AN ELLIPSOMETRIC STUDY OF THE ANODIC PASSIVATION OF IRON-NICKEL ALLOYS IN ACID-SOLUTION

Steady state polarization curves of several iron-nickel alloys were measured in acid sulphate solution, pH 1.3. The apparent optical constants of the active alloys, and the thickness and the complex refractive index of the passivating films as a function of electrode potential and addition of chloride were determined by ellipsoreflectometry. The results are compared to a theory for the relation between the rate constants of the charge transfer reactions of the components across the interface and their transport coefficients in the film, and the compositions at the interfaces and within the film. According to the optical constants, the composition of the passivating film changed little with the composition of the alloy and was similar to the film on pure nickel. The film thicknesses grew with the nickel content and were larger than for both pure iron and pure nickel. After addition of chloride, the current density increased slightly and the thickness and the refractive index of the passivating film decreased slowly with time before pits started to grow. © 1990.