Anodization of molybdenum in glycol-borate electrolyte— A peculiar kinetics of insulating film formation

Abstract

Anodic film formation on Mo in a glycol-borate electrolyte was studied at a temperature of 1.5°C and in a wide range of current densities (20μA/cm 2< J<50 mA/cm 2). The anodization process was found to be affected not only by the usual anodizing parameters ( J, T and electrolyte composition), but also by the convective conditions in the electrolyte and (at low J) by visible light irradiation. Under galvanostatic and isothermal conditions and in unstirred electrolyte the anodization always begins with an induction period during which the electrode potential remains low and almost constant. After that the electrode potential increases abruptly to high voltages. The anodizing kinetics was found to be different from the kinetics of insulating film formation on other metals, known to the author. A qualitative film formation mechanism, involving the formation of semiconductor film and the conversion of this film into an electronic insulator, is proposed to explain the anodic behaviour of Mo.