Analysis on Restoration Rate of Chromate Film Formed on Steel and Galvanized Steel by 3D Impedance Spectroscopy

Abstract

Shintomi, Three-dimensional （3D） impedance spectroscopy was applied to the evaluation of film growth rate of a chromate film formed on a steel and a galvanized steel. Impedance spectra were plotted on a 3D diagram with real, imaginary, and time axes. Scratched electrodes of three types were used for 3D impedance measurements to investigate the film growth rate of the chromate film （1） with a scratch reaching to the substrate steel of the chromated steel （C/Fe Fe ） , （2） with a scratch reaching to the substrate steel of the chromated galvanized steel （C/Zn/Fe Fe ） , and （3） with a scratch reaching to the zinc layer of the chromated galvanized steel （C/Zn/Fe Zn ） . Subscripts of C/Fe Fe , C/Zn/Fe Fe and C/Zn/Fe Zn denote the layer to which the scratch reached. The 3D impedance of the C/Fe Fe and C/Zn/Fe Fe demonstrated that the impedance was decreased drastically at the initial stage, and that it increased remarkably with time. The 3D impedance of C/Zn/Fe Zn indicated that the impedance was decreased gradually at the initial stage, and that it increased slightly with time. The instantaneous impedance at an arbitrary time was determined from the cross-section of the 3D impedance plot. The film resistance R df of each electrode was estimated by curve fitting of the instantaneous impedance using the equivalent circuit. Results indicate that the increasing rate of R df until the steady-state condition is in the order of C/ Zn/Fe Zn < C/Zn/Fe Fe < C/Fe Fe . Keywords