Ellipsometric study on the film formation of nickel in phosphate solutions

Ellipsometry was used to study in situ corrosion and passivation of nickel in phosphate solutions with pH values 9.1 and 4.5. The passive films formed in both solutions possessed the same optical constants but differed in gowth behavior. The film formed in pH 9.1 solution reached a steady state in a few minutes while that in pH 4.5 solution followed either logarithmic law or inverse-logarithmic law when the film was thin (less than 20 Å) and followed parabolic law when the film was thick. After prolonged polarization, additional films were formed on top of the passive film. Experiments conducted at open circuit potential in pH 9.1 solution produced a Ni(OH) 2 film. At higher potentials this film was probably partially dehydrated and decreased in thickness. Experiments conducted in pH 4.5 solution at open circuit potential as well as at potentials more negative than the critical passive potential produced films which were identical to those films grown on top of passive film at prolonged passive polarization.