An Investigation of Corrosion Mechanisms of Prepainted GI and GA Steels by A.C. Impedance Methods

摘要

The most commonly observed corrosion problems of prepainted steels are due to corrosion-induced delami-nation or undermining process, especially initiating from porous regions. A.C. impedance measurements were considered as powerful techniques in the determination of corrosion and delamination mechanisms for prepainted steels. It has been observed that PVDF-painted galvanized (GI) and galvalume (GA) steels exhibit a very capacitive character and show less blistering during long-term immersion testing. However, silicone modified polyester (SMP) painted GI steel in NaCl solution and SMP-painted GA steel in acid rain water exhibit a greater film delamination, a swelling of blisters and a decline of pore resistance on the paint film. Two major charge transfer characterizations of A.C. impedance spectrum were suggested to interpret the corrosion mechanisms of prepainted steels under different corrosive environments.