AN IMPROVEMENT TO THE MANSFELD METHOD FOR COMPUTING ELECTROCHEMICAL PARAMETERS FROM POLARIZATION DATA

A theory aimed at improving the Mansfeld method for computing electrochemical parameters through the introduction of a convergence criterion and giving an objective significance to the determination of the Tafel slopes has been developed. Examination of some theoretical curves has shown that the convergence of the actual method depends on the corrosion current density, for when the value of I-c is lower than a given boundary, a decrease can be observed in the convergence radius. This difficulty, which requires a suitable choice of the initial values of B-a and B-c, can be overcome by multiplying the function i(Delta E) by an appropriate factor. It is also proven that the evaluation of B-a and B-c depends on the choice of the scheme adopted to compute the polarization resistance. The analysis of some experimental curves concerning the behaviour of iron in H2SO4 and HCl solutions at different temperatures was performed using their analytical representation with a polynomial of the fourth degree. The best-fitting, which accurately represented the experimental data over the Delta E interval, [-50, 50] mV, was used also to calculate R(p) This procedure has shown that, independently of the choice of the initial values and of any graphic comparison, the method proposed provides correct information when I-c is greater than approx. 0.140 mA cm(-2). The method, however, may not be effective when I-c is less than 0.140 mA cm(-2). In the latter case, the numerical sequences used for the computation of B-a and B-c may have an oscillating behaviour, even if the experimental values of i(Delta E) are multiplied by a constant.