Toshio Shibata

Semiconductive behavior of passive films formed on pure Cr and Fe–Cr alloys in sulfuric acid solution

Photoelectrochemical current response and capacitance were analyzed for passive films formed on pure Cr and Fe–Cr alloys in sulfuric acid solution. The photocurrent was plotted as a photoelectrochemical action spectrum that could be separated into two components, which were mainly derived from Cr2O3 (Eg=3.6–3.7 eV) and Cr(OH)3 (Eg=2.5 eV). The band gap energy, Eg, of each component was almost constant for various applied potentials and polarization periods. The photoelectrochemical response showed negative photocurrent for most potentials in the passive region, which means that the photocurrent apparently exhibited p-type semiconductor behavior. On the other hand, Mott–Schottky plots for pure Cr exhibited positive slope, which means that passive films on Cr behave as n-type semiconductors. Furthermore, the passive films on Fe–18Cr alloy exhibited both positive and negative slopes, which means that passive films on Fe–18Cr in sulfuric acid solution have both p- and n-type semiconductive properties. These apparently conflicting results can be rationally explained, if we assume that passive films on Cr and Fe–Cr alloys formed in the sulfuric acid solution are composed of an outer hydroxide layer with an n-type semiconductor property and of a p-type inner oxide layer.

Semiconductive Properties of Passive Films Formed on Fe-18Cr in Borate Buffer Solution

Passive films formed on Fe-18Cr alloy in a borate buffer solution were studied using impedance spectroscopy and photoelectro-chemical responses. The passive film was composed of an inner oxide and covering hydroxide layer. Both layers exhibited n-type semiconductor properties with an optical bandgap energy, E g , of 3.4-3.5 and 2.4 eV, respectively. Based on the X-ray photoelectron spectroscopy results of the film structure, an electronic energy band structure model for the bilayer semiconductor was proposed. Photoirradiation altered the energy band structure, which resulted in a photocurrent transient and changes in the capacitance of the space charge layer. The differences between passive films on Fe-Cr alloys formed in a borate buffer solution and a sulfuric acid solution are discussed in terms of film growth kinetics and corrosion behavior.

Applicability of Chemical Cleaning Process to Steam Generator Secondary Side, (I) Outline of the Investigation and Cleaning Effectiveness

The application of the chemical cleaning process to dissolve and remove scales and sludge by chemicals is being planned at the Japanese pressurized water reactor (PWR) plant in order to maintain a high heat transfer condition and to prevent the steam generator tube degradation. In this paper, fundamental characteristics and cleaning effectiveness of the EPRI process and the KWU process, which are typical cleaning processes, were investigated. Both processes showed a satisfactory cleaning effectiveness for sludges and scales, and the cleaning effectiveness of the standard KWU process was improved by tailoring it to crevice cleaning conditions.

Applicability of Chemical Cleaning Process to Steam Generator Secondary Side, (II)

The application of a chemical cleaning process to dissolve and remove scale and sludge by chemicals is being planned for Japanese pressurized water reactor (PWR) plants in order to maintain a high heat transfer condition and to prevent the SG tubing degradation. In this paper, the effectiveness of the EPRI process and the KWU process to maintain integrity of SG tubing were investigated under accelerated IGSCC conditions. IGSCC susceptibility and propagation ratio of degraded SG tubing with chemical cleaning, were the same as those of tubing not subjected to the chemical cleaning process. Neither the EPRI nor the KWU process adversely affected tubing integrity.

Applicability of Chemical Cleaning Process to Steam Generator Secondary Side, (III): Effect of Chemical Cleaning on Long Term Integrity of Steam Generator Tube after Chemical Cleaning Process

The application of the chemical cleaning process to dissolve and remove scales and sludge by chemicals is being planned at the Japanese pressurized water reactor (PWR) plant in order to maintain a designed heat transfer condition and to prevent the steam generator (SG) tube degradation. In this paper, the affects of the EPRI process and the KWU process on the long term integrity of SG tubing were investigated under the simulated SG condition using a SG model boiler test facility. No adverse effect of the both chemical cleaning processes on the long term integrity of SG tubing were observed.

Applicability of Chemical Cleaning Process to Steam Generator Secondary Side, (IV): Comprehensive Applicability Evaluation of Chemical Cleaning and Its Effect on Integrity of Other Structural Materials Other than Steam Generator Tubes

The application of chemical cleaning for dissolving and removing scale and sludge is being planned in the Japanese pressurized water reactor (PWR) plant in order to maintain high heat transfer performance and to prevent steam generator (SG) tube degradation. In this paper, the effectiveness of the Electric Power Research Institute (EPRI) and German Kraftwerk Union (KWU) processes on the integrity of structural materials other than SG tubes and the comprehensive applicability of chemical cleaning are discussed. The integrity of structural materials such as carbon steel, low-alloy steel and stainless steel was maintained after the EPRI and KWU processes. KWU chemical cleaning tailored for crevice cleaning has been studied to improve its cleaning effectiveness in crevices and to control the corrosion depth of structural materials less than the criterion for corrosion depth.

Atmospheric Corrosion of Electroplated Cu Thin Film in Moist Oxygen Environment

Atmospheric corrosion of electrodeposited Cu thin film in water vapour with oxygen was studied using a quartz crystal microbalance. The adsorption of water with N2 carrier gas without oxygen was reversible and roughly proportional to the relative humidity. The corrosion weight gain exhibited an initial linear rate law region followed by a parabolic rate law one. The rate for both regions was influenced by both relative humidity and O2 partial pressure. The electrochemical oxidation process of Cu might occur under an adsorbed water layer. It is considered that the water adsorbed as clusters and corrosion progressed under the water cluster, the thickness and surface coverage of which varied with relative humidity. The rough estimation revealed that the kinetic constants for both first linear rate and parabolic rate laws is proportional to the ratio of surface coverage by water cluster. However, the rate constant is independent of R.H., if normalized by the area of water clusters.