Ryoichi Ichino

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.

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.

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.

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.

Improvement of corrosion resistance of AZ31 Mg alloy by anodizing with co-precipitation of cerium oxide

Anodizing of AZ31 Mg alloy in NaOH solution by co-precipitation of cerium oxide was investigated. The chemical composition and phase structure of the coating film were determined via optical microscopy, SEM and XRD. The corrosion properties of the anodic film were characterized by using potentiodynamic polarization curves in 17 mmol/L NaCl and 0.1 mol/L Na2SO4 solution at 298 K. The corrosion resistance of AZ31 magnesium alloy is significantly improved by adding cerium oxide to alkaline solution. In addition, the surface properties are enhanced and the film contains no crack.

Gold Leaching in an Alkaline Thiourea Solution

Gold leaching by thiourea has been investigated in an alkaline solution. In the presence of both Na 2 SO 3 and Na 2 S 2 O 8 , the chemical dissolution of gold can be accomplished in the alkaline thiourea solution. It was found that used as a buffer, SO 2- 3 can prevent thiourea from further decomposing into some insoluble species in contact with the oxidant S 2 O 2- 8 and that the mixed potential of gold is an important factor influencing thiourea decomposition and the rate of gold dissolution. Time dependence of the mixed potential and the dissolution mass for gold were also evaluated during the leaching process. This alkaline thiourea solution can be applied to gold extraction from the gold resources containing silver, copper, nickel, and iron, because these metals normally do not dissolve in the solution.

Photocatalytic reduction of Cr(VI) on TiO2 film formed by anodizing

Abstract Application of photocatalytic reduction to waste treatment of aqueous solutions is of interest comparing to the usage of chemical agents sometimes emanating a secondary waste. Photoreduction of dilute Cr(VI) ions less than 10 ppm in aqueous acidic solutions has been investigated using titanium oxide films formed by the anodizing process. Titanium plates with surface area of 5 cm 2 were anodized in a sulfuric acid solution of pH 2 at 293 K. Photoreduction tests of Cr(VI) ions were carried out in 10 ppm K 2 Cr 2 O 7 aqueous solution adjusted pH 2 by H 2 SO 4 , for 7 days, the change in the concentrations of Cr(VI) and Cr(III) ions was evaluated by capillary electrophoresis analysis. The TiO 2 film prepared with anodization at 200 V was an anatase type. Concentration of Cr(VI) ions was reduced from 10 to 0.03 ppm and concentration of Cr(III) ions was increased to approximately 9 ppm, for 7 days by the reaction on TiO 2 film under irradiation with 350 W Hg–Xe lamp.

Characterization of the chemical conversion films that form on Mg−Al alloy in colloidal silica solution

Chemical conversion treatment of Mg−Al alloy (AZ91) using colloidal silica as an alternative to chromate conversion was investigated as a function of solution pH, temperature, solution conditions, and treatment time. The solution used for the colloidal silica coating consisted of colloidal silica, titanium sulfate, and cobalt ions to maintain good anti-corrosion and adhesion properties. Adding CoSO4 to the colloidal silica solution enhanced the adhesion force between the silica film and magnesium substrate. The optimum conditions for the chemical conversion treatment solution were pH 2, 90-sec treatment, and 25°C.

A Comparative Electrochemical Study of AZ31 and AZ91 Magnesium Alloy

A comparative study has been carried out on AZ31 and AZ91 magnesium alloys in order to understand the electrochemical behavior in both alkaline and chloride containing solutions. The open circuit potential (OCP) was examined in 1 M NaOH and 3.5 mass % NaCl solutions. AZ31 magnesium alloy shows several potential drops throughout the immersion in 1 M NaOH solution, though AZ91 does not show this phenomenon. The specimens were anodized at a constant potential of 3 V for 30 minutes at 298 K in 1 M NaOH solution. The anticorrosion behavior of the anodized specimens was better than those of nonanodized specimens. The anodized AZ91 has better corrosion resistance compared to nonanodized specimen and anodized AZ31 magnesium alloy.