Abstract:
A chromate free conversion coating for Al based metals and methods of use. The compositions comprise (a) water soluble fluoacids of Group IVB metals, (b) fluoboric acid, (c) boric acid, (d) gluconic acid and, optionally (e) an aminosilane adhesion promoter or an organophosphonate corrosion inhibitor. In the method, the requisite metal part is contacted by the composition such as by immersion or spraying or the like.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates generally to non-chromate coatings for aluminum and aluminum based alloys that improve the adhesion of siccative coatings to the aluminum surfaces and provides corrosion protection, while maintaining the bright appearance of the metal.  
         BACKGROUND OF THE INVENTION  
         [0002]    The current practice for treating aluminum and aluminum alloy surfaces, such as automotive wheel surfaces, requires a chromate based process to effect good paint adhesion and corrosion resistance. These chromate based treatments result in an aesthetically pleasing appearance as the machined aluminum wheel surface maintains its bright, metallic luster. However, as is well known, chromate solutions are carcinogenic and represent an environmental liability and safety concern to those who handle these solutions. Moreover, the costs associated with disposal of spent chromate baths and chromate laden rinse waters are high.  
           [0003]    Non-chromate alternatives have been provided to enhance the adherence of paints, lacquers, inks, vanishes, resins, etc. (hereinafter “siccative” coatings) and to provide corrosion inhibition. However, many of these treatments are not suitable in the aluminum wheel market as they result in a dulling or discoloration of the substrate metal and are unacceptable, especially when the wheels are coated with the now popular clear coat paints.  
         SUMMARY OF THE INVENTION  
         [0004]    The inventors have endeavored to discover a chromate free conversion coating composition and method that provides corrosion protection and siccative coating adherence comparable to conventional chromate based systems.  
           [0005]    In accordance with the invention, an acidic aqueous treatment solution comprising (a) a water soluble fluoacid of a Group IVB metal or mixtures of such acids (b) fluoboric acid (c) boric acid and (d) gluconic acid or salt thereof is provided. The metal surface is contacted with this treatment solution and, optionally, with a topping agent (e) that is selected to provide enhanced adhesion of siccative coatings (i) and/or enhanced corrosion resistance (ii).  
           [0006]    Typically, an aminosilane may be used as component (e)(i) and an organophosphonate may serve as the component (e)(ii). Despite the low pH of the treatment solutions, the incorporation of the fluoboric acid and boric acid components help to minimize aluminum etch, thereby maintaining the bright appearance of machined and polished metal surfaces.  
           [0007]    In a typical treatment scenario, the aluminum or aluminum alloy part is first cleaned in a mild alkaline cleaning solution, such as those conventional in the art, to remove surface contaminants and to assure that the metal is receptive to the coating. Care must be taken in cleaning the part so as not to dull or discolor the metal.  
           [0008]    Also, as is known in the art, after cleaning, the metal part is rinsed with water and then treated with a chemical deoxidizer to remove excess aluminum oxide and to remove alloying elements from the metal surface. This deoxidizer can be a strong acid solution typically comprising sulfuric or nitric acid combined with an oxidizing species such as ferric ion. The parts are again rinsed before treatment with the non-chromate conversion coating treatment of the invention.  
           [0009]    After treatment with the inventive compositions and methods, the parts are typically rinsed to prevent puddling and the like. The quality of the water used for the conversion coating treatment and subsequent rinse must be good to avoid undesirable accumulation of soluble salts on the metal surface.  
           [0010]    The parts are then dried and coated with the desired siccative coating such as paint, lacquer, varnish, ink, etc.  
         DETAILED DESCRIPTION  
         [0011]    In accordance with the invention, a concentrated aqueous solution of: (1) water soluble fluoacid of a Group IVB metal or metals or mixtures of such fluoacids; (2) fluoboric acid; (3) boric acid; (4) gluconic acid or salt thereof and (5) pH regulators such as nitric acid and ammonium hydroxide is prepared. This concentrate is then diluted to make an aqueous solution comprising about 1-10% v/v of the concentrate. To this bath is optionally added the desired adhesion promoter, preferably an aminosilane, an amount of about 50 to 500 ppm. In addition to or in lieu of the adhesion promoter, a corrosion inhibitor enhancer, such as an organophosphonate may be added. Alternatively, these latter components may be added directly to the concentrate.  
           [0012]    The thus formed conversion coating solution can be applied to the requisite aluminum surface by any suitable method. For example, the surface can be immersed in the solution, or the coating solution can be applied via spray techniques. Additionally, flow-coating techniques can be employed where convenient. Typically, the treatment temperature ranges from about 70° F. to about 170° F.  
           [0013]    Preferably, the temperature of the coating solution can be adjusted to above about 100° F., and the contact time for the treatment solution to the metal substrate is normally between about 15 seconds to 2 minutes. As above stated, the thus coated substrate is rinsed with water and then dried, typically in an oven having forced circulation of hot air. After drying, the desired siccative coating is applied.  
           [0014]    With regard to the fluoacid of a Group IVB metal, fluozirconic acid H 2 ZrF 6 , and fluotitanic acid H 2 TiF 6  are preferred. A combination of H 2 ZrF 6  and H 2 TiF 6  is preferred. These acids may be present in a molar amount of 1:3 to 3:1 of H 2 ZrF 6 :H 2 TiF 6 . Presently, a 1:1 molar ratio is preferred.  
           [0015]    Fluoboric acid and boric acid are also added, as necessary to minimize etching of the aluminum. To maintain the bright appearance of aluminum articles such as wheels, it is required that aluminum etch be minimized.  
           [0016]    Overall, after dilution of the concentrate and addition of the aminosilane adhesion promoter and/or organophosphate corrosion inhibitor, the pH of the diluted, working solutions will be on the order of about 0.5-5 with a range of about 1-3 even more preferred.  
           [0017]    As to the aminosilane compounds that may be used, it is desirable to use gamma-aminopropyltriethoxysilane (7-APS) due to its efficacy and commercial availability. However, other alkoxylated aminoalkylsilanes such as aminopropyltrimethoxy silane, etc., can also be mentioned. U.S. Pat. No. 6,203,854 can be reviewed for a more complete listing of the alkoxylated aminoalkylsilanes.  
           [0018]    With regard to the organophosphonates that may be employed, amino tri (methylene phosphonic acid) (ATMP) is presently preferred due to commercial availability, but other organophosphonates such as 1-hydroxy-1,1-diphosphonic acid; ethylene diamine tetra(methylene phosphonic acid); hexamethylene diamine tetra (methylene phosphoic acid) and diethylenetriamine penta(methylene phosphonic acid) can be mentioned.  
           [0019]    Concentrate compositions in accordance with the invention include the following:  
                                                                     Exemplary   Preferred                                        Fluoacid of Group IVB metal    1-15    4-12           or mixtures of such acids           HNO 3     0.1-10    4-8           NH 4 OH   0.1-10    2-6           Fluoboric acid    1-10   1-6           Boric Acid   0.1-5     1-3           Gluconic Acid    1-10   1-3           H 2 O   remainder   remainder                      
 
           [0020]    At present, the concentrate preferred for use is  
                                                       100 wt % total               H 2 ZrF 6  (45%)   5.2% w/w           H 2 TiF 6  (60%)   3.2           HNO 3  (42°Be)   6.0           NH 4 OH (26°Be)   3.2           Fluoboric acid (48%)   4.0           Boric acid   2.0           Gluconic Acid (50%)   2.4           deionized water   remainder                      
 
           [0021]    As stated above, the concentrates are diluted in deionized, distilled, reverse osmosis, or other suitably high purity water to about 0.5-10% v/v solutions, preferably about 1-2% v/v of the concentrate (concentrate/total solution). To this, the aminosilane may be added in small amounts. For instance, γ-APS is added in an amount of about 5 to 500 ppm into the diluted working solution.  
           [0022]    The organophosponate may be added in amounts similar to those of the addition levels of the aminosilane.  
           [0023]    The bath, or working solution, that is used to contact the aluminum part thus includes the following active components given in terms of ppm.  
                                                                     Exemplary   Preferred                                        Group IV metal fluoacid   20-9000 ppm   200-1400 ppm           HNO 3      5-7000 ppm   300-1200 ppm           NH 4 OH    2-2000 ppm    60-340 ppm           Fluoboric Acid   25-5000 ppm    50-600 ppm           Boric Acid    5-5000 ppm    100-600 ppm           Gluconic Acid   25-5000 ppm    50-300 ppm                      
 
       
    
    
     EXAMPLES  
       [0024]    The invention will be further described in conjunction with the following examples which are included for illustrative purposes and should not be viewed to limit the invention. Protocols Used  
         [0025]    1. Wheel Section Preparations Sections of cast aluminum wheels (supplied by various wheel manufacturers) are treated per the following process sequence:  
                                                       preclean:   hot water rinse, 30 s       clean:   mild alkaline cleaner, 60 s spray at 15 psi and 130° F.       rinse:   tap water spray for 60 s       deox:   acid deoxidizer, 90 s spray at 100° F.       rinse:   tap water for 30 s, followed by DI water for 30 s       treat:   various treatments spray or dip applied for 90 s at           100-120° F.       rinse:   DI water spray for 60 s            shake off excess water            warm air dry:   conveyor oven at 150° F. air temperature, 8 min duration                  
 
         [0026]    Treated sections are allowed to cool overnight and then powder painted. The wheel sections are baked in accordance with the paint manufacturers&#39; recommendations. Typical powder curing conditions include a metal temperature of 320° F. for 17 min (typically 30-40 min in oven set at 360° F.). Dry film thickness is 2.5 mils on average.  
         [0027]    Painted wheel sections are allowed to “age” for at least three days prior to any performance testing.  
         [0028]    2. Filiform Corrosion Resistance  
         [0029]    The procedure is similar to GM 9682P.  
         [0030]    Wheel sections are scribed using a carbide tip scribe as prescribed in ASTM D 1654. The scribe is made with the aid of a straight edge and using a moderate, even pressure over the length of the ˜10 cm scribe. The scribe is made perpendicular to the machining marks. Wheel sections are placed into the CAASS chamber within 30 minutes of being scribed.  
         [0031]    Vertically scribed wheel sections are placed so that the scribe is 30° from vertical. A Singleton corrosion test chamber (Model # Q-FOG/SP 1100), is used, running under CAASS condition as specified by ASTM B 368-97. The wheel sections are exposed to this test for 6.0±0.3 hours.  
         [0032]    Upon removal from the CAASS chamber, the sections are rinsed by immersion in deionized water. The wheel section is immersed straight into the water, rotated a quarter turn right and then a half turn left and pulled from the water; the total dip time taking ˜3 s.  
         [0033]    The wheel sections are then placed in a humidity chamber. A Blue M Model # FRS 09C maintained at 140° F. and 80% RH is used. The sections are placed in the chamber with an orientation similar to that of the CAASS exposure. Wheel sections are exposed in a chamber for at least two weeks, more typically three weeks. Filiform is rated by recording the maximum filament length (to the nearest 0.5 mm), and the quantity of visible filaments along the scribe. For sections providing better performance, we further rate the wheel section by developing a histogram of the quantity of filaments in 0.5 mm length increments and by noting any design flaws.  
         [0034]    3. Copper Accelerated Acid Salt Spray  
         [0035]    This test is run in accordance with ASTM B 368-97.  
         [0036]    Scribing and exposure conditions are the same as described in the corrosion inoculation portion of the filiform test. The duration of the test is 16813 hours. After rinsing the wheel sections in deionized water, we rate the average and maximum blister size along the scribe and along design edges.  
         [0037]    4. Thermal Shock  
         [0038]    This test is run in accordance with GM 9525P.  
         [0039]    This procedure is used to determine the resistance to coating adhesion loss of coated surfaces of aluminum when subjected to a wet steam blast similar to that produced by vehicle wash equipment. The test consists of cooling the wheel part to minus 29° C. for three hours, then scribing the painted part with an X scribe and subjecting this area to a steam blast. Paint loss, or paint blushing (whitening, loss of gloss) and the average distance of paint adhesion loss from the scribe line is reported.  
         [0040]    Test Solutions  
         [0041]    The following treatment baths were used in evaluating performance of powder painted aluminum wheels.  
                                                                                                 A   B   C   D   E   F                                    fluozirconic acid (45%)   11.6   6.1   12.8   0.0   0.7   5.0       fluotitanicp acid (60%)   7.0   3.2   0.0   1.6   0.8   0.0       nitric acid (42 Be)   6.6   7.1   7.5   1.5   1.5   0.0       ammonium hydroxide (26 Be)   3.5   3.7   3.9   0.8   0.8   3.4       fluoboric acid (48%)   8.8   4.7   5.8   1.0   1.0   0.0       boric acid   0.0   2.3   2.5   0.5   0.5   0.0       gluconic acid (50%)   2.7   2.8   3.0   0.6   0.6   0.0       chromic acid   0.0   0.0   0.0   0.0   0.0   4.0       Water   59.8   70.1   64.5   94.0   94.1   67.6                  
 
       Example 1  
       [0042]    Formulations A, B, and C were diluted to 1% v/v in deionized water, pH adjusted to 3.0 using ammonium hydroxide or left at native solution pH of 2.0, and further modified by the addition of 150, 300, or 450 ppm γ-APS. These solutions were used to treat aluminum wheels as set forth under Protocol #1 above. Treated wheels were painted with a clear coat acrylic powder from PPG and performance tested by CAASS and filiform test conditions.  
                                                                                                                         paint performance evaluation                filiform corrosion resistance**   CAASS***       solution   (max/# along scribe/# design failures)   (max/avg)                     1   F (chrome controls)   2.5/7.8/1   3.0/9.9/0   —   1.0/0.5                    pH   γ-                                   APS                       (ppm)            2   B   2   150   2.0/5.9/2   2.5/6.0/1   4.5/8.1/0   1.0/0.5        3       2   450   2.0/6.0/1   2.5/8.2/0   4.0/8.9/0   1.0/0.5        4       3   150   2.0/9.8/3   3.0/11.1/0    3.5/14.8/1    2.0/1.0        5       3   450   3.5/13.9/2    3.5/14.4/1    —   1.5/1.0        6   C   2   300   2.5/8.8/0   3.5/10.0/0    3.5/10.8/2    1.0/1.0        7*       2   300   2.5/5.5/0   3.0/9.4/0   3.5/1.04/2    2.0/1.0        8   A   2   150   2.5/5.2/0   6.5/12.2/2    —   2.5/1.5        9       2   450   1.5/6.9/1   2.0/7.3/0   2.5/11.5/0    1.0/0.5       10       3   150   3.0/7.6/0   3.5/9.7/2   4.0/4.5/0   2.5/1.5       11       3   450   3.0/10.9/0    3.0/11.3/0    3.5/9.1/0   3.0/2.0                                          
 
       Example 2  
       [0043]    Cast and polished aluminum wheel sections and panels were prepared in accordance with Protocol #1 above using solutions D, E, and chrome controls F. Non-chrome treatments were evaluated with and without the addition of γ-APS or Dequest 2000. The wheel sections were painted with a clear coat acrylate powder from PPG.  
                                                                                                   paint performance evaluation                        filiform corrosion resistance   CAASS       #   treatment   pH   (max/# along scribe/# design failures)   (max/avg)                    1   5% F (chrome   —   1.0/2.9/0   —   —   2.0/1.5           controls)       2.5/5.4/0           2   5% D + 300 ppm γ-   2.2   2.0/6.9/0   4.0/11.8/0    2.0/7.9/1   1.5/1.0           APS       4.0/9.4/0   4.5/9.6/2   3.0/1.8/0           3   5% D + 300 ppm   3.0   3.0/5.9/0   3.5/7.7/0   2.5/2.5/0   1.0/1.0           ATMP       4.0/4.7/0   4.5/4.4/0   3.0/5.0/0           4   5% E   2.5   2.0/8.6/0   1.0/6.5/1   2.5/4.9/0   1.5/1.0                   3.5/8.5/0   4.5/7.4/0   3.0/4.9/0           5   5% E + 300 ppm γ-   2.5   4.0/12.6/0    3.0/9.5/0   2.0/6.0/0   1.0/0.5           APS       4.0/8.8/0   4.5/7.4/3   2.0/7.4/0           6   5% E + 300 ppm   3.0   1.5/2.7/0   1.5/3.5/0   1.0/0.8/0   2.0/1.5           ATMP       4.5/3.0/0   4.0/6.6/0   1.5/4.1/0                  
 
       Example 3  
       [0044]    Basic formulations were tested in order to assess the performance of and optimal concentrations of H 2 ZrF 6 , HBF 4  and H 3 BO 3  components. Wheel sections were pretreated in accordance with Protocol #1 and painted with clear coat powder acrylic from PPG.  
         [0045]    The basic formulation that was varied as shown was as follows:  
                                                                                                                                                                                                         G   H   I   J   K   L                        fluozirconic acid (45%)   1.3   1.3   1.3   1.3   5.3   5.3       nitric acid (42 Be)   1.5   1.5   1.5   1.5   1.5   1.5       ammonium hydroxide (26 Be)   0.8   0.8   0.8   0.8   0.8   0.8       fluoboric acid (48%)   0.5   0.5   2.0   2.0   0.5   0.5       boric acid   0.0   1.0   0.0   1.0   0.0   1.0       gluconic acid (50%)   0.6   0.6   0.6   0.6   0.6   0.6       Water   95.3   94.3   93.8   92.8   91.3   90.3                        M   N   O   P   Q                            fluozirconic acid (45%)   5.3   5.3   2.6   0.0   1.3           fluotitanic acid (60%)           0.0   1.6   0.8           nitric acid (42 Be)   1.5   1.5   1.5   1.5   1.5           ammonium hydroxide (26 Be)   0.8   0.8   0.8   0.8   0.8           fluoboric acid (48%)   2.0   2.0   1.0   1.0   1.0           boric acid   0.0   1.0   0.5   0.5   0.5           gluconic acid (50%)   0.6   0.6   0.6   0.6   0.6           Water   89.8   88.8   93.0   94.0   93.5                            filiform corrosion resistance   thermal shock               (max/#/# design failures)   adhesion loss   CAASS            #   Solution   wheel type 1   wheel type 2   (mm)/blush   max/avg               1     5% F   3.0/4.0/0   2.5/5.8/0   0.0/none   3.0/1.5               4.0/4.4/0   4.0/10.7/0            2   2.5% G   4.5/5.1/0   3.0/9.0/0   0.0/none   2.0/1.0               5.0/6.6/0   3.5/10.7/0            3   2.5% H   6.5/6.8/0   4.0/7.4/0   0.0/none   2.0/1.5               7.0/13.4/10    5.0/8.0/0           4   2.5% I   0.5/3.5/0   3.0/7.8/0   2.8/none   2.5/1.5               3.5/6.1/0   5.5/5.6/0           5   2.5% J   1.5/5.3/0   2.5/5.8/0   0.0/none   1.5/1.0               3.0/4.3/1   6.0/10.0/0            6   2.5% K   1.0/3.8/1   2.0/11.1/0    0.0/none   2.0/1.5               2.5/4.5/0   4.5/8.4/0           7   2.5% L   3.5/4.0/0   3.0/9.5/0   0.0/none   1.5/1.0               4.0/3.3/0   3.5/8.5/0           8   2.5% M   2.5/7.0/1   3.0/7.5/0   0.0/none   —               3.0/7.9/0   4.5/7.7/0           9   2.5% N   3.0/4.8/0   —   0.0/none   —               3.5/8.2/0   —                  
 
       Example 4  
       [0046]    Tests were conducted using formulation 0, modified by the addition of either 300 ppm γ-APS or 300 ppm of an acrylic acid/vinyl phosphonic acid co-polymer.  
         [0047]    A variety of cleaners, deoxidizing acid and rinses were employed as pretreatment, but these showed little effect on filiform performance. The pretreated wheel samples were painted with acrylic clear coats from PPG and from Ferro.  
                                                                                                     treatment conditions   filiform (max/#/# design failures)            #   treatment   PPG acrylic   Ferro acrylic                    production samples   2.0/5.8/1   0.5/0.6/1             1   5% F   —/—/0   2.5/7.1/0        2       3.0/5.9/0   3.0/12.7/0        3       2.0/4.6/0   2.0/8.7/0        4       2.0/4.6/1   3.0/10.4/0        7       2.5/5.7/1   2.0/6.8/1        8       2.5/3.2/1   2.5/9.7/1        5       —   2.5/4.4/0        6       2.5/6.1/0   2.0/5.2/3        9   2.5% O +   4.0/5.5/1   2.5/9.5/1       10   300 ppm AA/VPA copolymer   —/—/2   3.0/9.5/2       11       2.0/6.9/0   3.0/11.1/0       12       —/—/3   3.0/8.3/0       13       3.0/6.9/0   4.0/12.5/1       14       4.0/7.3/4   6.0/15.4/5       15       —/—/0   3.5/11.6/1       16       4.0/4.5/2   2.0/8.3/0       17   2.5% O +   2.0/8.5/0   4.0/12.4/1       18   300 ppm γ-APS   2.0/7.2/1   2.5/8.1/2       19       —/—/0   3.0/8.8/2       20       2.5/10/0   2.5/7.2/0       23       —/—/1   2.0/7.7/2       24       2.0/6.6/1   2.0/9.0/1       21       3.5/7.5/2   2.0/8.7/0       22       —/—/1   2.0/10.0/2                  
 
       Example 5  
       [0048]    Sections of cast aluminum wheels and cast and polished aluminum wheels were treated in accordance with pretreatment Protocol #1. Wheel sections were painted with clear coat acrylic powder paint and evaluated by CAASS and filiform corrosion resistance.  
                                                                                                                 process conditions   performance evaluation                    conc   temp   filiform corrosion resistance   CAASS       #   treat   (%)   (F.)   (max/#/# design failures)   (max/avg)                    1   no treatment   4.5/14.1/1    4.0/11.7/2    7.0/5.0            2   F   5.0   100   1.5/5.8/0   3.0/5.1/1   —       3   P   1.0   100   3.5/10.7/0    4.0/9.1/7   7.0/4.0       4   P   1.0   140   3.0/6.2/0   3.0/9.6/2   3.5/2.0       5   P   5.0   140   2.5/8.3/3   3.5/11.9/1    5.0/3.5       6   P   5.0   100   1.5/2.4/1   2.5/7.2/1   6.0/4.0       7   Q   2.5   120   3.0/4.2/0   4.0/7.4/0   —                       2.5/9.6/1   3.5/6.4/0   7.0/4.0                       3.0/10.4/0    4.0/9.8/0   —       8   O   1.0   100   2.5/8.4/0   3.5/10.7/1    3.0/2.5       9   O   1.0   140   2.5/3.6/0   4.0/5.8/0   4.0/3.0       10   O   5.0   140   3.0/8.7/0   5.5/7.4/1   4.0/3.0       11   O   5.0   100   3.5/7.1/0   5.0/8.3/1   2.5/1.5                  
 
       Example 6  
       [0049]    Sections of cast aluminum wheels were treated in accordance with Protocol #1. The treated wheel sections were painted with clear coat acrylic powder. A variety of additives, as shown, were added to the basic formulation listed in Example 5.  
                                                                                                                                                                   paint performance evaluation                    thermal           process conditions   filiform corrosion resistance   shock   CAASS                    300   (max/#/# design failures)   adhesion   (max/            #   sol&#39;n   ppm   wheel type 1 (Mazda)   wheel type 2 (Ford)   loss/blush   avg)                    1     5% F   none   —   —   2.5/9.4/0   3.0/8.2/0   0.0/none   2.0/1.5       2   2.5% O   none   4.0/11.6/3    —   3.0/7.4/0   3.5/9.5/0   0.0/none   3.0/2.0       3   2.5% O   γ-APS   2.5/7.6/1   3.0/11.7/0    2.0/7.9/0   2.5/4.6/0   0.0/none   2.0/1.5       4   2.5% O   AA/AM   4.0/11.4/0    4.5/10.3/0    3.0/6.0/0   4.0/12.2/2    0.0/none   6.0/4.0       5   2.5% O   ATMP   4.0/4.8/0   5.0/3.1/0   2.0/6.0/0   4.0/4.0/0   0.5/none   2.0/1.5