Nippon Paint Compnay, Ltd.

Asao Mochizuki

Minoru Ishida

Isao Kawasaki

Haruo Kojima

A method for treating a metal surface with zinc phosphate of this invention can treat an iron-based surface, a zinc-based surface, an aluminum-based surface as well as a metal surface having two kinds or more of these surfaces with the same zinc phosphate treating solution, and even if treating times increases, the method makes it possible to form a coating film having superior adhesion and high corrosion resistance under a stable condition, and also, it prevents a precipitate formed by a metal ion eluted from a metal surface to be treated, especially, by an aluminum ion. A complex fluoride compound contained in said zinc phosphate treating solution of coming in contact with a metal surface is added so that a concentration (unit: g/l) converted into at least one of a hexafluorosilicic acid group (SiF.sub.6.sup.2-) and a tetrafluoroboric acid group (BF.sub.4.sup.-), with an aluminum ion concentration (unit: g/l) contained in said treating solution, is satisfactory for the following equation (I). ##EQU1##

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Aichi, Japan

Asao Mochizuki