Plates of Al–Si alloy were anodized in a sulfuric acid solution. This treatment provides aSi–Al2O3coating growing at a rate of 0.14 &mgr;m/min. The Si particles had sizes between 1 and 10 &mgr;m, as seen by scanning electron microscopy. Optical measurements showed a continuous decrease of reflectance with increasing film thickness. The reflectance of theSi–Al2O3coated aluminum could be understood from a four flux radiative transfer theory. In order to explain our measurements, it was found necessary to include a free-carrier term in the dielectric permittivity of Si. The free carriers are probably due to doping with Al. Hence, the relaxation time of the free carriers is determined by scattering from the charged Al impurities. ©1997 American Institute of Physics.