Nobuyoshi Hara

Microelectrochemical Measurements of Dissolution of MnS Inclusions and Morphological Observation of Metastable and Stable Pitting on Stainless Steel

The electrochemical dissolution of MnS inclusions and pit initiation processes on type 303 stainless steel were investigated by an electrochemical microcell technique. In 1 M Na 2 SO 4 , the electrochemical dissolution of MnS inclusions started at 0.1-0.3 V vs Ag/AgCl(3.33 M KCl), which was followed by the initiation of a hemispherical and smooth wall pit at MnS/matrix boundary. It was suggested that the dissolution products of MnS inclusions induced the formation of a salt film, which caused localized electropolishing. In 0.1 M NaCl, the onset potential of electrochemical dissolution of MnS inclusions was 0.3-0.4 V and the stable pit growth occurred at a potential of around 0.5 V. Field-emission scanning electron microscopy observations revealed that many metastable pits with a diameter of approximately 1 μm were formed at MnS/matrix boundary on the specimen, of which the polarization measurement was stopped immediately after stable pit initiation. From a morphological point of view, the metastable pits were found to be very similar to flat-walled etch pits. The dissolution products of MnS inclusions and chloride ions would attack the metal surface newly exposed by dissolution of the inclusions. It is proposed that the chemistry of dissolved species from MnS inclusions have a substantial influence on pit initiation at MnS inclusions.

Development of a Wide Range pH Sensor based on Electrolyte-Insulator-Semiconductor Structure with Corrosion-Resistant Al2 O 3 ­ Ta2 O 5 and Al2 O 3 ­ ZrO2 Double-Oxide Thin Films

In order to develop a pH sensor having a wide pH response range from strong acid to strong alkali, electrolyte-insulator-semiconductor capacitors using corrosion-resistant Al 2 O 3 -Ta 2 O 5 and Al 2 O 3 -ZrO 2 double-oxide films as the insulator were fabricated and their response characteristics to H + and alkali metal ions were examined. The sensors with Al 2 O 3 -Ta 2 O 5 and Al 2 O 3 -ZrO 2 films having an Al 3+ cationic fraction of about 0.4 showed a wide linear pH response range of -1.5 to 13.0 and a small alkali metal ion sensitivity. The use of the Al 2 O 3 -Ta 2 O 5 film is suitable for increasing pH sensitivity and that of the Al 2 O 3 -ZrO 2 film is promising for increasing corrosion resistance against strong alkali solutions.

Hydrogen Gas Sensor Using Pt- and Pd-Added Anodic TiO2 Nanotube Films

TiO 2 nanotube films were formed on pure Ti, Ti-Pt, and Ti-Pd thin films by anodization in a mixture of glycerol and water (1:1 volume ratio) containing 0.5 wt % NH 4 F and were applied as a resistance-type hydrogen gas sensor. A transmission electron microscopy analysis of the TiO 2 nanotube films revealed that the nanosized particles of Pt and Pd were dispersed in the wall of the nanotube. These dispersed Pt or Pd particles effectively improved the performance of the hydrogen gas sensor perhaps due to the acceleration of hydrogen chemisorption on the wall of the nanotube. Pt- and Pd-added TiO 2 nanotube sensors showed a two-order decrease in resistance upon exposure to 1000 ppm H 2 at 290°C and had little or no response to 1000 ppm CH 4 , 1000 ppm CO, and 1 ppm SO 2 .

Ellipsometric analysis of growth process and corrosion resistance of Nb[sub 2]O[sub 5] films formed by MOCVD

The formation process of Nb[sub 2]O[sub 5] films by metallorganic chemical vapor deposition using pentaethoxy niobium (PEN: Nb(OC[sub 2]H[sub 5])[sub 5]) as a vapor source and O[sub 2] as reactant gas has been analyzed by in situ ellipsometry at substrate temperatures of 200 to 500 C, and at O[sub 2] flow rates of 0 to 2,000 cm[sup 3]/min. According to the changes in the refractive index and the growth rate of the films, the formation process can be divided into three stages, that is, the nucleation and coalescence, the homogeneous growth, and the surface roughness evolution stages. The refractive index and the growth rate of the films were found to depend on the substrate temperature and the O[sub 2] flow rate. The films deposited at 200 to 400 C had amorphous structures, and those deposited at 450 to 500 C contained a [delta]-Nb[sub 2]O[sub 5] phase in an amorphous matrix. The corrosion resistance of the films in a buffered HF solution depended on both the deposition temperature and the O[sub 2] flow rate. The film deposited at 450 C in the absence of O[sub 2] showed the highest corrosion resistance against the solution.

Microelectrochemical Investigation of Anodic Polarization Behavior of CrS Inclusions in Stainless Steels

The anodic dissolution behavior of CrS inclusions was compared with that of (Mn,Cr)S inclusions containing around 10 atom Cr using a microelectrochemical cell in NaCl solutions. CrS inclusions were resistant to pit initiation compared with (Mn,Cr)S inclusions. The dissolution potential of CrS inclusions was in the transpassive region of stainless steels. Stable pits were initiated in this potential region. (Mn,Cr)S inclusions dissolved in the passive region of stainless steels, providing lower pitting potential than those of CrS-containing stainless steels. Auger electron spectroscopy analysis indicated that chromium-containing oxide film were formed on the CrS inclusions and grew with electrode potential, which suggested that the oxide films on CrS inclusion prevented the anodic dissolution of the inclusions, thereby leading to an increased resistance to pit initiation.

In Situ Ellipsometric Analysis of Growth Processes of Anodic TiO2 Nanotube Films

The growth process of TiO 2 nanotube films formed on Ti by anodization in NH 4 H 2 P0 4 -NH 4 F electrolytes has been studied by electrochemical measurements, field emission-scanning electron microscopy (FE-SEM) observations, and ellipsometric analyses. The results of electrochemical measurements and FE-SEM observations showed that the change in current density during anodization partly reflects the formation process of nanotube films but does not give detailed information on the film growth process. Ellipsometry could be successfully applied to monitor the growth of nanotube films in situ. Ellipsometric data (Δ and ψ) obtained during anodization in 0.5 wt % NH 4 F revealed that the growth process of TiO 2 nanotubes can be divided into four stages, that is, the barrier layer growth (1st), the outer nanoporous layer formation and inner nanotube layer growth (2nd), the pure nanotube growth (3rd), and the tube mouth dissolution (4th) stages. The outer nanoporous layer in the 2nd stage is formed by F - -induced breakdown of the initially grown barrier layer. The final 4th stage is characterized by an inward spiral of the Δ-ψ plot and a significant reduction of the amplitude of a cyclic change in A. On the basis of these characteristics of ellipsometric data, one can control the end point of anodization for preparing ordered TiO 2 nanotubes.

Corrosion characteristics of Fe2O3Cr2O3 artificial passivation films under potentiostatic control

Fe 2 O 3 -Cr 2 O 3 composite oxide thin films, that is, artificial passivation films, with various Cr 2 O 3 content were formed on Pt substrates by a MOCVD method. The rate of change of film thickness as a function of potential was examined in 1 kmol m -3 H 2 SO 4 using in-situ ellipsometry under potentiostatic control. The rate of thinning can be divided into four characteristic ranges according to potential ; i.e. the range of the reduction dissolution of Fe 2 O 3 component, the range of no dissolution, the range of the field-assisted dissolution of Fe 2 O 3 component, and the range of the oxidation dissolution of Cr 2 O 3 component. The range of no dissolution of Fe 2 O 3 Cr 2 O 3 composite films was named the intrinsic passivity. The anodic polarization curve for an Fe-18Cr alloy in passive and transpassive potentials was also divided into four characteristic ranges which correspond to the ranges for the artificial passivation films. The dissolution behavior of passive and transpassive films can be explained based on the origin of the four ranges. It was presumed that passive films on Fe-Cr alloys exhibit their intrinsic nature in the potential range of the intrinsic passivity.

Initiation and Propagation of Stress Corrosion Cracking of Stainless Steel Canister for Concrete Cask Storage of Spent Nuclear Fuel

Abstract Resistance to atmospheric stress corrosion cracking (SCC) and crevice corrosion was examined for various candidate canister materials in the spent fuel dry storage condition using concrete casks. A constant load SCC test was conducted on the candidate materials in air after deposition of simulated sea salt particles on the specimen gauge section. UNS S31260 and UNS S31254, highly corrosion-resistant stainless steels (SS), did not fail for more than 46,000 h at 353 K with relative humidity of 35%, although UNS S30403 and UNS S31603 SS failed around 500 h by SCC. Crack growth measurement was done to explain this result. Cracking on S31603 propagated around 3 × 10−10 m/s at K values larger than 10 MPa·m0.5 at 353 K with RH = 35%. S31260 SS showed a crack growth rate of 4 × 10−13 m/s at the same condition. Crevice corrosion potentials of S31260 and S31254 SS became larger than 0.9 V vs. saturated calomel electrode (SCE) in synthetic seawater at temperatures below 298 K, while those of S30403 and S316...

In Situ Ellipsometry Analysis on Formation Process of Al2O3-Ta2O5 Films in Ion Beam Sputter Deposition

Al 2 O 3 -Ta 2 O 5 thin films are known as highly corrosion-resistant materials for coatings. Here the formation process of Al 2 O 3 -Ta 2 O 5 films was analyzed by in situ ellipsometry during deposition by ion beam sputtering. Composite targets composed of Al 2 O 3 and Ta 2 O 5 plates were used for sputter deposition. During deposition, the formation process of thin films was monitored by a single-wavelength rotating analyzer ellipsometer attached to the sputtering system. The composition, the chemical binding states of constituent elements, and the depth profile of the elements of the films were analyzed by inductively coupled plasma-atomic emission spectroscopy, X-ray photoelectron spectroscopy (XPS), and auger-electron spectroscopy (AES), respectively. Ellipso-metric analysis showed that the thickness of the film increased linearly with time after short induction periods. The refractive index of the films increases with increasing tantalum cationic fraction. The extinction coefficient of the films was larger than those of single Al 2 O 3 and Ta 2 O 5 films. The XPS analysis showed that the preferential deposition of the Ta 2 O 5 component occurred in the induction period and the codeposition of Ta 2 O 5 and Al 2 O 3 in the linear growth stage. The AES analysis showed that the content of constituent elements was homogeneous from the top to the bottom of the films in the linear growth stage.

Corrosion and Electrochemical Properties of Sn-8% Zn Alloy-Coated Steel in Methanol Containing H 2 O , NaCl, and HCOOH

Corrosion and electrochemical properties of Sn-8% Zn alloy-coated steel prepared by a melt-dipping method have been investigated in methanol containing H 2 O, NaCl, and HCOOH, which are known as contaminants in methanol fuel. The coated steel showed high corrosion resistance in methanol containing 0.1-30% H 2 O and 0.1-30% H 2 O + 0.1% NaCI. However, the coated steel showed slight corrosion in methanol containing 0.1-30% H 2 O + 0.1% HCOOH. Polarization curves measured in methanol containing 0.1-30% H 2 O + 0.1% NaCl showed that the corrosion potential of the coated steel was close to that of Zn, while the pitting potential of the coated steel was close to that of Sn. Polarization curves of the coated steel measured in methanol containing 0.1-30% H 2 O + 0.1% HCOOH showed that both anodic and cathodic currents monotonously increased from corrosion potentials. This behavior was very similar to that of Zn. According to Auger electron spectroscopy and X-ray photoelectron spectroscopy analyses, the surface film of the coated steel after an immersion corrosion test in methanol containing 0.1% H 2 O + 0.1% NaCI was composed of a large amount of oxide and hydroxide of Zn and a small amount of those of Sn. It was presumed that the corrosion of the coated steel in methanol containing H 2 O, NaCI, and HCOOH proceeded through the selective dissolution of the Zn component in the coating layer.