An aqueous composition for applying to and treating metallic substrates that does not include chrome or chromium-containing compounds, does not include an organic solvent, and promotes adhesion of overcoat layers that are applied to the metallic substrate. The composition is an aqueous solution that includes of two hydrolyzed silanes, and water, wherein the composition has a pH of less than or equal to 5 and is substantially free of organic solvent and any chromium-containing compound. Also provided is a method for applying a chromium-free treatment on a metallic substrate comprising mixing a cross-liking agent with the above-described composition and applying the resulting mixture to the metallic substrate, and drying the metallic substrate.