Process for after-treating metal surfaces coated with crystalline anticorrosion layers

Metals such as Zn, Al and ferrous materials which have been coated with an inorganic crystalline coating such as a phosphate layer are after treated with a solution containing an aliphatic phosphonic acid containing more than five carbon atoms and then dried. The aliphatic phosphonic acid which may be eicosane-, decane-, octane-, or hexane phosphonic acid is present in a concentration of 10-5-10-1 mol per litre of solution and the solvent may be water and/or an organic solvent such as an aliphatic alcohol containing 1-5 carbon atoms and/or a ketone such as acetone or isopropanol.