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 investigate the protecting properties of organic coatings for metals. The possible defects and limitations of coatings are detectable with the use of such experimental techniques. The metal coated system is generally fairly complex and consists of a metal substrate (mild steel, aluminium, zinc, magnesium, stainless steel, etc.), a surface pretreatment (chromatation, phosphatation, or mechanical surface preparations, etc.) and some layers of paint with different chemical and physical properties (adhesion promoters, primers, intermediate layers and top coatings, etc.). All these parameters can influence the electrochemical behaviour measured by EIS, and therefore also the electrical models (equivalent electrical circuits) used to explain the impedance results. The aim of this work is to introduce the approach of electrochemical impedance data interpretation by equivalent electrical circuits on organic coated metals showing some examples relevant to different metal substrates or coating cycles.