This study aims at investigating the transport through free-films of organic coatings derived from vegetable oils (crambe oil and castor oil) plus pre-polymer based on hexamethylene diisocyanate and castor oil, which were used in our previous studies to protect aluminum alloy (AA1200) against corrosion in NaCl solution [1]. The free-films were characterized by Raman, NMR, FTIR and AFM (roughness and texture), and the permeability to KCl in aqueous solutions was investigated by electrochemical impedance spectroscopy (EIS) and open circuit potential (E_OCP) measurements as function of time. Different coating formulations were tested to demonstrate the influence of their structure and chemical composition on electrolyte permeability. The good corrosion resistance of these coatings encouraged us to investigate the KCl permeability to assist in understanding the mechanism of corrosion protection of the aluminum alloy surface. The measurements were done by using a setup similar to that employed by Armstrong et al. [2] in 0.1 mol L^-1 KCl aqueous solution, at 25 ^oC and atmospheric pressure. Raman, NMR, and FTIR spectra, and AFM images were taken before and after four months of immersion in the electrolyte and no significant differences between spectra or images were found. These results allowed concluding that no compositional and / or chemical structural and topological changes have occurred in the free-films. EIS and E_OCP measurements were conducted with the free-film of organic coatings immersed in 0.1 mol L^-1 KCl aqueous solutions for four months at 25 ^oC and atmospheric pressure. The modulus of impedance remained between 1.2 to 1.7 GΩ cm² suggesting that the coating is not degraded by the solution and K⁺ and Cl^- ions permeability is practically insignificant.

References

[1] J.V. Nardeli, D.V. Snihirova, C.S. Fugivara, M.F. Montemor, E.R.P. Pinto, Y. Messaddecq, A.V. Benedetti, Localised corrosion assessement of crambe-oil-based polyurethane coatings applied on the ASTM 1200 aluminum alloy, Corros. Sci. 111 (2016) 422-435.

Acknowledgments

Process FAPESP: 2015/10554-9.