Masazumi Okido

Anodizing of Mg alloys in alkaline solutions

Abstract The electrochemical behaviors of 99.95 mass% magnesium and its alloys, i.e. AZ31 and AZ91, were investigated in NaOH alkaline solution. Mg and Mg alloy specimens were anodized for 10 min at 3, 10 and 80 V in 1 mol/dm3 NaOH alkaline solution. The films anodized at 3 V on Mg and Mg alloys had the best effective corrosion resistance and these films consisted of comparatively thick magnesium hydroxide. Corrosion resistances of the films anodized at 80 V on Mg and Mg alloys were higher than those at 10 V and lower than those at 3 V, from results of the anodic polarization measurement in 0.1 mass% NaCl solution.

Corrosion Resistant Performances of Alkanoic and Phosphonic Acids Derived Self-Assembled Monolayers on Magnesium Alloy AZ31 by Vapor-Phase Method

Alkanoic and phosphonic acid derived self-assembled monolayers (SAMs) were formed on magnesium alloy by the vapor phase method. AFM and XPS studies showed that SAMs were formed on Mg alloy. The chemical and anticorrosive properties of the SAMs prepared on magnesium alloys were characterized using contact angle measurements, X-ray photoelectron spectroscopy (XPS), and electrochemical measurements. Water contact angle measurements revealed that, although SA and ISA have the same headgroup to anchor to the magnesium alloy surface, the packing density on the magnesium alloy surface could be considerably different. The contact angle hysteresis of SAMs with a carboxylate headgroup is much larger than that of SAMs with a phosphonic acid group. The XPS O 1s peaks indicated more likely a mix of mono-, bi-, or tridentate binding of phosphonic acid SAM to the oxide or hydroxide surface of the Mg alloy. The electrochemical measurements showed that the phosphonic acid derived SAM had better corrosion resistance compared to alkanoic acid derived SAM. The chemical stability of SAMs modified magnesium alloy was investigated using water contact angle and XPS measurements. The water contact angle and XPS measurements revealed that the molecular density of OP and PFEP on magnesium alloy would be higher than those of SA and ISA on magnesium alloy.

Electrodeposition and Thermoelectric Characterization of Bi2Te3

Bismuth and tellurium were electrodeposited from acidic aqueous solutions of Bi 2 (SO 4 ) 3 and K 2 TeO 3 at constant potentials in order to produce a Bi 2 Te 3 film for miniaturized thermoelectric devices. Electrodeposition behavior was examined using cathodic polarization and electrochemical quartz crystal microbalance techniques. The solution composition and the cathodic potential affected the chemical composition, preferred crystal orientation, and thermoelectric characteristics of the deposits. Films electrodeposited at potentials more positive than -100 mV vs Ag/AgCl had a dense, smooth surface and high crystallinity. The film composition depended more strongly on the solution composition than on the applied potential. A stoichiometric Bi 2 Te 3 film could be obtained from a solution containing 2 mM Bi(III) and 2.6 mM Te(IV). The Seebeck coefficient increased with Te content of the film and was inversely proportional to the electrical conductivity. The highest power factor (7.4 X 10 -4 Wm -1 K -2 ) was obtained in the case of the Bi 2 Te 3 film electrodeposited at 0 V, 293 K, pH 0.5, in a sulfuric acid solution containing 2 mM Bi(III) and 2 mM Te(IV); the film contained 57 mol % Te, corresponding to Bi-rich and a carrier concentration of 6.6 × 10 20 cm -3 .

Hydroxyapatite coating on titanium by means of thermal substrate method in aqueous solutions

Hydroxyapatite (HAp) films were formed on a titanium substrate in aqueous solutions by the thermal substrate method controlling the substrate temperature and a cathodic electrolysis method supplying hydroxide ions. A local increase in temperature on substrate and the supply of calcium, phosphate, and hydroxide ions near the substrate accelerate the HAp film formation on the substrate in aqueous solutions at ambient temperature and pressure. The HAp can be directly coated only on the substrate quickly by heating the substrate in an aqueous solution. In the cathodic process, reduction of hydrogen peroxide forms hydroxide ions, which results in the formation of a flat, plate-like HAp film.

The mechanism of hydrogen evolution reaction on a modified Raney nickel composite-coated electrode by AC impedance

The hydrogen evolution reaction (HER) on a modified Raney nickel electrode, consisting of Ni-Al-Cu-Cr alloy, was investigated with ac impedance measurements and by dc polarization current behavior in 5.36 M KOH solution at 70 C. Comparative measurements also were carried out on commercial Raney nickel (Ni-Al). The rate constants of the forward and backward reactions of Volmer, Heyrovsky, and Tafel steps were estimated by a nonlinear fitting method. The modified Raney nickel composite-coated electrode provides a lower overpotential for the HER than the commercial Raney nickel-coated electrode. The main pathway for the HER is Volmer-Tafel with Volmer as the rate determining step.

Preparation of Cu nanoparticles with NaBH4 by aqueous reduction method

Cu nanoparticles were prepared by reducing Cu2+ ions with NaBH4 in alkaline solution. The effects of NaBH4 concentration and dripping rate on the formation of Cu nanoparticles were studied. The optimum conditions are found to be 0.2 mol/L Cu2+, solution with pH=12, temperature of 313 K and 1% gelatin as dispersant, to which 0.4 mol/L NaBH4 is added at a dripping rate of 50 mL/min. NH3·H2O is found to be the optimal complexant to form the Cu precursor. A series experiments were conducted to study the reaction process at different time points.

Preparation of Cu nanoparticles with ascorbic acid by aqueous solution reduction method

Cu nanoparticles were prepared by reducing Cu 2+ ions with ascorbic acid through aqueous solution reduction method. The effects of solution pH and average size of Cu2O particles on the preparation of Cu nanoparticles were investigated. Cu particles were prepared at pH 3, 5 or 7, with the smallest Cu particles obtained at pH 7. However, Cu particles could not be prepared at pH 9 or 11. The average size of Cu2O particles can affect that of Cu particles. Larger Cu2O particles result in larger Cu particles. In addition, experiments were conducted to explore the reaction process by measuring the X-ray diffraction (XRD) patterns of specimens collected at different time points during the reaction. It was found that Cu(OH)2 was initially formed as a precursor, followed by the formation of Cu2O, which was finally reduced to Cu particles.

Hydroxyapatite coating of titanium implants using hydroprocessing and evaluation of their osteoconductivity.

Many techniques for the surface modification of titanium and its alloys have been proposed from the viewpoint of improving bioactivity. This paper contains an overview of surface treatment methods, including coating with hydroxyapatite (HAp), an osteoconductive compound. There are two types of coating methods: pyroprocessing and hydroprocessing. In this paper, hydroprocessing for coating on the titanium substrate with HAp, carbonate apatite (CO3–Ap), a CO3–Ap/CaCO3 composite, HAp/collagen, and a HAp/gelatin composite is outlined. Moreover, evaluation by implantation of surface-modified samples in rat tibiae is described.

Corrosion protection property of colloidal silicate film on galvanized steel

Abstract Chemical conversion treatment of galvanized steel using colloidal silica was investigated as an alternative treatment to chromate conversion. The solution used for the colloidal silicate coating consisted of colloidal silicate, titanium sulfate, and nitrate ions to maintain good corrosion resistance. Adding CoSO 4 to the colloidal silicate solution enhanced the adhesion between the silicate film and the zinc coat on the steel. The corrosion of galvanized steel with and without this treatment was investigated in 3 mass% NaCl solution using electrochemical techniques, such as electrochemical impedance spectroscopy, open circuit potential, and the salt spray test (SST). With immersion in NaCl solution, rust appeared on the surface of galvanized steel after 15–20 days, and after 40–50 days on silicate-conversion-coated specimens. In the SST, rust appeared on chromate-coated specimens more quickly than on those with a colloidal silicate film surface.

An electrochemical study of cathodic protection of steel used for marine structures

Impressed current cathodic protection can result in hydrogen embrittlement, which can cause trouble with high-strength steels, particularly at welds. Therefore, the limiting potential for hydrogen embrittlement should be examined in detail as a function of the cathodic protection potential. This study investigated the effects of post-weld heat treatment (PWHT) on marine structural steels from an electrochemical viewpoint. In addition, the slow strain rate test (SSRT) was used to investigate both the electrochemical and mechanical effects of PWHT on impressed current cathodic protection. According to the SSRT, the optimum cathodic protection potential was −770 mV [with a saturated calomel electrode (SCE)]. SEM fractography analysis showed that the fracture morphology at an applied cathodic protection potential of −770–850 mV (SCE) was a dimpled pattern with ductile fractures, while a transgranular pattern was seen at potentials below −875 mV (SCE). Therefore, the cathodic protection potential range should be −770–850 mV (SCE).