Izumi Muto

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.

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 .

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.

Improvement of Pitting Corrosion Resistance of Type 316L Stainless Steel by Potentiostatic Removal of Surface MnS Inclusions

The beneficial effect of the removal of MnS inclusions on the pitting of stainless steels has been demonstrated in two ways. (1) High-purity Type 316L stainless steel with no inclusions was used as a specimen in the measurement of anodic polarization curves in 0.5 M NaCl and (2) commercial Type 316L stainless steel with MnS and slag-related inclusions was first polarized at different potentials for 30 min in 1 M Na2SO4 of pH 3 and then anodic polarization measurements were taken in 0.5 M NaCl. Pitting did not occur in the passive or transpassive region of the high-purity steel. The polarization treatment dissolved MnS and some oxide inclusions (CaO and SiO2) on the surface of the commercial steel. An increase in pitting potential of the commercial steel was noted after treatment at potentials above 0.2 V. At the same time, the number of current spikes due to metastable pits decreased significantly. These results are more likely due to the beneficial effect of removing MnS inclusions from the steel surface rather than the modification effect of the chemical composition of passive films on the surface.

Cut Edge Corrosion Inhibition by Chromate in Primer of Prepainted 55% Al–Zn Alloy Coated Steel

The effect of chromate in primer layers on the cut edge corrosion of prepainted 55% Al―Zn alloy coated (galvalume) steels has been examined under simulated atmospheric corrosion conditions in a marine environment. The chromate released from the primer layers in effect prolonged and expanded the sacrificial protection of the 55% Al―Zn layers. The corrosion potentials, measured by a Kelvin probe, indicated that the cut edges were maintained at low potentials. The galvanic current measured for the 55% Al―Zn/steel couple specimen with the chromate-containing primer was lower than that of the specimen with the chromate-free primer. This concurs with the results of conventional polarization measurements, indicating that the cathodic oxygen reduction on steel substrates is inhibited by the presence of chromate. The corrosion potential variations across the interface between 55% Al―Zn layers and steel substrates were measured using Kelvin probe force microscopy (KPFM). Almost negligible variations in the KPFM potential were found at the interface in the cut edges with the chromate-containing primer. Our findings indicate that the prolonged and expanded sacrificial protection effect of the 55% Al―Zn layers is likely due to the release of chromate from the primer layers, which makes it possible for the 55% Al―Zn layers to dissolve slowly.

Nanoporous Copper Dealloyed from a Nanocrystallized Ticu Alloy

Nanoporous copper (NPC) was fabricated through dealloying nanocrystallized TiSubscript text50Cu50 ribbon alloy under a free immersion condition in HF solutioSubscript textns at 25 °C. Multimodal nanoporous structure was formed due to the presence of Ti3Cu4 phase, which was co-precipitated with Ti2Cu during the heat treatment at T = 400 °C (Italic textTSubscript textg Italic textT Subscript textx). The presence of multiphases in tItalic texthe starting material caused the different behavior in the evolution of nanoporosity. In 0.03 mol/L HF solution, the bimodal nanoporous copper with a pore size of 54 nm and 184 nm was obtained in different regions where the composition differed. The ligament scale lengths in two regions were confirmed to be 54 nm and 203 nm, respectively. In 0.13 mol/L HF solution, the difference in the pore size and phase separation became weak, accompanying with the evolution of larger pores and smaller ligaments. The residue after dealloying was confirmed to be fcc Cu, indicated by the presence of Cu (111), (200), (220) and (311) in XRD patterns and TEM selective area diffraction pattern. The microstructure of the starting materials for dealloying, such as intermetallic phases, played a key role in the formation of the final multimodal nanoporous structure.

Microelectrochemistry on CrS and MnS Inclusions and Its Relation with Pitting Potentials of Stainless Steels

The anodic dissolution behavior of CrS inclusions was compared with that of MnS inclusions containing around 10 at% Cr, (Mn,Cr)S, 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 found to be 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 potentials than those of CrS-containing stainless steels. AES analysis of the surface films on CrS inclusions indicated that chromium-containing oxide films were formed on the CrS inclusions and grew with electrode potential, which suggested the oxide films on CrS inclusions prevented the anodic dissolution of the inclusions, thereby leading to an increased resistance to pit initiation.

Corrosion Propagation Behavior of Magnesium Alloys under Atmospheric Conditions

Magnesium has excellent characteristics such as low specific gravity and a high strength to weight ratio. Thus, it has been widely used for the bodies of cellular phones and notebook-type PCs to meet their light weight requirements. The application of magnesium to the aerospace and automotive industries can be expected in the future. However, there are drawbacks to its application such as low corrosion resistance compared with that of other light metals, especially in chloride containing environments. In the aerospace and automotive industries, high safety and reliability are essential. Therefore, it is necessary to understand the atmospheric corrosion mechanism of magnesium alloys and to improve the corrosion resistance of the alloys. The objective of this study was to examine the corrosion behavior of magnesium alloys under atmospheric conditions.

Formation of Pt Skin Layer on Ordered and Disordered Pt-Co Alloys and Corrosion Resistance in Sulfuric Acid

AbstractThe formation of the Pt-enriched layer (Pt skin layer) formed at the surface of the ordered and disordered Pt-Co alloy specimens by a dealloying treatment and its corrosion resistance under potential cycling in sulfuric acid were examined to clarify the dissolution behavior of the Pt skin layer in polymer electrolyte fuel cell-operating conditions. The ordered and disordered Pt-Co specimens were obtained by heat treatment at 1073 and 1473 K, respectively. Co at the alloy surfaces dramatically dissolved in an early phase of the dealloying treatment in naturally aerated 0.5 M H2SO4 at 298 K, and Pt skin layers were formed. Pt skin layers ca. 2 monolayer in thickness were formed on the Pt-Co alloy specimens by the dealloying treatment in 0.5 M H2SO4. The cyclic voltammetry measurements of the Pt-Co specimens showed the existence of Pt skin layers after the dealloying treatment and the inhibition of the Pt oxide formation on the Pt skin layers. The Pt oxide formation for ordered Pt-Co was more suppressed than that for the disordered Pt-Co. The Pt skin layers on the Pt-Co specimens exhibited a higher corrosion resistance than pure Pt, and the dissolution of the Pt skin layer for ordered Pt-Co was more inhibited than that for disordered Pt-Co under potential cycling in the potential range of 0.6–1.4 V in 0.5 M H2SO4 (dissolution test). A thin, continuous Pt skin layer remained at the surface of the ordered Pt-Co specimen after the dissolution test. The formation of a uniform Pt skin layer seems to provide high oxidation resistance to the surface, leading to high corrosion resistance. Graphical AbstractThe dissolution of Pt skin layer for the ordered Pt-Co was highly suppressed compared with that for the disordered Pt-Co and pure Pt. The suppression of Pt-O formation seems to be the main reason for the high corrosion resistance. The formation of a thin and continuous Pt skin layer is important to prevent the dissolution of Pt.