PASSIVATION OF NI-CR-MO ALLOYS IN CHLORIDE SOLUTION A NEW KINETICS MODEL

In studying the current Ia decay versus time under anodic polarization during the passive film growth the experiment data are found to obey the relation: Ia = A t-b exp(Ct). The C coefficient accounts for the observed deviations, characterized by downward or upward bendings observed on the logIa=f(logt) plots. The potential dependencies of the A and C kinetics parameters allows a better understanding of the nickel base alloys behaviour in chloride solutions, in which the pit initiation competes with the passivation process for potentials higher than a critical value Ec, lower than the pitting potential. A new kinetics model of the film growth is then proposed, leading to the final expression of the metal cations concentration including the electrical migration transposed in the finite oxide medium. This model pointed out the high effect of the electrostatic field on the current responses of the studied systems. © 1990.