IMPEDANCE SPECTROSCOPY AND OTHER ELECTROCHEMICAL IN-SITU INVESTIGATIONS OF THE PHOSPHATING PROCESS

Equivalent circuits for the phosphate layer were obtained from electrochemical impedance spectra. The dominant time constant is due to the transfer resistance R(t) and the double-layer capacitance C-dl. An initial high frequency pseudo-inductive time constant L(pseudo)/R(2) is followed by a capacitive time constant R(2)C(2) as the phosphating time increases. The results obtained on varying the reference electrode position and the bath composition suggest that L(pseudo)/R(2) is due to the electrical coupling between cell elements while R(2)C(2) is due to an amorphous precipitation of FePO4 in the pores of the phosphate layer. Since electrochemical impedance spectroscopy is restricted to t > 200 s, ac conductance transients were recorded continuously to determine the coverage. Corrosion potential transients were measured simultaneously. These methods were used to estimate the effect of oxidants on the phosphating kinetics with respect to the coverage. The results obtained by the various methods were compared with ex-situ measurements from a previous study.