Shiro Haruyama

Anodic Oxidation of Reductants in Electroless Plating

The anodic oxidation of reductants (hypophosphite, formaldehyde, borohydride, dimethylamine borane, and hydrazine) was studied on different metal electrodes (Au, Pt, Pd, Ag, Cu, Ni, and Co) at various temperatures, with special interest in the catalytic aspect of electroless plating. The rate of the anodic oxidation strongly depended on the pH value, the concentration of reductants, and the nature of the metal electrode. The catalytic activities of the metals for the anodic oxidation of different reductants were evaluated by the potentials at a reference current density. The order of the catalytic activity with metal varied depending on the nature of the reductants. The catalytic activity series thus obtained can be utilized for choosing the reductant suitable for the metal to be deposited. Arrhenius plots of the anodic currents on different metals at a reference potential yielded their respective straight lines. Some correlations were observed between the catalytic activity and the activation energy. The catalytic activity series was discussed in connection with that for hydrogen electrode reaction.

Impedance studies of the inhibitive effect of benzotriazole on the corrosion of copper in sodium chloride medium

Abstract Impedance studies of a copper electrode in chloride medium with or without benzotriazole (BTA) were made over a wide frequency range (10 −2 −10 2 KHz). The impedance characteristics of the Cu-BTA surface film were established under various experimental conditions. The results indicate that the film was of a dielectric nature having a dielectric constant of ca . 20. It was found that the film thickness increased and the corrosion rate decreased by increasing the BTA-concentration or with ageing. It was assumed that, at the stationary state, the rate of dissolution of Cu-BTA film blanced that of the slow transport of Cu ions through the film. Good support for these results was obtained from the data of potentiostatic polarization and weight loss measurements.

Passive film formed on nickel in a neutral solution

Abstract A bright nickel was passivated in the pH 8·39 boric acid-borate buffer solution. The oxide formation reactions were mainly investigated by means of polarization experiments, alternating anodic and cathodic polarization, potential decay experiments and colorimetric analysis. From the results, the chemical composition and structure of passive films formed on nickel were discussed. The experimental results are summarized as follows. 1. A considerable amount of nickel ions dissolved during the anodic formation of passive films. The dissolution ceased because of the formation of NiO 2 . 2. The passive films on bright nickel has a duplex structure consisting of NiO and Ni 3 O 4 . At higher potentials, NiO 2 was produced on them. NiO and Ni 3 O 4 were formed directly from Ni, and NiO 2 was transformed from Ni 3 O 4 . 3. NiO and NiO 2 were reversibly produced and reduced, but Ni 3 O 4 was very difficult to be reduced. 4. It was observed that there was a reversible charging and discharging layer at the potential where Ni 3 O 4 formed. The electric capacity was calculated to be about 100 μF/cm 2 , assuming that roughness factor was 2. It would be reasonable to think that a space charge was established on nickel surface.

The polarization behaviour of chromium in acidic sulphate solutions

Abstract The polarization behaviour of chromium was studied in solutions of various pH, with particular regard to the hydrogen evolution reaction and chromium dissolution. Hydrogen evolution occurred on active and passive chromium and the exchange current density was ca. 10 −6 A/cm 2 at pH 1.0 on both electrodes. The rate of active dissolution of chromium did not depend on pH, an observation that indicated the absence of a hydroxyl-containing intermediate in the rate determining step. The transpassive dissolution of chromium was strongly dependent on pH, and it was suggested that the rate was controlled by a base-accelerated anodic dissolution of an oxide film.

The polarization behaviour of FeCr alloys in acidic sulphate solutions in the active region

Abstract The polarization behaviour of six Fe Cr alloys was studied in sulphate solution of variou pH, with emphasis on the hydrogen evolution, active dissolution, and evaluation of active corrosion current, in comparison with results on the individual components. The exchange current densities of hydrogen evolution on Fe Cr alloys were found to increase with increasing Cr content up to 30Cr%. The rate of the anodic dissolution of low Cr alloys ( 10%Cr) was independent of pH and the Tafel slope was 60 mV/decade, similar to that of pure Cr, thus there is a threshold Cr content above which the dissolution of the alloy is controlled by the Cr component. The peak current at the active-passive transition decreased with increasing pH or Cr content. It is highly probable that the pre-passive state commences with the formation of chromic hydroxide, which promotes the formation of a ferrous oxide to shift the Flade potential in the negative direction.

The dissolution of magnetite in acidic perchlorate solutions

Abstract The reduction dissolution of magnetite was investigated in the acidic perchlorate solutions of various pH values by means of the potential sweep method and impedance measurement, with special interest in the mechanism of oxide dissolution. In a certain potential region, the current potential diagram exhibited a peak, both the positive and negative branch of which obeyed a Butler—Volmer type relation. The rate of the dissolution in this potential region was controlled by a transfer of ion across the Helmholtz layer. The positive branch of the peak depended on the pH of solution. The rate of dissolution was found to be controlled by an anion-removal process. The negative branch of the peak was independent of pH and the transfer of ferrous ion was rate-controlling. From the impedance measurement, the faradaic formation of a surface layer with non-stoichiometric composition was suggested. The impedance behaviour at low frequencies corresponded well to the stationary dissolution behaviour.

Mechano-electrochemical dissolution of copper at reversible electrode potential

Abstract The mechano-electrochemical dissolution of copper during yielding at constant strain rate has been studied in acidic cupric sulphate solutions, restraining the electrode potential to the reversible potential by a potentiostat. It has been found that the mechano-electrochemical current undergoes a marked rise at the beginning of the plastic region and then shows an asymptotic tendency with increasing strain. It has been proved that the initial slope of the current-strain diagram is independent of strain rate and gives a direct measure of the anodic current density on the active surface formed by yielding. The initial slope increased with increasing either temperature or the concentration of cupric ion. It seems likely that slipping occurs at yielding, and the emerging active surface which has a high adatom activity provide an anodic dissolution at the reversible potential. The theory based on the model agrees well with the experimental results.

A resistometric study of the passive film on an evaporated iron electrode

Abstract Cathodic reduction mechanisms of the passive film formed on an evaporated iron electrode in a neutral solution were studied by chronopotentiometry, colorimetric analysis and measurement of the electrical conductance of the film electrode (resistometry). By galvanostatic cathodic reduction, the outer γ-Fe 2 O 3 layer was reduced to ferrous ion and dissolved with nearly 100% current efficiency without showing any change in the conductance of the film. The inner magnetite layer was partly reduced to metallic iron and the residual part was dissolved as ferrous ion. The latter reaction was predominant in low pH. From the increase in conductance in the initial part of cathodic reduction, it was concluded that adsorbed oxygen fortified the passivity by lowering the conductance of the passive film.

Passivation and its stability on copper in alkaline solutions containing carbonate and chloride ions

Abstract The anodic polarization curve of copper in borate buffer solution at pH 11 shows two anodic current peaks at −0.26V(Pa a1) and −0.03V(P a2) and a passive region with a small hump at a more positive potential. It is found from the result of a channel flow electrode that P a1 corresponds to the formation of Cu 2 O, while P a2 seems to be related with the formation of Cu(OH) 2 . The presence of carbonate ion hinders protective Cu(OH) 2 passive layer formation. The addition of chloride ion (>0.5 mol/l) causes pitting corrosion at a potential close to oxygen evolution. The stability of the passive film in alkaline solution is lowered by the presence of carbonate or chloride ion. The anodic polarization curve obtained in concentrated carbonate-chloride solution (>1mol/l) at pH11 shows two Tafel regions, the slope of which has 60 and 40 mV/decade, proceeding P a1 and P a2 , respectively. Active dissolution model in both Tafel regions have been proposed.

The cathodic reduction of oxygen on stainless steels in a neutral solution

Abstract Cathodic reduction of oxygen on a passivated Fe-18%Cr alloy was studied in borate solutions by using a rotating disk electrode. The effect of pH and the addition of hydrogen peroxide on the cathodic reduction were examined in order to establish the oxygen reduction mechanism. The cathodic polarization curves obeyed the Tafel relation with a slope of 0.06 V/decade at low current densities, exhibiting a limiting current at high current densities. Since the limiting current was in proportion to the partial pressure of oxygen and independent of the rotation rate of electrode, it is likely that a chemical reaction on the electrode controls the reaction rate. The reduction current decreased with increasing pH in the Tafel region but the pH dependence of the limiting currents was the reverse. Since oxygen reduction took place on the passive films on FeCr alloys, it is probable that a film property such as semiconductivity strongly affects the surface reaction. The reverse pH dependence of the limiting current could be explained by a change in the film nature.