Electrochemical and structural properties of a polyurethane coating on steel substrates for corrosion protection

Electrochemical impedance spectroscopy (EIS) has been employed in the investigation of the corrosion characteristics of polyurethane polymeric film applied on metallic substrates during immersion in 0.5 M NaCl aqueous solution. Galvanized steel and mild steel coupons were studied. EIS has made it possible to monitor the penetration of the surrounding electrolyte into the organic coating and determine the extent of the coating breakdown. The EIS spectra were modeled in terms of equivalent electric circuits to explain the impedance results. The time course of the impedance parameters was used to establish the effectiveness of the anticorrosive properties of the various paint-substrate systems. Pull-off adhesion and microhardness tests were conducted on the samples both before and after exposure to the corrosive environment, in order to investigate the existence of a relationship between the changes in mechanical properties of the coating/metal system and the degradation process as monitored by EIS measurements. Polyurethane coating shows higher adhesion and enhanced anticorrosion protection characteristics when applied on carbon steel than on galvanized steel.