Disclosed and claimed is a novel protective coating for metal surfaces. The protective coating includes one part by weight of the organosilane-modified silica nanoparticles made by the disclosed process and further includes from 0.3 to 3 parts by weight of the reaction product of (1) a compound comprising a polyamine in which a plurality of amine groups are bonded to at least one radical selected from the group consisting of hydrocarbon and hydrocarbon ether groups that separate nitrogen atoms of said amine groups by at least four intermediate atoms in a chain, and (2) a silane which carries a plurality of silicon-bonded hydrolyzable groups and a silicon-bonded organic group that is covalently reactive to and which bonds with said amine group, said silane also comprising hydrolyzable groups, to provide a reaction product molecule which comprises an average of 2.5 to 3.5 silane groups per molecule.