DIRECT AND THROUGH-FILM DISSOLUTION OF NI DURING ANODIC POLARIZATION IN AQUEOUS HF MEDIA

Anodic dissolution of Ni in aqueous HF solutions over a wide concentration range 0.1-20.0 M was investigated using cyclic voltammetry, chronoamperometry and scanning electron microscopy (SEM). In low HF concentrations, anodic dissolution of Ni proceeds by a dissolution-precipitation mechanism through a thin oxide layer of Ni. However, from 5.0 M HF onwards, another dissolution process through the pits of the protective oxide layer of Ni is noticed before the onset of through-film dissolution stated above. In 20.0 M HF the dissolution current due to pits exceeds that due to the other process. The voltammetric responses suggest similar mechanisms for both processes. The salt film dissolves by an electrochemical mechanism during reverse sweep. Current-time transients and SEM studies also substantiate this overall mechanistic pathway.