Method for applying a conversion coating with an aquous pickling composition comprising an aminophosphonic acid and a polymer

A method for coating a metallic substrate with a conversion coating comprising the steps of:

• A) pickling the metallic substrate by contacting the metallic substrate with an aqueous pickling composition having a pH value at 55° C. in the range of from 5 to 9, said composition comprising:
• a) at least one amino organophosphonic acid derivative of formula (I)
• wherein:
• residues R independently of each other are CH2—PO(OR″)2,
• residues R′ independently of each other are alkylene residues with 2 to 4 carbon atoms,
• residues R″ independently of each other are H, Na, K, Li or NH4;
• and
• n is an integer from 0 to 4;
• b) at least one copolymer, which is water-soluble or water-dispersible and selected from the group consisting of at least partially neutralized poly(meth)acrylic acids and polyvinylpyrrolidones;
• and
• c) from 80 to 99.5 wt. % water, based on the total weight of the composition, wherein the aqueous pickling composition does not comprise enzymes, followed by
• B) a step of coating the thus pickled metallic substrate with a conversion coating composition, optionally followed by
• C) a step of applying an electrodeposition coating composition;
• and optionally followed by
• D) one or more steps of applying one or more further coating composition(s).