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 thermostimulated current (TSC) method to investigate the effect of fillers on the barrier properties of a polyurethane-based film deposited on galvanized steel. Three systems were studied: a clear coat and two pigmented coatings (with and without chromates). From the impedance measurements carried out for different immersion times in a 0.5 M NaCl, it was shown that, for pigmented systems, the size and/or the number of pores in the coatings significantly increased with the exposure time. This effect was lower for the chromated system. 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 explain the higher reproducibility of the experiments obtained for the pigmented systems in comparison with the clear coat because pigments increase the amount of hard phase in the coating due to their interactions with the urethane groups. Impedance results gave clear evidence of the beneficial effect of chromate for the corrosion resistance of galvanized steel.

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