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Metallic magnesium is chemically corroded by the 2HF‐KF mixture at 80°C, with the formation of a passivating microporous magnesium fluoride layer as revealed by electron scanning micrograph and cyclic voltammetry. A more compact layer can be obtained electrochemically. The corrosion mechanism has been studied by ac impedance measurements. The reaction pathway involves oxidation, desorption, diffusion, and precipitation steps. Kinetic parameters (charge and mass transfer) have been determined by a computerized simulation method.

The electrochemistry of titanium in the LiC1-KC1 eutectic melt has been investigated by means of pulse techniques and ac impedance measurements. The Ti3 § electrochemical system appears reversible while Ti2+/Ti 3 § is quasi reversible and metallic titanium deposition from Ti 2 § is irreversible. Kinetic constants related to these redox systems have been determined, as have the thickness .of the diffusion layer, the double layer capacitance, and the electrolyte resistance.The electrodeposition of metallic titanium from titanium (IV) chloride in molten chloride melts has aroused a lot of interest since the early fifties. Nevertheless, the important discrepancies between the various results available in literature led us to perform a systematic study of the thermodynamic stability of titanium chlorides and oxides in a particular bath, the molten eutectic LiC1-KC1 at 743 K.First, we determined the standard potentials of the three different titanium redox systems of titanium [Ti(0)/Ti(II), Ti(II)/Ti(III), and Ti(III)/Ti(IV)] and the solubility products of all the titanium oxides (1). Second, because of the lack of information abQut the kinetic parameters of the electrochemical systems of titanium, we have undertaken their determination by using pulse and ac impedance techniques. This is the subject of this paper. TechnicalPreparation of melt and apparatus.--The eutectic melt (45 weight percent (w/o) LiC1-55 w/o KC1) was contained in a Pyrex crucible placed inside a Pyrex reactor. This mixture was fused under vacuum, as it was described earlier (2-4), and then maintained at the working temperature (743 K) under an inert atmosphere of dry argon. Heating was accomplished by means of a Renat furnace connected to a temperature-controlled device (Microcor Chauvin-Arnoux).Products and electrodes.--Lithium and potassium chlorides (minimum purity 99%) were supplied by Merck. Solid titanium(III) chloride (purity -> 98%), from Alpha Ventron, was kept in a dry glove box until its use. TIC13 was added into the melt by means of a "powder burette" (3).The working and counterelectrodes were tungsten wires (surface area S = 0.24 cm 2) and the reference electrode was the silver-silver chloride (0.75 mol 9 kg -1) reference previously described (3). Metallic wires were supplied by Johnson-Matthey (W purity: 99.98%; Ag: 15 ppm of impurities). Silver chloride was a Merck product (99%). Potentials reported in this paper are.referred to the chlorine (1 atm)chloride electrode whose potential is + 1.023V vs. our reference electrode.Pulse techniques.--Chronoamperograms and chrono-' potentiograms have been recorded by using a PAR Model 176 potentiostat-galvanostat connected, via a Model 273 interface, to an Apple IIe microcomputer. Experimental data were stored onto 51/4 in. flexible disks and plotted with a 7470 A Hewlett-Packard plotter.AC impedance measurements.--Experiments were performed with a Tacussel Z Computer system coupled to a Hewlett-Packard 9826 microcomputer. Measurements have been carried out using a 15 mV amplitude sine wave sig...

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