Masahiko Tachibana

Effects of Hydrogen Peroxide on Intergranular Stress Corrosion Cracking of Stainless Steel in High Temperature Water, (I), Effects of Hydrogen Peroxide on Electrochemical Corrosion Potential of Stainless Steel

In order to determine effects of hydrogen peroxide on stress corrosion cracking of structural materials in the primary cooling systems of boiling water reactors, a high temperature high pressure water loop with controlled hydrogen peroxide concentrations and lower possible oxygen concentrations has been fabricated. Test specimens are installed in a stainless steel autoclave which has poly tetra-fluoro-ethylene (PTFE) inner liner to prevent decomposition of hydrogen peroxide on the autoclave surfaces. Hydrogen peroxide is injected into the autoclave inlet through the injection line which also has PTFE inner liner. The concentration of hydrogen peroxide is measured at the autoclave outlet by sampling water via the PTFE-lined sampling line. More than 65% of the injected hydrogen peroxide remains at the autoclave outlet at elevated temperature (288°C). Electrochemical corrosion potential (ECP) of stainless steel is then measured in the autoclave while changing hydrogen peroxide and oxygen concentrations. From these measurements it is concluded that, at the same oxidant concentration: (1) ECP of stainless steel exposed to hydrogen peroxide is higher than that exposed to oxygen; (2) ECP is much affected by specimen surfaces; and (3) ECP shows a hysteresis pattern for on its concentration dependence. ECP of stainless steel with an oxidized surface formed under high hydrogen peroxide concentration is much higher than that with a mechanically polished surface and it is less affected by oxidant species and their concentrations.

Effects of Hydrogen Peroxide on Intergranular Stress Corrosion Cracking of Stainless Steel in High Temperature Water, (IV) Effects of Oxide Film on Electrochemical Corrosion Potential

In order to determine the effects of hydrogen peroxide on electrochemical corrosion potential (ECP) of type 304 stainless steel (SUS304), ECPs were measured using a high temperature, high pressure water loop with polytetrafluoroethylene (PTFE) inner liner at controlled hydrogen peroxide concentration. It is observed that the ECP of SUS304 exposed to hydrogen peroxide is higher than that when exposed to oxygen at the same oxidant concentration. The ECP shows a hysteresis pattern for its concentration dependency. Those results were attributed mainly from the chemical form of oxide film on stainless steel specimens. The oxide film was affected by the corrosive circumstances. Hematite (α-Fe2 O3) was observed for the specimens exposed to hydrogen peroxide, while Fe3O4 was a main oxide when exposed to oxygen. The difference of the anodic polarization curves between O2 and H2O2 environments was caused by the difference of the stability between α-Fe2O3 and Fe3O4. Since the α-Fe2O3 is reduced to the Fe2+ when hydrogen is added to water, the ECP decreases with decreasing oxidant concentration without showing the hysteresis that keep the ECP higher value.

Effects of Hydrogen Peroxide on Intergranular Stress Corrosion Cracking of Stainless Steel in High Temperature Water, (III): Crack Growth Rates in Corrosive Environment Determined by Hydrogen Peroxide

The stress corrosion cracking (SCC) of structural materials used in boiling water reactors has been studied at relatively low hydrogen peroxide (H2O2) concentrations, around lOppb, which was assumed to be representative of the corrosion environment formed in hydrogen water chemistry (HWC). The 1/4T compact tension specimen was used for measurement of crack growth rates (CGRs) of sensitized type 304 stainless steel in high temperature and high purity water. Crack length was monitored by a reversing direct current potential drop method. Since H2O2 is easily decomposed thermally, a polytetrafluoroethylene-lined autoclave was used to minimize its decomposition on the autoclave surface. The CGR in the H2O2 environment differed from that in the O2 environment even though the electrochemical corrosion potential (ECP) for both conditions was the same. The data implied that the ECP could not be used as a common environmental deterministic parameter for SCC behavior at higher potentials for different oxidant conditions. The corrosion current density was found to play an important role as an environmental index for SCC, which was given as just the current density at the ECP at a specific oxidant concentration. The CGRs were found to be written as CGR = (3.8±0.6)xl0-3 icor +(l-5±1.6) x 10-8mm/s using the calculated corrosion current density icorbelow 10-4 A-cm-2.

Effects of hydrogen peroxide on intergranular stress corrosion cracking of stainless steel in high temperature water, (V) characterization of oxide film on stainless steel by multilateral surface analyses

The difference in electrochemical corrosion potential of stainless steel exposed to high temperature pure water containing hydrogen peroxide (H2O2) and oxygen (O2)is caused by differences in chemical form of oxide films. In order to identify differences in oxide film structures on stainless steel after exposure to H2O2 and O2 environments, characteristics of the oxide films have been examined by multilateral surface analyses, e.g., X-ray diffraction (XRD), Rutherford back scattering spectroscopy (RBS), secondary ion mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS). Preliminary characterization results of oxide films confirmed that the oxide film formed under the H2O2 environment consists mainly of hematite (α-Fe2O2), while that under the O2 environment consists of magnetite (Fe3O4). Furthermore oxidation at the very surface of the film is much more enhanced under the H2O2 environment than that under the O2 environment. It was speculated that metal hydroxide plays an important role in oxidation of stainless steel in the presence of H2O2. The difference in electric resistance of oxide film causes the difference in anodic polarization properties. It is recommended that several anodic polarization curves for specimens with differently oxidized films should be prepared to calculate ECP based on the Evans diagram.

Hydrazine and Hydrogen Co-injection to Mitigate Stress Corrosion Cracking of Structural Materials in Boiling Water Reactors, (I) Temperature Dependence of Hydrazine Reactions

Hydrazine and hydrogen co-injection into reactor water is considered a new mitigation method of stress corrosion cracking in BWRs. Fundamental data such as the thermal decomposition of hydrazine, the reaction of hydrazine with oxygen and with hydrogen peroxide at temperatures ranging from 150 to 280°C are needed to evaluate suitability of this method. Reactions in bulk water were studied in a polytetrafluoroethylene pipe to separate surface reaction effects. The results were as follows. (1) The orders of the apparent reaction rate of hydrazine with oxygen were 1 and 0.5 for hydrazine and oxygen concentrations, respectively . Arrhenius parameters were k 0=69.0 s−1.μM−0.5 and (2) The orders of apparent reaction rate of hydrazine with hydrogen peroxide were each 0.5 for hydrazine and hydrogen peroxide concentrations kC 0.5 N2H4 C 0.5 H2O2 . Arrhenius parameters were k 0=1.42x 106 s−1, Ea =78.8 kJ.mol−1. Based on these data, the applicability of hydrazine and hydrogen co-injection into BWRs was considered. Hydrazine introduction to reactor water was confirmed to be accompanied by only 1% decomposition. The concentration of oxygen, which is injected to suppress the flow-assisted corrosion of carbon steel in current BWR operation, would decrease due to the reaction of hydrazine with oxygen. However oxygen concentration in feed water could be maintained at the required level if the concentration of oxygen injected in condensate water was at most doubled compared to the current operating concentration.

Effects of hydrogen peroxide on corrosion of stainless steel (II) ; Evaluation of oxide film properties by complex impedance measurement

A high temperature high pressure water loop, which can control H2O2 concentration with minimal oxygen (O2) coexistence, has been fabricated. In order to evaluate the effects of hydrogen peroxide (H2O2) on intergranular stress corrosion cracking. Not only static responses, i.e., electrochemical corrosion potential (ECP), of the stainless steel specimens exposed to H2O2 and O2 at elevated temperatures but also their dynamic responses, i.e., frequency dependent complex impedances (FDCI), were measured. The conclusions obtained by the experiments are as follows. 1. The ECP measured for the SUS 304 specimen exposed to 100ppb H2O2 reached the saturated level in 50h, showed a larger value than the specimen exposed to 200 ppb O2 and kept the same ECP level when the H2O2 concentration was decreased to 10ppb. 2. The FDCI measured for the specimen exposed to 100 ppb H2O2 showed saturation in the low frequency semicircles; this behavior was determined by the electric resistance of the oxide film and caused by saturation of oxide film thickness. Behavior for the specimen exposed to 200 ppb O2 was determined by the resistance of oxide dissolution, which was much larger than that for the specimen exposed to H2O2 3. The ECPs of the specimens exposed to 200 ppb O2 after 200-h exposure to 100 ppb H2O2 were higher than those exposed to only 200 ppb O2 due to memory effects on oxide films. The specimens with pre-exposure to 200 ppb O2 did not show these memory effects.

Effects of hydrogen peroxide on intergranular stress corrosion cracking of stainless steel in high temperature water, (II) optimization of crack propagation rate measurement system

In order to determine a crack propagation rate of less than 10-8 mm/s in a 24-hour integrated measurement, major parameters of a coupled system of a constant tension specimen and crack depth measurement, based on potential drop method, have been optimized. Influences of sensor geometry, location for detecting potential drop and data processing of the ratio of signal to noise (S/N) were optimized by applying Taguchis Method. Then a suitable sensor geometry and data processing method were proposed to get a robust measurement system with higher sensitivity and lower susceptibility for geometrical and procedural fluctuations. By applying the optimal crack propagation rate measurement system, it was confirmed that a crack propagation rate of lxlO-8 mm/s can be measured under a low concentration condition of hydrogen peroxide with less than a 20% error by a 24-hour integrated measurement.

Effects of Hydrogen Peroxide on Corrosion of Stainless Steel, (III): Evaluation of Electric Resistance of Oxide Film by Equivalent Circuit Analysis for Frequency Dependent Complex Impedances

Corrosive conditions in BWRs are determined mainly by hydrogen peroxide (H2O2). Then, a high temperature, high-pressure H2O2 water loop was fabricated to identify the effects of H2O2 on corrosion and stress corrosion cracking of stainless steel. By changing concentrations of H2O2 and O2, in situ measurements of electrochemical corrosion potential (ECP) and frequency dependent complex impedance (FDCI) of test specimens were carried out and then characteristics of oxide film on the specimens were evaluated by analyzing FDCI data based on the equivalent circuit analysis. The following points were experimentally confirmed. 1. The ECP and FDCI data of the specimens exposed to 100 ppb H2O2 were not affected by co-existing O2 with the same level oxidant concentration and they were also not affected by pre-exposure to 200 ppb O2. From the viewpoint of ECP, this meant that corrosive conditions of hydrogen water chemistry were the same as those of normal water chemistry. 2. The low frequency semi-circles of the FDCI data for the specimens exposed to 100 ppb H2O2 reached a saturation value which was much smaller than saturation values for specimens exposed to 200 ppb O2 and to 10 ppb H2O2. 3. Smaller oxide dissolution resistance and larger electric resistance of the oxide film were obtained for the specimens exposed to 100 ppb H2O2. This caused ECP to increase by shifting the anodic polarization curve of stainless steel to the high potential side.

Effects of Noble Metal Deposition upon Corrosion Behavior of Structural Materials in Nuclear Power Plants, (I) : Effect of Noble Metal Deposition with an Oxide Film on Type 304 Stainless Steel under Simulated Hydrogen Water Chemistry Condition

The effects of noble metal deposition under hydrogen water chemistry (HWC) condition on the features of oxide film formed on structural components in a reactor were studied. Noble metal-deposited type 304 stainless steel specimens with an oxide film were exposed to the simulated HWC condition, including co-existing Co radioactivity. Relationships between features of the oxide film which had two layers and the accumulation and distribution of Co radioactivity in the oxide film were established. The outer layer of the oxide film which consisted of α-Fe2O3, Fe3O4 and NiFe2O4 was dissolved by noble metal deposition and exposure to the HWC condition. The reasons for this were as follows. Solubility of α-Fe2O3, Fe3O4 and NiFe2O4 increased with the decrease of electrochemical corrosion potential. Dissolution of these compounds was accelerated by the anodic reaction of hydrogen which is catalyzed by noble metal. Co radioactivity was mainly incorporated into the inner layer. This was caused by the substitution of radioactive Co ions for ferrous ions in the oxide film, based on the observation that growth of the oxide film and oxidation of base metal stopped in the HWC condition. The inner layer consisted of FeCr2O4 which is stable at low ECP and it did not dissolve. Co radioactivity was not incorporated into the outer layer because it dissolved.

Hydrazine and Hydrogen Co-injection to Mitigate Stress Corrosion Cracking of Structural Materials in Boiling Water Reactors (IV) : Reaction Mechanism and Plant Feasibility Analysis

A calculation model has been developed in order to evaluate effectiveness of hydrazine and hydrogen co-injection (HHC) into reactor water for mitigation of intergranular stress corrosion cracking of structural materials used in boiling water reactors (BWRs). The HHC uses the strong reducing power of hydrazine radical, which is produced in the downcomer region under irradiation by γ-rays and neutrons. Some reactions and their reaction rate constants were determined based on experiments which were carried out in aerated water, hydrogenated water, and deaerated water. The calculated results were in good agreement with experimental data by a factor of two. The model was applied to a BWR and it was found that the HHC cut oxygen and hydrogen peroxide amounts dissolved in reactor water more effectively than hydrogen water chemistry alone. Thus, the required amount of hydrogen for hydrazine injection was much lower than that for hydrogen water chemistry. Consequently, electrochemical corrosion potential of structural materials could be lowered below–0:1V vs. SHE without any increase of MS line dose rate, which has been a limitation of the conventional hydrogen water chemistry. The HHC was predicted to decrease crack growth rate of structural materials by a factor of 10.