Patent Application
20070095436
This invention relates to aqueous compositions and to the process of using said compositions for treating aluminum substrates with effective amounts of an acidic aqueous solution comprising at least one fluorometallate compound, cationic or divalent zinc compounds, water soluble surfactants or wetting agents and water soluble thickeners. This invention relates to compositions and to the process for treating aluminum substrates to improve the adhesion bonding and corrosion resistant properties of the substrates. The process comprises treating aluminum and its alloys, including anodized aluminum and its alloys with a composition comprising an acidic aqueous solution having a pH ranging from 2.5 to 5.5 which contains effective amounts of at least one water soluble fluorometallate, at least one water soluble cationic or divalent zinc compound, and effective amounts of water soluble thickeners and/or surfactants.
More specifically, this invention relates to non-chromium aqueous compositions and the processes used to deposit coatings onto aluminum, aluminum alloys and anodized aluminum. These coatings are known as conversion coatings e.g. pretreatments or post-treatments designed to improve paint adhesion, corrosion resistance, and electrical resistance of the substrates. This invention also relates to the compositions and process used to deposit a film or coating onto aluminum and its alloys that have been anodized and therefore have a porous surface that requires a post-treatment to enhance corrosion protection and avoid degrading the adhesion-bonding characteristics of the anodized surface. Current conversion coatings or pretreatments for aluminum and aluminum alloys comprise hexavalent chromium or chromate chemistry. Hexavalent chromium is highly toxic and a known carcinogen. As a result, the compositions used to deposit these coatings and the coatings per se are toxic. However, these coatings provide outstanding paint adhesion, and corrosion resistance, and can be applied by various methods including immersion, spray or wipe-on-techniques.
High performance post-treatments or sealers for anodized aluminum are based also on hexavalent chromium chemistry. These coatings provide outstanding paint adhesion and corrosion resistance to the anodized aluminum. Typically, these coatings or films are deposited onto the anodized aluminum at elevated temperatures (over 50° Celsius) and are usually applied by immersion processes. These post-treatments generally are required by the military and commercial users to follow specifications that govern each substrate being treated. As such there is not a unique “post-treatment” specification for anodized aluminum as there is for “conversion coating” aluminum.
Moreover, environmental laws, executive orders, and local occupational, safety, and health (OSHA) regulations are driving the military and commercial users to search for chromate-free treatments. In the case of anodized aluminum, the anodize film and base metal are relatively non-toxic. With the addition of the chromate treatment, the coatings become toxic. There are other coatings, however, such as nickel acetate and fluoride-containing compositions that do not contain chromium, but their technical performance is inferior to chromate-based coatings. In addition, the use of chromate treatments is becoming more expensive as regulations tighten. Costs can become prohibitive with future PEL (personal exposure limits) restrictions imposed by OSHA. Certain processes like spraying chromate-based solutions are forbidden at most facilities thereby forcing the use of less-than-optimum alternative processes. Although, chromium compositions yield good corrosion protection, certain DOD and commercial entities are seeking alternative coatings that do not contain chromium. Even though chromate conversion coatings and the chromate post-treatment of anodized aluminum provide outstanding technical performance, from a life-cycle cost, environmental, and OSHA perspective, non-chromium alternatives are highly desirable. Research is being directed to develop alternative coatings for aluminum and its alloys that are technically equivalent or even superior to chromium coatings. One important shortcoming of the present commercially available non-chromium coatings is the poor corrosion-resistance properties. These non-chromium coatings generally are inferior to treatments using hexavalent and trivalent chromium, showing little to no improvement in corrosion resistance compared to an untreated aluminum substrate. There are no adequate non-chromium coatings available that offer the advantages of improved corrosion resistance, an adhesion base for subsequent coatings, non-chromium formulations, practical color change, and robust processing capability.
This invention relates to chromium-free compositions and the process for preparing corrosion-resistant coatings for aluminum, aluminum alloys, anodized aluminum and its alloys which comprises treating these aluminum substrates with an acidic aqueous solution comprising at least one fluorometallate compound, cationic or divalent zinc compounds, and optionally thickeners such as cellulose-based thickeners and surfactants. The compositions of this invention are referred to herein as “NCP” for “Non-Chromium Post-Treatment” or “Non-Chromium Process”.
Therefore, it is an object of this invention to provide an acidic aqueous solution comprising fluorometallates such as the hexafluorozirconates, cationic or divalent zinc compounds for coating substrates of aluminum, aluminum alloys, and anodized aluminum to improve the adhesion and corrosion-resistance properties of the substrates.
It is another object of this invention to provide a stable acidic aqueous solution having a pH ranging from about 2.5 to 5.5 comprising an alkali metal fluorometallate and cationic or divalent zinc salts for post-treating anodized aluminum and aluminum alloys.
It is still another object of this invention to provide a treatment for aluminum substrates to provide color change and improved corrosion resistance.
It is still a further object of this invention to provide an acidic aqueous solution comprising fluorometallates and cationic or divalent zinc salt for treating aluminum and anodized aluminum substrates at temperatures ranging up to 212° F. wherein said acidic solution is substantially free of chromium.
These and other object of the invention will become apparent by reference to the detailed description when considered in conjunction with the accompanying
This invention relates to aqueous compositions and to the process of using said compositions which comprises an acidic aqueous solution having a pH ranging from about 2.5 to 5.5, and preferably from about 3.7 to 4.0 for treating aluminum, aluminum alloys, anodized aluminum and anodized aluminum alloys to improve the adhesion bonding (e.g. paint adhesion) and corrosion-resistance properties of said aluminums. The process comprises preparing the pre- or post-treatments by using an acidic aqueous solution at temperatures ranging from ambient up to 160° F. or higher e.g. up to 212° F. The composition comprises, per liter of solution, from about 0.1 to 22 grams and preferably about 1.0 to 12.0 grams e.g. 6.0 to 8.0 grams of at least one or more fluorometallate selected from the group consisting of hexafluorozirconates, tetrafluoroborates, hexafluorosilicates, hexafluorotitanates, and heptafluorotantalates, from about 0.1 up to the solubility limit and preferably 0.1 to 12 grams of a cationic or divalent zinc compound, from 0.0 to 10 grams and preferably from 0.5 to 10 grams of a water soluble thickener, and from 0.0 to 10 grams and preferably 0.5 to 10 grams of a surfactant. The composition forms a coating on the aluminum that has a practical color change, imparts good paint adhesion and improves the corrosion resistance of the aluminum.
More specifically, the post-treatments were applied to anodized aluminum substrates, for example, by the following method: Coupons were anodized via conventional anodizing techniques, immediately rinsed and then immersed directly into an aqueous composition such as Example 4. Coupons were allowed to dwell in NCP-6 (Example 4) for approximately 10 minutes, removed, and thoroughly rinsed in deionized water. Coupons were allowed to dry in a rack at ambient conditions. The aqueous compositions of this invention can be applied at about room temperature and applied to the aluminum substrate via immersion, spray or wipe on techniques. Solution dwell time ranges from about 1 to 60 minutes depending on the anodize process.
Table 2 provides examples of the performance of the compositions of this invention (NCP) in comparison to untreated aluminum alloys, alloys treated with commercially available hexavalent chromium pretreatments, trivalent chromium pretreatments (TCP), and commercially available chromium-free pretreatments. In addition, Table 2 provides dry and wet-tape adhesion data for non-chromium post-treatment (NCP) compared to trivalent chromium control (TCP). The dry tape adhesion test (DTA) was conducted in accordance with ASTM D3359, procedure A. The wet-tape adhesion (WTA) test was conducted in accordance with ASTM D3359, procedure B.
In preparing the post-treatment for anodized aluminum, a unique feature is the ability to treat the anodized films without blocking the pores and still maintaining the excellent corrosion resistance. Generally, anodize aluminum coatings that are to be painted or adhesively bonded are not sealed by conventional methods, since the sealing process plugs the pores of the coating and degrades paint adhesion and adhesive-bonding strengths. As a result, designers trade-off corrosion performance for increased bonding performance. The treatments or coatings of this invention allows the user to post-treat the anodize film to increase corrosion resistance while maintaining excellent paint adhesion. This is especially attractive, since there is an overall objective to replace high-VOC, solvent borne chromated primers with low-VOC, water borne non-chromated compositions. The post-treatment set forth in this invention provides added corrosion resistance to the coating, potentially making the implementation of a more environmentally preferred primer easier to accomplish while maintaining technical performance compared to the current systems. Another benefit is the improved wear characteristics imparted to the anodize coating. Generally, all post-treatments or sealers soften or reduce the hardness of anodize coatings. Chromate sealers are the least detrimental, while water sealing causes up to a 50% reduction in coating wear resistance. This softening effect is due to the hydration of the anodize coating caused by the sealers. Since the solutions of this invention can be carried out at about room temperature, little or no reduction in wear resistance occurs. This is especially important in anodizing where wear and corrosion resistance are critical.
The acidic solutions or compositions of this invention contain at least one cationic or divalent zinc compound to provide color and to improve the corrosion protection of the aluminum when compared to compositions that do not contain cationic zinc. The amount of the zinc compound can be varied to adjust the color imparted to the coating, from as little as about 0.1 gram per liter up to the solubility limit of the zinc compound depending on the temperature of the solution e.g. 0.1 to 12 grams of Zinc2+cation. The cationic or divalent zinc can be supplied by various chemical compounds e.g. salts that dissolves in water and are compatible with the other components in the acidic solution. Cationic or divalent zinc compounds that are water soluble at the required concentrations i.e. up to the solubility limits of the compounds, preferably include, zinc acetate, zinc telluride, zinc tetrafluoroborate, zinc molybdate, zinc hexafluorosilicate, zinc sulfate and various other zinc salts and any combination thereof in any ratio.
The pretreatment or post-treatment of the aluminum can be carried out at various temperatures including the temperature of the solution which ranges from ambient e.g. from about room temperature up to about 160° F. or higher e.g. up to about 212° F. Room temperature up to 100° F. is preferred, however, in that this eliminates the necessity for heating equipment. The subsequent coating may be air dried by any of the methods known in the art including, for example, oven drying, forced-air drying, exposure to infra-red lamps, and the like. For purposes of this invention, the term aluminum includes alloys of aluminum containing small but effective amounts of various other metals.
The following Examples illustrate the stable solutions of this invention, and the method of using the acidic solutions in providing color recognition, improved adhesion bonding and corrosion-resistant for aluminum, aluminum alloy and anodized aluminum substrates.
A stable acidic aqueous solution having a pH ranging from about 3.7 to 4.0 for post-treating an anodized aluminum to provide a corrosion-resistant coating thereon comprises, per liter of solution, from about 1.0 to 12 grams of potassium hexafluorozirconate, about 0.1 to 12 grams of zinc sulfate and from about 0.01 to 10 grams of an organic thickener.
A stable acidic aqueous solution of deionized water for treating an aluminum alloy substrate to form a corrosion-resistant coating thereon comprises, per liter of solution, about 4.0 grams of potassium hexafluorozirconate, about 3.0 grams of zinc sulfate, and about 1.0 gram of methyl cellulose.
A NCP solution was prepared by mixing about 10 grams per liter of solution of potassium hexafluorozirconate, about 10 grams per liter of a water soluble divalent zinc salt and about 0.5 grams of cellulose.
An NCP-6 solution was prepared by mixing 4.0 grams per liter of potassium hexafluorozirconate and 3.0 grams per liter of zinc sulfate in deionized water. The NCP-6 solution can then be used as is or diluted as appropriate.
A sprayable concentrate of NCP was produced by mixing 8.0 grams per liter of potassium hexafluorozirconate and 6.0 grams per liter of zinc sulfate in deionized water. About 0.1 gram per liter of Methocel F4M can be added to enhance wetting and reduce evaporation of water during spraying.
More specifically, the conversion coatings were applied onto 2024-T3 aluminum as follows: Test coupons made from 2024-T3 aluminum were cleaned in a standard alkaline cleaner (Turco NC-LT) at 120° F. for 20 minutes. Coupons were double rinsed in warm tap water and immersed in Tuco Smut-Go NC for 5 minutes. Coupons were double rinsed in cold tap water and then immediately (without drying) immersed into the composition set forth in Example 4. Coupons were allowed to dwell in the NCP-6 for approximately 10 minutes, removed, and thoroughly rinsed in deionized water. Coupons were then allowed to dry in a rack at ambient conditions. The resulting coating was a dark brown/tan in color, easily visible from across the laboratory. This is critical for quality control during processing so that processors have a way of knowing that a coating is present. This process can be used for any aluminum to produce an acceptable conversion coating with variations on immersion times depending on the aluminum or alloy leading to various color intensities, shades and coating weights.
In preparing the acidic solutions of this invention, water soluble surfactants can be added to the acidic solutions in amounts ranging from about 0.0 to 10 or 0.5 to 10 grams per liter and preferably from about 0.5 to 1.5 grams or 1.0 gram per liter. The surfactants are added to the aqueous solution to provide better wetting properties by lowering the surface tension thereby insuring complete coverage, and a more uniform film on the aluminum substrates. The surfactants include at least one water soluble compound selected from the group consisting of non-ionic, anionic, and cationic surfactants. These surfactants include the monocarboxyl imidoazolines, alkylsulfate sodium salts (DUPONOL®), tridecyloxypoly(alkyleneoxy ethanol), ethoxylated or propoxylated alkylphenols (IGEPAL®), alkylsulfonamides, alkaryl sulfonates, the alkylaryl polyether alcohols such as octylphenoxypolyethoxy ethanol (TRITON®), sorbitan monopaimitate (SPAN®), polyoxyethylenealkylphenyl ethers, dodecylphenyl polyethyleneglycol ether (TERGITOL®), alkyl pyrrolidones, polyalkoxylated fatty acid esters, alkylbenzene sulfonates and mixtures thereof. Other water soluble surfactants include the alkylphenol alkoxylates, preferably the nonylphenol ethoxylates, and adducts of ethylene oxide with fatty amines; see the publication: “Surfactants and Detersive Systems”, published in Kirk-Othmer's Encyclopedia of Chemical Technology, 3rd Ed.
When large surfaces do not permit immersion or where vertical surfaces are to be sprayed, thickening agents can be added to retain the aqueous solution on the surface for sufficient contact time. The thickeners employed are preferably the organic water soluble thickeners added to the coating solutions at sufficient concentrations ranging from about 0.0 to 10 grams or 0.5 to 10 grams per liter and preferably 0.5 to 1.5 grams or 1.0 gram per liter of the acidic solution. Specific examples of some preferred thickeners include the cellulose compounds, such as hydroxypropyl cellulose (Klucel), methyl or ethyl cellulose, hydroxyethyl cellulose, hydroxymethyl cellulose and mixtures thereof. Other water soluble inorganic thickeners include colloidal silica, clays such as bentonite, starches, gum arabic, tragacanth, agar and various combinations.
The aqueous solution can be applied to the aluminum substrate via immersion, spray or wipe-on techniques. The solutions can be used at temperatures ranging from about ambient up to about 160° F. or higher e.g. from about room temperature up to about 212° F. and optimally applied via immersion to further improve the corrosion resistance of the coatings. Solution dwell time ranges from about 1 to 10 minutes and as high as 60 minutes or more, and preferably, for example, from about 5 to 15 minutes at about 80° F. After dwelling, the remaining solution is then thoroughly rinsed from the alloy with tap or deionized water. No additional chemical manipulations of the deposited films are necessary for excellent performance. In addition, the aqueous solutions may be sprayed from a spray tank apparatus designed to replace immersion tanks. This concept also reduces active chemical volume from about 1,000 gallons to about 30 to 50 gallons.
While this invention has been described by a number of specific examples, it is obvious that there are other variations and modifications which can be made without departing from the spirit and scope of the invention as particularly set forth in the appended claims.
The invention described herein was made by employee(s) 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.
