Compositions: coating or plastic – Coating or plastic compositions – Corrosion inhibiting coating composition
Reexamination Certificate
2003-01-23
2003-12-30
Green, Anthony J. (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Corrosion inhibiting coating composition
C106S014210, C106S014410, C148S247000, C148S267000, C148S275000, C428S472000, C428S472100
Reexamination Certificate
active
06669764
ABSTRACT:
ORIGIN OF THE INVENTION
This invention described herein was made by an employee of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the protection and pretreatment of aluminum and aluminum alloys and to the surface treatment of aluminum and its alloys with novel compositions and to the process of using these compositions for the pretreatment of aluminum and aluminum alloys at ambient and higher temperatures. The pretreatment coatings provide color recognition, improved corrosion resistance, adhesion of overlaying coatings e.g. paints and maintains low electrical contact resistance in a corrosive environment. More specifically, this invention relates to corrosion resistant compositions and to the process of using the compositions on aluminum and aluminum alloys which comprises an acidic aqueous solution containing effective amounts of at least one water soluble trivalent chromium salt, an alkali metal hexafluorozirconate, at least one fluoro-compound, at least one zinc compound and at least one water soluble thickener.
2. Description of Prior Art
It is current practice to improve the corrosion resistance and bonding of subsequent coatings to aluminum and its alloys by initially coating or pretreating the metal surface with protective films. The conversion coating enhances the corrosion resistance of the untreated metal surface and prepares the surface for a finish coating including paints and the like. These conversion or pretreatment coatings are most often applied by using hexavalent chromium-containing solutions. While these coatings provide good corrosion resistance, attempts have been made to provide a more acceptable non-chromate derived coating because of concern regarding the occupational, safety, health and environmental effects of hexavalent chromium. Hexavalent chromium is highly toxic and is a known carcinogen. Therefore, the solutions used to deposit these protective films, and the films per se are toxic and carcinogenic. Chromate films, however, provide outstanding adhesion and corrosion resistance and are easy to apply by various methods including immersion, spraying or by the wipe-on technique. Moreover, the environmental laws and OSH regulations are forcing the military and commercial users to find other non-toxic, non-chromate pretreatments. In addition, the use of chromate conversion coatings is becoming more expensive as the regulations are being enforced and costs become prohibitive because of the restrictions being imposed by the EPA. For example, certain processes such as spraying chromate conversion coatings are forbidden because of OSH regulations, thereby forcing the use of less than optimum alternative methods.
It is generally known that aqueous chromate solutions contain chemicals that partially dissolve the surface of the metal e.g. aluminum and form insoluble films known as a pretreatment or a chromate conversion coating. These coatings are corrosion resistant and protect the metal from various elements which cause corrosion. Although these conversion coatings enhance corrosion resistance and improve the bonding or adhesion properties, the coatings have serious drawbacks, i.e., the toxic nature of the solutions from which they are made and the presence of hexavalent chromium in the applied films. This is a serious problem for the operators handling the solution e.g. disposing the used chromate solution, the chromate-contaminated rinse water, and the coating systems contaminated with chromates. These problems, however, can be avoided by eliminating hexavalent chromium from the process. However, this method is expensive and can be a major cost factor in the overall metal treating process. Therefore, it is highly desirable to provide processes and protective coatings which are free of hexavalent chromium, but at the same time capable of imparting corrosion resistant and bonding properties which are comparable to those imparted by conventional chromate-based conversion coatings. Of particular interest is the use of chromate conversion coatings for aluminum and its alloys e.g. the coating of large objects such as aircraft. Therefore, it would be desirable to provide a protective coating for aluminum and its alloys utilizing relatively non-toxic chemicals that serve as an alternative to the toxic hexavalent chromium coatings.
SUMMARY OF THE INVENTION
This invention relates to an acidic aqueous trivalent chromium pretreatment (TCP) of aluminum and aluminum alloys and to the process for providing color recognition, and identification of the coating and to improve the adhesion and corrosion resistant properties of the aluminum surfaces. Specifically, this invention relates to pretreating aluminum and to the composition and process of pretreating aluminum and aluminum alloys at ambient or higher temperatures ranging up to about 200° F. The pretreatment solutions comprise an acidic aqueous solution having a pH ranging from about 2.5 to 5.5 and preferably from 3.7 to 4.0, and contain per liter of said solution, from about 0.01 to 22 grams of a water-soluble trivalent chromium compound, about 0.01 to 12 grams of an alkali metal hexafluorozirconate, about 0.0 to 12 grams and preferably 0.01 to 1.2 e.g. 0.12 to 0.24 grams of at least one fluoro-compound selected from the group consisting of an alkali metal tetrafluoroborate, an alkali metal hexafluorosilicate and various combinations or mixtures thereof, from about 0.001 to 10 grams of at least one zinc compound, from about 0.0 to 10 grams per liter and preferably 0.5 to 1.5 grams of at least one water-soluble thickener, and from 0.0 to 10 grams per liter and preferably 0.5 to 1.5 grams of at least one water-soluble non-ionic, cationic or anionic surfactant.
One shortcoming of the compositions and processes described by the prior TCP coatings is the lack of a significant color change or color recognition in the as deposited coating, especially when used as a pretreatment for aluminum alloys. Chromate-based coatings typically have an iridescent gold to brown color that is easily identified by processors, quality control personnel, and other users in the field. A gold color on aluminum components generally means that a chromate pretreatment is present and the color is useful for this type of quality control irrespective of technical coating performance such as resistance to corrosion or paint adhesion. The TCP coatings described heretofore typically have a light bluish to purplish to tan color, depending on the process conditions, that is very difficult to see in mixed light such as inside a production facility, in a repair depot or manufacturing plant. TCP on some surfaces like clad aluminum alloys is virtually colorless. Therefore, to provide a means of color identification or recognition of the coating, this invention relates to compositions and processes for preparing a corrosion-resistant coating for aluminum and aluminum alloys. The compositions comprise treating the aluminum substrates with an acidic aqueous solution containing trivalent chromium sulfate basic, an alkali metal hexafluorozirconate, and optionally a cellulose-based thickener and an alkali metal tetrafluoroborate and/or an alkali metal hexafluorosilicate stabilizer. In addition, the composition must also contain a zinc-based compound e.g. divalent salt to impart color recognition to the as produced coating.
It is therefore an object of this invention to provide pretreated aluminum and its alloys by treating said aluminum with an aqueous solution comprising a trivalent chromium compound, an alkali metal hexafluorozirconate, effective amounts of an alkali metal tetrafluoroborate and/or hexafluorosilicate and at least one zinc compound to improve the electrical resistance, adhesion, and corrosion resistance properties and to provide color recognition for identification of the coated aluminum.
It is another object of this invention to provide a proc
Matzdorf Craig A.
Nickerson, Jr. William C.
Billi Ron
Green Anthony J.
The United States of America as represented by the Secretary of
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