Abstract: The passivation behavior and the pitting process at the fixed potentials for Ni electrode in Na_2SO_4-borax-borate buffer solution have been studied with alternating current(AC) impedance technique. For the passivation in the Cl~--free buffer solution the maximum of impedance moduli |Z| and the minimum of capacitances C_(dl) appeared at the potential of -0.2 V (vs. Hg/HgO), because both the thickness of passive film and the rate of anodic dissolution increased with rising potential. As the pitting went on in the Cl~--containing buffer solutions at the fixed potentials, |Z| at low fretrations of 0.1, 0.05, 0.01 mol·L~(-1) and the fixed potentials of 0.2, 0.7, 0.7 V, respectively, are discussed.

Key words: Ni electrode, Passivation, Pitting corrosion, Alternating Current impedance techniques, Borax-borate solution