ИСТИНА

Anodic dissolution of tungsten in alkaline solutions is a widely used process that underlies electrochemical polishing and electrochemical machining of the metal, electrochemical fabrication of sharp tips for scanning probe microscopes and tools for nanoelectrochemical machining. A number of studies have been devoted to the investigation of anodic dissolution of tungsten in the alkaline solutions. At low polarizations, the Tafel portions are observed in the anodic polarization curves. At higher anodic potentials, the dissolution rate is controlled by the mass transfer in the near-electrode layer, and the limiting current is observed. From the experiments with a tungsten disk electrode, it was concluded that this diffusion limiting current is associated with the rate of delivery of OH ions, which are involved the dissolution. The diffusion limiting currents, which were calculated by the analytical equation derived for this case, agree well with the experimental data [1, 2]. In - the calculations, the diffusion coefficient of OH ions was taken to be equal to that for infinitely diluted solution. The migration mechanism of mass transfer was ignored. - To check the above results, in this work, the problem of limiting current of tungsten anodic dissolution in alkaline solution is solved numerically taking into account the concentration dependences of diffusion coefficients of ions, viscosity and density of solution, the migration mechanism of transfer. The equations of ionic transfer were solved simultaneously with the hydrodynamic equations. The distributions of dimensionless concentrations of all four types of ions and dimensionless potential are calculated with and without r egard for the concentration dependences of aforementioned characteristics. It is concluded that the concentration dependence of diffusion coefficients of ions plays the leading role. In addition, a similar problem of the limiting current of tungsten anodic dissolution in alkaline solution is solved under the conditions of natural convection. The calculated and experimental limiting currents are compared.