| Chitosan, a derivative of naturally abundant biopolymer chitin, was used as the basis for corrosion resistant coating. Chitosan suffers from two inherent weaknesses as a coating material, namely its high hydrophilicity and its poor adhesive strength with Al 2024 T3 alloy. In the present study, the chitosan structure was modified using epoxy functional silane and vanadate. Two epoxy functional silanes (3-Glycidoxypropyl)-trimethoxysilane (GPTS) and (2-(3,4-epoxycyclohexyl)-ethyltrimethoxysilane (ECET) were tested. The performance of different coatings was tested using electrochemical impedance spectroscopy, adhesion testing and salt spray testing. Addition of GPTS resulted in improvement in corrosion resistance and adhesive strength. Chitosan-GPTS-vanadate coatings prepared using chitosan-GPTS solution at viscosity 0.96 pa-s and post treated in NaVO3 solution at pH 6-8 demonstrated the highest corrosion resistance. The best salt spray performance was observed in case of chitosan-GPTS-vanadate coatings, which lasted 450 hours in salt spray chamber.; Detailed fundamental characterization was carried out related to the structure, chemistry and properties of chitosan-based coatings using optical spectroscopy. FTIR spectra of chitosan gel showed adsorption of vanadate at protonated amine sites of chitosan. Chitosan showed a maximum in the vanadate adsorption capacity when treated in NaVO3 solution at pH 3-5. GPTS reacted with amine functional group of chitosan and, at the same time, formed a hydrophobic siloxane network with the Al alloy substrate. Formation of a siloxane network with the Al substrate provided the observed increase in corrosion and adhesive strength of the coatings.; UV/Visible spectroscopy measurements showed release of vanadate by chitosan increases with increasing solution pH, increasing chloride concentration and polarizing the sample cathodically. Structured experiments have been used to show that vanadate is reversibly bound to chitosan. Adsorption and release have been found to depend strongly on the pH of the aqueous solution contacting the chitosan coating. When the solution pH is readjusted to a lower value, chitosan can re-adsorb released vanadate. Further, a direct electrochemically triggered release of inhibitor was demonstrated by cathodically polarizing the coated sample. Release of vanadate under different conditions demonstrated the on demand inhibitor release capability of the coatings. This capability of coating is useful to provide a self-healing effect. |
