Patent Publication Number: US-4220485-A

Title: Process for sealing phosphatized metal components

Description:
This invention relates to a composition and method for sealing phosphatized metal components to improve corrosion resistance and paint adhesion. 
     More particularly, this invention relates to a composition and method for sealing phosphatized metal components with a non-chromic acid based material. 
     It has been common practice to seal the surface of phosphatized metal components with a chromic acid rinse prior to painting. Hexa-valent chromium is highly toxic and environmental considerations have resulted in a search for a less toxic substitute which provides corrosion protection for the metal components being treated. 
     Accordingly, it is an object of this invention to provide a less-toxic sealant rinse for phosphatized metal components. 
     It is a further object of this invention to provide a sealant rinse for phosphatized metal components that increases the coating weight of the phosphate coating and increases paint adhesion. 
     It is an additional object of this invention to provide a sealant rinse for phosphatized metal components that provides improved corrosion resistance. 
     It is still a further object of this invention to provide a sealant rinse for phosphatized metal components which may be applied over a wide range of temperatures, i.e. room temperature to 180° F. 
     These and other objects of this invention are accomplished by the novel composition and method disclosed herein. The composition of the present invention consists essentially of phosphoric acid, a zinc compound(s), a heavy metal accelerator and/or crystal refiner, a phosphonate corrosion inhibitor and sufficient water to dilute the composition to its desired strength. 
     The components of the composition of the present invention are present in the following amounts: 
     
         ______________________________________                                    
                 Weight Percent                                           
Component          Broad      Preferred                                   
______________________________________                                    
Phosphoric Acid     5 to 80   22.5 to 60                                  
Zinc Compound(s)   1 to 16     3 to 12                                    
Heavy Metal Accelerator                                                   
and/or Crystal Refiner                                                    
                   0.1 to 10  2 to 7                                      
Phosphonate        1 to 80    10 to 30                                    
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     The phosphoric acid component of the composition may be of any suitable grade, however, 75% by weight phosphoric acid is the preferred material. Similarly, while zinc oxide is the preferred form of zinc, any acid-soluble form of the zinc ion, such as the nitrate or chloride, may be used. 
     Heavy metal accelerators useful in the compositions of the instant invention include compounds of such metals as vanadium, titanium, zirconium, tungsten and molybdenum. The compounds utilized most frequently are the molybdates. In combination with or in place of accelerators, an optional crystal refiner, such as acid-soluble salts of nickel, cobalt, magnesium and calcium, may be utilized in the compositions of the instant invention. 
     Suitable phosphonates include those of the formula: ##STR1## wherein R is ##STR2## M is H, NH 4 , alkali metal or combinations thereof; n is 0 to 6; and 
     x is 1 to 6; and those of the general formula: ##STR3## wherein X is --OH or --NH 2  and 
     R is an alkyl group of from 1 to 5 carbon atoms. 
     The most preferred compounds, however, are amino tris(methylene phosphonic acid) and hydroxyethylidene-1,1-diphosphonic acid (HEDP) and water-soluble salts thereof. 
     The zinc sealant rinse composition of the present invention may be applied by conventional immersion or spray processes. Typical processes which may be used include a three-stage process which comprises a cleaning and phosphatizing step, a water rinse step and the zinc sealant rinse step. Better coatings may be obtained by using a five-stage process which comprises an alkaline cleaning step, a water rinse step, a phosphatizing step, an additional water rinse step and the zinc sealant rinse step. The zinc sealant rinse step is carried out at temperatures of from 55° to 180° F. and contact times of from 10 seconds to 2 minutes. 
     Both the three- and five-stage process may be controlled manually or automatically. Automatic control is, however, preferred because it permits more accurate control of the concentration of the coating compositions, thereby resulting in a more uniform coating on the metal surfaces being treated. 
     The compositions of the present invention may be prepared by conventional liquid blending techniques and when used in spray or immersion processes, their concentration in water should be at least 1/4 oz./gallon of water. 
    
    
     The following examples are representative of the compositions of the present invention: 
     
                       EXAMPLE 1                                                   
______________________________________                                    
Component       Amount (Weight Percent)                                   
______________________________________                                    
Water           24.5                                                      
Zinc Oxide      7                                                         
Sodium Molybdate                                                          
                0.5                                                       
75% Phosphoric Acid                                                       
                65                                                        
50% Amino tri(methylene-                                                  
phosphonic acid)                                                          
                3                                                         
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                       EXAMPLE 2                                                   
______________________________________                                    
Component       Amount (Weight Percent)                                   
______________________________________                                    
Water           33                                                        
Zinc Oxide      2                                                         
75% phosphoric Acid                                                       
                50                                                        
50% Amino tri(methylene-                                                  
phosphonic acid)                                                          
                10                                                        
Ammonium Metavanadate                                                     
                5                                                         
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                       EXAMPLE 3                                                   
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Component       Amount (Weight Percent)                                   
______________________________________                                    
Water           20                                                        
Zinc Oxide      5                                                         
75% Phosphoric Acid                                                       
                50                                                        
50% Amino tri(methylene-                                                  
phosphonic acid)                                                          
                20                                                        
Nickelous Nitrate - 6H.sub.2 O                                            
                5                                                         
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                       EXAMPLE 4                                                   
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Component       Amount (Weight Percent)                                   
______________________________________                                    
Water           21                                                        
Zinc Oxide      4                                                         
75% Phosphoric Acid                                                       
                40                                                        
50% Amino tri(methylene-                                                  
phosphonic acid)                                                          
                30                                                        
Calcium Nitrate - 4H.sub.2 O                                              
                0.5                                                       
Sodium Molybdate                                                          
                4.5                                                       
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                       EXAMPLE 5                                                   
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Component       Amount (Weight Percent)                                   
______________________________________                                    
Water           17                                                        
Zinc Oxide      1                                                         
75% Phosphoric Acid                                                       
                10                                                        
50% Amino tri(methylene-                                                  
phosphonic acid)                                                          
                70                                                        
Cobaltous Nitrate - 6H.sub.2 O                                            
                1                                                         
Sodium Molybdate                                                          
                1                                                         
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     EXAMPLE 6 
     Metal panels were evaluated in salt tests using a 5% salt spray at 95° F. for 120 hours in accordance with the procedures set forth in ASTM Procedure B117-64 and the panels were evaluated by ASTM Procedure D1654-61 for corrosion creepage from a scratch as well as the degree of body blisters on the test area. The ratings are based on a scale of 1 to 10, with 10 being the best possible rating and 1 being the least. A representative composition of the instant invention was compared to prior art compositions in the results set forth in Table I. 
     
                                           TABLE I                                 
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                 0.05% Chromic                                            
                         A at 3                                           
                              B at 3                                      
                                   B at 5                                 
                                        B at 7                            
Panel Description                                                         
          Water Rinse                                                     
                 Acid    oz./gal.                                         
                              oz./gal.                                    
                                   oz./gal.                               
                                        oz./gal.                          
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Blister Creepage                                                          
          1/2to 3/4&#34;                                                      
                 1/8to 1/4&#34;                                               
                         to 1/4&#34;                                          
                              to 1/16&#34;                                    
                                   to 1/32&#34;                               
                                        to 1/64&#34;                          
 ASTM D-1654-61                                                           
 Schedule #1                                                              
 Rating   1      5       4    7    8    9                                 
Body Blisters                                                             
          None   None    None None None None                              
 ASTM D-1654-61                                                           
 Schedule #2                                                              
 Rating   10     10      10   10   10   10                                
Body Pinhole                                                              
Rusting   None   None    None None None None                              
Moisture Penetra-                                                         
tion of Paint                                                             
          Nil    Nil     Nil  Nil  Nil  Nil                               
Paint Thickness                                                           
          1.3 mil                                                         
                 1.3 mil 1.3 mil                                          
                              1.3 mil                                     
                                   1.3 mil                                
                                        1.3 mil                           
Paint Coverage                                                            
          Good   Good    Good Good Good Good                              
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 A is a composition consisting of 5% by weight sodium molybdate, 50% by   
 weight of 75% phosphoric acid, 20% by weight of Dequest 2000 [amino      
 tris(methylene phosphonic acid)] and 25% by weight water.                
 B is a composition consisting of 5% by weight zinc oxide, 5% by weight   
 sodium molybdate, 50% by weight of 75% phosphoric acid, 20% by weight of 
 Dequest 2000 [amino tris(meythylene phosphonic acid)] and 20% by weight  
 water.                                                                   
 
    
     The results set forth in Table I demonstrate the improvements obtained when using the compositions of the instant invention.