Characterisation 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. Impedance measurements were performed at room temperature in Na2HPO4 solutions of concentrations from 5 x 10(-4) to 0.1 mol L-1 on steel and on electrodeposited 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 and the ability to strongly differentiate electrochemical behaviour of phosphated and unphosphated materials. In this solution, ratios of charge transfer resistance and of interfacial capacitance for phosphated and unphosphated materials can be used for the quality evaluation, however, they cannot be assigned to real porosity of the phosphate coatings. (C) 1997 Elsevier Science Ltd.