Maja D. Obradović

A contribution to the knowledge of electrochemical deposition of W induced by Ni

Abstract It was found that codeposition of Ni and W from an ammonia and citrate-containing electrolyte, is very sensitive to changes in operating conditions. The Ni–W alloy deposit exhibited sensitivity of chemical composition and phase structure on the composition of the electrolyte, pH in particular, as well as on the current density (cd), hydrodynamic conditions and quantity (thickness) of the deposit. Thus, small changes in pH, which arise during the process of bath utilization, are found to change the relationships between peaks in anodic dissolution voltammograms reflecting the existence of different phases. The anodic linear sweep voltammograms (ALSV) of the alloys deposited at medium cds, pH lower than 8.4 and a high rate of rotation of the RDE contained two peaks. The first one could be ascribed to some tungsten oxide species while the appearance of the second one basically reflects the presence of solid solution of W in Ni, but also points to some other phases. The increase in pH, increase in cd, and the decrease in the rate of rotation of the RDE all lead to ALSV with a single peak with a peak potential shifted in the negative direction, i.e. towards the dissolution peak of pure Ni. Analysis confirmed a decrease of W content in that shift. Still a somewhat more noble behavior than that of Ni points out that some W still remains in the Ni.

Hydrogen evolution at Zn-Ni alloys

Zn-Ni alloys were electrodeposited from a borate-citrate and TEPA bath in potentiostatic and galvanostatic regimes. Potential of electrodeposition or current density were varied and chemical composition and phase structure were determined. It was found that both chemical composition and phase structure of alloys electrodeposited at the same potential or current density but in the two electrolytes were different. Differences were found even when one and the same chemical composition was achieved. Hydrogen evolution kinetics at the Zn-Ni alloys deposited from the two baths was investigated in alkaline solution (1 M NaOH). Substantial differences in the reaction rates at the alloys with the same chemical but different phase composition were found.

Oxygen reduction on a duplex stainless steel

O2 reduction on duplex stainless steel was investigated in neutral and alkaline solutions with and without the presence of chloride ions. All polarization curves showed hysteresis with higher reaction rate in positive going sweep than in the negative one. Tafel slopes of about −90 mV dec−1 in neutral and alkaline solutions without chloride ions and about −150 mV dec−1 in NaCl solution were obtained. Reaction rate is almost independent on pH in solutions of pH < 12 while in more alkaline solutions reaction order with respect to OH− ions of about −1 was obtained. Reaction rate and kinetic parameters for O2 reduction on duplex stainless steel were compared to data for O2 reduction on pure Fe and great similarity was found.