Process for selective phosphating of a composite metal construction

Abstract

The present disclosure also relates to a corresponding zinc phosphation composition, to a concentrate for production thereof, to a corresponding composite metal construction and to a method of using thereof.

1 . A method of chemical pretreatment and selective phosphation of a composite metal construction comprising at least a portion made of aluminum and at least a portion made of zinc and optionally a further portion made of iron, wherein the method comprises:
 (I) treating the composite metal construction with an aqueous zinc phosphation composition that results in a formation of a surface-covering crystalline zinc phosphate layer having a coat weight in a range from 0.5 to 5 g/m 2  on the parts made of iron and zinc, but does not produce a zinc phosphate layer having a coat weight of at least 0.5 g/m 2  on the aluminum parts, and then—with an intervening water rinse operation—   (II) applying an aqueous acidic passivation composition having a pH in a range from 2.0 to 5.5 to the composite metal construction, where the aqueous acidic passivation composition removes not more than 50% of the crystalline zinc phosphate on the parts made of zinc and iron, but forms a passive layer on the aluminum parts that does not constitute a surface-covering crystalline phosphate layer,   wherein the aqueous zinc phosphation composition in step (I) comprises a content of sodium and/or potassium ions and of aluminum ions,   wherein the aqueous zinc phosphation composition in step (I) has a temperature in a range from 20 to 65° C. and comprises a free fluoride content of at least 5 mg/l but not greater than 200 mg/1,   where wherein at least 0.04 g/l but not more than 3.2 g/l of boron in a form of water-soluble inorganic compounds, calculated as BF 4 , is present in the aqueous zinc phosphation composition, and   wherein a points number of free acid with added KCl in the aqueous zinc phosphation composition is at least 0.6 points.   
     
     
         2 . The method according to  claim 1 , wherein the aqueous zinc phosphation composition in step (I) has a free fluoride content in a range from 10 to 200 mg/l. 
     
     
         3 . The method according to  claim 1 , wherein the aqueous zinc phosphation composition in step (I) has a concentration of boron in the form of water-soluble inorganic compounds calculated as BF 4  in a range from 0.08 to 2.5 g/l. 
     
     
         4 . The method according to  claim 1 , wherein the content of sodium and/or potassium ions (calculated as sodium) in the aqueous zinc phosphation composition in step (I) is in a range from 1 to 4 g/l, and the free fluoride content is in the range from 50 to 150 mg/l. 
     
     
         5 . The method according to  claim 1 , wherein a content of inorganic compounds comprising silicon (calculated as SiF 6 ) in the aqueous zinc phosphation composition in step (I) is below 25 mg/l. 
     
     
         6 . The method according to  claim 1 , wherein the aqueous zinc phosphation composition in step (I) has a points number of free acid with added KCl in a range from 0.6 to 3.0 points. 
     
     
         7 . The method according to  claim 1 , wherein for the aqueous zinc phosphation composition in step (I) a dimensionless quotient
   (A value×T)/(F tot ×[B]×1000)
   is in a range from 0.13 to 22.5.   
     
     
         8 . The method according to  claim 1 , wherein the aqueous zinc phosphation composition in step (I) additionally includes one or more of the following cations:
 0.3 to 2 g/l of manganese(II),   0.3 to 2 g/l of nickel(II),   0.001 to 0.2 g/l of iron(III).   
     
     
         9 . The method according to  claim 1 , wherein a pH of the aqueous acidic passivation composition in step (II) is in a range from 2.5 to 5.5. 
     
     
         10 . The method according to  claim 1 , wherein the aqueous acidic passivation composition in step (II) comprises Ti, Zr and/or Hf in complexed form and optionally further metal ions in a form of water-soluble compounds. 
     
     
         11 . The method according to  claim 1 , wherein the aqueous acidic passivation composition in step (II) additionally comprises at least one organosilane and/or at least one hydrolysis product thereof and/or at least one condensation product thereof. 
     
     
         12 . A zinc phosphation composition for selective phosphation of steel, galvanized steel and/or alloy-galvanized steel surfaces in a metallic composite construction comprising a portion made of aluminum according to  claim 1 , wherein the zinc phosphation composition has a points number of free acid with added KCl of at least 0.6 points, and
 (a) 5 to 50 g/l of phosphate ions calculated as P 2 O 5 ,   (b) 0.3 to 3 g/l of zinc ions,   (c) at least 5 mg/l but not more than 200 mg/l of free fluoride, and   (d) at least 0.04 g/l but not more than 3.2 g/l of boron in the form of water-soluble inorganic compounds, calculated as BF 4 ,   
       where the zinc phosphation composition additionally comprises a content of sodium and/or potassium ions and of aluminum ions. 
     
     
         13 . A concentrate from which the zinc phosphation composition according to  claim 12  can be obtained by diluting with a suitable solvent and/or dispersion medium and optionally adjusting the pH. 
     
     
         14 . A composite metal construction comprising at least a portion made of aluminum and at least a portion made of zinc and optionally a further portion made of iron, wherein it is obtainable by a method according to  claim 1 . 
     
     
         15 . A method of using the composite metal construction according to  claim 14 , the method comprising incorporating the composite metal construction into a component used in an automotive supplier industry, in bodywork construction in automobile manufacture, in agricultural machine construction, in shipbuilding, in a construction sector or for production of white goods. 
     
     
         16 . The method according to  claim 2 , wherein the aqueous zinc phosphation composition in step (I) has a free fluoride content in a range from 20 to 150 mg/l. 
     
     
         17 . The method according to  claim 2 , wherein the aqueous zinc phosphation composition in step (I) has a free fluoride content in a range from 20 to 120 mg/l. 
     
     
         18 . The method according to  claim 2 , wherein the aqueous zinc phosphation composition in step (I) has a free fluoride content in a range from 50 to 120 mg/l. 
     
     
         19 . The method according to  claim 2 , wherein the aqueous zinc phosphation composition in step (I) has a free fluoride content in a range from 70 to 120 mg/l. 
     
     
         20 . The method according to  claim 3 , wherein the aqueous zinc phosphation composition in step (I) has a concentration of boron in the form of water-soluble inorganic compounds calculated as BF 4  in a range from 0.08 to 2 g/l.