INTERPRETING NYQUIST RESPONSES FROM DEFECTIVE COATINGS ON STEEL SUBSTRATES

Interpretation of AC impedance plots for coated metal substrates can be complicated, particularly when small defects are present in the coating. Theoretically, the Nyquist plot for a defective coating undergoing active corrosion should contain two semicircular responses, a high frequency response due to the coating and a lower frequency response due to the corrosion cell formed at the defect. Experience has shown, however, that a number of situations can lead to the resolution of a single semicircle the origin of which is often difficult to establish. The paper describes a programme of work, the purpose of which was to obtain a greater understanding of the factors which influence impedance plots, in particular the effect of variables such as defect size, electrolyte conductivity and coating thickness. It also describes a protocol which has been developed for distinguishing between a coating and corrosion response in situations where a single Nyquist semicircle occurs.