The electrodeposition of polypyrrole films on an aluminium. substrate in acid nitric solution has been studied by means of cyclic voltammetry and potential step chronoamperometry methods. The coating appears to be initially deposited at flaws in the passive aluminium oxide film. The initial step in the formation of the film is an instantaneous nucleation and tridimensional growth under diffusion control. At high polarisation potentials, the formation of degraded PPy takes place. SEM and open-circuit potential measurements revealed that localised corrosion occurs at the coated aluminium in the presence of chloride ions.