Toshiyasu Nishimura

Electrochemical Behavior of Rust Formed on Carbon Steel in a Wet/Dry Environment Containing Chloride Ions

Abstract The iron rust phase was analyzed by using the in-situ x-ray diffraction (XRD) and alternating current (AC) impedance methods after a wet/dry corrosion test using sodium chloride (NaCl) solution, which is the main composition of airborne saline particles. The corrosion content of the carbon steel depended on the concentration of Cl ions in the environment of the test chamber. As the concentration of Cl ions increased, the content of β-FeOOH increased in iron rust phases. The transition of β-FeOOH from the green rust I (GRI) was observed directly by in-situ XRD. The amount of GRI depended on the concentration of Cl ions, and β-FeOOH was transformed from GRI automatically in the dry process of the test. AC impedance showed that the resistance of the rust (Rrust) increased with the number of cycles in the corrosion test, and that the structural factor of the rust became predominant in Rrust. With the increase of the amount of rust, the resistance corresponding to the corrosion rate (Rt decreased, whi...

Effect of Co and Ni on the corrosion behavior of low alloy steels in wet/dry environments

Abstract The iron rust phase formed on low alloy steels has been analyzed by using EPMA, XPS, TEM and alternating current (AC) impedance methods after wet/dry corrosion tests using 0.5 mass% NaCl solution. Steel containing Co or Ni from 1 to 3 mass% showed higher corrosion resistance than carbon steel in the test. Cobalt was identified in its trivalent state (Co 3+ ) in the rust of Co-bearing steel by EPMA and TEM, which means that Co was incorporated mainly into FeOOH. On the other hand, nickel was identified in its bivalent state (Ni 2+ ) in the rust of Ni-bearing steel by XPS and TEM, which implies that Ni was involved in the formation of spinel oxide in rust. AC impedance demonstrated that the resistance of rust ( R rust ) corresponds to the structural factor of the rust. The high value of R rust for Co-bearing steel indicated high resistance of FeOOH-type rust against the penetration of Cl ion. The high R rust value was observed at the early stage of corrosion cycles. The R rust value of Ni-bearing steel increased as the amount of rust on the steel increased, implying that Ni helped to convert spinels into a dense and fine structure, and that its effectiveness increased after the formation of the inner rust layer.

Clarification of chemical state for alloying elements in iron rust using a binary-phase potential–pH diagram and physical analyses

A binary-phase potential–pH diagram has been investigated to evaluate the chemical stability of various kinds of double oxide rusts (Fe–X) to get a principle for alloy design enhancing the corrosion resistance of steels. It was found that there are the following types of alloying elements enhancing the corrosion resistance of steels in the rust: (1) iron substitution type (Ni), (2) oxide formation type (Al), (3) metallic type (Ru), and (4) oxygen-acid salt type (WO4). X-ray photoelectron spectroscopy and transmission electron microscopy analyses have been conducted on the rust formed on the low alloy steel in a saline environment. The analytical results were discussed using potential–pH diagrams. The iron substitution type and the oxide formation type elements make spinel double oxides with iron. In the corrosion tests, steels added with Ni or Al had high corrosion resistance. Thus it is possible to obtain high corrosion resistance by the creation of spinel double oxide such as Fe2NiO4 and FeAl2O4 in an inner layer. On the other hand it was found that the metallic type and the oxygen-acid salt type elements were not contained into the iron rust. In particular the oxygen-acid salt elements were excluded from the iron rust and concentrated into the defects of the rust. It is suggested that insoluble salts like FeWO4 are formed on the base metal in the defects to act as an anodic inhibitor. Thus, the addition of a small quantity of W gives high corrosion resistance. The penetration of Cl ions can be prevented by the spinel double oxide in an inner layer and the oxygen-acid salt in the defects. In this way, the high corrosion resistance by the addition of these elements can be understood from the potential–pH diagram and the physical analyses.

Corrosion resistance of Al-based metal matrix composites

Abstract Corrosion characteristics of Al 6061-based metal matrix composites (MMCs) containing carbon fibres, alumina fibres or silicon carbide whiskers (SiCw) and the effects of ageing heat treatment on the stress corrosion cracking resistance of SiCwAl 7075 MMC were studied in chloride solutions. MMCs were prepared by the squeeze casting method in the laboratory. The pitting potentials for Al 6061 and the three Al 6061-based MMCs were similar. The resistance of SiCwAl 6061 MMC to pit initiation is the same as that of the monolithic alloy. Once pits initiate, damage by pitting corrosion would be greater owing to the formation of crevices between the reinforcement and the matrix by the dissolution of the latter in the pits. The cathodic current of the SiCwAl 6061 is slightly larger than that of the matrix alloy, possibly owing to an interfacial layer observed by transmission electron microscopy. Ageing of SiCwAl 7075 MMC at 170 °C for more than 1 h following ageing at 110 °C makes the MMC resistant to stress corrosion cracking in 3.5% NaCl and increases the pitting potential.

Nanocomposite Coatings on Biomedical Grade Stainless Steel for Improved Corrosion Resistance and Biocompatibility

The 316 L stainless steel is one of the most commonly available commercial implant materials with a few limitations in its ease of biocompatibility and long-standing performance. Hence, porous TiO(2)/ZrO(2) nanocomposite coated over 316 L stainless steels was studied for their enhanced performance in terms of its biocompatibility and corrosion resistance, following a sol-gel process via dip-coating technique. The surface composition and porosity texture was studied to be uniform on the substrate. Biocompatibility studies on the TiO(2)/ZrO(2) nanocomposite coatings were investigated by placing the coated substrate in a simulated body fluid (SBF). The immersion procedure resulted in the complete coverage of the TiO(2)/ZrO(2) nanocomposite (coated on the surface of 316 L stainless steel) with the growth of a one-dimensional (1D) rod-like carbonate-containing apatite. The TiO(2)/ZrO(2) nanocomposite coated specimens showed a higher corrosion resistance in the SBF solution with an enhanced biocompatibility, surpassing the performance of the pure oxide coatings. The cell viability of TiO(2)/ZrO(2) nanocomposite coated implant surface was examined under human dermal fibroblasts culture, and it was observed that the composite coating enhances the proliferation through effective cellular attachment compared to pristine 316 L SS surface.

Corrosion behavior of tungsten-bearing steel in a wet/dry environment containing chloride ions

Abstract The chemical state of tungsten in the rust of low-alloy steel was analyzed by electroprobe x-ray microanalysis (EPMA) and x-ray photoelectron spectroscopy (XPS), and the electrochemical behavior of the rust was investigated by the electrochemical impedance spectroscopy (EIS) method after a wet/dry cyclic corrosion test with chloride ions. The 1.0 mass% tungsten-bearing steel showed high corrosion resistance compared to carbon steel (SM) in the corrosion test. EPMA and XPS showed that tungsten existed as WO4 compounds in the rust of tungsten-bearing steel. An Fe-W binary potential (E)-pH diagram was made to investigate the thermodynamic stability of Fe-W oxides, and this showed that WO4 was stable in the rust, which confirmed the results of the physical analysis. An EIS measurement was taken to find the corrosion reaction resistance (Rt) of the rust. Rt of tungsten-bearing steel was much larger than that of SM. The corrosion of the tungsten-bearing steel was suppressed by the insoluble FeWO4 forme...

Corrosion Behavior of Carbon Steel in Bicarbonate (HCO 3 ) Solutions

Carbon steel is considered in Japan the most promising candidate material for overpacks in high-level radioactive waste disposal. Effects of bicarbonate solutions on the corrosion behavior and corrosion products of carbon steel were investigated by electrochemical measurements; FT-IR spectra and XRD pattern analyses. The results of the anodic polarization measurements showed that bicarbonate (HCO 3 ) accelerates the anodic dissolution and the outer layer film formation of carbon steel in the case of high concentrations, whereas it inhibits these processes in the case of low concentrations. The FTIR and XRD analyses of the anodized film showed that siderite (FeCO 3 ) was formed in 0.5 to 1.0mol/L bicarbonate solution, and Fe 2 (OH) 2 CO 3 in 0.1 to 0.2mol/L bicarbonate solution, while Fe 6 (OH) 12 CO 3 was formed in 0.02 to 0.05mol/L bicarbonate solution. In all cases the pH value was around 8.3. The stability of these chemical compositions was discussed using a potential – pH diagram for the Fe-H 2 O-CO 2 system.

Prevention of Delamination during Drilling of Composite Material Using Vibration

Prevention method of delamination at adhesive layer during drilling of composite material using vibration is examined. First, this method is examined experimentally. As vibrational load, relatively low frequency vibration and ultrasonic vibration are used. It is concluded occurrence of delamination is less when vibration is used during drilling. Next, experimental results are examined by analytical method. Adhesive layer is assumed to be damping component. Condition of occurrence of delamination is proposed.

Galvanic corrosion behaviour of iron coupled to aluminium in NaCl solution by scanning eectrochemical microscopy

Scanning electrochemical microscopy (SECM) was used to sense the concentration of ions in 0.1M NaCl aqueous solution at the iron-aluminium couple. The SECM measured the concentration of ions relevant to the corrosion processes. The electrochemical behaviour of galvanic Fe/Al coupling was investigated as a function of time using SECM microelectrode tip. SECM amperometric line scan curves were obtained over the Fe/Al at a constant distance. In the first case the chemical species participating in the corrosion reactions at the sample are detected at the SECM-tip by applying appropriate potential values to the microelectrode. The release of Al3+ into the solution from local anodic surface, as well as the consumption of dissolved oxygen at the corresponding cathodic surface was successfully monitored. The results revealed that the galvanic couple where Fe/Al is close to each other will show lower corrosion rate due to the formation of corrosion products on the metal surface with further increase in exposure times.

Localized Electrochemical Impedance Spectroscopy Observation on Scratched Epoxy Coated Carbon Steel in Saturated Ca(OH)2 with Various Chloride Concentration

The in-situ local corrosion behavior of scratched epoxy coated carbon steel is investigated in sat. Ca(OH)2 with varying concentration of Cl- ions by localized electrochemical impedance spectroscopy (LEIS). The localized corrosion process and mechanism of coated steel (scratch area) is measured by LEIS plots and 3D topographic images. The LEIS responses measured at the defect are attributed to the pore impedance with defect in the highfrequency range and an interfacial corrosion reaction in the low-frequency range of corroding steel at the base of defect within 1-10 h immersion. The continuous decrease in |Z| at the scratch is due to the higher extent of dissolution of Fe with increase of Cl- ion concentration. However, the resistance values of coated steel in sat. Ca(OH)2 with each concentration of Cl- ions are not changed significantly with increase in immersion time from 1-10 h. On the other hand, LEIS Nyquist plots clearly showed that the measured impedance at high frequency is related to corrosion products formed at the defect which acts as anodic zones and the low frequency part are related to corroding of carbon steel with immersion of 1-5 days. 2D topographic images clearly showed that corrosion occurs at scratch and followed by coating degradation at scratch front as well as away from scratch due to cathodic reactions (reduction of O2) leads to coating delamination. No significant change in corrosion resistance is observed for 0 and 0.0085 M/L of Cl- ions containing solution for 5 days of immersion as well as 1-10 h immersion. This is due the formation of better passive film on the steel surface (defect) in which the competition between the aggressive Cl- ions and the inhibitive OH- ions determines the rate of corrosion. A significant decrease in corrosion resistance is observed with higher concentration of Cl- ions (0.17 and 0.51 M) due to the preferential adsorption of Cl- ions at the defect site.