Electrochemical Impedance Spectroscopy of Bis-[Triethoxysilypropyl]Tetrasulfide on Al 2024-T3 Substrates

Thin films of the hydrolyzed silane bis-[triethoxysilylpropyl]-tetrasulfide (sulfane) were deposited on alkaline-cleaned Al 2024-T3 panels and investigated by electrochemical impedance spectroscopy (EIS) using the noncorrosive electrolyte 0.5 M potassium sulfate (K₂SO₄) aqueous solution. The effects of continuously immersing the films in this electrolyte and curing the films at room temperature or at 100°C in air were studied. Three equivalent circuits were proposed that fit the experimental data very well. A continuous increase of impedance and the appearance of an additional time constant with respect to time in EIS spectra during immersion, curing, and aging processes were explained by the hydrolysis of the ester groups to silanol groups and condensation of the latter to siloxane bonds in the film, and by the formation of an unknown interfacial phase between the cross-linked silane film and the aluminum oxide (Al₂O₃). Reflection absorption Fourier transform infrared spectroscopy (RAIR) was used to enhance the interpretation of the EIS results. It was concluded that EIS in a noncorrosive electrolyte is a useful method for studying the stability of silane films formed on metals and can contribute to determining the optimum conditions for depositing silane films on metals for corrosion protection. The results of potentiodynamic polarization tests and salt spray testing (ASTM B117) confirmed the conclusion drawn from EIS that a fully cured sulfane-silane film provides excellent corrosion protection on Al 2024-T3.