The anodic dissolution of tungsten

The anodic dissolution of tungsten was studied in H2SO4-K2SO4 solutions (pH----2 to 2.5) and K2SO4-K2CO3-KOH solutions (pH = 11.5 to 14.5) at 25 ~ Faradaic efficiency studies in basic solutions showed the metal to be oxidized to the +6 state. In acid solutions, a thick protective film of yellow WO3 was formed. Polarization curves possessed linear Tafel regions (slope ~ 2.3 RT/F) at potentials slightly more positive than the rest potentials. An anodic dissolution mechanism is proposed which involves a surface film of W205 that is further oxidized to WO3 and dissolves by hydrolyzation. Tungsten (1) is one of the so-called "valve metals" which are distinguished by their abil ity to form highly protective oxide films upon anodic polarization in many aqueous media. Studies of its electrochemical behavior have appeared infrequently in the l iterature for over 100 years. A bibliography and brief review of much of the early work appears in a paper by Koerner (2). Koerner also reported extensive open-circuit potential measurements of the W electrode and observations of its anodic behavior in various acids and bases. The potentials were dependent on the type of W used, the electrolyte pH, and also showed some variation with the nature of the electrolyte. The former dependence was attributed to the presence of appreciable oxides in the metal and the latter to the effect of specific ions on the solubility of the dissolved W species, primari ly W(OH)6. This compound was believed to break down to tungstic cations in acidic electrolytes and anions in basic. Upon commencing polarization in acid electro-lytes, films were formed on the W surface whose co lo r changed through a definite sequence. These colors were associated with tungsten oxides of progressively higher oxygen content, i.e