SCC) behavior and tensile properties of 310S stainless steel were studied by slow strain rate tensile (SSRT) tests, which were carried out in oxygenated supercritical water at the temperature of 400, 550, and 620 °C, respectively. The distribution of chemical composition of oxide, tensile properties, fracture morphology of the materials were analyzed for evaluating the SCC susceptibility of 310S stainless steel, respectively. The results show that both the elongation and tensile strength decreased significantly with the increase of the temperature, implying that a higher temperature may lead to a higher SCC susceptibility. The remarkable changes of the morphology of the fracture surface and gauge surface of 310S stainless steel were observed at different temperatures, that is, from an entire ductile fracture at lower temperature to a brittle fracture at higher temperature. The temperature has significant effects on morphologies and thickness. Oxides with double layer were observed. TEM analysis revealed that the inner oxide layer is composed of Cr and O, and Ni‐enrichment can be observed to segregate at the metal‐oxide interface.

Influences of oxygen on corrosion characteristics of TiO2/316L stainless steel in supercritical water

Supercritical water gasification (SCWG) is a promising technology for converting organic wastes to hydrogen. Less amount of oxygen is beneficial for increasing hydrogen generation rate. However, the corrosion rate of reactor material would be accelerated. TiO2 coating with a thickness of 0.1 mm was prepared on the surface of 316L stainless steel (SS316L) to improve its corrosion resistance in supercritical water (SCW). The corrosion performances of TiO2/SS316L were tested in a bath SCW reactor at 400 °C, 25 MPa. The influences of oxygen concentration (0–1000 mg/L) on surface morphologies and corrosion depths were studied thoroughly. Results indicated that the surface of TiO2/SS316L exhibited cracks and pores after exposed in SCW. And the average corrosion rates accelerated at higher oxygen concentrations. The interface between the coating and medium was relatively smooth and there was no obvious change in the thickness of the coating with oxygen concentration of 0 and 500 mg/L. While for that with 1000 mg/L oxygen, the surface of TiO2/SS316L exhibited reticulate crack. The cross section showed a serrate structure, and only 0.08 mm thick of the coating was remained. In addition, the corrosion mechanism of coating was discussed.

Corrosion behavior of alumina-forming and oxide dispersion strengthened austenitic 316 stainless steel in supercritical water

The corrosion behavior of alumina-forming austenitic (AFA) 316 stainless steel (SS) and oxide dispersion strengthened (ODS) 316 SS in aerated supercritical water (SCW) at 650 °C/25 MPa was investigated. The results show that the weight gain curves of AFA and ODS 316 SS follow near-parabolic law. Both AFA and ODS 316 SS show higher corrosion resistance in SCW than normal 316 SS, which is because that Al-Cr rich oxide and Cr-rich oxide layer are formed on AFA and ODS 316 SS, respectively. The corrosion process and mechanism of AFA and ODS 316 SS are discussed.

Oxidation behaviour of Super 304H stainless steel in supercritical water

ABSTRACTOxidation of Super 304H steel in supercritical water at 600°C and 25 MPa was investigated, for the oxidation time of up to 1000 h. The oxidation kinetics approximately followed a parabolic law. The composition and microstructure of the oxide were investigated using the SEM equipped with EDS, EBSD and XRD. The phase compositions of the oxide evolve with increasing time. Cr2O3 was present at the initial stage and not detected while Fe2O3 appeared at longer time. EBSD indicated that continuous Cr2O3 formed at the interface of oxide and matrix for HR3C at 600°C for 40 h but only scattered Cr2O3 can be observed for 200 h. After the formation of a thin Fe–Cr–Ni oxide film on the Super 304H steel surface, the Fe-rich outer nodular oxide begins to grow and forms a continuous layer after 1000 h. The growth process of Super304H steel in supercritical water was discussed.

Stress Corrosion Crack Growth Behavior of Type 310S Stainless Steel in Supercritical Water

Stress corrosion crack (SCC) behavior of Type 310S stainless steel (SS) in supercritical water was evaluated at 400°C to 550°C. Crack growth rates (CGR) were obtained as a function of temperature and corrosion potential. Results show that Type 310S SS exhibits 100% intergranular SCC, and creep plays an important role in the overall growth rate. The SCC CGR increases with temperature between 400°C and 550°C, and is higher at a medium-low corrosion potential (argon deaerated water) than in water with dissolved O2 or dissolved H2. A mechanism for SCC of Type 310S SS in supercritical water is proposed to explain the combined effect of SCC and creep.

Stress corrosion cracking susceptibility of thermally-aged Type 310S stainless steels in supercritical water

The effect of thermal ageing on the stress corrosion cracking (SCC) susceptibility of Type 310S stainless steel in supercritical water (coolant for a supercritical water cooled reactor design concept) was evaluated. SCC was evaluated using the slow strain rate testing technique, straining samples to 10% in a flowing autoclave test facility using 25 MPa SCW at 500 °C with 20 ppm dissolved oxygen. The localized deformation model for SCC initiation and a void-assisted film-cleavage model for SCC propagation can be used to explain the SCC susceptibility exhibited by the thermally-aged Type 310S stainless steel in supercritical water.

Corrosion and Stress Corrosion Cracking Susceptibility of Type 347H Stainless Steel in Supercritical Water

The corrosion resistance and stress corrosion cracking (SCC) susceptibility of Type 347H stainless steel (SS) in supercritical water (SCW) were investigated. The general corrosion behavior was investigated by exposure test and the specimens after testing were characterized utilizing scanning electron microscopy (SEM), Auger and x-ray diffraction analysis, optical microscopy, and energy dispersive spectroscopy (EDS). The results show that with the increase of testing temperature, the corrosion rate of the materials is greatly enhanced. The corrosion process is analyzed and the formation of oxide islands on the surface of the corroded sample is attributed to the higher diffusion rate of Cr along the grain boundary. The effects of temperature and dissolved oxygen on SCC susceptibility were investigated by slow strain rate tensile test. The fractographs were characterized by SEM and the cross-section morphologies were characterized with back-scattered electron imaging, SEM, and EDS. The results indicate that, as temperature is increased, the tensile strength and strain of materials is greatly reduced, while the oxygen content in SCW has a limited effect on the mechanical properties. Intergranular cracking and ductile fracture are the main fracture modes for Type 347H SS tested in SCW, independent of temperature and oxygen content. The implications of the results to the mechanisms of SCC are discussed.

Comparative study on corrosion characteristics of Al2O3/316L and TiO2/316L stainless steel in supercritical water

Al2O3 and TiO2 coatings were fabricated on 316L stainless steel by atmospheric plasma spraying to improve the corrosion resistance of 316L stainless steel in supercritical water. The corrosion characteristics of the samples were evaluated in a batch reactor at 500 °C and 25 MPa with an oxygen concentration of 1000 mg/L for 80 h. The adhesive strengths of the coated samples were tested, and the weight changes, morphologies and elements distributions of the fresh and corroded samples were analyzed. Results showed that the bond strength of TiO2/316L was 1.5 times than that of Al2O3/316L (26.639 N/mm2). The surface morphology of Al2O3/316L showed gully erosion with much pores and cracks after exposed in SCW, which provided channels for oxygen and SCW to get into the substrate and also the elements in substrate to diffuse to the surface of the coating. The corroded Al2O3/316L suffered significant weight loss, and most of the coatings were peeled off. However, the surface morphology of TiO2/316L was relatively dense and the thickness of the coating was not found to decrease obviously.

The corrosion and stress corrosion cracking behavior of a novel alumina-forming austenitic stainless steel in supercritical water

The general corrosion and stress corrosion behavior of Fe-27Ni-15Cr-5Al-2Mo-0.4Nb alumina-forming austenitic (AFA) steel were investigated in supercritical water under different conditions. A double layer oxide structure was formed: a Fe-rich outer layer (Fe2O3 and Fe3O4) and an Al-Cr-rich inner layer. And the inner layer has a low growth rate with exposing time, which is good for improvement of corrosion resistance. Additionally, some internal nodular Al-Cr-rich oxides were also observed, which resulted in a local absence of inner layer. Stress corrosion specimens exhibited a combination of high strength, good ductility and low susceptibility. The stress strength and elongation was reduced by increasing temperature and amount of dissolved oxygen. In addition, the corresponding susceptibility was increased with decreased temperatures and increased oxygen contents.

Stress corrosion cracking behavior of annealed and cold worked 316L stainless steel in supercritical water

The supercritical water reactor (SCWR) is one of the more promising designs considered by the Generation IV International Forum due to its high thermal efficiency and improving security. To build this reactor, standardized structural materials used in light water reactors (LWR), like austenitic stainless steels, have been proposed. These kind of materials have shown an optimum behavior to stress corrosion cracking (SCC) under LWR conditions except when they are cold worked.It is known that physicochemical properties of water change sharply with pressure and temperature inside of the supercritical region. Owing to this situation, there are several doubts about the behavior of candidate materials like austenitic stainless steel 316L to SCC in the SCWR conditions.In this work, alloy 316L was studied in deaerated SCW at two different temperatures (400°C and 500°C) and at 25MPa in order to determine how changes in this variable influence the resistance of this material to SCC. The influence of plastic deformation in the behavior of alloy 316L to SCC in SCW was also studied at both temperatures.Results obtained from these tests have shown that alloy 316L is susceptible to SCC in supercritical water reactor conditions where the susceptibility of this alloy increases with temperature. Moreover, prior plastic deformation of 316L SS increased its susceptibility to environmental cracking in SCW.

Effect of Thermal Pretreatment on the Corrosion of Stainless Steel in Flowing Supercritical Water

The effect of high-temperature microstructure degradation (thermal ageing) on the corrosion resistance of austenitic stainless steels in supercritical water (SCW) was evaluated in this study. Mill-annealed (MA) and thermally treated (TT) samples of Type 316L and Type 310S stainless steel were exposed in 25 MPa SCW at 550°C with 8 ppm dissolved oxygen in a flowing autoclave testing loop. The thermal treatments applied to Type 316L (815°C for 1000 hr + water quench) and Type 310S (800°C for 1000 hr + air cool) were successful in precipitating the expected intermetallic phases in each alloy, both within the grains and on the grain boundaries. It was found that a prolonged time at relatively high temperature was sufficient to suppress significant compositional variation across the various intermetallic phase boundaries. This paper presents the results of the gravimetric analysis and oxide scale characterization using scanning electron microscopy (SEM) coupled with X-ray energy-dispersive spectroscopy (EDS). The role played by the fine precipitate structure on formation of the oxide scale, and thus corrosion resistance, is discussed. The combined role of dissolved oxygen and flow (revealed by examining the differences between Type 316L samples exposed in a static autoclave and in the flowing autoclave loop) is also addressed. It was concluded that formation of intermetallic phase precipitates during high-temperature exposure is not likely to have a major effect on the apparent corrosion resistance because of the discontinuous nature of the precipitation.