Patent Publication Number: US-2007099022-A1

Title: Non-chromium post-treatment for aluminum coated steel

Description:
ORIGIN OF INVENTION  
      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.  
     BACKGROUND OF THE INVENTION  
     Field of the Invention  
      This invention relates to aqueous compositions and to the method of using said compositions for post-treating aluminum and aluminum alloys deposited on steel substrates with effective amounts of an acidic aqueous solution comprising at least one hexafluorozirconate, cationic or divalent zinc compounds, water soluble surfactants or wetting agents and/or water soluble thickeners. More specifically, this invention relates to compositions and to the process for post-treating aluminum and aluminum alloys deposited on steel to improve the aluminum&#39;s adhesion bonding and corrosion resistant properties. The process comprises treating aluminum and its alloys coated on steel substrates with a chromium-free composition comprising an acidic aqueous solution having a pH ranging from about 2.5 to 5.5 and containing an effective amount of at least one water soluble hexafluorozirconate, at least one water soluble cationic or divalent zinc compound, and effective amounts of water soluble thickeners and/or surfactants.  
      The invention relates to chromium-free compositions and the process to deposit a coating onto aluminum or aluminum alloy coated steel such as Alumiplate, IVD-aluminum, and sputtered aluminum. The as-deposited aluminum coatings on steel require a post-treatment for enhanced corrosion protection and paint adhesion. This post-treatment produces a film or coating which imparts improved corrosion resistance and paint adhesion as compared to an untreated sacrificial coating with or without a subsequent coating.  
      Generally, post-treatment coatings on sacrificial coatings contain chromium compounds. Currently, chromate or chromium post-treatments are used on sacrificial coatings i.e. aluminum deposited onto steel substrates. These post-treatments are applied by spraying and immersion techniques thereby generating large amounts of hazardous waste and personnel exposure. Additionally, since current processes contain hexavalent chromium, the coating deposited from the post-treatment also contains hexavalent chromium which is classified as toxic. Environmental laws, executive orders, and local occupational, safety, and health (OSHA) regulations are requiring military and commercial users to find alternatives to chromate post-treatments without sacrificing performance. The additional impact of the tighter regulations concerning the use of hexavalent chromium compounds is the reason for the increase in cost to reduce personnel exposure and disposal fees. Due to further planned reductions in PEL (personal exposure limits), as set forth by OSHA, the cost of using hexavalent chromium compounds is going to become prohibitive. The non-post treated aluminum coated substrates per se are non-toxic, however, the application of chromate coatings requires that these substrates be listed as hazardous materials. Moreover, the current commercially available non-chromated post-treatment coatings are inferior when compared to the chromated coatings with the exception of certain trivalent chromium coatings. Trivalent chromium coatings while not an issue for environmental and OSHA concerns, due to their low toxicity, still pose a greater life-cycle cost impact than the non-heavy metal materials. While existing hexavalent and trivalent chromium post-treatments are acceptable from a performance standpoint, from the perspective of life-cycle, environment, and OSHA impact, the non-chromium alternatives are highly desirable. Compositions and processes comprising trivalent chromium offers excellent technical performance compared to hexavalent chromium compositions. However, both of these compositions still contain chromium and although the chromium may be in its low toxicity trivalent state and in significantly lower concentration, industry is still pushing toward the elimination of chromium from their facility.  
      Therefore, post-treatment coatings without chromium that maintains the performance of conventional chromate post-treatments are highly desirable. Specifically, current non-chromium post-treatments have shown poor corrosion resistance when applied to aluminum coatings on steel. These non-chromium post-treatments are generally inferior to hexavalent and trivalent chromium post-treatments. Thus, there is need for a chromium-free post-treatment that provides excellent adhesion bonding and corrosion resistance in addition to improved processes.  
     SUMMARY OF THE INVENTION  
      This invention relates to preparing a non-chromium composition and the process of depositing a film or coating made from the composition on aluminum and its alloys deposited on steel substrates. The composition is referred to herein as “NCP” for “Non-Chromium Post-Treatment” or “Non-Chromium Process”. The composition comprises an acidic aqueous solution containing a fluoride source, such as 0.1 to 22 grams per liter of solution of an alkali-metal complex fluoride, e.g. potassium hexafluorozirconate, and depending on the temperature of the solution, from about 0.1 gram up to the solubility limit of a water-soluble cationic or divalent zinc compound e.g. about 577 grams of zinc sulfate per liter of solution at 25° C. A preferred composition (NCP 6) contains about 4.0 grams per liter of solution of sodium or potassium hexafluorozirconate and a cationic zinc source, most effectively about 3.0 grams per liter of solution of a zinc salt. If a thickening agent is needed to enhance solution application, about 0.0 to 10 grams per liter of solution of a cellulose thickener is effective. The components of the composition are mixed together in de-ionized or distilled water and used with no further chemical manipulation.  
      Therefore, it is an object of this invention to provide an acidic aqueous solution comprising hexafluorozirconates and cationic or divalent zinc compounds for post-treating aluminum and its alloys deposited on steel to improve the aluminum&#39;s adhesion and corrosion-resistance properties.  
      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 hexafluorozirconate and divalent zinc salts for post-treating an aluminum alloy coated steel.  
      It is another object of this invention to provide a post-treatment for aluminum coated steel that has a practical color change and improved corrosion resistance.  
      It is still a further object of this invention to provide an acidic aqueous solution having a pH ranging from about 3.7 to 4.0 comprising hexafluorozirconates and cationic or divalent zinc salts for treating aluminum coated steel at about room temperature and higher 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  FIGS. 1-3  (photos). 
    
    
     DESCRIPTION OF THE DRAWINGS  
       FIG. 1  (photos) shows burnished untreated panel A (IVD-Aluminum) in comparison to the burnished NCP treated panel B (IVD-Aluminum).  
       FIG. 2  (photos) shows panel A (Alumiplate™ on 4130 Steel) post-treated with a composition of this invention (NCP 6) after 4,200 hours in the salt fog test (ASTM B117) in comparison to panel B (Alumiplate™) post-treated with NCP 6 as-deposited.  
       FIG. 3  (photos) shows panel A (Alumiplate™ on 4130 Steel) post-treated with a commercial product (Alodine™ 1200S) in the salt fog test after 4,200 hours in comparison to panel B (Alumiplate™ on 4130 Steel) post-treated with the commercial product (Alodine™ 1200S), as-deposited. 
    
    
     DETAILED DESCRIPTION OF THE INTENTION  
      This invention relates to chromium-free aqueous compositions and to the process of using 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 post-treating aluminum and aluminum alloy coated steel substrates to improve the adhesion bonding and corrosion-resistance properties of the aluminum coated steel substrates. The process comprises preparing the post-treatment coatings by using an acidic aqueous solution at temperatures ranging up to about 120° F. or higher e.g. up to 160°-200° F. and 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 hexafluorozirconate, and depending on the temperature of the solution from about 0.1 up to the solubility limit of a water soluble divalent or cationic zinc compound and preferably from 0.1 to 12 or 1.0 to 6.0 grams of a divalent zinc salt, 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 or 0.5 to 1.5 grams of a surfactant. The process forms a post-treatment coating on the aluminum coated steel that has a practical color change, good paint adhesion and improved corrosion resistance.  
      More specifically, the post-treatment coatings were applied onto aluminum coated  4130  steel substrates by the following method: The aluminum coated steel coupons were cleaned in a standard alkaline cleaner (Turco HTC) at 140-160° F. for about 10 minutes. Coupons were then rinsed and immersed directly into a composition of this invention (Example 2). Coupons were allowed to dwell in the NCP-6 composition 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 dark gray in color, see  FIGS. 1-3  (photos A and B).  
      For coatings of aluminum-on-steel, the aluminum surface may or may not be mechanically treated by methods such as glass-bead burnishing. Once the aluminum surface is ready to be post-treated; cleaning, deoxidizing, or activating the aluminum surface is performed by chemical techniques. For example, the aqueous composition can be used at about room temperature or higher and applied to the aluminum-coated steel substrate via immersion, spray or wipe-on techniques similar to methods generally used for other aluminum treatments. Solution dwell time ranges from about 1 to 60 minutes. A 1 to 15 minute dwell time yields an optimum film or coating for color change, paint adhesion, and corrosion resistance. These dwell times yield appreciable color change to the as-deposited coating that ranges from blue-gray to gray depending on the specific composition of the aqueous solution. The remaining unreacted solution is then thoroughly rinsed from the coated substrate with tap or deionized water. No additional post-treatment is necessary prior to use. The coating is allowed to dry thoroughly before subsequent painting.  
      In comparison, examples of untreated glass-bead burnished IVD aluminum showed severely red rust in neutral salt fog within 90 days whereas after 150 days the NCP post-treated specimen of this invention still showed no signs of red rust. Burnished IVD-aluminum coupons (A and B) were tested in accordance with ASTM B117 (neutral salt fog testing).  FIG. 1  shows the positive effect of the NCP post-treatment (coupon B) after 11 days in neutral salt fog in comparison to coupon A.  FIG. 2  shows the positive results of panel A, post-treated with NCP 6 after 4,200 hours in the salt fog test in comparison to panel B also post-treated with NCP 6, as-deposited (no salt fog exposure).  FIG. 3  shows panel A post-treated with Alodine™ 1200S, after 4200 hours of salt fog test in comparison to panel B post-treated with Alodine™ 1200S, as-deposited.  
      In some processes, depending on the physical characteristics of the aluminum coated steel substrate e.g. the physical size of the substrate, the addition of a thickener to the solution aids in optimum film formation during spray and wipe-on applications by slowing down solution evaporation. This also mitigates the formation of powdery deposits that degrade paint adhesion. The addition of thickeners, also aids in proper film formation during large area applications and mitigates the diluent effect of rinse water that remains on the substrate during processing from previous steps. This addition to the process yields films that have no streaks and are an improvement in coloration and corrosion protection. Water-soluble thickeners such as the cellulose compounds can be added to the acidic aqueous solution, as an option, in amounts ranging from about 0.0 to 10 or 0.5 to 10 grams per liter and preferably 0.5 to 1.5 grams e.g. about 1.0 gram per liter of the aqueous solution.  
      In addition, depending on the characteristics of the aluminum coated steel substrate, as an option, an effective but small amount of a water-soluble surfactant or wetting agent may be added to the acidic solution in amounts ranging from about 0.0 to 10 or 0.5 to 10 grams and preferably from 0.5 to 1.5 grams e.g. 1.0 gram per liter of the acidic solution. There are many water soluble surfactants known in the prior art and therefore, for purpose of this invention, one or more water soluble surfactants can be selected from the group consisting of non-ionic, cationic and anionic surfactants.  
      The acidic solution contains at least one divalent or cationic zinc compound not only to provide color, but also to improve corrosion protection of the aluminum coated steel as compared to compositions that do not contain 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 grams per liter of solution up to the solubility limit of the particular water soluble compound depending on the temperature of the solution. For example, about 0.1 to 12 grams or 1.0 to 6.0 grams per liter of solution of a Zinc 2 +cation. The divalent cationic zinc can be supplied by any chemical compound e.g. zinc salt that dissolves in water and is compatible with the other components in the acidic solution. Cationic zinc compounds that are water soluble at the required concentrations preferably include, for example, zinc acetate, zinc telluride, zinc tetrafluoroborate, zinc molybdate, zinc hexafluorosilicate, zinc sulfate and various other zinc salts and combination thereof in any ratio.  
      The post-treatment of an aluminum coated steel 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 200° F. Room temperature 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 aluminum alloys e.g. aluminums that contain small but effective amounts of various other metals to form the alloy.  
      The Examples illustrate the stable solutions of this invention, and the method of using the acidic solutions to provide color recognition, improved adhesion bonding and corrosion-resistant coatings for aluminum and aluminum alloys coated on steel substrates.  
     EXAMPLE 1  
      A stable acidic aqueous solution having a pH ranging from about 3.7 to 4.0 for post-treating an aluminum coated steel substrate to provide a corrosion-resistant and color recognized coating thereon comprises, per liter of solution, from about 1.0 to 12 grams of potassium hexafluorozirconate and from about 0.1 to 12 grams of zinc sulfate.  
     EXAMPLE 2  
      A stable acidic aqueous solution of deionized water for treating an aluminum alloy coated steel 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.  
     EXAMPLE 3  
      A stable acidic aqueous solution having a pH ranging from about 2.5 to 5.5 for post-treating an aluminum alloy coated steel to provide a corrosion-resistant and color recognized coating thereon comprises, per liter of solution, about 8.0 grams of potassium hexafluorozirconate, about 6.0 grams of divalent zinc sulfate and about 0.1 gram of methyl cellulose.  
      In preparing the acidic solutions of this invention, the water soluble surfactants can be added to the acidic solutions in amounts ranging from about 0.0 to 10 grams 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 coated steel. The surfactants include water soluble compounds selected from the group consisting of non-ionic, anionic, and cationic surfactants. Some known surfactants include the monocarboxyl imidazolines, 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 monopalmitate (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&#39;s Encyclopedia of Chemical Technology, 3 rd  Ed.  
      When large surfaces do not permit immersion or where vertical surfaces are to be sprayed, thickening agents are 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 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.  
      After preparing the aluminum or aluminum alloy coated steel to be coated via conventional techniques, the aqueous solution can be applied via immersion, spray or wipe-on techniques. The solutions can be used at ambient or elevated temperatures ranging from 120° F. up to 160° F. to 200° F. and optimally applied via immersion to further improve the corrosion resistance of the coatings. Solution dwell time ranges from about 1 to 60 minutes, and preferably from 5.0 to 15 or 10 to 30 minutes at about 80° F. or higher. 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. Moreover, 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.