CHROMIUM PHOSPHATE CONVERSION COATINGS ON ZINC ELECTROPLATES - CATHODIC FORMATION AND CHARACTERIZATION

The formation of a conversion coating under cathodic conditions has be en studied in a chromating bath containing phosphate. The experimental techniques used include cyclic voltammetry on a glassy carbon electro de to investigate the electrochemical behaviour of the system and anal ysis techniques (XPS, EDAX, FTIR and analytical methods) to determine the composition of the conversion film on zinc-coated samples. The thi ck chromium phosphate (Cr-P) coating has two distinct layers: an outer porous layer and an inner and thinner pore-free adherent one. Both la yers contain chromium phosphate as the main constituent and, to a less er extent, zinc phosphate species whose concentrations decrease from t he interface metal/coating toward the external surface of the layer. ( C) 1998 Elsevier Science S.A.