O 4 r o T o e

Dospecies so affecting the ‘‘active protection’ ’ of CCCs also called self-healing. In this frame different hypothesis have been suggested to explain the associated loss of corrosion protection.5-9 Among those, structural changes occurring into the CCC and related to de-hydration were evidenced.10 As for the mechanism of formation and growth of CCCs only very few papers have addressed both the role of the redox couples, usually added to the commercial conversion bath to speed up the growth of the coating, and the mechanism of initial formation of CCCs during the conversion process.3,4 Brown et al.3 suggested that on a very pure Al surface a uniform coating develops due to a mechanism of electron tunneling injection through the chemically thinned alumina films always present on aluminum metal. This hypothesis could account only for the very first instants of formation of CCC and the authors did not account for the growth of thicker coatings as occurring in long lasting im-mersion experiments. We believe that both the role of the various redox couples in accelerating the coating growth and the mechanism of growth itself require a better knowledge of the solid-state prop-erties and morphology of thick CCCs grown on well defined Al surface. In order to gain a better understanding of these different aspects, we have undertaken a photocurrent spectroscopy ~PCS! study of different conversion coatings grown on pure Al surfaces.11 derived which appears compatible with the compositional analysis derived by other techniques. These results will be used in future works to monitor in situ by PCS technique possible changes in solid-state properties of CCCs, grown on Al and its alloys, as a function of aging process