Environmentally friendly
and sustainable methods to protect hot-dip
galvanized (HDG) steel from corrosion are extensively studied. Films
of the biopolymer polyelectrolyte chitosan were ionically cross-linked
in this work with the well-known corrosion inhibitors phosphate and
molybdate. Layers on this basis are presented as components in a protective
system and could, e.g., be applied in pretreatments similar to a conversion
coating. For the preparation of the chitosan-based films, a procedure
involving sol–gel chemistry and wet-wet application was utilized.
Homogeneous films of few micrometers thickness were obtained on HDG
steel substrates after thermal curing. Properties of chitosan-molybdate
and chitosan-phosphate films were compared with purely passive epoxysilane-cross-linked
chitosan, and pure chitosan. Delamination behavior of a poly(vinyl
butyral) (PVB) weak model top coating studied by scanning Kelvin probe
(SKP) showed an almost linear time dependence over >10 h on all
systems.
Delamination rates were 0.28 mm h–1(chitosan-molybdate)
and 0.19 mm h–1(chitosan-phosphate), ca. 5% of
a non-cross-linked chitosan reference and slightly higher than of
the epoxsyilane cross-linked chitosan. Immersion of the treated zinc
samples over 40 h in 5% NaCl solution yielded a 5-fold increase of
the resistance in the chitosan-molybdate system, as evidenced by electrochemical
impedance spectroscopy (EIS). Ion exchange of electrolyte anions with
molybdate and phosphate triggers corrosion inhibition, presumably
by reaction with the HDG surface as well described in the literature
for these inhibitors. Thus, such surface treatments have potential
for application, e.g., in temporary corrosion protection.