Abstract:
A process for coating metal surfaces in which an uncured resin coating is formed on the surface of a metal by contacting the metal surface with an autodepositing coating composition that contains acid, oxidizing agent, and water-dispersible or water-soluble organic film-forming resin, and the uncured coating is thereafter dried, produces a strongly adherent, highly corrosion-resistant autodeposited coating when the uncured coating is brought, prior to its drying, into contact with an aqueous solution that has a pH adjusted to 7 to 11 and that contains at least one selection from the following inorganic acids and salts thereof: molybdic acid, tungstic acid, polyphosphoric acid, and polysilicic acid.

Description:
TECHNICAL FIELD 
     The invention relates to a process for coating metal surfaces. More specifically, the invention relates to a process that produces highly corrosion-resistant and strongly adherent resin coatings on metal surfaces. This process is applied to the uncured resin coatings that are formed on metal surfaces when the surface of a metal is brought into contact with an autodepositing coating composition that contains acid, oxidizing agent, and water-dispersible or water-soluble organic film-forming resin. The process consists of contacting an uncured resin coating of this type with an aqueous solution that has a pH adjusted to 7 to 11 and that contains at least 1 selection from the following inorganic acids and salts thereof: molybdic acid, tungstic acid, polyphosphoric acid, and polysilicic acid. 
     BACKGROUND ART 
     Autodepositing coating compositions as referred to in this invention are acidic coating compositions that contain an organic film-forming resin and that produce a resin coating on the surface of metal brought into contact with the composition. Compositions of this type are disclosed in Japanese Patent Publication Numbers Sho 47-17630  17,630/1972!, Sho 48-14412  14,412/1973!, Sho 52-21006  21,006/1977!, Sho 52-35692  35,692/1977!, Sho 53-15093  15,093/1978!, Sho 53-16010  16,010/1978!, Sho 53-44949  44,949/1978!, and Sho 54-13435  13,435/1979! and in Japanese Patent Application Laid Open  Kokai or Unexamined! Numbers Sho 60-58474  58,474/1985!, Sho 61-168673  168,673/1986!, and Sho 61-246267  246,267/1986!. 
     A characteristic feature of these known coating compositions is that immersion of a clean metal surface in the coating composition produces a resin coating whose thickness or weight increases with increasing immersion time. In addition, part of the metal ion eluted from the metal surface by the chemical etching of the coating composition is taken into the coating; this makes possible the efficient formation of a resin coating on the metal surface without utilizing externally supplied electricity as is used in the electrodeposition of coatings. 
     The conventional coating compositions, however, do not themselves always exhibit a satisfactory adherence or corrosion resistance, and as a result various treatment processes have been examined for the purpose of improving the adherence and corrosion resistance of autodeposited coatings on metal surfaces. For example, Japanese Patent Application Laid Open Number Sho 52-56142  56,142/1977! teaches the use of a rinse in autodepositing coating processes that consist of application of a waterborne coating composition to a metal substrate followed by baking. This rinse is applied to the uncured autodeposited film between the deposition step and baking step and consists of an aqueous solution or waterborne dispersion whose main component is at least one selection from the group comprising the silicofluorides, borofluorides, titanofluorides, aluminofluorides, and nitrites. The application of this process makes possible the production, without the use of rust-preventing pigments, of autodeposited films that have a good corrosion resistance. 
     Japanese Patent Application Laid Open Number Sho 52-68240  68,240/1977! teaches a rinse that, as in Japanese Patent Application Laid Open Number Sho 52-56142, is applied to the uncured autodeposited coating between the deposition step and the baking step. In this case, however, the rinse consists of an aqueous solution or waterborne dispersion whose main component is at least one selection from the group comprising amines, the amine salts of carboxylic acids, amino salts, melamine, and amides. The practice of this process is again reported to make possible the production of good corrosion-resistant autodeposited films without the use of rust-preventing pigments. Japanese Patent Application Laid Open Number Sho 60-58474 teaches a process that improves the water resistance of the coatings. In this process, an aqueous alkali metal or ammonium hydroxide solution is applied to the uncured autodeposited film on the metal surface prior to drying. This makes the dried film impermeable to water and thereby improves the water resistance. 
     U.S. Pat. Nos. 3,647,567 and 4,030,945 and Japanese Patent Application Laid Open Number Sho 61-168673 teach the preparation of good corrosion-resistant autodeposited films by a pre-drying rinse of the uncured autodeposited film on the metal surface with a chromium compound-containing aqueous solution. 
     In recent years, higher added value has been sought from the painting of metal surfaces, and in particular a higher corrosion resistance and higher adherence are desired. The art described hereinbefore, however, is not at a level that can consistently satisfy the required performance. 
     DISCLOSURE OF THE INVENTION 
     Problems to Be Solved by the Invention 
     The invention takes as its object the introduction of a process for coating metal surfaces that is able to coat metal surfaces with an autodeposited coating that has an adherence and corrosion resistance superior to those of the autodeposited coatings afforded by the prior art. 
     SUMMARY OF THE INVENTION 
     An autodeposited coating with an adherence and corrosion resistance far superior to the corresponding values provided by the prior art can be produced by the application before drying of a specific type of aqueous solution to the uncured resin coating formed on metal surfaces by contact with an autodepositing coating composition. 
     In specific terms, in the metal surface coating process in which an uncured resin coating is formed on the surface of a metal by contacting the metal surface with an autodepositing coating composition that contains acid, oxidizing agent, and water-dispersible or water-soluble organic film-forming resin, and the uncured coating is thereafter dried, the invention provides a process which is characterized in that said uncured coating is brought, prior to its drying, into contact with an aqueous solution that has a pH adjusted to 7 to 11 and that contains at least one selection from the following inorganic acids and salts thereof: molybdic acid, tungstic acid, polyphosphoric acid, and polysilicic acid. 
     Suitable metal substrates for the invention are specifically exemplified by ferriferous metals, zinciferous metals, aluminiferous metals, and the like. 
     The known autodepositing coating compositions are highly suitable for use as the autodepositing coating composition employed in the present invention. Some examples thereof are the coating compositions disclosed in Japanese Patent Publication Number Sho 54-13453  13,453/1979!, one of which consists of approximately 5 to 550 g/L as solids of a dispersed resin, approximately 0.4 to 5.0 g/L of hydrofluoric acid as acid component, approximately 0.3 to 3.0 g/L of hydrogen peroxide as oxidizing agent, and, optionally, water-dispersible pigment. Other examples are the coating compositions disclosed in Japanese Patent Publication Number Sho 52-35692, one of which consists of approximately 5 to 550 g/L as solids of a dispersed resin, approximately 0.4 to 5.0 g/L of hydrofluoric acid as acid component, no more than approximately 3.0 g/L of hydrogen peroxide as oxidizing agent, approximately 1 to 50 g/L of ferric fluoride as a metal compound component and co-oxidizing agent, and, optionally, water-dispersible pigment. 
     The inorganic acids used by the present invention are molybdic acid, tungstic acid, polyphosphoric acid, and polysilicic acid. The salts usable by the invention include the sodium salts, ammonium salts, calcium salts, potassium salts, and the like. The invention uses at least one selection from these inorganic acids and salts thereof. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The preferred content of the total of the inorganic acid(s) characteristic of the invention and salt(s) thereof in an aqueous solution used for treatment according to the invention preferably is, with increasing preference in the order given, at least 0.01, 0.05, 0.10, 0.30, 0.50, 0.70, 0.80, 0.90, or 1.0 grams per liter (hereinafter usually abbreviated as &#34;g/L&#34;) and independently preferably is, with increasing preference in the order given, not more than 50, 40, 30, 20, 15, 13, 11, 9.0, 8.5, 8.0, 7.5, 7.0, 6.8, or 6.6 g/L. The effect from addition is usually insignificant at concentrations below 0.01 g/L, while additions in excess of 50 g/L are undesirable from a quality standpoint because such additions degrade the appearance of the coating and also its adherence and corrosion resistance. 
     The pH of the aqueous solution used by the invention is preferably adjusted before use into and maintained during use at a value that is, with increasing preference in the order given, not less than 7.00, 7.05, 7.10, 7.15, or 7.18 and independently preferably is, with increasing preference in the order given, not more than 11, 10.7, 10.4, 10.2, 10.1, 10.0, 9.9, or 9.8. An improved adherence and corrosion resistance can not normally be expected when the pH is substantially outside this range. Alkaline materials, for example, can be used to adjust the pH into this preferred range. While no narrow restrictions apply to alkaline materials usable in the present invention, ammonium bicarbonate and ammonium hydroxide are preferred. In addition to bases, inorganic acids, organic acids, and their salts may also be used to adjust the pH of the aqueous solution; for example, hydrofluoric acid, nitric acid, sulfuric acid, phosphonic acid, phosphinic acid, citric acid, oxalic acid, tartaric acid, and the like may be used for pH adjustment. The aqueous solution may contain an oxidizing agent, such as hydrogen peroxide and the like, as an optional component. 
     The wet uncured coatings are preferably rinsed with plain water before being treated according to the invention, but should not be allowed to dry before being contacted with the characteristic aqueous treatment solutions of this invention. After such contact with the aqueous treatment solutions according to this invention, the coating preferably is not rinsed before being dried. 
     The method for applying the aqueous solution to the uncured autodeposited coating is not critical to the invention, but immersion is preferred. The application temperature of the aqueous solution is also not critical, although for convenience, a normal ambient human comfort temperature of 20°±5° C. The contact time preferably is, with increasing preference in the order given, at least 0.08, 0.16, 0.30, 0.50, 0.60, 0.70, 0.80, 0.90, or 0.95 minutes (hereinafter usually abbreviated &#34;min&#34;) and independently preferably is, with increasing preference in the order given, not more than 10, 7, 5, 4, 3, 2, 1.8, 1.6, 1.4, 1.2, or 1.1 min. Drying conditions also are not critical to the invention, but preferably the maximum temperature during drying is, with increasing preference in the order given, at least 30°, 50°, 70°, 90°, 100°, or 105° C. and independently preferably is, with increasing preference in the order given, not more than 250°, 230°, 220°, 210°, 200°, 190°, 185°, or 181° C. for acrylate type organic resins and not more than 120°, 115°, 114°, 113°, 112°, 111°, or 110° C. for poly{vinylidene chloride} resins. Independently, the time at these temperatures preferably is, with increasing preference in the order given, at least 0.5, 1.0, 2.0, 4.0, 7.0, 10, 12, 14, 16, 18, or 19 min and independently preferably is, with increasing preference in the order given, not more than 60, 50, 40, 30, 28, 26, 24, 22, or 21 min. 
     The crux of the invention is that application of the subject aqueous solution to the uncured autodeposited coating is able to produce a strongly adherent, highly corrosion-resistant autodeposited coating, possibly due to the formation of a passivation coating on the metal surface. Such a passivation coating could be produced through the formation of complexes between anion supplied from the molybdic acid, tungstic acid, polyphosphoric acid, and/or polysilicic acid and/or salts thereof present in the aqueous solution and metal cations taken up by the autodeposited coating and particularly metal cations present on the metal surface by virtue of the uncured autodeposited film. Corrosion may also be inhibited through the formation of complexes between said anions that have been taken up by the coating and the metal cations eluting during metal corrosion. 
     The invention is illustrated in greater detail below through working and comparative examples. The test methods used to evaluate performance are also explained below. 
    
    
     EXAMPLES 
     Evaluation of coating adherence by the crosscut/tape peel test 
     A crosshatch pattern of one hundred 1 millimeter (&#34;mm&#34;)×1 mm squares was cut in the test panel and peeled with adhesive tape, and the number of remaining coating film squares was counted. This test was conducted both before and after immersion in water at 40° C. for 240 hours. 
     Evaluation of coating adherence by the Du Pont impact test 
     A 1 kg impacter with a diameter of 1.27 centimeters (&#34;cm&#34;) was dropped on the test panel and the impact zone was peeled with adhesive tape. The value measured was the highest impacter drop-height in cm at which film peeling did not occur. This test was conducted both before and after immersion in water at 40° C. for 240 hours. 
     Evaluation of coating adherence by the Erichsen test 
     The test surface of the test panel was extruded 5 mm using an Erichsen test instrument, and the cupped region was then peeled with adhesive tape. Film peeling was evaluated according to the scale given below: 
     +++: no film peeling 
     ++: area of film peeling is less than 2% 
     +: area of film peeling is less than 10% 
     x: area of film peeling is at least 10%. 
     This test was conducted both before and after immersion in water at 40° C. for 240 hours. 
     Corrosion resistance test 
     A cross was scribed in the coating on the test panel to reach the base metal. The test panel was then subjected to salt spray testing (500 hours for cold-rolled steel, galvannealed hot-dip zinc-coated steel, and electrogalvanized steel; 1,000 hours for aluminum), and tape peeling was conducted after this exposure. The evaluation consisted of measurement of the peel width (one side, in mm) from the cross scribe line. 
     Examples 1 to 6 
     Preliminarily cleaned panels (70×150×1 mm) of cold-rolled steel, galvannealed hot-dip zinc-plated steel, electrogalvanized steel, and aluminum were immersed for 180 seconds in autodepositing coating composition A: 
     
         ______________________________________Component                Concentration______________________________________Rhoplex ™ WL-91 acrylic resin latex, 41.5% solids                    120.50 g/L(supplied by the Rohm and Haas Company)Hydrofluoric acid        0.50 g/LHydrogen peroxide        0.05 g/LFerric fluoride          3.00 g/LDeionized water          balance.______________________________________ 
    
     This composition was maintained at a bath temperature of approximately 20° C. to 22° C. After a water rinse, the panels were immersed at ambient temperature for 60 seconds in aqueous treating solution A, B, C, D, or E, with compositions shown in Table 1 below. This was followed by oven drying for 20 minutes at 180° C. and then testing. The results of performance testing on the test panels are reported in Table 2. 
     
                                           TABLE 1__________________________________________________________________________Characteristic of the Aqueous        Value of the Characteristic for Aqueous Treating Solution        Composition Designated:Treating Solutions        A  B  C  D  E  F  G  H  I  J  K  L__________________________________________________________________________Molybdic acid, g/L        5.00           5.00Calcium molybdate, g/L           1.50Ammonium Molybdate, g/L                 5.00        0.80                                5.00Tungstic acid, g/L 0.15Sodium tungstate, g/L              5.00     1.00Polyphosphoric acid, g/L 0.25Sodium polyphosphate, g/L                    3.50Sodium polysilicate, g/L       3.00Ammonium Hydroxide, g/L        5.00           4.50              4.50  6.50                       5.50                          5.50                             6.50                                2.00  5.00                                         5.00Ammonium Bicarbonate, g/L                 3.00              3.00Nitric acid, g/L                     5.00Phosphinic acid, g/L        0.20Sodium phosphonate, g/L  0.65Citric acid, g,/L     0.80Ammonium Oxalate, g/L             0.50Hydrogen peroxide, g/L        0.01                 0.01pH           8.5           9.8              8.5                 7.2                    7.5                       8.0                          8.5                             9.0                                6.0                                   7.5                                      10.5                                         12.0__________________________________________________________________________ 
    
     
                                           TABLE 2__________________________________________________________________________                        Adherence Test Results:                                          Corro-                   Coating                        Crosscut/                              DuPont                                    Erichsen                                          sion      Type of Panel Coated:                   Add-On,                        Peel  Impact                                    Test  Re-AC:     ATC:      CRS         GHD            EG              Aluminum                   gm.sup.2                        Pre-I                           Post-I                              Pre-I                                 Post-I                                    Pre-I                                       Post-I                                          sistance__________________________________________________________________________ExampleNumber1    A  A  .check mark. 22   100                           100                              40 30 +++                                       +++                                          1.02    A  B  .check mark. 24   100                           100                              40 40 +++                                       +++                                          1.03    A  C  .check mark. 23   100                           100                              40 40 +++                                       ++ 1.54    A  D  .check mark. 22   100                           100                              40 30 +++                                       +++                                          1.05    A  E     .check mark.                   21   100                           100                              40 40 +++                                       ++ 1.56    A  E          .check mark.                   20   100                           100                              50 40 +++                                       +++                                          0.57    B  F  .check mark. 18   100                           100                              50 40 +++                                       +++                                          1.58    B  G     .check mark.                   17   100                           100                              40 40 +++                                       +++                                          1.09    B  G        .check mark.                   20   100                           100                              30 30 +++                                       ++ 1.510   B  H  .check mark. 18   100                           100                              40 30 +++                                       ++ 1.511   B  H          .check mark.                   19   100                           100                              50 40 +++                                       +++                                          0.5Com-parativeExampleNumber1    A  I  .check mark. 22   100                           100                              30 20 +  x  4.02    A  J     .check mark.                   24   100                           100                              20 20 +  x  3.53    A  K        .check mark.                   23   100                           100                              30 20 +  x  3.54    A  L  .check mark. 24   100                            95                              20 20 +  x  4.55    A  M  .check mark. 20   100                           100                              30 20 +  x  3.06    B  J     .check mark.                   17   100                           100                              30 20 +  +  3.57    B  K          .check mark.                   18   100                           100                              40 30 ++ +  2.58    B  L  .check mark. 20   100                            95                              30 20 +  x  5.5__________________________________________________________________________ Notes for Table 2 &#34;AC&#34; means &#34;Autodepositing Composition&#34;; &#34;ATC&#34; means &#34;Aqueous Treatment Composition&#34;; &#34;CRS&#34; means &#34;ColdRolled Steel&#34;; &#34;GHD&#34; means &#34;Galvannealed HotDipped  ZincCoated Steel!&#34;; &#34;EG&#34; means &#34;Electrogalvanized  Steel!; &#34;PreI&#34; means &#34;PreImmersion&#34;; &#34;PostI&#34; means &#34;PostImmersion&#34;. 
    
     Examples 7 to 11 
     Preliminarily cleaned panels (70×150×1 mm) of cold-rolled steel, galvannealed hot-dip zinc-coated steel, electrogalvanized steel, and aluminum were immersed for 180 seconds in autodepositing coating composition B: 
     Autodepositing coating composition B 
     
         ______________________________________Component                 Concentration______________________________________Daran ™ SL-143 vinylidene chloride copolymer resin la-                     90.90 g/Ltex, 41.5% solids (supplied by W. R. Grace &amp; Company)Hydrofluoric acid         0.50 g/LHydrogen peroxide         0.05 g/LFerric fluoride           3.00 g/LDeionized water           balance.______________________________________ 
    
     This composition was maintained at a bath temperature of approximately 20° C. to 22° C. After a water rinse, the panels were immersed for 60 seconds in aqueous treating solution F, G, or H with compositions reported in Table 1 at ambient temperature. This was followed by oven drying for 20 minutes at 110° C. and then testing. The results of performance testing on the test panels are reported in Table 2. 
     Comparative Examples 1 to 4 
     Preliminarily cleaned panels (70×150×1 mm) of cold-rolled steel, galvannealed hot-dip zinc-coated steel, and electrogalvanized steel were immersed for 180 seconds in Autodepositing Coating Composition A as described for Examples 1 to 6. The composition was maintained at a bath temperature of approximately 20° to 22° C. After a water rinse, the panels were immersed for 60 seconds in an aqueous solution I, J, K, or L with compositions as shown in Table 1 at ambient temperature. This was followed by oven drying for 20 minutes at 180° C. and then testing. The results of performance testing on the test panels are reported in Table 2. 
     Comparative Example 5 
     Preliminarily cleaned panels (70×150×1 mm) of cold-rolled steel were immersed for 180 seconds in the autodepositing coating composition A described for Examples 1 to 6. The composition was maintained at a bath temperature of approximately 20° C. to 22° C. After a water rinse, the panels were immersed for 60 seconds at ambient temperature in chromium-containing aqueous solution containing 30 g/L of Palene® 60 concentrate, supplied by Nihon Parkerizing Company, Ltd. This was followed by oven drying for 20 minutes at 180° C. and then testing. The results of performance testing on the test panels are reported in Table 2. 
     Comparative Examples 6 to 8 
     Preliminarily cleaned panels (70×150×1 mm) of cold-rolled steel, galvannealed hot-dip zinc-plated steel, and aluminum were immersed for 180 seconds in the autodepositing coating composition B described for Examples 7 to 11. The composition was maintained at a bath temperature of approximately 20° to 22° C. After a water rinse, the panels were immersed for 60 seconds in aqueous solution J, K, or L, with compositions as reported in Table 1 at ambient temperature. This was followed by oven drying for 20 minutes at 110° C. and then testing. The results of performance testing on the test panels are reported in Table 2. 
     The conclusions set forth below can be drawn on the basis of Examples 1 to 11 and Comparative Examples 1 to 8. 
     1. In Examples 1 to 11, both autodepositing coating compositions A and B gave very good results for corrosion resistance and for film adherence as evaluated by the crosscut/tape peel test, Du Pont impact test, and Erichsen test. 
     2. In contrast to this, Comparative Examples 1,2, 3, and 4 gave poorer results for film adherence (as evaluated by the Du Pont impact test and Erichsen test) and corrosion resistance than were obtained in Examples 1 to 11. The pH 6.0 used in Comparative Example 1 was too low, while the pH 12.0 used in Comparative Example 4 was too high. Comparative Examples 2 and 3 used only base without the addition of an inorganic acid or salt thereof as specified for the invention. Comparative Example 4 also gave poorer results in the crosscut/tape peel test. 
     3. Comparative Example 5 used a chromium compound-containing aqueous solution from the prior art. While the film adherence as measured by the Du Pont impact test was only slightly poorer in Comparative Example 5 than in Examples 1 to 11, the film adherence as measured by the Erichsen test and the corrosion resistance were substantially poorer. 
     4. Comparative Examples 6 and 8 used autodepositing coating composition B in place of autodepositing coating composition A, but otherwise corresponded to Comparative Examples 2 and 4, respectively. The trends observed for Comparative Examples 6 and 8 were the same as for Comparative Examples 2 and 4, and the former thus exhibited a film adherence and corrosion resistance inferior to Examples 1 to 11. 
     5. Comparative Example 7 corresponded to Comparative Example 3, except that the former used autodepositing coating composition B in place of autodepositing coating composition A and used an aluminum panel as the coated panel in place of an electrogalvanized steel panel. Nevertheless, Comparative Example 7 still had a film adherence as measured by the Du Pont impact and Erichsen tests and a corrosion resistance inferior to those of Example 11. 
     BENEFITS OF THE INVENTION 
     The process for coating metal surfaces in accordance with the present invention forms an autodeposited coating whose adherence and corrosion resistance are superior to those of coatings produced by the heretofore known processes.