The present invention relates to an acidic aqueous composition for treating of zirconium-pretreated metal-based substrate surfaces such as steel metals or aluminium surfaces, a process for treating the substrate surfaces with the composition and the use of the composition as post-treatment of zirconium-pretreated metal-based substrate surfaces for subsequent electrocoating of the surfaces to increase corrosion resistance of said metal-based surfaces prior to electro-coating (e-coat) applications and increases detergent and chemical resistance of treated surfaces used in the white goods industry. The acidic aqueous composition comprises trivalent chromium ions; and hexafluorozirconate ions; characterized in that the source of trivalent chromium ions is a trivalent chromium nitrate salt.