Multiple step conversion coating process

Abstract

A process for the chemical pretreatment, before an organic coating, of composite metal structures that contain aluminum or aluminum alloy portions together with zinc or zinc alloy portions, and steel, galvanized steel and/or alloy-galvanized steel portions, such that the phosphate level and the operating temperature of the zinc phosphate solution is reduced, and the nitrite accelerator level of the solution is increased. The process of the present invention reduces the sludge produced by the process, the temperature at which the process operates, and the need for monitoring the process. Furthermore, the pretreatment process of the present invention forms a conversion layer on all exposed metal surfaces that is suitable as a corrosion-preventing paint substrate, especially before a cathodic electro-dipcoating.

1 . A process for chemical pretreatment, before an organic coating, of a composite metal structure that contains at least one aluminum or aluminum alloy portion, at least one zinc or zinc alloy portion, and at least one steel, galvanized steel or alloy-galvanized steel portion, the process comprising: 
 (I) treating the composite metal structure with a zinc phosphating solution having less than 5 g/l of phosphate ions and more than 0.2 g/l of nitrite ions, the zinc phosphating solution forming a surface-covering crystalline zinc phosphate layer having a coating weight in the range from 0.5 to 5 g/m 2  on the zinc or zinc alloy, steel, galvanized steel, or alloy-galvanized steel portions, but without forming a zinc phosphate layer on the aluminum or aluminum alloy portion; and    (II) subsequently, contacting the composite metal structure with a treatment solution that does not dissolve more than 60% of the crystalline zinc phosphate layer formed on the zinc or zinc alloy, steel, galvanized steel, or alloy-galvanized steel portions, but does produce a conversion layer on the aluminum or aluminum alloy portion.    
     
     
         2 . A process as recited in  claim 1 , wherein: 
 in step (I) the zinc phosphating solution has a temperature in a range from 20° C. to 40° C.; and    in step (II) the treatment solution does not dissolve more than 25% of the crystalline zinc phosphate layer deposited in step (I).    
     
     
         3 . A process as recited in  claim 2 , wherein in step (II) the treatment solution does not dissolve more than 10% of the crystalline zinc phosphate layer deposited in step (I).  
     
     
         4 . A process as recited in  claim 1 , wherein in step (I) the zinc phosphating solution has a temperature in a range from 30° C. to 35° C.  
     
     
         5 . A process as recited in  claim 1 , wherein the treatment solution used in step (II) has a pH in a range from 3.5 to 5.5 and comprises from 0.3 to 1.5 g/l of one of hexafluorotitanate ions, hexafluorozirconate ions, or both.  
     
     
         6 . A process as recited in  claim 1 , wherein the treatment solution used in step (II) further includes from 0.01 to 0.1 g/l of copper ions.  
     
     
         7 . A process as recited in  claim 1 , wherein the treatment solution used in step (II) has a pH in a range from 3.5 to 5.8 and contains from 10 to 500 mg/l of organic polymers selected from poly-4-vinylphenol molecules that conform to the general formula (I):  
       
         
           
           
               
               
           
         
       
       wherein n is an integer between 5 and 100, each of X and Y independently of each other denotes hydrogen or a CRR 1 OH moiety in which each of R and R′ independently is hydrogen or an aliphatic or an aromatic moiety with 1 to 12 carbon atoms.  
     
     
         8 . A process as recited in  claim 7 , wherein the treatment solution used in step (II) comprises a condensation reaction product of: 
 (i) polyvinyl phenol having an average molecular weight in a range from 1000 to 10,000;    (ii) one of formaldehyde or paraformaldehyde; and    (iii) at least one secondary organic amine.    
     
     
         9 . A process as recited in  claim 8 , wherein the secondary organic amine is selected from the group consisting of methylethanolamine, N-methylglucamine, and mixtures thereof.  
     
     
         10 . A process as recited in  claim 9 , wherein the treatment solution has a pH in the range from 3.3 to 4.8, contains 100 to 5000 m g/l of the condensation reaction product, and further comprises: 
 from 10 to 2000 mg/l of phosphate ions;    from 10 to 2500 mg/l of hexafluorotitanate ions, hexafluorozirconate ions, or both; and    from 10 to 1000 mg/l of manganese ions.    
     
     
         11 . A process as recited in  claim 1 , wherein the treatment solution used in step (II) has a pH in the range from 3.3 to 5.8 and comprises from 250 to 1500 mg/l of organic polymers selected from the group consisting of homopolymers and copolymers of acrylic acid, methacrylic acid, and esters of acrylic and methacrylic acids.  
     
     
         12 . A process as recited in  claim 1 , wherein the zinc phosphating solution further comprises: 
 zinc cations having a concentration between 0.30 g/l to 2.0 g/l; and    manganese(II) cations having a concentration between 0.05 g/l to 2.0 g/l.    
     
     
         13 . A process as recited in  claim 12 , wherein the zinc phosphating solution further comprises: 
 at least one of: (i) about 0.1 to 1.5 g/l of nickel(II) cations, or (ii) about 0.0011 to about 0.025 of copper cations; and    fluorine-containing anions having a stoichiometric equivalent as fluoride that is from about 0.1 to 1.5 g/l.    
     
     
         14 . A process as recited in  claim 13 , wherein the zinc phosphating solution has a Free Acid value of about −0.5 to about 1.80 points.  
     
     
         15 . A process as recited in  claim 1 , wherein the zinc phosphating solution further comprises: 
 zinc cations having a concentration between about 1.47 g/l and about 1.85 g/l;    manganese(II) cations having a concentration between about 0.37 g/l and about 0.41 g/l;    nickel(II) cations having a concentration of about 0.46 g/l to about 0.51 g/l;    fluorine-containing anions having a stoichiometric equivalent as fluoride that is from about 1.0 to about 1.1 g/l; and    nitrate ions having a concentration of about 5.8 g/l to about 6.0 g/l.    
     
     
         16 . A process as recited in  claim 1 , wherein the zinc phosphating solution further comprises: 
 zinc having a concentration from 0.6 to 1.1 g/l and phosphate having a concentration from 3 to 5 g/l, wherein the weight ratio Zn/PO 4  is approximately less than or equal to 0.21.    
     
     
         17 . A process as recited in  claim 1 , wherein the surface-covering crystalline zinc phosphate layer has a coating weight in the range from 2 to 3 g/m 2  on the zinc or zinc alloy, steel, galvanized steel, or alloy-galvanized steel portions.  
     
     
         18 . A process as recited in  claim 17 , wherein the zinc or zinc alloy portion, and the steel, galvanized steel, or alloy-galvanized steel portion have a weight loss of approximately 1.7 g/m 2 , and the aluminum or aluminum alloy portion has a weight loss of approximately 0.1 to 0.6 g/m 2 .  
     
     
         19 . A process as recited in  claim 1 , wherein step (I) oxidizes the at least one aluminum or aluminum alloy portion to remove oxides.