ChemInform Abstract: The Effect of F- on the Anodic Passivation of Nickel

ChemInform Abstract

has been studied in Na2SO4/NaF solutions (pH 3.0 and 5.0) by electrochemical and surface analytical techniques. The presence of F- interferes with the long-term establishment of passivity and the anodic currents only fall to the mA range. In the absence of any prior cathodic reduction, potentiostatic immersion of electropolished Ni into the F- solution results in an initial decay of the current toa fairly low value followed by an increase to higher steady-state values. The delay in establishing steady-state conditions depends on the pH and the delay time increases with increasing pH. Similar delays are observed on cathodically reduced (oxide-free) surfaces if the anodization potential is sufficiently high. The delays are especially noticeable in potential switching experiments, and a stable and unstable state of the surface can be defined for each potential. On the basis of AES results it is suggested that F- is incorporated into the surface oxide film and that the amount of F- incorporated increases with increasing anodization potential.