Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
2002-04-29
2004-06-15
Oltmans, Andrew L. (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S247000, C148S273000, C148S279000, C106S014110, C106S014210, C252S387000
Reexamination Certificate
active
06749694
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to coating compositions for pretreating metal surfaces. More particularly, the present invention is directed to aqueous coating compositions for providing durable, adhesive and corrosion-inhibiting coatings, as well as a method for pretreating metal surfaces with such coating compositions.
BACKGROUND OF THE INVENTION
The use of protective coatings on metal surfaces for improved corrosion resistance and paint adhesion characteristics is well known in the metal finishing arts. Conventional techniques involve pretreating metal substrates with a phosphate conversion coating and chrome-containing rinses for promoting corrosion resistance. The use of such chromate-containing compositions, however, imparts environmental and health concerns due to the toxic nature associated with chromium compounds.
As a result, chromate-free conversion coatings have been developed to overcome the need for chromate-containing compositions. Such chromate-free coatings are generally based on chemical mixtures that in some way will react with the substrate surface and bind to it to form protective layers.
Chromate-free conversion coatings typically employ a Group IVB metal such as titanium, zirconium or hafnium, a source of fluoride ion and a mineral acid to regulate the pH.
For example, U.S. Pat. No. 4,338,140 to Reghi discloses a conversion coating for improved corrosion resistance which includes zirconium, fluoride, and tannin compounds, and optionally phosphate ions. U.S. Pat. No. 5,759,244 discloses conversion coatings for metal substrates including a Group IVB metal in an acidic solution with one or more oxyanions, and which specifically excludes fluoride ions from the composition.
It has been suggested to include Group IA and/or Group IIA elements into such conversion coatings. For example, U.S. Pat. No. 5,441,580 to Tomlinson discloses the use of a Group IVB metal such as titanium, zirconium or hafnium, and Group IA metal such as potassium, and a source of fluoride ions, and U.S. Pat. No. 5,380,374 to Tomlinson discloses coatings based on such Group IVB metals including a Group IIA metal such as calcium at a concentration of 50 ppm to 1300 ppm. As is recognized in the art, for example in U.S. Pat. No. 5,964,928 to Tomlinson, coatings including Group IIA metals such as calcium generate considerable scaling from alkali metal precipitates, which may inhibit formation of the continuous metal oxide matrix. Such Group IIA metals are therefore generally used in lower concentrations. Also, as recognized in the U.S. Pat. No. 5,964,928, such compositions including Group IA or Group IIA metals likely provide little if any long-range structure.
Accordingly, it would be desirable to provide a composition useful for coating metal substrates, particularly bare ferrous metals, which overcomes the environmental drawbacks of the prior art, which demonstrates excellent corrosion resistance and adherence of subsequently applied coatings, and which does not form a precipitate which may interfere with proper formation of the coating.
SUMMARY OF THE INVENTION
In accordance with the present invention, an aqueous composition for pretreating and depositing a coating on metal substrates is provided, which includes from about 1,500 to about 55,000 ppm based on the aqueous composition, of a Group IIA dissolved metal ion, such as calcium; from about 100 to about 200,000 ppm based on the aqueous composition, of a dissolved complex metal fluoride ion wherein the central atom is selected from Group IIIA, Group IVA, Group IVB, Group VA, and Group VB metals such as aluminum, silicon, zirconium, antimony, and niobium; and water, wherein the composition is substantially free of Group IIA metal fluoride precipitate. The aqueous composition desirably contains a complex-forming metal compound, such as a complex metal salt, which is different than the salt associated with the complex metal fluoride ion, with the complex metal salt being capable of complexing free fluoride ions to prevent a precipitation reaction with the Group IIA metal ion. The metal atom of the complex metal salt is desirably selected from zirconium and silicon, such as sodium metasilicate, polysilicate, Zeolites (aluminosilicates), zirconyl nitrate, titanyl sulfate, tetrafluorozirconate and tetrafluorotitanate.
In a further embodiment, the present invention includes a method of preparing an aqueous composition for treating metal substrates, which includes adding to water a complex metal fluoride compound wherein the central atom is selected from Group IIIA, Group IVA, Group IVB, Group VA and Group VB metals; adding a complex metal salt different from the complex metal fluoride compound in an amount capable of reacting with any free fluoride ions from the complex metal fluoride compound; and adding a Group IIA metal compound. The composition is substantially free of precipitated Group IIA metal fluoride.
Desirably, the Group IIA metal compound is provided in an amount of from about 2.0 to 10.0 g/L based on the aqueous composition, the complex metal fluoride compound is added in an amount of from about 1.0 to 80 g/L based on the aqueous composition, and the complex metal salt is added in an amount of from about 0.05 to about 6.0 g/L based on the aqueous composition.
DETAILED DESCRIPTION OF THE INVENTION
Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients or reaction conditions used in the specification and claims are to be understood as modified in all instances by the term “about”.
As indicated, the present invention is directed to aqueous compositions for pretreating and depositing crystalline and non-crystalline coatings on metal substrates. The compositions of the present invention may be utilized to improve the corrosion-inhibiting properties of metal surfaces such as iron, steel, zinc, magnesium, or aluminum, or their alloys. The compositions of the present invention can be used to replace conventional metal treatments such as iron phosphate, zinc phosphate and chromium conversion coatings.
In one embodiment of the invention, the aqueous coating composition includes a Group IIA dissolved metal ion, a dissolved complex metal fluoride ion with the central atom selected from selected from Group IIIA, Group IVA, Group IVB, Group VA, and Group VB metals, and water. The composition according to the present invention is substantially free of Group IIA metal fluoride precipitate.
The Group IIA dissolved metal ions referred to herein are those elements included in such group in the CAS Periodic Table of the Elements as is shown, for example, in the
Handbook of Chemistry and Physics
, 63rd Edition (1983). The Group IIA metal is, in particular, an alkaline earth metal. For example, the Group IIA metal may be calcium, magnesium, beryllium, strontium or barium. Calcium is particularly useful in connection with the present invention. The Group IIA metal may be provided from any compound or composition which is easily dissolved in the aqueous composition to provide a source of Group IIA metal ion. In particular, the Group IIA metal may be provided as any of the many inorganic hydroxides or salts available, including the nitrates, sulfates, chlorides, etc. Calcium hydroxide [Ca(OH)
2
], calcium nitrate [Ca(NO
3
)
2
], etc. are particularly useful, with calcium nitrate being particularly desirable in connection with the present invention.
The composition of the present invention further includes at least one metal compound which is capable of converting to a metal oxide upon application to the metal substrate. The metal compound which is the precursor of the formation of the metal oxide on the surface of the substrate can be any metal compound capable of converting to a metal oxide. For example, the metal compound may be selected from those elements included in Groups IIIA, IVA, IVB, VA, VB, and VIB of the CAS Periodic Table of the Elements. Examples of such useful metal compounds include silicon, boron, aluminum and tin. Additionally, the me
Greene Jeffrey Allen
Vonk Donald Robb
Altman Deborah M.
Oltmans Andrew L.
PPG Industries Ohio Inc.
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