The determination of tafel slopes by the successive approximation method

Some theoretical considerations are given concerning the shape of polarization curves and the determination of the polarization potential intervals over which the asymptotic Tafel law is valid. These mathematical arguments are useful to introduce the theory of the method of successive approximations, which allows calculation of the three electrochemical parameters, Ic, Ba and Bc defining the kinetics of a corrosion process under activation energy control. In particular, the technique proposed is shown to be readily obtainable from the asymptotic expressions of the current-voltage characteristic in the anodic and cathodic regions. The problem of the convergence of the numerical sequences generated by recurrence relations is then tackled and it is shown that under suitable hypotheses the problem always has a physical solution, provided that the information contained in the experimental polarization curves is not modified by random or systematic errors and the true kinetics of the corrosion process do not differ markedly from the ideal behaviour. Lastly, two experimental applications are discussed concerning the systems: Armco iron and H2SO4 solutions at different pH values and 25 °C, and Armco iron and l m HCl solutions at temperatures between 25 °C and 65 °C. The results obtained are very satisfactory. In the first case, the validity of the method is confirmed by the observation that the values of the electrochemical parameters are practically coincident with those obtained by the NOLI method, which is a well-established numerical technique. In the second case, the comparison of the direct and electrochemical determinations of the corrosion rate shows that the method provides a faithful representation of the actual behaviour of the corrosion process.