We study a model of corrosion and passivation of a metallic surface in contact with a solution using scaling arguments and simulation. The passive layer is porous, so the metal surface is in contact with the solution. The volume excess of the products may suppress the access of the solution to the metal surface, but it is then restored by a diffusion mechanism. A metallic site in contact with the solution or with the porous layer can be passivated with rate p and the volume excess diffuses with rate D. At small times, the corrosion front grows linearly in time, but the growth velocity shows a t^−1/2 decrease after a crossover time of order t_c~D/p², where the average front height is of order h_c~D/p. A universal scaling relation between h/h_c and t/t_c is proposed and confirmed by simulation for 0.000 05≤p≤0.5 in square lattices. The roughness of the corrosion front shows a crossover from Kardar–Parisi–Zhang (KPZ) scaling to Laplacian growth (diffusion-limited erosion—DLE) at t_c. The amplitudes of roughness scaling are obtained by the same kind of arguments as previously applied to the other competitive growth models. The simulation results confirm their validity. Since the proposed model captures the essential ingredients of different corrosion processes, we also expect these universal features to appear in real systems.