Coating processes – Immersion or partial immersion – Metal base
Reexamination Certificate
2001-04-30
2002-03-26
Anthony, Joseph D. (Department: 1714)
Coating processes
Immersion or partial immersion
Metal base
C427S435000, C252S389520, C252S389540, C252S392000, C252S400520, C252S400540, C252S403000, C148S437000, C148S420000, C148S441000
Reexamination Certificate
active
06361833
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a novel aqueous liquid composition, which is usually hereinafter called a “bath” for brevity, without any implication thereby that it must be used by immersion only, and to a process for treating a metal surface. The composition and process can provide the surfaces of various metals, especially aluminum, aluminum alloys, magnesium, magnesium alloys, and galvanized steel sheet, with an excellent corrosion resistance and excellent paint adherence.
The baths used to treat aluminum and aluminum alloy surfaces can be broadly classified into chromate-type baths and non-chromate-type baths. Chromic acid chromate conversion baths and phosphoric acid chromate conversion baths are typical examples of the chromate-type treatment baths.
Chromic acid chromate conversion baths first reached practical application in about 1950 and even now are widely used for the surface treatment of automotive heat exchangers, aluminum wheels, building materials, and aerospace materials. The main components in chromic acid chromate conversion baths are chromic acid and a fluoride reaction accelerator. This type of bath produces a conversion coating containing moderate amounts of hexavalent chromium on the metal surface.
Phosphoric acid chromate conversion baths originated with the invention disclosed in U.S. Pat. No. 2,438,877. The main components in phosphoric acid chromate conversion baths are chromic acid, phosphoric acid, and hydrofluoric acid. A conversion coating whose main component is hydrated chromium phosphate is formed by this type of bath on the metal surface. Since the resulting conversion coating does not contain hexavalent chromium, this type of bath is in wide use at the present time as an underpaint treatment for the body stock and lid stock of beverage cans.
While the conversion coatings generated by these chromate-type surface treatment baths exhibit an excellent corrosion resistance and an excellent adherence to paint films, these treatment baths also contain toxic hexavalent chromium, and the associated environmental problems have made it desirable to use treatment baths that are completely free of hexavalent chromium.
The treatment bath disclosed in Japanese Laid Open (Kokai or Unexamined) Patent Application Number Sho 52-131937 (131,937/1977) is an invention typical of the chromium-free non-chromate-type surface treatment baths. This surface treatment bath is an acidic (pH=approximately 1.5 to 4.0) aqueous coating solution that contains phosphate, fluoride, and zirconium or titanium or a mixture thereof. The treatment of metal surfaces with this surface treatment bath results in the formation on the metal surface of a conversion coating whose main component is an oxide of zirconium or titanium. This non-chromate-type surface treatment bath offers the advantage of not containing hexavalent chromium and for this reason is widely used at present for treating aluminum drawn-and-ironed, hereinafter usually abbreviated as “DI”, can surfaces. Unfortunately, the coating produced by this non-chromate-type surface treatment bath is less corrosion resistant than chromate coatings.
The treatment method disclosed in Japanese Laid Open (Kokai or Unexamined) Patent Application Number Sho 57-41376 (41,376/1982) comprises treating the surface of aluminum, magnesium, or an alloy thereof with an aqueous solution containing at least one selection from titanium salts and zirconium salts, at least one selection from imidazole derivatives, and an oxidizer selected from nitric acid, hydrogen peroxide, and potassium permanganate. While the corrosion resistance of the coatings produced by this treatment bath would have been considered acceptable 15 years ago, this level of corrosion resistance is not unequivocally satisfactory at the present time.
Japanese Laid Open (Kokai or Unexamined) Patent Application Number Sho 56-136978 (136,978/1981) teaches a conversion bath that characteristically comprises an aqueous solution containing a vanadium compound and at least one compound selected from the group consisting of titanium salts, zirconium salts, and zinc salts. However, the conversion coating formed by this treatment bath cannot be expected to have a corrosion resistance better than or even as good as that of a chromate film in the case of challenge by long-term anticorrosion testing.
Thus, as described above, the use of the aforementioned prior-art non-chromate-type surface treatment baths remains associated with problems with the corrosion resistance of the produced conversion coatings. It is for this reason that at present non-chromate-type surface treatment baths are little used on surface treatment lines where a particularly good corrosion resistance is required, for example, for aluminum alloy heat exchangers and aluminiferous metal coil and sheet stock.
In summary, then, there has yet to be established a bath for treating aluminum and aluminum alloy surfaces that does not contain hexavalent chromium, that has an excellent effluent treatability, and that has the ability to form highly corrosion-resistant, highly paint-adherent conversion coatings.
For treating magnesium surfaces and magnesium alloy surfaces, chromate treatments as typified by JIS (Japanese Industrial Standard) H-8651 and MIL M-3171 are in use for treating magnesium and magnesium alloy surfaces. The conversion coatings generated by these chromate-type surface treatment baths exhibit an excellent corrosion resistance and an excellent adherence to paint films, but these treatment baths also contain highly toxic hexavalent chromium. The associated environmental problems have made it desirable to use treatment baths that are entirely free of hexavalent chromium.
The process disclosed in Japanese Patent Publication Number Hei 3-6994 (6,994/1991) is an invention typical of the chromium-tree non-chromate-type surface treatment baths for magnesium and its alloys. This treatment process comprises a phosphate treatment followed by a silicate treatment and then execution of a silicone treatment after the silicate treatment. The phosphate treatment coating by itself provides a low level of corrosion resistance and paint adherence when used as an underpaint treatment for magnesium and magnesium alloy surfaces. This treatment method also requires a multistage treatment process, uses high treatment temperatures, and requires long treatment times.
The known phosphate-based surface treatment methods for magnesium and its alloys include methods that employ treatment baths based on zinc phosphate, iron phosphate, calcium phosphate, or zirconium phosphate. However, these methods are not believed to have consistently provided a corrosion resistance that is satisfactory at a practical level.
A manganese phosphate treatment is disclosed in category 7 of JIS H-8651. This treatment bath is not acceptable from a practical standpoint because it contains chromium, requires high treatment temperatures of 80° C. to 90° C., and requires long treatment times of 30 to 60 minutes.
Another example of the non-chromate-type technology is found in Japanese Laid Open (Kokai or Unexamined) Patent Application Number Hei 9-228062 (228,062/1997), which teaches a surface treatment process that uses an aqueous solution that contains at least one organometal compound selected from metal alkoxides, metal acetylacetonates, and metal carboxylates and at least one film-formation stabilizer or film-formation auxiliary selected from acids, bases, their salts, and organic compounds containing the hydroxyl group, carboxyl group, or amino group. This aqueous solution is applied to magnesium stock at from 0 to 50° C. Again, however, the conversion coating formed by this treatment bath cannot be expected to have a corrosion resistance better than or even as good as that of a chromate film in the case of challenge by long-term anticorrosion testing.
Thus, as described above, the use of the aforementioned prior-art non-chromate-type surface treatment baths for magnesium and its alloys remains associated with problems with the corrosion resi
Kawaguchi Motoki
Nakada Kazuya
Anthony Joseph D.
Harper Stephen D.
Henkel Corporation
Jaeschke Wayne C.
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