AN ELECTROCHEMICAL STUDY OF THE ATMOSPHERIC CORROSION OF MILD-STEEL .1. EXPERIMENTAL-METHOD

A conventional three-electrode measurement system using a Luggin capillary was successfully developed to study the corrosion of metals under thin films of electrolyte. The experimental arrangement also permitted condensation and evaporation of electrolyte directly on to the metal surface from a gas ambient of controlled composition and relative humidity. This arrangement was intended to allow conditions similar to those of atmospheric corrosion. Automatic resistive compensation in the electrolyte was carried out using a current interrupting potentiostat. Using this apparatus, good potentiodynamic data could be obtained with an electrolyte film thickness of 300 mum using an electrolyte of 0.2 M Na2SO4. Under these conditions, the cathodic kinetics were clearly controlled by the ambient O2 concentration. The oxygen reduction rate was found to increase significantly during evaporation of an electrolyte film which is consistent with increased oxygen transport across the reducing film thickness. However, measurement of the electrolyte film resistance showed a rising resistance with decreasing film thickness. This is consistent with ion transport in the plane of the electrolyte film parallel to the metal substrate.