Abstract:
An aqueous conversion coating composition for treating steel substrates to provide a conversion coating thereon. The conversion coating composition comprises a) a source of aluminum ions; b) hexafluorozirconic acid or its salts; c) at least one pH adjuster; and d) optionally, a surfactant. The conversion coating composition provides a corrosion resistant coating on the metal surface and improves the adhesion of subsequently applied layers.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to compositions and methods for the formation of protective, corrosion-inhibiting coatings on metals, or other materials coated with metals. 
       BACKGROUND OF THE INVENTION 
       [0002]    Metals such as steel tend to corrode rapidly in the presence of water due to their low oxidation-reduction (redox) potentials or ease of oxide formation. In addition, steel can also have a significant problem with paint adhesion, because the as-formed metal surfaces are typically very smooth, and tend to form weakly bound surface oxides, which do not normally provide a robust base on which subsequent applied paints can anchor themselves. 
         [0003]    One method of enhancing the corrosion resistance of steel is through the use of a conversion coating, which is a self-healing, corrosion-inhibiting layer formed during intentional exposure of the metal to a chemically reactive solution. Conversion coatings are particularly useful in surface treatment of metals such as steel. The conversion coating process forms an adherent surface containing an integral corrosion inhibitor that can provide protection to coating breaches. The metal is exposed to a compound that chemically alters the surface and forms a coating that provides a high degree of corrosion resistance. Thus, a chemical conversion coating applied to the surface of a less-noble alloy can reduce the extent and severity of aqueous corrosion, provide long-term property stability, and extend the useful life of the object of manufacture. 
         [0004]    A critical feature of effective conversion coatings is their ability to provide corrosion protection to the base metal in the presence of a coating breach. Conversion coatings grow on the metal without an externally applied electrical potential. The protective film is produced by a chemical reaction between the metal surface and the conversion coating solution. The film is composed both of an oxide and integral corrosion inhibitor species formed during exposure to the conversion coating solution. 
         [0005]    It has previously been common to apply these conversion or pretreatment coatings using hexavalent chromium-containing solutions. While these coatings provide good corrosion resistance, attempts have been made to provide more acceptable non-chromate derived coatings or coatings derived from trivalent chromium because of concern regarding the occupational, safety, health and environmental effects of hexavalent chromium, which is highly toxic and is a known carcinogen. Various efforts have been made to develop such coating and examples of these coatings can be found for example in U.S. Pat. No. 7,294,362 to Tanaka et al. in U.S. Pat. Nos. 6,375,726, 6,521,029 and 6,669,764 to Matzdorf et al., and in U.S. Pat. No. 7,294,211 to Sturgill et al., the subject matter of each of which is herein incorporated by reference in its entirety. 
         [0006]    The conversion-coated surface may be left bare or afforded further protection by the application of additional films or coatings. Conversion coatings need to adhere to the substrate and should also result in a surface that will promote the formation of a strong bond with subsequently applied coatings. Bonding with subsequently applied coatings is a function of the morphology and chemical composition of the conversion coating. Adhesion promoting surface treatments may exhibit corrosion inhibiting characteristics. Depending on the intended application, a conversion coating, as described herein, may also be considered to be an “adhesion promoter” and vice versa. 
         [0007]    Conversion coatings are usually formed by the application of a conversion coating solution to the metal surface. The solution can be applied by immersion, spray, fogging, wiping, or other similar means depending on the complexity of the surface of the substrate being treated. 
         [0008]    While various conversion coating chemistries have been proposed, the inventors of the present invention have determined that additional further improvement are still needed. 
       SUMMARY OF THE INVENTION 
       [0009]    It is an object of the present invention to provide an improved conversion coating composition that does not contain toxic hexavalent chromium but affords improved corrosion protection of the metal surface. 
         [0010]    It is another object of the present invention to provide an improved conversion coating composition that provides improved corrosion protection for steel and increases the adhesion of subsequently applied paint to the so treated steel. 
         [0011]    It is still another object of the present invention to provide a conversion coating composition that contains a stable solution for the treatment of metal substrates. 
         [0012]    To that end, the present invention relates generally to an aqueous conversion coating composition comprising: 
         [0013]    a) from 0 g/l to 300 g/l of aluminum ions; 
         [0014]    b) hexafluorozirconic acid or its salts; 
         [0015]    c) at least one pH adjuster; 
         [0016]    d) optionally, a surfactant. 
         [0017]    The present invention also relates to a method of using the aqueous conversion coating composition of the invention to treat metal substrates such as steel substrates to provide an improved conversion coating thereon. In one embodiment, the aqueous conversion coating composition of the invention is preferably substantially free of phosphorous and/or chromium. As used throughout this patent specification and claims, the word “steel” means both raw steel and steel coated with zinc or zinc alloys. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    In one embodiment, the present invention relates generally to acidic aqueous conversion coatings for the pretreatment of metals, such as steel, and to a process for improving the corrosion resistant properties of such metal substrates and the adhesion of subsequently applied paint. 
         [0019]    In one embodiment, the present invention relates generally to compositions for pretreating steel substrates and to a process of pretreating steel substrates at temperatures ranging from ambient temperatures up to about 200° F. These pretreatment compositions typically comprise an acidic aqueous solution having a pH in the range of about 0.5 to about 6 and preferably from about 4 to 5. 
         [0020]    The pretreatment composition of the invention is an aqueous solution that typically comprises: 
         [0021]    a) from 0 g/l to 300 g/l of aluminum ions; 
         [0022]    b) hexafluorozirconic acid or its salts; 
         [0023]    c) at least one pH adjuster; 
         [0024]    d) optionally, a surfactant. 
         [0025]    The composition is preferably essentially free of hexavalent chromium and sequestering agents for zinc. The composition is also preferably free of all chromium compounds. 
         [0026]    The composition preferably comprises aluminum ions. In one preferred embodiment, the source of aluminum ions is aluminum hydroxide. Although aluminum hydroxide is typically preferred, alkali hexafluoride aluminates such as sodium hexafluoride aluminate may also be used in the practice of the present invention. In another preferred embodiment, the source of aluminum ions is pure aluminum powder which is used in combination with hexafluorozirconic acid, whereby the aluminum powder is thus dissolved in the solution by the acidity of the hexafluorozirconic acid. The concentration of aluminum ions in the composition can be from 0 g/l to 300 g/l, but is preferably from 1 g/l to 50 g/l. 
         [0027]    The composition also comprises hexafluorozirconic acid or salts thereof. Water-soluble potassium, sodium, lithium, or ammonium salts of these anions are typical, with ammonium salts of these anions being preferred. 
         [0028]    The pH adjuster, which may include one or more alkali metal hydroxide, ammonium hydroxide, ammonium bicarbonate, ammonium carbonate, sodium carbonate, and sodium bicarbonate and combinations of one or more of the foregoing. Other compounds would also be known to those skilled in the art. Preferably the pH of the composition is maintained from 0.5 to 6, most preferably from 4 to 5. 
         [0029]    The composition may also comprise a surfactant. The inventors have found the following surfactants to be useful: TRITON® DF12, ethylene oxide-propylene oxide copolymers, DOWFAX® 63N10, polypropylene glycol, polyethylene glycol, and ethoxylated alcohols such as the LUTENSOL® series of surfactants. 
         [0030]    Another optional ingredient that may beneficially be included in some formulations prepared in accordance with the present invention is a preservative. One preferred preservative is hexahydro-1,3,5-tris(2-hydroxyethyl)-s-triazine, sold under the tradename Surcide P (available from Surety Laboratories, Cranford, N.J.). Other similar preservatives would also be known to those skilled in the art. 
         [0031]    Table 1 sets forth one preferred composition for Formula A prepared in accordance with the present invention. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Composition of Formula A 
               
             
          
           
               
                   
                 Component 
                 Grams/liter 
               
               
                   
                   
               
               
                   
                 hexafluorozirconic acid 
                 2.2 g/l 
               
               
                   
                 aluminum hydroxide 
                 0.1 g/l 
               
               
                   
                 Triton ® DF-16 
                 0.4 g/l 
               
               
                   
                 Water 
                 To 1 liter 
               
               
                   
                   
               
               
                   
                 Ammonium hydroxide (37%) to pH 4.5 
               
             
          
         
       
     
         [0032]    The metal substrate for use in the process of the present invention may be any substrate having a metal surface. Examples of usable substrates include those having a surface made of steel or steel coated with zinc or zinc alloys. Particularly preferred are steel sheet substrates. 
         [0033]    Examples of steel sheet substrates include hot-dip galvanized steel sheets, electrogalvanized steel sheets, iron-zinc alloy plated steel sheets, nickel-zinc alloy plated steel sheets, aluminum-zinc alloy plated steel sheets and the like. Also usable as steel sheet substrates are zinc-based metal plated steel sheets that have been subjected to chemical conversion treatment such as chromate treatment, zinc phosphate treatment or composite oxide film treatment. Further, a steel sheet assembly can be employed as a steel sheet substrate. 
         [0034]    The coating composition of the present invention can be applied to a metal substrate by any known process, such as dip coating, shower coating, spray coating, roll coating and electrocoating. The duration of contact ranges from about 30 seconds to about 5 minutes, but longer contact may be required if the solution is weak or if the temperature of the solution is relatively low. The aqueous solution temperature is normally below 100° C., for example in the range of 15° to 75° C., and more preferably at about 50° C. 
         [0035]    It is generally preferable that the composition be dried for about 2 seconds to about 30 minutes by heating under such conditions that the substrate reaches a maximum temperature of about 60 to 250° C. 
         [0036]    Prior to performing the pretreatment, the substrates may be treated by cleaning and/or activating as is generally well known in the art. The substrates may then be contacted with the composition of the invention for a sufficient period of time to form a conversion coating layer on the surface of the substrate and then allowed to dry. 
         [0037]    The treatment composition provided for herein, generally produces a blue conversion coating on steel with very good resistance to corrosion and adhesion to paint subsequently applied. 
         [0038]    While the invention has been described above with reference to specific embodiments thereof, it is apparent that many changes, modifications, and variations can be made without departing from the inventive concept disclosed here. Accordingly, it is intended to embrace all such changes, modifications, and variations that fall within the spirit and broad scope of the appended claims. All patent applications, patents, and other publications cited herein are incorporated by reference in their entirety.