We present numerical simulations of corrosion processes in a simple automata cellular model at a mesoscopic scale. We observe that small pieces of the material are detached from the surface and become part of the solution side of the corrosion front. The formation of these islands is important when the probability of dissolution of the material is small. This process is similar to that observed in real experiments and named chunk effect. This effect can be one of the reasons for the deviations of the observed apparent weight loss of the corroding material from that predicted by Faraday's law. The main result of our work is to show that the chunk effect exists even for the ideal surface free of defects and therefore the effect is generally present in corrosion processes. We analyze the simulation results within a mean field approach and describe quantitatively the combination of material dissolution and islands detachment. The effective rate of corrosion is largely determined by the mechanical processes that change the surface structure. (c) 2007 Published by Elsevier Ltd.