Effect of Mass Transport on the Determination of Corrosion Rates from Polarization Measurements

Abstract

Many methods have been proposed for the determination of the corrosion current density from polarization curves; the two most commonly used ones are the three‐point method and the curve‐fitting method. The basis of these methods is a polarization equation that (i) was derived for the complete absence of mass‐transport effect or (ii) considers the cathodic partial reaction to be under complete mass‐transport control. The mixed‐control case has been much less investigated, even though this case may quite frequently reflect practical situations. A completely generalized polarization equation is proposed that can be used as a basis for curve‐fitting data evaluation under any conditions. This equation was used to determine the error caused by the neglect of mass‐transport effect in conventional data evaluation. The error of the calculated corrosion rate was found to be the function of two dimensionless parameters: the ratio of the corrosion current density to the cathodic limiting current density , and the ratio of the Tafel slopes . It was shown that the error can be considerable, even for small , if the ratio is high. However, up to , which is the case for many corrosion systems, the maximum error of the conventional data evaluation methods is less than 25% even when the system is near or under cathodic mass‐transport control. The errors at high values can be avoided by using a curve‐fitting data evaluation method based on the proposed polarization equation to determine , and, if desired, .