CHARACTERIZATION OF PHOSPHATE COATINGS ON ZINC, ZINC-NICKEL AND MILD-STEEL BY IMPEDANCE MEASUREMENTS IN DILUTE SODIUM-PHOSPHATE SOLUTIONS

Electrochemical Impedance Spectroscopy (EIS) was used in seeking a non -aggressive test solution for evaluation of phosphate coatings. Impeda nce measurements were performed at room temperature in Na2HPO4 solutio ns of concentrations from 5 x 10(-4) to 0.1 mol L-1 on steel and on el ectrodeposited Zn and Zn-12%Ni. It was found that 0.005 M Na2HPO4 was most appropriate as a test solution owing to its low aggressiveness an d the ability to strongly differentiate electrochemical behaviour of p hosphated and unphosphated materials. In this solution, ratios of char ge transfer resistance and of interfacial capacitance for phosphated a nd unphosphated materials can be used for the quality evaluation, howe ver, they cannot be assigned to real porosity of the phosphate coating s. (C) 1997 Elsevier Science Ltd.