Surface treatment chemicals and bath for aluminum or its alloy and surface treatment method
This is a divisional of application Ser. No. 07/561,423 filed Aug. 1, 1990, now U.S. Pat. No. 5,296,052. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a chemicals or bath for surface-treating aluminum or its alloy, and more particularly to a surface treatment chemicals or bath suitable for the surface treatment of aluminum cans for drinks. 
     Aluminum and its alloy are conventionally subjected to a chemical treatment to provide them with corrosion resistance and to form undercoating layers thereon. A typical example of such chemical treatment is a treatment with a solution containing chromic acid, phosphoric acid and hydrofluoric acid. This method can provide a coating having high resistance to blackening by boiling water and high adhesion to a polymer coating film formed thereon. However, since the solution contains chromium (VI), it is hazardous to health and also causes problems of waste water treatment. Thus, various surface treatment solutions containing no chromium (VI) have already been developed. 
     For instance, Japanese Patent Publication No. 56-33468 discloses a coating solution for the surface treatment of aluminum, which contains zirconium, phosphate and an effective fluoride and has a pH of 1.5-4.0. Japanese Patent Laid-Open No. 56-136978 discloses a chemical treatment solution for aluminum or its alloy containing a vanadium compound, and a zirconium compound or a silicon fluoride compound. Further, Japanese Patent Publication No. 60-13427 discloses an acidic aqueous composition containing hafnium ion and fluorine ion. 
     With respect to the coating solution disclosed in Japanese Patent Publication No. 56-33468, it shows sufficient properties when it is a fresh solution, namely a newly prepared solution. However, after repeated use for chemical treatment, aluminum is accumulated in the solution by etching of the aluminum plates or sheets with fluorine. A conversion coating produced by such a coating solution does not show high resistance to blackening by boiling water which is used for sterilization, and it also has poor adhesion to a polymer coating film produced by paints, inks, lacquers, etc. 
     Further, the treatment solution disclosed in Japanese Patent Laid-Open No. 56-136978 needs a treatment at a relatively high temperature for a long period of time, preferably at 50°-80° C. for 3-5 minutes, and the formed conversion coating does not have sufficient resistance to blackening by boiling water and sufficient adhesion to a polymer coating film. In addition, since the formed conversion coating is grayish, it cannot be suitably applied to aluminum cans for drinks. 
     The composition disclosed in Japanese Patent Publication No. 60-13427 is also insufficient in resistance to blackening by boiling water and adhesion to a polymer coating film. 
     OBJECT AND SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a surface treatment chemicals for aluminum or its alloy free from the above problems inherent in the conventional techniques, which makes it possible to conduct a surface treatment at a low temperature for short time to provide a conversion coating excellent in resistance to blackening by boiling water and in adhesion to a polymer coating film formed thereon, and which suffers from little deterioration with time, so that it can provide a conversion coating having the above properties even when it is not a fresh one. 
     Another object of the present invention is to provide a surface treatment bath for aluminum or its alloy having such characteristics. 
     As a result of intense research in view of the above objects, the inventors have found that a combination of particular proportions of niobium ion and/or tantalum ion, and effective fluorine ion, and optionally zirconium ion and/or titanium ion, and phosphate ion can provide surface treatment chemicals and a bath free from any problems of the conventional techniques. The present invention is based on this finding. 
     Thus, the first surface treatment chemicals for aluminum or its alloy according to the present invention consists essentially of 10-1000 parts by weight of niobium ion and/or tantalum ion and 1-50 parts by weight of effective fluorine ion. 
     The second surface treatment chemicals for aluminum or its alloy according to the present invention consists essentially of 10-1000 parts by weight of niobium ion and/or tantalum ion, 10-500 parts by weight of zirconium ion and/or titanium ion, 10-500 parts by weight of phosphate ion, and 1-50 parts by weight of effective fluorine ion. 
     The first surface treatment bath for aluminum or its alloy according to the present invention consists essentially of 10-1000 ppm of niobium ion and/or tantalum ion, and 1-50 ppm of effective fluorine ion, and has a pH of 1.5-4.0. 
     The second surface treatment bath for aluminum or its alloy according to the present invention consists essentially of 10-1000 ppm of niobium ion and/or tantalum ion, 10-500 ppm of zirconium ion and/or titanium ion, 10-500 ppm of phosphate ion, and 1-50 ppm of effective fluorine ion, and has a pH of 1.5-4.0. 
     The first method of surface-treating aluminum or its alloy comprises the steps of applying to said aluminum or its alloy a surface treatment bath consisting essentially of 10-1000 ppm of niobium ion and/or tantalum ion and 1-50 ppm of effective fluorine ion, and having a pH of 1.5-4.0, at a temperature between room temperature and 50° C. 
     The second method of surface-treating aluminum or its alloy comprises the steps of applying to said aluminum or its alloy a surface treatment bath consisting essentially of 10-1000 ppm of niobium ion and/or tantalum ion, 10-500 ppm of zirconium ion and/or titanium ion, 10-500 ppm of phosphate ion and 1-50 ppm of effective fluorine ion, and having a pH of 1.5-4.0, at a temperature between room temperature and 50° C. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The surface treatment chemical of the present invention contains particular proportions of substances suitable for the surface treatment of aluminum or its alloy, and it is diluted to a proper concentration as a surface treatment bath. 
     Specifically, the first surface treatment chemical (first surface treatment bath) contains 10-1000 parts by weight of niobium ion and/or tantalum ion (10-1000 ppm as a concentration in a surface treatment bath, same in the following). When the content of niobium ion and/or tantalum ion is less than 10 parts by weight (10 ppm), a conversion coating-forming rate is extremely low, failing to produce a sufficient conversion coating. On the other hand, when it exceeds 1000 parts by weight (1000 ppm), further improvement due to the addition of niobium ion and/or tantalum ion cannot be obtained. Thus, from the economic point of view, 1000 parts by weight (1000 ppm) of niobium ion and/or tantalum ion is sufficient. The preferred content of niobium ion and/or tantalum ion is 15-100 parts by weight (15-100 ppm). 
     Sources of niobium ion and tantalum ion include hexafluoroniobates and hexafluorotantalates such as NaNbF 6 , NH 4  NbF 6 , NaTaF 6 , NH 4  TaF 6 , etc., and particularly the ammonium salts are preferable. 
     The first surface treatment chemical (first surface treatment bath) of the present invention further contains 1-50 parts by weight (1-50 ppm), preferably 3-20 parts by weight (3-20 ppm) of effective fluorine ion. When the content of effective fluorine ion is less than 1 part by weight (1 ppm), substantially no etching reaction of aluminum takes place, failing to form a conversion coating. On the other hand, when it exceeds 50 parts by weight (50 ppm), an aluminum etching rate becomes higher than a conversion coating-forming rate, deterring the formation of the conversion coating. In addition, even though a conversion coating is formed, it is poor in resistance to blackening by boiling water and adhesion to a polymer coating film. Incidentally, the term &#34;effective fluorine ion&#34; means isolated fluorine ion, and its concentration can be determined by measuring a treatment solution by a meter with a fluorine ion electrode. Thus, fluoride compounds from which fluorine ion is not isolated in the surface treatment solution cannot be regarded as the sources of effective fluorine ion. 
     The suitable sources of effective fluorine ion include HF, NH 4  F, NH 4  HF 2 , NaF, NaHF 2 , etc., and particularly HF is preferable. 
     The first surface treatment bath is generally produced by diluting the first surface treatment chemical to a proper concentration. The resulting first surface treatment bath should have a pH of 1.5-4.0. When the pH of the first surface treatment bath is lower than 1.5, too much etching reaction of aluminum takes place, deterring the formation of the conversion coating. On the other hand, when it exceeds 4.0, the etching reaction rather becomes too slow, deterring the formation of the conversion coating. The preferred pH of the first surface treatment bath is 2.5-3.3. 
     The second surface treatment chemical (second surface treatment bath) of the present invention further contains, in addition to niobium ion and/or tantalum ion and effective fluorine ion in amounts as described above, 10-500 parts by weight (10-500 ppm) of zirconium ion and/or titanium ion, and 10-500 parts by weight (10-500 ppm) of phosphate ion. By adding zirconium ion and/or titanium ion together with phosphate ion, the resulting coating has further improved resistance to blackening by boiling water and adhesion to a polymer coating film. 
     When the content of zirconium ion and/or titanium ion is less than 10 parts by weight (10 ppm), no improvement is obtained by the addition thereof. However, even though it exceeds 500 parts by weight (500 ppm), further effects cannot be obtained. Thus, from the economic point of view, it would be sufficient if it is up to 500 parts by weight (500 ppm). The preferred content of zirconium ion and/or titanium ion is 20-100 parts by weight (20-100 ppm). 
     When the content of phosphate ion is less than 10 parts by weight, no improvement is obtained by the addition of phosphate ion, and when it exceeds 500 parts by weight, the resulting coating rather has a poor resistance to blackening by boiling water and adhesion to a polymer coating film. The preferred content of phosphate ion is 25-200 parts by weight (25-200 ppm). 
     The sources of zirconium ion and titanium ion include complex fluorides such as H 2  ZrF 6 , H 2  TiF 6 , (NH 4 ) 2  ZrF 6 , (NH 4 ) 2  TiF 6 , Na 2  ZrF 6 , etc., nitrates such as Zr(NO 3 ) 4 , Ti(NO 3 ) 4 , etc., sulfates such as Zr(SO 4 ) 2 , Ti(SO4) 2 , etc., and particularly (NH 4 ) 2  ZrF 6  and (NH 4 ) 2  TiF 6  are preferable. The sources of phosphate ion include H 3  PO 4 , NaH 2  PO 4 , (NH 4 )H 2  PO 4 , etc., and particularly H 3  PO 4  is preferable. 
     The second surface treatment bath should-have a pH of 1.5-4.0, and the preferred pH is 2.5-3.3 for the same reasons as in the first surface treatment bath. 
     Incidentally, the pH of each surface treatment bath may be controlled by pH-adjusting agents. The pH-adjusting agents are preferably nitric acid, sulfuric acid, ammonium aqueous solution, etc. Phosphoric acid can serve as a pH-adjusting agent, but it should be noted that it cannot be added in an amount exceeding the above range because it acts to deteriorate the properties of the resulting conversion coating. 
     The surface treatment chemical (surface treatment bath) of the present invention may optionally contain organic chelating agents of aluminum derived from gluconic acid (or its salt), heptonic acid (or its salt), etc. 
     The surface treatment chemical of the present invention may be prepared by adding the above components to water as an aqueous concentrated solution, and it may be diluted by a proper amount of water to a predetermined concentration with its pH adjusted, if necessary, to provide the surface treatment bath of the present invention. 
     The application of the surface treatment bath to aluminum or its alloy can be conducted by any methods such as an immersion method, a spraying method, a roll coat method, etc. The application is usually conducted between room temperature and 50° C., preferably at a temperature of 30°-40° C. The treatment time may vary depending upon the treatment method and the treatment temperature, but it is usually as short as 5-60 sec. 
     Incidentally, aluminum or its alloy to which the surface treatment bath of the present invention is applicable includes aluminum, aluminum-copper alloy, aluminum-manganese alloy, aluminum-silicon alloy, aluminum-magnesium alloy, aluminum-magnesium-silicon alloy, aluminum-zinc alloy, alulminum-zinc-magnesium alloy, etc. It may be used in any shape such as a plate, a rod, a wire, a pipe, etc. Particularly, the surface treatment bath of the present invention is suitable for treating aluminum cans for soft drinks, alcohol beverages, etc. 
     By treating aluminum or its alloy with the surface treatment bath of the present invention, the aluminum is etched with effective fluorine ion, and forms a double salt with the niobium ion, tantalum ion and fluorine ion to produce slightly soluble coating made of aluminum fluoroniobate and/or aluminum fluorotantalate, thereby forming a strong conversion coating. Strong corrosion resistance and adhesion to a polymer coating layer appears to contribute to the improvement in resistance to blackening by boiling water. 
     The present invention will be explained in further detail by the following Examples and Comparative Examples. In Examples and Comparative Examples, (1) resistance to blackening by boiling water and (2) adhesion to a polymer coating film are evaluated as follows: 
     (1) Resistance to Blackening By Boiling Water 
     Each aluminum can treated with a surface treatment bath is dried, and a bottom portion is cut off from the can, and then immersed in boiling water at 100° C. for 30 minutes. After that, the degree of blackening is evaluated as follows: 
     Excel.: Not blackened at all. 
     Good: Slightly blackened. 
     Fair: Lightly blackened (No problem for practical purposes). 
     Poor: Considerably blackened. 
     Very poor: Completely blackened. 
     (2) Adhesion to Polymer Coating Film 
     Each aluminum can treated with a surface treatment bath is dried, and its outer surface is further coated with an epoxy-phenol paint (Finishes A, manufactured by Toyo Ink Manufacturing Co., Ltd.) and then baked. A polyamide film of 40 μm in thickness (Diamide Film 7000 manufactured by Daicel Chemical Industries, Ltd.) is interposed between two of the resulting coated plates and subjected to hot pressing. A 5-mm-wide test piece is cut off from the hot pressed plates, and to evaluate the adhesion of each test piece, its peel strength is measured by a T-peel method and a 180° peel method. The unit of the peel strength is kgf/5 mm. Incidentally, the adhesion measured on a test piece before immersion in boiling water is called &#34;primary adhesion,&#34; and the adhesion measured on a test piece after immersion in tap water at 90° C. for 7.5 hours is called &#34;secondary adhesion.&#34; 
    
    
     EXAMPLES 1-11 
     An aluminum sheet (JIS A 3004) is formed into a can by a Drawing &amp; Ironing method, and degreased by spraying an acidic cleaner (Surfcleaner NHC 100 manufactured by Nippon Paint Co., Ltd.). After washing with water, it is sprayed with a surface treatment bath having the composition and pH shown in Table 1 at 40° C. for 30 sec. Next, it is washed with water and then with deionized water, and then dried in an oven at 200° C. After drying, each can is tested with respect to resistance to blackening by boiling water and adhesion to a polymer coating film. The results are also shown in Table 1. 
     
                                           TABLE 1                                 
__________________________________________________________________________
Composition (ppm)                               Adhesion of Coating Film  
               Effective                Resistance to   180°-Peel  
Example                                                                   
     Niobium                                                              
          Tantalum                                                        
               Fluorine                                                   
                    Zirconium                                             
                          Titanium                                        
                               Phosphate                                  
                                     pH Blackening by                     
                                                T-Peel Method             
                                                        Method            
No.  Ion (1)                                                              
          Ion (2)                                                         
               Ion (3)                                                    
                    Ion (4)                                               
                          Ion (5)                                         
                               Ion (6)                                    
                                      (7)                                 
                                        Boiling Water                     
                                                Prim.                     
                                                    Sec.                  
                                                        Prim.             
                                                            Sec.          
__________________________________________________________________________
1    50    0   8    0     0    0     2.8                                  
                                        Fair    3.8 1.5 3.3 2.0           
2     0   50   8    0     0    0     2.8                                  
                                        Good    3.9 1.5 3.4 1.9           
3    25   25   8    0     0    0     2.8                                  
                                        Fair    3.8 1.4 3.5 2.0           
4    25    0   8    25    0    50    2.8                                  
                                        Excel.  4.6 2.3 4.0 2.8           
5     0   25   8    25    0    50    2.8                                  
                                        Excel.  4.9 2.4 4.2 3.0           
6    25    0   8    0     25   50    2.8                                  
                                        Excel.  4.5 2.2 3.9 2.8           
7     0   25   8    0     25   50    2.8                                  
                                        Excel.  4.7 2.3 4.0 2.9           
8    25   25   8    0     0    0     1.8                                  
                                        Fair    3.5 1.4 3.2 1.9           
9    25   25   8    0     0    0     3.5                                  
                                        Fair    3.8 1.6 3.2 2.0           
10   50    0   20   0     0    0     2.8                                  
                                        Fair    3.7 1.7 3.3 2.1           
11    0   50   20   0     0    0     2.8                                  
                                        Good    3.8 1.7 3.3 2.2           
__________________________________________________________________________
 Note                                                                     
 (1): Added as NH.sub.4 NbF.sub.6.                                        
 (2): Added as NH.sub.4 TaF.sub.6.                                        
 (3): Added as HF.                                                        
 (4): Added as (NH.sub.4).sub.2 ZrF.sub.6.                                
 (5): Added as (NH.sub.4).sub.2 TiF.sub.6.                                
 (6): Added as H.sub.3 PO.sub.4.                                          
 (7): Controlled with HNO.sub.3 and an ammonium aqueous solution.         
 
    
     COMPARATIVE EXAMPLES 1-8 
     For comparison, surface treatment baths having the compositions and pH shown in Table 2 are prepared. The same surface treatment of an aluminum can as in Example 1 is conducted by using each surface treatment bath, and the same tests as in Example 1 are conducted. The results are also shown in Table 2. 
     
                                           TABLE 2                                 
__________________________________________________________________________
Compara-                                                                  
     Composition (ppm)                          Adhesion of Coating Film  
tive           Effective                Resistance to   180°-Peel  
Example                                                                   
     Niobium                                                              
          Tantalum                                                        
               Fluorine                                                   
                    Zirconium                                             
                          Titanium                                        
                               Phosphate                                  
                                     pH Blackening by                     
                                                T-Peel Method             
                                                        Method            
No.  Ion (1)                                                              
          Ion (2)                                                         
               Ion (3)                                                    
                    Ion (4)                                               
                          Ion (5)                                         
                               Ion (6)                                    
                                      (7)                                 
                                        Boiling Water                     
                                                Prim.                     
                                                    Sec.                  
                                                        Prim.             
                                                            Sec.          
__________________________________________________________________________
1    5    0    8    0     0    0     2.8                                  
                                        Very Poor                         
                                                0.8 0.5 2.0 0.9           
2    0    5    8    0     0    0     2.8                                  
                                        Very Poor                         
                                                0.9 0.5 2.0 0.8           
3    25   25   0.3  0     0    0     2.8                                  
                                        Very Poor                         
                                                0.8 0.4 1.8 0.9           
4    25   25   8    0     0    0     1.3                                  
                                        Poor    2.3 0.7 2.1 1.6           
5    25   25   8    0     0    0     4.5                                  
                                        Poor    2.0 0.6 2.2 1.0           
6    0    0    8    50    0    50    2.8                                  
                                        Poor    2.1 0.7 2.4 1.5           
7    0    0    8    0     50   50    2.8                                  
                                        Poor    1.8 0.6 2.2 1.4           
8    No chemical treatment after degreasing.                              
                                        Very Poor                         
                                                0.7 0.3 1.9 0.6           
__________________________________________________________________________
 Note                                                                     
 (1): Added as NH.sub.4 NbF.sub.6.                                        
 (2): Added as NH.sub.4 TaF.sub.6.                                        
 (3): Added as HF.                                                        
 (4): Added as (NH.sub.4).sub.2 ZrF.sub.6.                                
 (5): Added as (NH.sub.4).sub.2 TiF.sub.6.                                
 (6): Added as H.sub.3 PO.sub.4.                                          
 (7): Controlled with HNO.sub.3 and an ammonium aqueous solution.         
 
    
     As is clear from the above results, in the case of treatment with the surface treatment bath of the present invention (Examples 1-11), the formed conversion coatings are good in resistance to blackening by boiling water and in adhesion to a polymer coating film. On the other hand, when the content of niobium ion and/or tantalum ion is less than 10 ppm (10 parts by weight) (Comparative Examples 1, 2, 6 and 7) or when effective fluorine ion is less than 1 ppm (part by weight) (Comparative Example 3), the formed conversion coatings suffer from poor resistance to blackening by boiling water and adhesion to a polymer coating film. When the pH of the surface treatment bath is less than 1.5 (Comparative Example 4), a conversion coating is not easily formed, and the formed conversion coating is slightly blackened and shows poor adhesion to a polymer coating film. On the other hand, when the pH exceeds 4.0 (Comparative Example 5), the treating bath becomes cloudy because of precipitation, and the resulting conversion coating is slightly poor in resistance to blackening by boiling water and also shows poor adhesion to a polymer coating film. 
     As described above in detail, with the surface treatment chemicals (surface treatment bath) of the present invention, a conversion coating having extremely high corrosion resistance can be formed on a surface of aluminum or its alloy at a low temperature in a very short time. The conversion coating thus formed is highly resistant to blackening even when immersed in boiling water, meaning that it has excellent resistance to blackening by boiling water even in a thin layer. In addition, when a polymer coating film is formed on the conversion coating by painting or printing, extremely strong adhesion between them can be achieved. Further, since the conversion coating shows good slidability, it is extremely advantageous in conveying. 
     Since the surface treatment chemical (surface treatment bath) of the present invention shows sufficient characteristics even though its concentration is varied, it is not required to strictly control the concentration of the surface treatment bath. 
     The surface treatment chemicals (surface treatment bath) having such advantages are highly suitable for the surface treatment of aluminum cans, etc.