Patent Publication Number: US-2007095437-A1

Title: Non-chromium conversion coatings for ferrous alloys

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 compositions and to the use of said compositions for pretreating ferrous alloys with effective amounts of an acidic aqueous solution comprising at least one fluorometallic compound such as an alkali metal hexafluorozirconate, divalent zinc compounds, water soluble surfactants or wetting agents and water soluble thickeners. This invention relates to compositions and to the process for treating ferrous alloys such as steel to improve the adhesion bonding and corrosion resistant properties of the alloys. More specifically, the process comprises treating ferrous or iron 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 metal hexafluorozirconate, at least one water soluble divalent zinc compound, and water soluble thickeners and/or water soluble surfactants.  
      Current surface preparation of ferrous alloys such as steel include a variety of mechanical and chemical processes, depending on the application. Phosphate coatings have been used historically to improve the adhesion of various outer coatings and corrosion resistance of steel. For example, grit blasting has been used to improve the adhesion when phosphate coatings were not practical to apply. In this case, the desired paint primer is applied directly to the grit-blasted steel which is referred to as the “direct-to-metal” process. Many organizations rely on the direct-to-metal technique as the use of phosphate coatings decline due to their reliance on hexavalent chromium “rinse” for optimum adhesion and corrosion resistance. While direct-to-metal coating is straightforward and does not rely on a chemically-produced coating, it does not provide an optimum-performing coating system. The absence of a chemical pretreatment lowers the overall corrosion resistance of the system by allowing undercutting of the paint primer and sometimes blistering of the primer. This is partially due to the near ubiquitous use of non-chromated primers for painting ferrous alloys such as steel. In addition, while grit-blasting imparts a sound mechanical base for paint adhesion it is labor intensive and requires application of the grit over all the steel surface generating large quantities of spent grit that must be collected and recycled or disposed.  
      Further, where there is no corrosion-resistant conversion coating on the ferrous alloys such as steel there is the problem of “flash rusting” that occurs after the steel parts are grit blasted or cleaned, but before the paint can be applied. The requirement to keep flash rust from occurring places a tremendous burden on scheduling and does not allow for components to sit for more than a few hours before being painted. This prevents the metal parts or components from being allowed to sit overnight or over the weekend and therefore prevents the work from being completed by the end of a shift where the parts cannot be painted before the end of a particular shift. This renders production less productive than it ordinarily could be. In addition, parts or components that are processed, but not yet painted that need to be shipped or moved before painting requires an elaborate and costly protective coating that must then be subsequently stripped before application of the outer coating such as paint. A chemical pretreatment for ferrous alloys that is analogous to an aluminum pretreatment is not available but would be desirable. Such a coating would impart good paint adhesion to the alloy e.g. steel without needing a grit-blasted surface and thereby increase the corrosion resistance of the painted steel. Moreover, the pretreatment of the alloy would serve also to prevent flash-rusting and allow for expanded handling times of the alloy parts before they need to be painted. Such a pretreatment could be produced by merely immersing the components or alloy parts in a tank of the pretreating solution or by spraying the solution onto the alloy or by wiping the solution onto the alloy.  
     SUMMARY OF THE INVENTION  
      This invention relates to compositions and the process for preparing conversion coatings on ferrous alloys at ambient temperatures e.g. ranging from room temperature up to about 160° F. or higher. More specifically, this invention relates to a process of preparing conversion coatings on ferrous alloys such as steel to improve its corrosion resistance and adhesion bonding properties. Specifically, this invention relates to preparing chromium and nickel-free compositions and the process to deposit a film or coating made from the compositions on ferrous alloys e.g. steel, especially non-stainless alloys that are prone to red rusting. The compositions are referred to herein as “NCP” for “Non-Chromium Post-Treatment” or “Non-Chromium Process”.  
      The compositions comprise an acidic aqueous solution having a pH ranging from about 2.5 to 5.5 which contains a fluoride source comprising between 0.1 and 22 grams/liter of an metal complex fluoride, such as potassium or sodium hexafluorozirconate, and from about 0.1 gram per liter up to the solubility limits of a water-soluble divalent zinc compound, and from 0.0 to effective amounts of thickening agents and surfactants.  
      It is therefore an object of this invention to provide an acidic aqueous solution comprising hexafluorozirconates, divalent zinc compounds for pretreating ferrous alloys to improve its 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 which comprises an alkali metal hexafluorozirconate and divalent zinc salts for pretreating ferrous alloys such as steel.  
      It is another object of this invention to provide a pretreatment for steel that has a practical color change and imparts good adhesion without a grit-blasted surface. The pretreatment process provides improved corrosion resistance of the painted alloy compared to the alloy painted by the direct-to-metal method.  
      It is 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 divalent zinc salts for treating ferrous alloys 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. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      This invention relates to 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 preparing a conversion coating on ferrous alloys such as steel to improve adhesion bonding and the corrosion-resistance properties of the alloys. The process comprises preparing the pretreatment coating by using an acidic aqueous solution at temperatures ranging from ambient up to about 160° F. or higher e.g. ranging from about 70° F. to 200° F. and comprises, per liter of solution, from about 0.1 to 22 grams and preferably about 1.0 to 12 grams e.g. 6.0 to 8.0 grams of at least one metal hexafluorozirconate, and from about 0.1 of a gram up to the solubility limits of a divalent zinc compound e.g. from about 0.1 to 12 or 1.0 to 6.0 grams, from 0.0 to 10 grams and preferably 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. An advantage of the process is the formation of a pretreatment coating on the alloy that has a practical color change and imparts good paint adhesion without a grit-blasted surface.  
      After cleaning and deoxidizing or pickling the alloy e.g. steel substrate via conventional mechanical or chemical techniques, the coating solution can be applied at about room temperature to the substrate via immersion, spray or wipe-on techniques similar to the process used for aluminum pretreatments. Solution dwell time ranges from about 1.0 to 60 minutes. With this solution, the 1.0 to 10 minute dwell time yields an optimum film for color change, paint adhesion, and corrosion resistance. More importantly for this invention, the 1.0 to 10 minute dwell time yields appreciable color change to the as-deposited coating that ranges from royal blue to blue-gray depending primarily on the chemical composition of the aqueous solution. The remaining unreacted solution is subsequently rinsed from the alloy with tap or deionized water. No additional post-treatment of the alloy is necessary. The pretreatment coating is allowed to dry thoroughly before subsequent painting.  
      In some processes, depending on the physical characteristics of the ferrous alloy i.e. steel substrate such as 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 feature of 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 are present in the acidic aqueous solution in amounts ranging from about 0.0 to 10 grams per liter and preferably 0.5 to 1.5 e.g., or about 1.0 gram per liter of the aqueous solution.  
      In addition, depending on the characteristics of the ferrous alloy, an effective but small amount of at least one water-soluble surfactant or wetting agent can be added to the acidic solution in amounts ranging from about 0.0 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 solution contains effective amounts of at least one divalent zinc compound to provide color and also to improve corrosion protection of the alloy when compared to other treatment and compositions that do not contain zinc. For example, the amount of the zinc compounds can be varied to adjust the color imparted to the coating, from as little as about 0.1 grams per liter up to 12 grams per liter e.g. 1.0 to 6.0 grams of Zinc 2 +cation. The divalent zinc can be supplied by any chemical compound e.g. salts that dissolves in the water in amounts ranging up to the zinc salts solubility limits and are compatible with the other components in the acid solution. Divalent zinc compounds that are water soluble at the required concentrations preferably include, for example, zinc acetate which is soluble in the solution up to 300 grams per liter at 20° C., zinc telluride, zinc tetrafluoroborate, zinc molybdate, zinc hexafluorosilicate, and zinc sulfate which is soluble in the solution up to 577 grams per liter at about 25° C. and any combination thereof in any ratio.  
      The pretreatment or coating of the ferrous alloys 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. Room temperature is preferred, however, in that this eliminates the necessity for heating equipment. The 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 “ferrous alloys” includes any iron alloy such as steel containing small but effective amounts of various other metals and non-metals such as carbon.  
      The Examples illustrate the stable solutions of this invention, and the method of using the solutions in providing color recognition, improved adhesion bonding and corrosion-resistant coatings for ferrous alloys.  
     EXAMPLE 1  
      A stable acidic aqueous solution having a pH ranging from about 3.7 to 4.0 for pretreating steel to provide a corrosion-resistant and a color recognized coating thereon comprises, per liter of solution, from about 1.0 to 12 grams of potassium hexafluorozirconate and about 0.1 to 12 grams of zinc sulfate.  
     EXAMPLE 2  
      A stable acidic aqueous solution of deionized water for treating steel to form a corrosion-resistant coating thereon comprises, per liter of solution, about 4.0 grams of sodium 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 treating steel to provide a corrosion-resistant and a 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 cellulose (Methocel F4M).  
      The conversion coatings were applied to 4130 steel by the following method:  
      The coupons were cleaned in a standard alkaline cleaner (Turco HTC) at 140-160° F. for 10 minutes. Coupons were then rinsed and immersed directly into the composition set forth in Example 1. 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 overnight. The resulting coating was a dark gray in color, easily visible from across the laboratory. This is critical for quality control during processing so that processors have an easy way to tell that a coating is present. In addition to the NCP coated 4130 steel, control sets of 4130 were prepared by cleaning in the same solution and treating with TCP-P (potassium hexafluorozirconate). A subset of these were grit blasted with alumina grit media to produce an average profile of 1.0 to 1.5 mils, standard for direct to metal painting. These coupons were then painted with MIL-C-53022 primer to approximately 1.0 mil thick. One sub-set was painted with Aqua Zen™ “wash primer” before the primer. Aqua Zen™ is an industry standard coating used to promote adhesion and improve corrosion performance of coating systems. The painted coupons sat for 14 days at ambient conditions to allow for the paint to cure. After curing, the coupons were subjected to paint adhesion and painted corrosion tests.  
      Table 1 describes paint adhesion results. Ratings of 4 and 5 are considered passing, 0 to 3, failing. As shown by the data, NCP provides excellent paint adhesion to the 4130 steel whether or not the steel is grit blasted. The NCP also shows better overall performance than the two controls which are commonly used on DOD equipment. In this case, the NCP is a better alternative to Aqua Zen™ and shows excellent performance with the MIL-C-53022 primer without grit blast, where the paint adhesion is very poor with only the primer.  
               TABLE 1                          COATINGS AND PAINT ADHESION RESULTS                             Coating   Paint Adhesion Results                                     Grit Blast   System   Dry   1-day Wet   4-day Wet   7-day Wet               No   MIL-C-53022   4   4   0   0           primer only       No   Aqua Zen   5   4   3   3           plus MIL-C-           53022 primer       No   TCP-P plus   5   4   4   4           MIL-C-53022           primer       No   NCP plus   5   5   5   NA           MIL-C-53022           primer       Yes   MIL-C-53022   5   5   5   3           primer only       Yes   Aqua Zen   4   4   3   1           plus MIL-C-           53022 primer       Yes   TCP-P plus   5   5   5   4           MIL-C-53022           primer       Yes   NCP plus   5   5   5   NA           MIL-C-53022           primer                  
 
      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 to 10 grams per liter and preferably 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 iron alloy substrates. The surfactants include at least one water soluble compound selected from the group consisting of non-ionic, anionic, and cationic surfactants. Some known water soluble surfactants include the monocarboxyl imidoazoline, 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 monopalnitate (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 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 thereof.  
      After preparing the iron alloy surface to be coated via conventional techniques, the solution can be applied via immersion, spray or wipe-on techniques. The solutions can be used at elevated temperatures ranging up to 160° F. or higher 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 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. Moreover, 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.