THE PASSIVITY OF NICKEL--EFFECTS OF APPLIED POTENTIAL, SOLUTION PH AND ENVIRONMENTAL TEMPERATURE

The passivity of nickel was studied by ellipsometric technique. Three important parameters that affect the passivity of metals--potential, solution pH, and temperature--were systematically investigated.;On the other hand, solution pH has a remarkable effect on the formation of the passive film. In basic solutions (pH 9.1 and 12.6), the passive film can be formed instantaneously and then remains unchanged for a couple of hours. In moderate acid solution (pH 4.5), the passive film grows with time. In strong acid solution (pH 1.5), a two-layer passive film is observed which evolves with time, too.;When a pre-passivated nickel specimen is immersed in a solution for prolonged time, a multilayer film is found to grow on top of the passive film that originally exists on the surface of the metal.;The film formation behavior of nickel in aerated pH 9.1 solution at open circuit potential exhibits a sharp contrast with that in deaerated solution. That is, in aerated solution, a Ni(OH)(,2) film is formed. This film then undergoes a partially hydration process at later stage. In deaerated solution at the open circuit potential, the film is formed in two stages: the first stage is the formation and growth of a partially hydrated NiO film and the second stage is the growth of a multilayer film on top of the first stage film.;It was found that, within the passive range, applied potential affects the thickness of the passive film but not the growth behavior of the film. This is also true for the effect of temperature on the passive film. Generally speaking, increasing applied potential and/or temperature increases the thickness of the passive film.