ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY AS A TOOL FOR INVESTIGATING UNDERPAINT CORROSION

Electrochemical Impedance Spectroscopy (EIS) has been widely utilised in the last few years as a tool, together with surface analysis, to in vestigate the protecting properties of organic coatings for metals. Th e possible defects and limitations of coatings are detectable with the use of such experimental techniques. The metal coated system is gener ally fairly complex and consists of a metal substrate (mild steel, alu minium, zinc, magnesium, stainless steel, etc.), a surface pretreatmen t (chromatation, phosphatation, or mechanical surface preparations, et c.) and some layers of paint with different chemical and physical prop erties (adhesion promoters, primers, intermediate layers and top coati ngs, etc.). All these parameters can influence the electrochemical beh aviour measured by EIS, and therefore also the electrical models (equi valent electrical circuits) used to explain the impedance results. The aim of this work is to introduce the approach of electrochemical impe dance data interpretation by equivalent electrical circuits on organic coated metals showing some examples relevant to different metal subst rates or coating cycles.