Electrolytic phosphating process

Abstract

This invention provides a phosphating process that can form a film suitable for a cold forging foundation within 60 seconds and preferably 30 seconds or less. The process uses a treatment bath that is formed of a phosphate ion solution (H 2 PO 4 − +Zn 2+ ), made by dissolving zinc in phosphoric acid, contains phosphoric acid (H 3 PO 4 ), phosphate ions, zinc ions and nitrate ions, may contain at least one kind of metal ion selected from the group consisting of nickel ions, cobalt ions, copper ions, manganese ions and iron ions, and further contains 0.5 g/l or below of metal ions other than the film forming components. The process involves electrolytic treatment by applying a voltage between a metal as a positive electrode and a treated article as a negative electrode and forms a phosphate film on the surface of the treated article. The phosphate ion solution prepared by dissolving zinc in phosphoric acid (H 2 PO 4 − +Zn 2+ ) is a solution obtained by dissolving 8 parts by mass to a maximum dissolution concentration of zinc in 100 parts by mass of phosphoric acid.

1 . An electrolytic phosphating process using a treatment bath that is formed of a phosphate ion solution (H 2 PO 4   − +Zn 2+ ), made by dissolving zinc in phosphoric acid, contains phosphoric acid (H 3 PO 4 ), phosphate ions, zinc ions and nitrate ions, may contain at least one kind of metal ion selected from the group consisting of nickel ions, cobalt ions, copper ions, manganese ions and iron ions, and further contains 0.5 g/l or below of metal ions other than said film forming components, said process comprising the step of conducting an electrolytic treatment by applying a voltage between a metal as a positive electrode and a treated article as a negative electrode and forming a phosphate film on the surface of the treated article.  
   
   
       2 . An electrolytic phosphating process according to  claim 1 , wherein said electrolytic treatment is carried out by using a treatment bath the pH of which is adjusted to 1.5 to 2.5 and the ORP (oxidation-reduction potential) of which is kept at 90 to 450 mV (silver/silver chloride electrode potential).  
   
   
       3 . An electrolytic phosphating process according to  claim 1 , which contains at least 15 g/l of phosphoric acid and phosphate ions, at least 15 g/l of zinc ions, at least 12.5 g/l of nitrate ions, and 0 to 3 g/l of at least one kind of metal ion selected from the group consisting of nickel ions, cobalt ions, copper ions, manganese ions and iron ions.  
   
   
       4 . An electrolytic phosphating process according to  claim 1 , wherein said positive electrode is selected from zinc or iron.  
   
   
       5 . An electrolytic phosphating process according to any of  claim 1 , wherein, after anodic electrolysis is carried out by using said treated article as the positive electrode and zinc or iron as the negative electrode, cathodic electrolysis is carried out by using said treated article as the negative electrode and zinc or iron as the positive electrode.  
   
   
       6 . An electrolytic phosphating process according to  claim 1 , wherein a voltage of not greater than 6 V is applied by connection to a D.C. power source.  
   
   
       7 . An electrolytic phosphating process according to  claim 1 , wherein said phosphate ion solution (H 2 PO 4   − +Zn 2+ ) dissolving zinc in phosphoric acid is a solution prepared by dissolving 8 parts by mass to a maximum dissolution concentration of zinc in 100 parts by mass of phosphoric acid.  
   
   
       8 . An electrolytic phosphating process according to  claim 1 , wherein said phosphate ion solution (H 2 PO 4   − +Zn 2+ ), made by dissolving zinc in phosphoric acid, is a solution prepared by dissolving 15 to 25 parts by mass of zinc in 100 parts by mass of phosphoric acid.  
   
   
       9 . An electrolytic phosphating process according to  claim 1 , wherein said phosphate ion solution (H 2 PO 4   − +Zn 2+ ), made by dissolving zinc in phosphoric acid, is a solution prepared by dissolving zinc oxide, zinc hydroxide or metallic zinc in a phosphate ion solution.  
   
   
       10 . An electrolytic phosphating process according to  claim 1 , wherein a ratio of the phosphate ion solution (H 2 PO 4   − +Zn 2+ ) and phosphoric acid (H 3 PO 4 ) in said electrolytic treatment bath has a ratio of 0.4 to 1 represented by the relation [phosphate ion solution in which zinc is dissolved (H 2 PO 4   − +Zn 2+ )]/[phosphate ion solution in which zinc is dissolved (H 2 PO 4   − +Zn 2+ )+phosphoric acid (H 3 PO 4 )].  
   
   
       11 . An electrolytic phosphating process according to  claim 1 , wherein said electrolytic treatment bath contains the phosphate ion solution (H 2 PO 4   − +Zn 2+ ) dissolving zinc in phosphoric acid, phosphoric acid (H 3 PO 4 ) and zinc nitrate, and may contain a metal nitrate constituted by at least one kind of nitrate selected from the group of nickel nitrate, cobalt nitrate, copper nitrate and manganese nitrate.  
   
   
       12 . An electrolytic phosphating process according to  claim 1 , wherein the electrolytic treatment is carried out at a cathodic electrolysis current density of 1 to 18 A/dm 2  to form a phosphate film on the surface of the treated article.  
   
   
       13 . An electrolytic phosphating process according to  claim 1 , wherein the electrolytic treatment is carried out under a condition free of any obstacles, that may impede the flow of a current between the positive electrode and the negative electrode, to form a phosphate film on the surface of the treated article.  
   
   
       14 . An electrolytic phosphating process according to  claim 1 , wherein a zinc phosphate film is formed by setting the electrolytic treatment time to 60 seconds or less.  
   
   
       15 . An electrolytic phosphating process according to  claim 1 , wherein two or more kinds of voltages and currents are applied to one treated article, depending on positions thereof, by using two or more power sources and electrodes.