Sealing composition and method for iron and zinc phosphating process

摘要

Method and composition for sealing of a phosphate conversion coating is disclosed. The method includes contacting a phosphate conversion coating with an acidic aqueous solution of substituted iminodimethylene diphosphonic acids represented, as a class, by the formula R1--N--(CH.sub. 2 --PO.sub.3 H.sub.2).sub.2 where R1 is a Z, alkyl or aryl moiety having a carbon chain up to the length where solubility in an acidic aqueous solution is lost. Z is ##STR1## where R2 and R3 are hydrogen, alkyl, aryl, or phosphono alkyl moieties, m is from 1 to 3 and Cn is a methylene carbon chain up to the length where solubility in an acidic aqueous solution is lost, and water soluble salts thereof. Preferably, R1 is methyl through octyl, benzyl, or diethylenediaminotrimethylene triphosphonic acid. The substituted iminodimethylene diphosphonic acid is applied as an aqueous bath with a pH of from about 3.3 to 4.0. The substituted iminodimethylene diphosphonic acid may dry in place or be followed by a deionized water rinse.