This is to synthesize our papers hitherto published on the electrochemical studies of electroless plating. Polarization experiments have been carried out on copper, nickel and cobalt electrodes in the appropriate electroless plating baths. Similar experiments have been made in the baths in the absence of either metallic ions or reductants, in order to simulate the partial cathodic or anodic reaction that would occur in the complete baths. The polarization curves in the complete baths were in many cases not entirely consistent with those expected from the superposition of the appropriate anodic and cathodic reactions. This means that the two reactions are interdependent when they occur simultaneously. The polarization resistance measured in the complete baths was proportional to the plating rate, exhibiting the individual proportional constants that were dependent on the nature of the electroless plating baths. These observations suggest that electroless plating takes place along an electrochemical mechanism according to the mixed potential theory. It is concluded that the polarization resistance method can be successfully used for monitoring the rate of electroless plating in both laboratory and industrial applications.

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Spectrophotometric studies have been carried out to elucidate the composition and the formation constants of nickel complexes formed in pyrophosphate tin-nickel alloy plating baths containing glycine. Continuous variation measurements revealed the formation of a mixed ligand complex, Ni(P₂O₇) (gly)^4-₂ in the bath. This finding suggests that addition of glycine to the bath leads to a stepwise complex formation to give Ni(P₂O₇)(gly)^3- and Ni(P₂O₇)(gly)^4-₂. The overall formation constants of these complexes were estimated and these were logβ₁=1.9 and logβ₂=5.0. From these values, the distribution of each complex species in the bath was calculated. In the bath of the optimum composition, approximately 50% of nickel ion forms the mixed ligand complexes, and the rest exists in the form of Ni(P₂O₇)^2-. Tin content of electrodeposits increased with decreasing glycine concentration or pH of the bath. When tin content is above 80at.%, β-tin is deposited. In this case, mixed ligand complexes are scarcely formed in the bath. Thus the formation of mixed ligand complexes is required to deposit tin-nickel alloy without preferential deposition of β-tin.

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The products formed during the electroreduction of a dilute chromic acid solution containing a small amount of sulfuric acid seem to be composed of four chemical species; chromic monomer, dimer, polymer and sulfate complex. They were separated by means of the ion-exchange chromatography. The formation of hydrated chromium oxide film and metallic chromium on the steel surface was investigated as a function of these chromic species. The formation of hydrated chromium oxide film, the incorporation of sulfate into the hydrated chromium oxide film and also the current efficiency of chromium deposition were not affected by the total amount of these chromic species but by the amount of chromic monomer. It is proposed that H⁺ released by proton-dissociation from the chromic monomer, which is produced by dissolution and decomposition of hydrated chromium oxide film on the electrode accelerates the reduction of HCrO₄^- to form the hydrated chromium oxide film.

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In the previous paper, authors reported that [Cr(H₂O)₆]³⁺ ion formed by the resolution of hydrated chromium oxide (Cr^ox) affected the formation of TFS film, which comprised metallic chromium and Cr^ox, in a dilute chromic acid electrolyte containing a small amount of sulfuric acid. The formation and resolution of Cr^ox and hydrogen evolution seemed to be connected with pH of the electrolyte near the cathode because these reactions occurred on the surface of the cathode. An attempt to measure the pH near the cathode during electrolysis was made using an Sb micro-electrode, which was recently applied to the pH measurement near the cathode. The results obtained were as follows: The pH measurement near the cathode during electrolysis in a dilute chromic acid electrolyte was successfully achieved using the Sb micro-electrode, and the pH value near the cathode ascended and the thickness of the cathodic layer increased by adding Cr³⁺ ion to the electrolyte. The pH value near the cathode ascended and the thickness of the cathodic layer increased with an increase of the current density. The pH value increased rapidly at about 15A/dm² and gradually at more than 20A/dm². The difference in the current efficiency of TFS film formation seemed to result from the change of pH near the cathode, of which degree depended on the difference of Cr³⁺ ion concentration.

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The mode of action of monocarboxylates and thallous nitrate in the chemical nickel bath was investigated. The monocarboxylates act as a buffer in the bath, and the buffering action increases in the following order; Propionate>Acetate>Formate. On the other hand, the internal stress of Ni-B coatings was decreased with the co-deposition of Tl, and the deposits gave rise to crack-free bright surface. The deposition rate of Ni-B coatings wasconfirmed to be about 5μm h^-1 for formate bath and 10μm h^-1 for acetate and propionate baths. The anodic oxidation of dimethylamine borane is inhibited strongly in the presence of formate to give the smaller deposition rate in the former bath. The fact that the lower pH gave the slower deposition rate of Ni-B coating and the higher rate of hydrogen evolution was fully explained by the analysis of partial cathodic and anodic polarization curves on the basis of the mixed potential theory.

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