Molybdate and Phosphate Cross-Linked Chitosan Films for Corrosion Protection of Hot-Dip Galvanized Steel

Environmentally friendlyand sustainable methods to protect hot-dipgalvanized (HDG) steel from corrosion are extensively studied. Filmsof the biopolymer polyelectrolyte chitosan were ionically cross-linkedin this work with the well-known corrosion inhibitors phosphate andmolybdate. Layers on this basis are presented as components in a protectivesystem and could, e.g., be applied in pretreatments similar to a conversioncoating. For the preparation of the chitosan-based films, a procedureinvolving sol-gel chemistry and wet-wet application was utilized.Homogeneous films of few micrometers thickness were obtained on HDGsteel substrates after thermal curing. Properties of chitosan-molybdateand chitosan-phosphate films were compared with purely passive epoxysilane-cross-linkedchitosan, and pure chitosan. Delamination behavior of a poly-(vinylbutyral) (PVB) weak model top coating studied by scanning Kelvin probe(SKP) showed an almost linear time dependence over >10 h on allsystems.Delamination rates were 0.28 mm h(-1) (chitosan-molybdate)and 0.19 mm h(-1) (chitosan-phosphate), ca. 5% ofa non-cross-linked chitosan reference and slightly higher than ofthe epoxsyilane cross-linked chitosan. Immersion of the treated zincsamples over 40 h in 5% NaCl solution yielded a 5-fold increase ofthe resistance in the chitosan-molybdate system, as evidenced by electrochemicalimpedance spectroscopy (EIS). Ion exchange of electrolyte anions withmolybdate and phosphate triggers corrosion inhibition, presumablyby reaction with the HDG surface as well described in the literaturefor these inhibitors. Thus, such surface treatments have potentialfor application, e.g., in temporary corrosion protection.