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Methanogens capable of accepting electrons from Fe

The effects of Cu addition on corrosion behavior of stainless steels are known to be controversy1,2, which depends on the corrosion environment or the amount of alloying Cu. In chloride environment, alloying Cu is known to prohibit the growth of pitting corrosion. The dissolved Cu2+ acted as an inhibiter inside the pits3. On the other hand, there are few studies about the effect of Cu2+ ions in the solution on the pitting corrosion of the stainless steels. Since the commercially made stainless steels commonly contains more than 0.1 mass% Cu, the Cu2+ ions which was dissolved from stainless steels made it difficult to evaluate the effect of Cu2+ ions on the pitting behavior of the stainless steels. Although the amount of dissolved Cu2+ was quite few, the Cu2+ concentration inside the pits was expected to be much higher than the solution bulk because the pitting corrosion was very localized phenomenon. Therefore, Cu2+ in the solution and dissolved Cu2+ should be analyzed separately. To clarify the effect of Cu2+ in the solution on the pitting corrosion of the stainless steels, the amount of alloying Cu is needed to be low enough not to affect the corrosion morphology. In this study, extra high purity Type 316 stainless steel (316EHP) was used as specimen. The Cu content of the specimen was 6.8 × 10-4 mass %. The specimens were heat-treated at 1373 K for 3.6 ks and then quenched in water. The surfaces of the specimens were polished down to 1 µm with a diamond paste. After that, specimens were ultrasonically cleaned in ethanol. Electrochemical measurements were conducted in deaerated 0.1 M NaCl or 0.098 M NaCl-1 mM CuCl2. Except the electrode area (ca. 10 × 10 mm), the surfaces of the specimens were insulated by an epoxy resin. The potential scan rate was 3.33 × 10-4 V s-1 (20 mV min-1). The reference electrode was an Ag/AgCl (Sat. KCl) electrode. All the potentials cited in this work refer to the Ag/AgCl (Sat. KCl) electrode. After the electrochemical measurements, the surfaces were analyzed by SEM/EDS and XPS. The inclusions of the specimen were characterized by SEM/EDS. The inclusions were identified as a Cr-oxide. The anodic polarization curves were measured in deaerated 0.1 M NaCl or 0.098 M NaCl-1 mM CuCl2. Although no pitting was observed in 0.1 M NaCl, pitting was occurred in 0.098 M NaCl-1 mM CuCl2. In both solutions, the dissolution of inclusions was not confirmed. Therefore, the inclusions were not thought to be the initiation site of pitting. This indicates that the pitting potential of 316EHP was decreased by Cu2+ in the solution. Cu2+ ions were thought to act as an accelerator of pitting corrosion. To analyze the effect of Cu2+ ions on the forming passivation film, potentiostatic polarization measurements at 0.4 V were carried out in 0.098 M NaCl-1 mM CuCl2. As a result, existence of a Cu signal on the surface was confirmed by XPS although it was not detected by SEM/EDS. This result indicatets that the few amounts of Cu compound was deposited on the surface in passive region of the 316 EHP.

Gamma radiolysis experiments on solutions of a mixture of sodium chloride (NaCl) and sodium bromide (NaBr) were conducted to confirm the validity of radiolysis calculations for simulated seawater s...

The effects of crystal textures and the potentials in the anodic oxidation of zirconium in a boiling nitric acid solution were investigated to study the stress corrosion cracking of zirconium in nitric acid solutions. The test specimen was machined such that the specimen surface was parallel to the rolling surface, arranged with a (0002) crystal texture. The potentials applied for the anodic oxidation of zirconium were set at 1.2, 1.4, and 1.5 V against a saturated KCl–Ag/AgCl electrode (SSE) in boiling 6 M HNO3. The growth of the zirconium oxide film dramatically changed depending on the applied potential at a closed depassivation potential (1.47 V vs. SSE in this study). At 1.5 V, the zirconium oxide film rapidly grows, and its growth exhibits cyclic oxidation kinetics in accordance with a nearly cubic rate law. The zirconium oxide film grows according to the quantity of electric charge and the growth rate does not depend on the crystal texture in the pretransition region before the cyclic oxida...

Radiolysis calculations of simulated seawater were conducted using reported data on chemical yields and chemical reaction sets to predict the effects of seawater constituents on water radiolysis. H...

It is well known that some certain metal ions in boiling nitric acid cause corrosion acceleration of stainless steel. Neptunium ion from fission products contained in reprocessing solution is known as corrosion accelerator of the stainless steel. Neptunium has several oxidation states (IV, V and VI) in nitric acid solution (up to 3M-HNO3). The metal ions with the REDOX reaction having near the potential of nitric acid promote corrosion of stainless steel but it is not clear why slight amount of the metal ion accelerates corrosion in boiling nitric acid solution. Simultaneous evaluating the coexisting oxidation state Np(IV), Np(V) and Np(VI) and the electrochemical behaviors on stainless steel in nitric acid solution is required in order to understand the corrosion acceleration mechanism. Specially designed electrochemical test cell integrated with optical cell for spectroscopic analysis was developed for this purpose. From results of electrochemical tests, cathode reaction on stainless steel was activated by Np ions. Np(VI) ion made corrosion potential shift nobler than Np(V) and nobler corrosion potential causes accelerating corrosion of stainless steel in nitric acid solution. From results of spectroscopic analysis for oxidation states of Np ion, Np(V) was easily oxidized to Np(VI) in nitric acid solution and Np(VI) was stable state in boiling 3M-HNO3. It was considered that role of Np ions was mediator to accelerate corrosion due to activating cathode reaction and re-oxidizing cycle in boiling 3M-HNO3.

An application of a cathodic protection method with an impressed current system to control the corrosion of austenitic stainless steel in a boiling nitric acid solution was studied to improve corrosion resistance and to extend the operation life of components in a fuel reprocessing plant. Plate-type specimens made of ultralow carbon type 304 stainless steel (SUS304ULC) were immersed in 3 mol·dm−3 boiling nitric acid solutions including 10 and 1.7 g·dm−3 vanadium ions. Electrochemical potentiostatic tests and cathodic protection tests were performed using electrochemical test cells. The selected protective potential was below the transition potential between the passive and trans-passive states based on anodic polarization measurement. Corrosion rates in the solution with and without the protection were measured by potentiostatic tests. Additionally, the outer surface of the tube-type specimen of SUS304ULC was studied under the same condition. From the obtained results, corrosion rates of plate-type specimens with cathodic protection were observed to decrease by 1/40 and 1/10 those of the specimens without cathodic protection in the solutions including 10 and 1.7 g·dm−3 vanadium ions, respectively. In the case of tube-type specimens, outer surface thickness loss was decreased from 24 to 3 μm by the protection, and platinum was chosen as the anode because it showed no corrosion loss like gold and no cracking like zirconium. Authors concluded that the cathodic protection method can be expected as one of the methods of maintaining components in a fuel reprocessing plant.

To evaluate the component life in a spent nuclear fuel reprocessing plant, a large-scale mock-up test apparatus of a reduced pressurized thermosiphon evaporator was constructed, and the corrosion mechanism of a heat transfer tube made of ultralow carbon type 304 stainless steel in boiling nitric acid solution was studied. The corrosion tests were conducted for about 36,000 h, and changes in the corrosion amount and rate in the test duration were discussed. The relationships between the amount of corrosion and tube surface temperature and heat flux were investigated, and the corrosion propagation mechanism considering intergranular penetration was studied based on the observations of morphologies of corrosion surfaces and the measurements of intergranular penetration depths. After a long duration, the increases in the corrosion amount and rate saturated when intergranular penetration and grain dropping occurred by turns. This result means that a linear estimation can be applied to the life prediction for c...

Raman spectra have obtained for {alpha}- and {beta}-NaFeO{sub 2}, Na{sub 3}FeO{sub 3}, Na{sub 8}Fe{sub 2}O{sub 7}, Na{sub 5}FeO{sub 4}, and Na{sub 4}FeO{sub 3} from room temperature up to 833 K in the powder state under an inert atmosphere. The comparison of the spectra showed good applicability of Raman spectroscopy to the identification of these sodium sodium iron double oxides. We have therefore investigated the corrosion reaction of a steel surface with Na{sub 2}O{sub 2} powder using in situ Raman spectroscopy. The obtained spectra showed that a corrosion reaction occurs under 833 K, and the corrosion product was identified as Na{sub 5}FeO{sub 4}. A reaction mechanism for the corrosion based on the above results i proposed as follows: the system of Fe + Na{sub 2}O{sub 2} produces corrosive melt under 833 K; it spreads over the surface of iron; the melts with a Na-rich composition produce Na{sub 5}FeO{sub 4} in the initial state; the corrosion products distribute homogeneously on the surface. The corrosion reaction in a homogeneous powder mixture of Fe and Na{sub 2}O{sub 2} has also been investigated using Raman spectroscopy with the help of XRD and DTA methods. The corrosion products were identified as double oxides whose compositions correspond to the stoichiometric molar ratio of Na to Fe in the starting materials, while the products from the surface reaction of steel with Na{sub 2}O{sub 2} tended to have a specified composition of Na{sub 5}FeO{sub 4}. The difference was reasonably explained using the mechanism presented above. It was found from the DTA measurements that Na{sub 2}O{sub 2}, in contrast to Na{sub 2}O and NaOH, has particularly strong corrosivity for iron. (author)

The microstructural changes of type 304 stainless steel in the creep-dominant condition under creep-fatigue loading were investigated and the representativeness of some metallographical damage parameters (MDPs) was studied. A series of creep-fatigue tests was conducted at low total strain range and long strain hold time. Some of these were interrupted during the periods of life cycle. Cross-section of all tests specimens were observed with a scanning electron microscope. Microstructures such as cavities, precipitate nucleation sites and internal microcrack initiation along grain boundaries were investigated. Cavity segregation phenomena which occur in an angular relationship between cavitated grain boundaries and the loading direction were also studied. Then the mechanism of cavity initiation and growth up to microcrack initiation was analysed. The number of cavities per unit area (NC), mean cavity area (MCA) and area fraction of cavity per unit area (AFC) were measured and studied in respect of their suitability to evaluate the damage under the present condition. Finally a more suitable MDP which should be possible to relate with both cavitation and grain boundary deformation quantitatively is discussed.

Titanium and Ti-5mass%Ta alloy has been utilized in nuclear fuel reprocessing plant material because of its superior corrosion resistance in nitric acid solutions. However, Ti alloy have been known to high susceptibility of hydrogen embrittlement. To evaluate properties of hydrogen absorption and hydrogen embrittlement of Ti alloys, cathodic polarization tests and slow strain rate tests (SSRT) under cathodic polarization were carried out. Results show titanium hydrides covered on the surface of metals and hydrides thickness were within 10μm. But hydride did not observed at inner part of metals. Ti and Ti-5%Ta did not show hydrogen embrittlement by SSRT under cathodic charging. These results suggested that Ti and Ti-5%Ta could absorb hydrogen. But hydrogen did not penetrate inner portion of the metals more than 10μm in depth because titanium hydrides act as barrier of hydrogen diffusion. It is considered that retardation of hydrogen diffusion hindered hydrogen embrittlement of Ti and Ti-5%Ta alloys.Copyright © 2013 by ASME

Fuel reprocessing plant is important for atomic energy cycle which is aimed to re-use uranium (U) and plutonium (Pu) from the spent nuclear fuels after barning-out in light water reactor. PUREX process, in which boiling nitric acid is used for dissolving fuel elements, is adopted in Japan. Many structural materials are used stainless steels protected by passive layers of the surface. At the beginning of developing the plant, many troubles of intergranular corrosion of stainless steels were experienced. According to the investigations of these problems, very low carbon type austenitic stainless steels were utilized for the materials. However, these stainless steels were corroded in some cases. For example, addition of hexavalent chromium (Cr (VI)) is heavily accelerated corrosion rate of these stainless steels in boiling nitric acid solutions. The corrosion potential of stainless steel in Cr (VI) added solution is much higher than that in pure nitric acid solution. The authors introduced that heptavalent vanadium (V (V)) was also accelerated corrosion rate of these stainless steels but the tendency of acceleration process was different from the case of containing Cr (VI). We analyzed the valence stats of Cr and V during corrosion test and showed that Cr (VI) reduced to Cr (III) with corrosion of stainless steel propagating. On the other hand, amount of V (V) was not changed during the corrosion test. It was concluded that V (IV) was easily re-oxidized in boiling nitric acid solution. Namely, V (V) is reduced to V (IV) on stainless steel surface by cathodic reaction of corrosion and then V (IV) is re-oxidized immediately to V (V) in the balk nitric acid solution. These data were obtained using non-radioactive solutions in the ordinary laboratory. The present work is aimed to compare the data of re-oxidation rate of actinides which are included in the actual solutions used in a reprocessing plant. We implemented a specially designed electrochemical test cell used for analyzing the valence states of actinide cations, i.e. neptunium (Np) and Pu, in a boiling nitric acid solution. The cell was set into glovebox in a hot laboratory which could be handled nuclear fuel materials. The cell was designed for minimizing the solution. The amounts of solution used are about 15 cm3 and sample size of working electrode is about 4 mm2. Electrochemical measurements and spectrum analyses were conducted with this apparatus. Analyzed cations were Cr (VI), V (V), Pu (VI) and Np (VI). Polarization curves showed that corrosion potentials of stainless steels changed to higher direction by addition of these cations and also corrosion current densities increased. The higher order of those is Cr (VI), Np (VI), V (V) and Pu (VI). Re-oxidation rates were obtained by the changes of absorption spectra with time using lower valence state ions added solutions, i.e. Cr (III), V (IV), Pu (IV) and Np (V). These species are re-oxidized to Cr (VI), V (V), Pu (VI) and Np (VI), respectively. Re-oxidation rates of them were very much different among them. Very fast re-oxidation rates were obtained by the V (IV) and Np (V) added solutions. A very low re-oxidation rate was obtained by the Cr (III) added solution. The Pu (IV) added solution was in the middle of them. The corrosion of stainless steel is controlled by corrosion potential of cation added solution at the initial stage and controlled by re-oxidation rate of cation as time passed. The element Np which indicates high corrosion potential and has fast re-oxidation rate influences too much the corrosion of stainless steel in boiling nitic acid solution among these elements in consideration.

For long-term creep-fatigue of Type 304SS, intergranular failure is dominant when significant life reduction occurs. This phenomenon has its origin in grain boundary sliding as same as that in cavity type creep failure. Accordingly a simplified procedure to estimate intergranular damages caused by grain boundary sliding is presented in connection with the secondary creep.In the conventional ductility exhaustion method, fracture ductility includes plastic strain, and damage estimation is based on primary creep recoverable during strain cycling. Therefore, the accumulated creep strain becomes a very large value, and is quite different from grain boundary sliding strain. As a new concept on ductility exhaustion, the products of secondary creep rate and time to rupture (e2tR) is applied to fracture ductility, and grain boundary sliding is estimated using the accumulated secondary creep strain. It was shown that the time fraction rule and the conventional ductility exhaustion method can be derived analytically from the new concept.Furthermore an advanced method on cyclic stress relaxation was examined. If cyclic plastic strain hardening is softened thermally during strain hold, softening of the primary and secondary creep can occur. And the unrecoverable accumulated primary creep strain causes hardening of the primary creep. The reduction of deformation resistance to the secondary creep accelerates the grain boundary sliding rate, and increases creep damage.The new concept ductility exhaustion method based on the above consideration gave good life prediction for the intergranular failure mode.

In PUREX process for spent fuel reprocessing plants, nitric acid solutions are treated in many components. Particularly, heating portions in the components are severely corroded in the boiling solution under heat transfer (HT) conditions. Although corrosion behavior under the conditions has been investigated by many researchers, corrosion mechanism has not been sufficiently understood. Consequently, because of its safety, we need to study dominant factors of corrosion mechanism and to develop a method to predict corrosion progress.Surface temperature, heat flux, concentration and compositions of the solution have been previously considered as important factors for corrosion rate (CR). In this paper, authors have focused on the effects of surface temperature and heat flux on CRs of stainless steels in boiling nitric acid under HT conditions. We performed experimental study in consideration of dissolvers and concentrators. Two types of cells for HT and immersion conditions were applied for corrosion tests and the effects were compared. Test solution used was 33 mol·m−3 (0.033 M) vanadium added to 3 kmol·m−3 (3 M) nitric acid solution. The solution was heated at boiling temperature under atmospheric pressure. Additionally, a boiling curve which was indicated the relation between heat flux and degree of superheating was investigated experimentally. Surface temperatures during corrosion tests were estimated from a boiling curve. The relationship among measured CRs, surface temperature and heat flux were studied.The results showed that CR did not depend on heat flux, but depended on surface temperature. Arrhenius plots on CRs indicated that CR was accelerated by solution boiling against non-boiling. The cause of the acceleration of CR under boiling nitric acid solution was discussed.Copyright © 2012 by ASME

Creep-fatigue is a fatal failure mode of the high temperature structural materials of liquid metal fast breeder reactors (LMFBRs). In this report, two important issues are discussed for creep-fatigue evaluation of normalized and tempered modified 9Cr1Mo (modified 9Cr1Mo(NT)) steel which is a promising structural material for the steam generator of large-scale LMFBRs in Japan. Several evaluation methods based on the ductility exhaustion concept are discussed for the prediction of tension strain hold creep-fatigue damage of this material. A time-fraction type of linear damage summation concept based on a new ductility exhaustion theory is proposed from the point of view of its appropriate conservatism for time extrapolation and its simplicity. Also, a life reduction mechanism of low cycle fatigue with strain hold at the compression side is discussed, based on the data observed by a scanning type electron microscope. Creep damage or the tension mean stress caused by compression strain hold hardly reduce the low cycle fatigue life of this material. A new concept based on the location of oxidation on the test specimen surface can explain the reduction in low cycle fatigue life of modified 9Cr-1Mo(NT) steel.

The pilot system on the distributed database for advanced nuclear materials is named “Data-Free-Way”. The project started in April 1990, and the material database including metals, ceramics and composites will be established within five years under the cooperation of the National Research Institute for Metals (NRIM), Japan Atomic Energy Research Institute (JAERI) and Power Reactor and Nuclear Fuel Development Corporation (PNC). For the pilot system, the method of sharing data, data structure and examples of the easily accessible search of materials properties are shown. Furthermore, the future trials of attractive/sophisticated utilization focused on the issues relating to fusion materials are described.

The long term creep-fatigue life of type 304 stainless steel was evaluated by the creep-fatigue life prediction method based on a linear damage fraction rule. The displacement controlled creep-fatigue tests were carried out, and the time to failure of longer than 10000 hours was obtained. The creep damage of long term creep-fatigue was evaluated by taking into account the stress relaxation behavior with elastic follow-up during the hold period. The relationship between life reduction of creep-fatigue and fracture mode was provided by the creep cavity growth. The results of this study are summarized as follows;(1) The long term creep-fatigue data can be reasonably evaluated by the present method. The predicted lives were within a factor of 3 of the observed ones.(2) The present method provides the capability to predict the long term creep-fatigue life at lower temperatures as well as that at the creep dominant temperature.(3) The value of creep damage for the long term creep-fatigue data increased by elastic follow-up. The creep-fatigue damage diagram intercepted between 0.3 and 1 can represent the observed creep-fatigue damages.(4) The cavity growth depends on the hold time. The fracture of long term creep-fatigue is caused by the intergranular cavity growth. The intergranular fracture of creep-fatigue is initiated by the cavity growth and followed by the microcrack propagation along grain boundaries starting from creep cavities.

The structural materials of the instruments used in Fukushima Daiichi nuclear power station are exposed to the corrosive condition under the gamma-ray irradiation. In order to determine the effects of the temperature of water under gamma-ray irradiation on the corrosion potential of stainless steel, the measurements of the corrosion potential under gamma-ray irradiation condition was performed. The dissolved oxygen concentration was aerated condition in each temperature in this study. And the corrosive condition under the irradiation was evaluated by the water radiolysis calculation and the measurements of the included hydrogen peroxide in the water after the irradiation. As parameters, the temperature of water and the gamma-ray dose rate were changed. The major results are listed as follows; (1) The corrosion potentials rose about 400 mV by the gamma-ray irradiation in all temperature conditions. (2) The potential was low with higher temperature under irradiation with the aerated condition. It was assumed that the lower dissolved oxygen concentration with higher temperature will cause the dependency of the corrosion potential on the temperature under irradiation. (3) The generation of some ppm of hydrogen peroxide was confirmed by the measurements of hydrogen peroxide concentration. It was indicated that the increase of the corrosion potential by the irradiation was caused by the generation of the high concentration of hydrogen peroxide. (4) It was confirmed that the generation of hydrogen peroxide was lower in higher temperature condition due to the lower dissolved oxygen concentration in high temperature. It was indicated that lower potential in high temperature water was caused by not only lower dissolved oxygen but also lower generation of hydrogen peroxide.

In order to clarify the acceleration effect of corrosion with small amount of oxidizing ion addition in boiling nitric acid solution, Cr(VI) and V(V) ions added solution were examined Cr(VI) and/or V(V) added nitric acid solutions accelerated corrosion rates of SUS304ULC. The acceleration effect of Cr was larger than that of V. Although the acceleration effect of Cr rapidly decreased, that of V stayed constant. Concentration of Cr(VI) in the solution was decreased correspond to the corrosion rate. Re-oxidation of V is faster than that of Cr. Cr(VI) ions are reduced to Cr(III) by corrosion reaction of SUS340ULC but a little amount of them are re-oxidized. On the other hand, V(V) ions are also reduced to V(IV) and almost of all of them are re-oxidized to V(V). So, corrosion rate of V added solution stayed constant and that of Cr added decreased.