A mathematical model for simulating a passive aluminium (Al) surface with a pit in which active electrochemical metal dissolution occurs has been developed. The model includes hydrolysis products of Al and the species obtained as a result of homogeneous reactions between chloride and Al3+ ions and Al hydrolysis products. The model does not assume the equilibrium state in solution: all terms in homogeneous reactions are treated explicitly using kinetic constants taken from the literature. The validity of assuming reaction equilibrium has been addressed. Solution potential values and species concentrations are predicted for different dissolution current densities. The acidity in the pit is explained by the hydrolysis of Al3+; an analytical expression for the pH values at the pit bottom for a given dissolution current density is presented.The model is applied to a real capillary geometry used in electrochemical microcell experiments. It was found that for rcap/rpit