Effect of the Degree of Hydrolysis and Condensation of Bis-[Triethoxysilylpropyl] Tetrasulfide on the Corrosion Protection of Coated Aluminum Alloy 2024-T3

Trialkoxysilanes have recently shown great potential for replacing toxic chromates as corrosion protective treatments for metal substrates in a variety of industries. The mechanism of the corrosion protective properties afforded by these coatings depends on various factors (pH, concentration, hydrolysis, time) of the parent silane solution. In this study, the effect of hydrolysis time on the corrosion protective properties of coatings derived from bis-[triethoxysilylpropyl] tetrasulfide (bis-sulfur silane) has been investigated. Silane films were coated on AA2024-T3 (UNS A92024) substrates after different hydrolysis times and their performance was tested using various electrochemical and characterization techniques, including direct current (DC) polarization, electrochemical impedance spectroscopy (EIS), contact angle measurement, and ellipsometry. Structural changes occurring during the hydrolysis of the bis-sulfur silane were monitored as a function of time using reflection absorption infrared reflectance spectroscopy and liquid-state ²⁹Si nuclear magnetic resonance (NMR). The results indicate that coating performance was directly related to the hydroxide (Si-OH) content of the parent silane solution. An optimal hydrolysis time was found to produce coatings exhibiting excellent corrosion protection properties. A model based on the Si-OH content and adhesion capabilities of the coating is presented.