Characterization of a thin protective coating on galvanized steel by electrochemical impedance spectroscopy and a thermostimulated current method

Electrochemical impedance spectroscopy (EIS) was coupled with a thermostimu lated current (TSC) method to investigate the effect of fillers on the barr ier properties of a polyurethane-based film deposited on galvanized steel. Three systems were studied: a clear coat and two pigmented coatings (with a nd without chromates). From the impedance measurements carried out for diff erent immersion times in a 0.5 M NaCl, it was shown that, for pigmented sys tems, the size and/or the number of pores in the coatings significantly inc reased with the exposure time. This effect was lower for the chromated syst em. TSC showed that chromates stabilize the structure of the coating due to electrostatic interactions. Thus the formation of pores and the diffusion of water and ions through the coating is slowed down. TSC measurements expl ain the higher reproducibility of the experiments obtained for the pigmente d systems in comparison with the clear coat because pigments increase the a mount of hard phase in the coating due to their interactions with the ureth ane groups. Impedance results gave clear evidence of the beneficial effect of chromate for the corrosion resistance of galvanized steel. (C) 1999 Else vier Science Ltd. All rights reserved.