Impedance analysis method for painted and unpainted metals

Abstract

An interactive computer analysis program, Z_CALC, is described for the analysis of electrochemical impedance data obtained from both unpainted and painted metal/solution interfaces. This program consists of two main subprograms, AUTOGUESS and CURVEFIT. The subprogram AUTOGUESS is used to obtain initial guess values for parameters of the model chosen to represent the unpainted or painted metal/solution interface. It is a semi-automatic routine requiring only a little operator iteration to identify impedance data belonging to a semicircle. AUTOGUESS is based on a non-iterative linear least squares curve fitting procedure for a Nyquist depressed semicircle, and on the method of subtraction. Subprogram CURVEFIT uses the initial guess values from AUTOGUESS and improves their accuracy using an iterative, complex, non-linear least squares method. Three such non-linear least squares algorithms (Gauss-Newton, Levenberg-Marquardt, Modified Gauss-Newton), are compared, using corrupted theoretical data, for their ability to converge and speed of execution. Z_CALC is also compared with two commercially available impedance analysis programs—EUQIVCRT and LEVM. These comparisons indicate what computers the programs run on, the ability of each program to provide parameter initial guess values, the types of equivalent circuit models available, curve fitting ability (convergence properties and number of iterations) and user-friendliness. Examples are given of the application of the Z_CALC program for the analysis of impedance data from unpainted (a 55%Al-Zn Atmospheric Corrosion Monitor with 0.5 mm water films exposed to laboratory air containing 0.05 ppm [SO 2]) and painted (silicone modified polyester painted ZINCALUME (55%Al-Zn alloy coating on a steel substrate) immersed in 5% NaCl solution at 50°C) metals.