Corrosion protection of galvanized steel by polyimide coatings: EIS and SEM investigations

Coatings of two types of polyimides (PI), as poly(4,4′-oxydiphtalic anhydride-co-2,5-bis(4,4′-methylenedianiline)-1,4-benzoquinone) (AQ) and poly(pyromellitic dianhydride-co-4,4′-oxydianiline) (PM), were synthesized on galvanized steel panels and studied and compared in terms of chemical structure, microstructure and corrosion performance in 0.5 M NaCl solution. Infra-red spectroscopy, scanning electron microscopy and in situ electrochemical impedance spectroscopy were employed in the investigations. The results showed that, although both studied PI coatings provided the galvanized steel substrate with corrosion protection during the test period, there were evident differences in electrochemical behaviour of the coatings, which could be primarily explained by the different nanostructures. FE-SEM examinations revealed AQ PI coating to be heterogeneous and discontinuous but PM PI coating homogeneous and continuous in nanostructure. Electrochemical behaviour of AQ PI coated galvanized steel obeyed that of a defect-containing coating and indicated gradual decrease in protectivity. In contrast, PM PI coating behaved like a defect-free coating and it provided the galvanized steel substrate with effective corrosion protection all through the 960 h test. The explanations for these observations and the mechanisms of coating damage are discussed in this paper.

► Two types of polyimide (PI) coatings were synthesized on galvanized steel panels. ► These were characterized by IR-spectroscopy, SEM and in situ EIS in 0.5 M NaCl solution for 960 h. ► A new amino-quinone PI was structurally heterogeneous and acted like a defect-containing coating. ► AQ PI coating showed a significant decrease in protectivity during the test. ► Conventional PI coating was homogeneous in structure and behaved like a defect-free coating.