Abstract: A deeply and uniformly black coating with excellent adhesion and corrosion resistance can be formed on a wide variety of substrates by coating them with an aqueous composition containing an amount of chromium trioxide to give a stoichiometric equivalent of from about 30 to about 150 g/L of Cr.sup.6+ ; from about 20 to about 100 g/L of Cr.sup.3+ ; a component to provide a stoichiometrically equivalent amount of from about 0.5 to about 50 g/L of total Fe, Co, and Ni ions; from about 5 to about 200 g/L of dispersed or dissolved organic film forming polymer; and the balance water, with a weight ratio of Cr.sup..alpha.6 to Cr+3 between about 5 and about 1; and heating the coated substrate to a temperature between 100.degree. and 350.degree. C. for 5 seconds to 10 minutes.