Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
1992-03-11
2002-01-29
Koehler, Robert R. (Department: 1775)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S261000, C148S262000, C148S273000, C148S283000
Reexamination Certificate
active
06342107
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an acidic aqueous phosphate solution and a process for phosphating a metal surface with said solution. More particularly, it relates to a solution and a process for forming a phosphate film especially suitable for cationic electrocoating, and is particularly applicable to metal surfaces which include an iron-based surface and a zinc-based surface such as an automobile body.
2. Description of Related Art
Japanese Patent Publication (unexamined) No. 107784/1980 (laid-open to the public on Aug. 19, 1980) discloses a process for treating a metal surface by dip treatment, followed by spray treatment, with an acidic aqueous phosphate solution containing from 0.5 to 1.5 g/l of zinc ion, from 5 to 30 g/l of phosphate ion, and from 0.01 to 0.2 g/l of nitrite ion and/or from 0.05 to 2 g/l of m-nitrobenzenesulfonate ion. Said process is reported to be capable of providing a phosphate film which is effective for forming a coating by cationic electrocoating having excellent adhesion and corrosion-resistance on complicated articles having many pocket portions like car bodies.
Japanese Patent Publication (unexamined) No. 145180/1980 (laid-open to the public on Nov. 12, 1980) discloses a process for treating a metal surface by spray treatment with an acidic aqueous phosphate solution containing from 0.4 to 1.0 g/l of zinc ion, from 5 to 40 g/l of phosphate ion, from 2.0 to 5.0 g/l of chlorate ion, and from 0.01 to 0.2 g/l of nitrite ion. Further, Japanese Patent Publication (unexamined) No. 152183/1980 (laid-open to the public on Nov. 27, 1980) discloses an acidic aqueous phosphate solution containing from 0.08 to 0.20 wt. % of zinc ion, from 0.8 to 3.0 wt. % of phosphate ion, from 0.05 to 0.35 wt. % of chlorate ion, from 0.001 to 0.10 wt. % of nitrite ion, and complex fluoride ion in an amount calculated by the formula: 0.4≧y≧0.63x−0.042, wherein x is the concentration in wt. % of zinc ion and y is the concentration in wt. % of the complex fluoride ion. These prior art processes are reported to be capable of providing excellent adhesion and corrosion-resistance to the coating by cationic electrocoating.
However, in a recent development in the automobile industry there has come to be used for car bodies steel component plated on one surface only with zinc or alloyed zinc, with the object of further improving corrosion-resistance after the application of the siccative coating. It has however come to be recognized that, when the above prior art compositions and processes are applied to such materials (i.e. to metal surfaces which include both an iron-based surface and a zinc-based surface), on the iron-based surface a phosphate film suitable as a substrate to be coated by cationic electrocoating can be formed as desired, but a phosphate film formed on a zinc-based surface is significantly inferior to that formed on the iron-based surface.
There has been developed a composition and process to solve the above-mentioned problems which occur on zinc-based surfaces in components which include both an iron-based surface and a zinc-based surface. This is the invention disclosed in Japanese Patent Publication (unexamined) No. 152472/1982 (laid-open to the public on Sep. 20, 1982). Said invention is characterized by subjecting the metal surface to treatment with an acidic aqueous phosphate solution comprising from 0.5 to 1.5 g/l of zinc ion, from 5 to 30 g/l of phosphate ion, from 0.6 to 3 g/l of manganese ion, and a phosphating accelerator.
However, in the zinc phosphate treatment of galvanized steel, there often results an abnormal coating with white spots, each spot having a diameter of from 1 mm to 2 mm. This is particularly true with an electroplated galvanized steel and especially in the dip treatment thereof. These white spots will cause craters in subsequent treatment, thereby resulting in inferior coatings. The mechanism by which white spots are formed is believed to be as follows: In a first stage, there appear many pits, at the edge portions of which the galvanized layer is gradually dissolved in the form of concentric circles through an excessive etching reaction. As the growth of each pit continues, zinc phosphate is precipitated in the center portion thereof, However, at the peripheral portions, the iron surface is exposed, which forms a galvanic cell with the zinc metal, thereby continuing the dissolution of the zinc.
As a consequence, an excess quantity of zinc phosphate crystals are precipitated and accumulate as “snow” at the peripheral portions of the spots, which can readily be observed by the naked eye.
Unfortunately, no solution to this problem has been found heretofor to consistently avoid the formation of such white spots.
DESCRIPTION OF THE INVENTION
The present invention represents a further improvement in the above techniques for phosphating as a substrate treatment under cationic electrocoating.
Accordingly, an object of the present invention is to provide an acidic aqueous phosphate solution which can give a phosphate film capable of providing excellent adhesion and corrosion-resistance to coatings from cationic electrocoating.
Another object of the present invention is to provide an acidic aqueous phosphate solution which provides excellent phosphate films on metal surfaces which include an iron-based surfaces, a zinc-based surface, and/or an aluminum-based surface.
Another object of the present invention is to provide an acidic aqueous phosphate solution which will not cause any white spots or at least any significant white spots on galvanized steel even in the dip treatment thereof.
Another object of the present invention is to provide an acidic aqueous phosphate solution which can give said phosphate film by treatment at low temperature.
A further object of the present invention is to provide a process for forming a phosphate film with said acidic aqueous phosphate solution.
A further object of the present invention is to provide a process by which a phosphate film can be satisfactorily formed on an article having a complicated shape like a car body.
A further object of the present invention is to provide an aqueous concentrated composition for formulating said acidic aqueous phosphate solution.
Other objects and advantages of the present invention will become apparent from the following disclosure.
It has now been found that phosphating compositions which are chlorate-free or at least substantially chlorate-free and which have a chloride ion level below 0.5 g/l provide excellent phosphate-coatings on iron, zinc, and aluminum-based surfaces, without the formation of deleterious white spots. It is important to the beneficial results of the present invention that the chloride ion level be consistently maintained below 0.5 g/l, which means that not only the chloride ion itself, but also the chlorate ion should not be added to the phosphating compositions, since the chlorate ion will be reduced to the chloride ion as the phosphating composition is used.
As stated above, the metal surfaces treated in accordance with the present invention include iron-based surfaces, zinc-based surfaces, aluminum-based surfaces, and their respective alloy-based surfaces. These metal surfaces can be treated either separately or in combination. The advantage of the present invention is most prominently exhibited when the treatment is carried out on metal surfaces which include both an iron-based surface and a zinc-based surface, as, for example, in a car body. Examples of zinc-based surfaces include galvanized steel plate, galvanealed steel plate, electrogalvanized steel plate, electro zinc-alloy plated steel plate, complex electrogalvanized steel plate, electro zinc-alloy plated steel plate, complex electrogalvanized steel plate, etc.
The acidic aqueous phosphate solutions of the invention contain:
a) from about 0.1 to about 1.5 g/l, preferably from about 0.5 to about 1.4 g/l of zinc ion;
b) from about 5 to about 50 g/l, preferably from about 10 to about 30 g/l, of phosphate ion;
c) from about 0.
Miyamoto Satoshi
Nagatani Masamichi
Connolly Bove & Lodge & Hutz LLP
Henkel Corporation
Koehler Robert R.
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