The effects of pulse parameters on the surface morphology, structure and electrodeposition behavior of Zn-Ni alloys were investigated by galvanostatic pulse plating The Ni content of Zn-Ni electrodeposits changed with average puls current density (Ia). This behavior was similar to that found in DC electrodeposition At current density of 1A/dm2 or less, Ni deposition and H2 evolution occured, while above 1A/dm2, Zn was deposited and abnormal Zn-Ni co-deposition began. Above 10A/dm2, Ni content increased with increases in Ia Pulse-plated Zn-Ni crystals were smaller than those obtained by DC electrodeposition Pulse period (T) also had a major effect on crystal size Off time (toff) affected the Ni content and the structure of the Zn-Ni electrodeposits With Ni content and γ phase increasing, while η phase decreased with increases in toff Polarization curves showing variations in pulse duty (ton/T) reflected Zn-Ni electrodeposition behavior The limited current density of abnormal electrodeposition (about 08A/dm2) gradually decreased with lower pulse duty Diffusion-controlled current densities above 10A/dm2 increased in accordance with pulse duty Initial deposits during pulse plating contained a Zn-enriched Zn-Ni layer, results which correspond to the theory of abnormal co-deposition
The anodic polarization curve of cobalt in borate solution is divided into four potential regions: active dissolution, primary passivation, transpassivation, and secondary passivation. These regions remain almost unchanged, whereas the current density increased with ammonia concentration, probably due to the formation of cobalt ammine complex of high solubility, especially in the active dissolution region. Oxyhydroxide and/or oxide deposition occurred on the metal surfaces at potentials more positive than ca. -0.5V vs. Ag/AgCl. In solutions containing ammonia, these oxide layers combined with ammonia to form a cobalt oxide ammonia complex as the reaction intermediate, and then dissolved at an accelerated rate that was dependent on the ammonia concentration.
The preparation of amorphous Ni-Mo alloy films by electrodeposition from ammonia citrate baths with the addition of ammonia as a complexing agent was studied, and their corrosion resistance was determined. Amorphous Ni-Mo alloy films were deposited from baths of a pH of 9.0 containing nickel sulfate (0.13∼0.18mol/L) and sodium molybdate (0.08∼0.13mol/L) such that total metallic (Ni2++Mo6+) ion concentration was a constant 0.26mol/L with 0.32mol/L of sodium citrate and 0.53mol/L of ammonia as a complexing agent at a pH of 9.0 and current densities of 8∼20A/dm2. Electrolytic aging produced an increase in the molybdenum content of the electrodeposited film to 37wt% as well as arise in cathodic current efficiency. Peeling of the films was reduced, and no cracking occurred. Films with a molybdenum content of about 28wt% or more exhibited an amorphous structure. Electrochemical measurements and immersion tests were curried out at 30°C in both 1N H2SO4 and 1N HCl solutions, and the results indicated that amorphous Ni-Mo alloys had greater self-passivation ability and lower corrosion rates than did crystalline alloys, and showed better corrosion resistance than SUS 304 stainless steel in the above mentioned solutions.
Smoothening of the aluminum surfaces during the growth of barrier anodic oxide films has been investigated by transmission electron microscopy of the stripped films and ultramicrotomed sections. It was found that three factors are responsible for the smoothening: (1) the anodic oxide films are amorphous, so that they can follow the continuously changing and reducing aluminum surfaces; (2) the oxide films should be of uniform thickness in the direction perpendicular to the local metal surfaces as required by the oxide growth kinetics; and (3) part of the oxide growth occurs at the oxide/electrolyte interface by the outward diffusion of Al3+ ions.
The effects of MnS inclusions and of electrolytic treatment in the active and passive region on the pitting resistance of 18-8 stainless steels have been studied in 3%NaCl solution mainly by electrochemical means. The pitting resistance of the specimens depended on the electrolytic potential. In the specimens electrolyzed in the active region directly below the passivation potential, pitting potential had a tendency to become noble, and this tendency was pronounced in specimen containing a small quantity of MnS or having a small Mn/S ratio. Cr concentration on the steel surface was increased by the electrolytic treatment, and the concentration had a tendency to inrease as electrolytic potential became more noble. Although there appears to be little correlation between Cr concentration and pitting potential, there does appear to be a correlation between the morphology of corrosion and the pitting potential. That is to say, electrolyzing just below the passivation potential at which dissolution of the matrix and MnS occure, let to greatly improve pitting resistance. This was due to the dissolution of MnS, which acts as the pit initiation site, and to Cr enrichment on the steel surface, which occurrs simultaneously.