Investigation of coatings by standard and advanced electrochemical techniques

Abstract

The barrier properties of coatings are vitally important in the protection of metals against corrosion. Water and ion ingression (e.g. Cl-), which penetrate to the metal/coating interface, generate electrochemically active sites and corrosion follows. Presently various experimental techniques, such as salt spray and electrochemical tests, are used to study corrosion protection properties of coatings and to predict their performance. Salt spray tests only provide a qualitative estimation of the protective properties of coatings without any information on the mechanism of protection or failure. Electrochemical Impedance Spectroscopy is widely used, not only to examine and rank the corrosion protection performance of coatings, but also to obtain important information on the kinetics of coating degradation and corrosion activity during exposure to a corrosive media. Localised scanning methods like the Scanning Vibrating Electrode Technique can be used to acquire localised electrochemical details that cannot be obtained by conventional techniques, can shed yet more light on coating performance. Although the above techniques have all been used to investigate coatings, correlation between the results has not been conducted comprehensively. Therefore the objective of this paper is to fully correlate results obtained by salt spray, Electrochemical Impedance Spectroscopy and Scanning Vibrating Electrode Technique for three different types of coatings; chromate, organosilane and sol-gel on aluminium AA2024.