The effect of IR-drop on corrosion rates calculated from low polarization data

Abstract

Corrosion rates calculated from low polarization data can have substantial errors due to uncompensated solution/surface film resistance, R, even in solutions of moderate conductivity. The resulting uncompensated IR-drop has the effect of increasing the calculated values of polarization resistance which by itself would be expected to make calculated corrosion rates too low. However, calculated corrosion rates can be too high due to an uncompensated IR-drop in cases where both polarization resistance, R p and anodic Tafel slope for the metal, b a, M , are calculated from low polarization data. This is true for metals corroding under conditions where the cathodic reaction(s) are limited by diffusion, because there is a greater positive error in calculated values of b a, M than in R p . These findings are confirmed experimentally, and techniques for measuring IR-drop are discussed. Electrochemically calculated corrosion rates corrected for both IR-drop and close approach between corrosion and metal equilibrium potentials are compared with those calculated from solution analysis.