A modified version of the foil penetration technique based on an optical detection of the full penetrating intergranular defects through thin foils was applied to AA2024 samples of various thicknesses in contact with a dilute chloride solution. As the rear side of the foil was free during the experiments, it was straightforward to measure the pH of the electrolyte trapped inside the grain boundary network leaking through the emerging defects on the rear side of the foils. The penetration regime in the longitudinal direction was determined for various exposure and electrochemical conditions confirming the decrease of the propagation kinetics as function of the depth of the intergranular defects. pH measurements were used to support mass transport simulation inside a pore electrode mimicking the propagation of the anodic head of grain boundaries. After validation under potentiostatic conditions, the same modelling approach was applied under free corrosion conditions, to discuss the location of cathodic areas during immersion and droplet exposures. (C) The Author(s) 2018. Published by ECS.