A cohydrolyzate and/or (partial) cohydrolytic condensate obtained from hydrolysis of (i) an epoxy-containing silane coupling agent, (ii) an organosilicon compound having at least two hydrolyzable silyl groups, and (iv) another hydrolyzable silyl group-containing organosilicon compound in the presence of (iii) an aqueous silica sol is dissolved and/or dispersed in water to form an aqueous siloxane coating composition having an alcohol content of up to 5% by weight. The composition is stable during storage at room temperature and able to form a cured coat which exerts satisfactory water resistance and corrosion control.