Waterborne chitosan–epoxysilane hybrid pretreatments for corrosion protection of zinc

Biopolymer-based systems are extensively studied as green alternatives for traditional polymer coatings, e.g., in corrosion protection. Chitosan–epoxysilane hybrid films are presented in this work as a chitosan-based protective system, which could, e.g., be applied in a pretreatment step. For the preparation of the chitosan–epoxysilane hybrid systems, a sol–gel procedure was applied. The function of the silane is to ensure adhesion to the substrate. On zinc substrates, homogeneous thin films with thickness of 50–70 nm were obtained after thermal curing. The hybrid-coated zinc substrates were characterized by infrared spectroscopy, ellipsometry, and x-ray photoelectron spectroscopy. As model corrosion experiments, linear polarization resistance was measured, and cathodic delamination of the weak polymer coating poly(vinylbutyral) (PVB) was studied using scanning Kelvin probe. Overall, chitosan–epoxysilane hybrid pretreated samples showed lower delamination rates than unmodified chitosan coatings and pure PVB. Electrochemical impedance spectroscopy confirmed a reduced ion permeability and water uptake by chitosan–epoxysilane films compared to that of a nonmodified chitosan coating. Even though the coatings are hydrophobic and contain water, they slow down cathodic delamination by limiting ion transport.