Anodic Passivation of Nickel in Sulfuric Acid Solutions

The potential decay curve of a passive nickel electrode on open circuit was followed in solutions of p H ranging from 0.4 to 1.4. The three arrest potentials observed in the decay curve correspond to the equilibrium potentials for the , , and the electrodes, respectively; the second potential is called the Flade potential, and the passivation occurs beyond this potential. A process for formation of the film is proposed that can be represented by the successive reactions, , . Then the critical potential beyond which the oxide formation takes place is calculated, under conditions when the solution in the vicinity of the surface becomes saturated with corresponding to the solubility product of , and is found to be equal to the Flade potential. The proposed mechanism can also explain the ability of the passive film to protect the metal against active dissolution due to breakdown of the film. The mechanisms of dissolution of nickel in the passive and overpassive potential regions are discussed from measurements of the Tafel constant and the p H dependence of the dissolution rate . The results are summarized as follows: passive region; .