Growth of a passive film on iron in a neutral borate solution by three-parameter ellipsometry

The growth of passive film on iron has been traced during potential sweep at various sweep rates in a neutral borate solution at pH 8.4. The transient thickness increase from a stationary state at 0.0 V (vs. AgAgCl) was measured by automated ellipsometry by which the ellipsometric parameters Ψ and Δ, and the reflectivity change ΔR/R, are simultaneously obtained. The transient thickness change and the complex refractive index, N = n − jk, of the film were calculated by the three parameters. The complex refractive index of the film during the transient growth is estimated to be N = 2.18 – 0.352j for light of 632.8 nm wave length. The linear potential increase initially causes an increase of the potential difference at the film/solution interface, where the film thickness remains constant and the observed current increases gradually. In the latter stage, where the interfacial potential difference reaches a constant value, steady growth of the film thickness takes place and the current reveals an almost constant value. The interfacial potential difference is a function of the potential sweep rate, and accordingly the current at the steady film growth stage is also a function of the sweep rate. From the transient change of the film thickness and current during the potential sweep at various sweep rates, the interfacial ionic transfer reactions and the mechanism of the film growth are discussed.