Corrosion resistance of multilayer silane sol-gel coatings on titanium Grade 2 and Ti6Al4V alloy

Vinyltrimethoxysilane (VTMS)-based coatings are known for their potential in improving the corrosion resistance of metallic surfaces, particularly titanium alloys. While numerous studies have examined silane systems containing corrosion inhibitors or nanoparticles, the role of pure multilayer VTMS architectures remains poorly understood. This study investigates how the number of VTMS layers (ranging from 1 to 6), applied via the sol-gel dip-coating method, affects the electrochemical behavior and microstructural characteristics of coatings on titanium Grade 2 and Ti6Al4V alloy. All multilayer configurations were first tested in 0.5 mol L^-1 Na₂SO₄ solution to assess their barrier performance. Based on polarization resistance and corrosion potential results, the three-layer configuration (3VTMS) was selected as optimal and was subsequently tested in four simulated physiological solutions: Simulated Body Fluid (SBF), Hank's solution, artificial saliva, and Ringer's solution. The findings demonstrate that 3VTMS offers the most effective combination of thickness, uniformity, and corrosion resistance. Comparative SEM imaging and potentiodynamic polarization tests confirm that multilayer stacking leads to improved barrier properties and enhances anodic protection, even in the absence of chemical additives. The added value of this study lies in demonstrating that additive-free VTMS multilayer coatings can achieve corrosion protection levels comparable to hybrid silane systems, relying solely on optimized structural layering. This work addresses an existing gap in the literature and provides a foundation for future investigations into biofunctional, low-toxicity, and energy-efficient silane coatings for biomedical titanium applications.