J. Thonstad

Studies on the corrosion and the behavior of inert anodes in aluminum electrolysis

The corrosion rates of inert anodes based on tin oxide and nickel ferrite cermet materials were studied as a function of some operating parameters. To reach a better understanding of the corrosion mechanism, the behavior of the anodes was observed under some specific conditions, such as in pure cryolite, at high current densities, at different potentials, and at varying cathode surface areas. It was confirmed that low alumina concentrations led to catastrophic corrosion of the anodes and that high current densities and high as well as low NaF/AlF3 molar ratios were also detrimental. The corrosion rate of tin oxide based anodes showed a minimum (so-called “normal corrosion”) at anodic potentials of 2.2 to 2.4 V with respect to aluminum. The normal corrosion is due to chemical dissolution of the anode material and reduction of the corrosion products into the cathode metal. The corrosion rate increased with increasing cathode surface area. At potentials higher than ∼2.5 V, the anodes showed catastrophic corrosion. Catastrophic corrosion can be ascribed to decomposition of the anode material by depletion of alumina at the anode surface provoked by low bulk concentration of alumina and/or high current density.

On the anode effect in cryolite-alumina melts—I

Abstract The behaviour of graphite anodes in cryolite-alumina melts prior to and during the occurrence of the anode effect (AE) was studied by potential-sweep and galvanostatic measurements. In melts with low alumina contents a potential jump was observed, corresponding to the beginning of co-discharge of fluoride ions. The AE appears to be preceded by a depletion of oxygen-containing ions followed by codeposition of fluorine, which then provokes the AE. The i/V curves during the AE have a complicated shape with maxima at 45 and 95 V. Sparks were seen in the ranges below 45 V and from 90 V until an arc was lit at 140 V. Anodes maintained at 20–45 B vecame polished.

On the anode effect in cryolite-alumina melts—II the initiation of the anode effect

Polarization curves obtained with graphite anodes in cryolite-alumina melts by steady state, potential sweep and galvanostatic measurements exhibited three steps at low alumina concentrations, due to the formation of CO2 and CF4, prior to the anode effect (AE), and probably of F2 after the occurrence of the AE. Formation of CF4 started at ~2·8 V positive to the aluminum electrode, while the potential and the critical current density (ccd) at which the AE occurred were 3·5 V and 0·11 A/cm2 resp. in purified cryolite and 2·7 V and 2·1 A/cm2 at 1 wt per cent Al2O3. The proportion of CF4 in the gas reached 40 per cent in purified melts, but decreased sharply on the addition of alumina. At > 2 wt per cent Al2O3 the ccd decreased when the external pressure was lowered approximately in proportion to the volume of escaping gas. It is concluded that the AE occurs when the melt adjacent to the anode is depleted with respect to oxygen-containing ions, and the rate of fluoride ion discharge exceeds the rate of CF4 formation.

Electrical Conductivity of Molten Cryolite‐Based Mixtures Obtained with a Tube‐type Cell Made of Pyrolytic Boron Nitride

A pyrolytic boron nitride tube-type cell was used to measure the electrical conductivity for molten cryolite, for binary mixtures of cryolite with Al2O3, AlF3, CaF2, KF, Li3AlF6, and MgF2, and for ternary mixtures Na3AlF6 - Al2O3 - CaF2 (MgF2) and Na3AlF6 - AlF3 - KF (Li3AlF6). The cell constant was about 40 cm−1. The temperature and concentration dependence of the conductivity in the investigated concentration range was described by the equation

Electrodeposition of titanium diboride from fused salts

\begin{gathered} \ln \kappa =1.977-0.0200\left[ {A{{l}_{2}}{{O}_{3}}} \right]-0.0131\left[ {Al{{F}_{3}}} \right]-0.0060\left[ {Ca{{F}_{2}}} \right] \hfill \\ \quad \;\;\;-0.0106\left[ {Mg{{F}_{2}}} \right]-0.0019\left[ {KF} \right]+0.0121\left[ {LiF} \right]-1204.3/T \hfill \\ \end{gathered}

Gas induced bath circulation in aluminium reduction cells

where T represents the temperature in K, and the brackets denote the concentration of the additives in wt%.

Electrochemical properties of metal-molten salt mixtures

Abstract When a constant cathodic current is applied to an aluminium electrode in a Na 3 AlF 6 Al 2 O 3 melt at 1010°C, the potential decreases gradually and linearly with the square root of time to more negative values. Pronounced potential oscillations occurred at cds above 2A/cm 2 and the evolution of sodium gas was observed at very high cds . Steady state measurements yielded straight η vs log i plots with a slope of −0.23V/decade, the overvoltage being −0.19V at 1A/cm 2 . The overvoltage decreased markedly when the melt was stirred. Potential decay measurements yielded linear log time plots with slopes of 0.01–0.5V/decade. The charge-transfer resistance was determined by double pulse measurements to be 0.0031 ohm cm 2 . Ac impedance measurements gave a similar result. The charge transfer overvoltage accounts for only about two percent of the total overvoltage, the rest is apparently diffusion controlled. The cathodic overvoltage in industrial aluminium cells is of similar magnitude as found in the laboratory investigations.

Electrical conductivity of molten cryolite-based mixtures obtained with a tube-type cell made of pyrolytic boron nitride

Electrodeposition of TiB2 has been performed in cryolite-based electrolytes at 960°C and in KF-KCl melts at 800°C. As electroactive species either boron oxide and titanium oxide or potassium tetrafluoroborate and potassium hexafluorotitanate were used. Preparation of coatings from cryolite-based electrolytes containing K2TiF6 and KBF4 was not successful. Coatings prepared from cryolite-based electrolytes containing B2O3 and TiO2 were not coherent. Owing to the relatively high temperature both types of electrolytes undergo thermal decomposition. Electrolysis in potassium fluoride-chloride electrolytes containing KBF4 and K2TiF6 provides coherent coatings with good adhesion to the substrate.