Anti-corrosion performance studies of organic coatings have been normally carried out by accelerated
weathering (saltspray, humidity cabinet, Kesternich,
etc.) or outdoor exposure of coated panels. Both
methods do not provide information about the
failure mechanism of the coatings and the former
methods can give unpredictable results which cannot
very well be compared with the actual outdoor performance of the coatings. Electrochemical Impedance Spectroscopy (EIS) has been used for some
years now in the study of the performance of organic
coatings. It was observed that the technique has
promising possibilities in this field, however the
interpretation was mostly carried out in terms of
changes in the curves in the Nyquist plot of the
impedance (eg[l, 21) and the accuracy of the measurements was not very high. Nowadays it is possible to perform a computer aided analysis and
modelling of the impedance measurement results[3,
43. Also modem electronics increased the accuracy
of the equipment a great deal.
In literature the impedance of barrier coatings was
often evaluated by the visual or graphical evaluation
of the Nyquist plot[l], or by judging the dcresistance of the coating[5]. It was commonly considered that when the Nyquist or Bode plots of the