Corrosion rate monitoring by the linear polarization method

The characteristics of the electrochemical system and the criteria for a correct application of the method of polarization resistance aimed at the evaluation of the corrosion current density are carefully discussed. The mathematical theory of the validity of the linear approximation of the kinetic law, over a suitable interval round the mixed potential, is based on the integral representation of the remainder of the series expansion of i( ΔE). This scheme makes it possible to compare the contribution of the linear term with that due to the non-linear remainder by introducing a criterion which permits the goodness of the approximation to be verified. Numerical examples have shown that the interval width, where the system could exhibit an approximate linear behaviour, depends on the three parameters α, β and ϵ and is independent of the bending radius of the curve i = i( ΔE) at ΔE = 0. Furthermore, the absolute values of the left and right bounds ΔE 1 and ΔE T are usually different. They are equal only when α = β. Examination of some experimental data concerning the behaviour of Armco iron in H 2SO 4 I N solutions inhibited with n-decylamine has demonstrated that the choice of the proportionality factor ( α + β) −1 plays an important role when measurements are aimed at a quantitative evaluation of the corrosion rate.