An assessment of A/C impedance as a basic research and routine testing method for studying corrosion of metals under paint films

A brief survey is given of electrochemical test methods available for testing the protective properties of paint and other organic coatings over metals in corrosive environments, so as to establish the reasons for choosing A/C impedance as a method for both basic research and routine testing work.

A description is given of the technique and the apparatus used. For research purposes, a transfer function analyser (TFA) is used to apply a small amplitude a/c signal to a cell containing the painted test panel in a corrosive liquid (3.5% aqueous sodium chloride solution as standard). The apparatus automatically programmes a frequency sweep between 10 KHz and 10 mHz and analysesthe response and presents the results as Nyquist or Bode plots. Typical results for studies on a series of epoxy resin paints are presented. Results are also presented showing the use of a commercially available A/C impedance meter for use as a routine testing instrument, as well as studies on damaged paint systems (scribed paint panels).

Finally, an assessment is given of our experience with A/C impedance as both a background research and a routine testing technique for monitoring corrosion of metals under paint films.