Oxidative dissolution and anion-assisted solubilisation in the transpassive state of nickel-chromium alloys

The transpassive state of pure Ni, Cr and two Ni-Cr alloys (10 and 20 wt.% Cr) in 14.8 M H₃PO₄ is studied by voltammetric, rotating ring-disc electrode and impedance spectroscopic measurements. The results indicate that the rate of transpassive dissolution of the alloys in a major part of the transpassive range is higher than the rate of this process for both pure metals. A kinetic model retaining the essential features of the corresponding models for the pure metals proposed in the literature has been found to reproduce both the steady-state current versus potential curves and the ac impedance spectra in the transpassive range. The parameters of the model are determined by a fitting procedure and the relevance of their values for the relative importance of two possible pathways in the transpassive state—oxidative dissolution and anion-assisted solubilisation—are discussed.     

Electrodeposition of refractory metals (Ti,Zr, Nb,Ta) from molten salt electrolytes

A selective review of the anodic and cathodic processes associated with electroextraction, electrorefining and plating of refractory metals (Ti, Zr, Nb and Ta) from molten salt-based electrolytes is presented with regard to the results obtained during the last two decades. Special attention is paid to the mechanism of cathodic reduction and its rate-limiting steps, as well as to the parallel chemical reactions of disproportionation and complexing. The anodic processes in corrosion and electrodissolution of Ti, Zr, Nb and Ta in molten electrolytes are also discussed. As a conclusion, a few contradictory subject-related questions are outlined and some new trends of investigation are suggested.     

Corrosion of nickel,iron, cobalt and their alloys in molten salt electrolytes

The processes of high-temperature corrosion, anodic dissolution and passivation of nickel, iron, cobalt and their alloys are reviewed to reveal the progress in understanding the reaction mechanisms defined in the last two decades. In the first part, the procedures of thermodynamical analysis of corrosion processes by potential — pO^2– diagrams are outlined. The second part is devoted to the electrochemical corrosion, anodic dissolution and passivation of the metals studied, the reaction mechanisms and composition of the corrosion layers formed. The effect of the alloying elements on the corrosion resistance and anodic behaviour of the base metal is treated in the third part. A brief summary of the kinetics of the so-called hot corrosion of the studied metals and their alloys in contact with thin molten salt films and aggressive atmospheres is then given. Finally, some conclusions are drawn and some future trends of investigation are indicated.     

An electrochemical and analytical characterization of surface films on AISI 316 as electrode material for pulse electrolysis of water

Pulse electrolysis of water is a highly efficient method of production of hydrogen and hydrogen/oxygen gas mixtures, sometimes called hydroxygen. In conditions of pulse electrolysis, the process rate is reported to increase in comparison to the dc regime, which poses more stringent requirements to the corrosion resistance of the electrode materials. The processes of their corrosion and degradation are expected to depend on the electrical characteristics of the pulse (nominal current/voltage, frequency, duty cycle). The aim of the present paper is to investigate the effect of pulse characteristics on the electrochemical properties of surface films formed on AISI 316 stainless steel using voltammetry and electrochemical impedance spectroscopy. An attempt to correlate these properties with the surface state obtained from microscopic observations and X-ray photoelectron spectroscopic estimations of the surface film composition is also made.