CHARACTERIZATION OF CHROMATE-PHOSPHATE CONVERSION LAYERS ON AL-ALLOYS BY ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY (EIS) AND OPTICAL MEASUREMENTS

Chromate conversion layers of a thickness ranging from 20 to 250 nm on Al-alloys are characterized by EIS in 0.1 M Na2SO4 and optical measurements. A model is derived from these data which describes the corrosion mechanism of these samples. The main components of the equivalent circuit can be attributed to the diffusion barrier and to defect sites within the layer. Optical spectroscopy and electrogenerated chemoluminescence confirm these main factors independently of EIS.