Waterborne top coatings for aluminum heat exchangers

Abstract

Disclosed are methods for coating aluminum alloy heat exchangers that include the use of a waterborne top coating. By using a waterborne top coating, relative to a solvent borne, useful advantages can be achieved, such as improved penetration depth of the top coating. An example method includes cleaning a surface of a heat exchanger; contacting the surface of the heat exchanger with a first mixture to provide a passivation layer on the surface of the heat exchanger; contacting the passivation layer with a second mixture in a bath and applying a positive charge to the bath to provide an electro-coating on a surface of the passivation layer; and contacting the electro-coating with a third mixture, the third mixture including water and a polymer resin having anti-corrosion and ultraviolet protection properties, to provide a top coating including the polymer resin on a surface of the electro-coating.

What is claimed is: 
     
         1 . A method for coating an aluminum alloy heat exchanger, the method comprising:
 cleaning a surface of a heat exchanger, the heat exchanger comprising an aluminum alloy;   contacting the surface of the heat exchanger with a first mixture, the first mixture comprising a trivalent chromium salt, to provide a passivation layer derived from the trivalent chromium salt on the surface of the heat exchanger;   contacting the passivation layer with a second mixture in a bath, the second mixture comprising a paste and a first resin, and applying a positive charge to the bath to provide an electro-coating derived from the paste and the first resin on a surface of the passivation layer; and   contacting the electro-coating with a third mixture, the third mixture comprising water and a second resin having anti-corrosion and ultraviolet protection properties, to provide a top coating comprising the second resin on a surface of the electro-coating.   
     
     
         2 . The method of  claim 1 , wherein the cleaning comprises contacting the surface with an alkaline mixture. 
     
     
         3 . The method of  claim 1 , wherein the first mixture further comprises a salt of hexafluorozirconic acid. 
     
     
         4 . The method of  claim 1 , wherein the trivalent chromium salt comprises trivalent chromium sulfate. 
     
     
         5 . The method of  claim 1 , wherein the passivation layer has a weight of at least 0.05 g/m 2 . 
     
     
         6 . The method of  claim 1 , wherein the surface of the heat exchanger is contacted with the first mixture for less than or equal to 12 minutes. 
     
     
         7 . The method of  claim 1 , wherein the heat exchanger acts as a cathode when the positive charge is being applied. 
     
     
         8 . The method of  claim 1 , wherein the electro-coating has a thickness of about 5 μm to about 50 μm. 
     
     
         9 . The method of  claim 1 , wherein the heat exchanger is washed, heated, or both after applying the electro-coating. 
     
     
         10 . The method of  claim 1 , wherein the third mixture comprises a solids content of less than 35% by weight of the third mixture. 
     
     
         11 . The method of  claim 1 , wherein the second resin of the third mixture comprises an acrylic resin, a polyurethane resin, or a combination thereof. 
     
     
         12 . The method of  claim 1 , wherein the first mixture, the third mixture, or both further comprise an adhesion promoter. 
     
     
         13 . The method of  claim 1 , wherein the top coating penetrates the heat exchanger at least 0.5 mm. 
     
     
         14 . The method of  claim 1 , wherein the third mixture is sprayed onto the electro-coating. 
     
     
         15 . The method of  claim 1 , wherein the third mixture comprises less than or equal to 75 g/L of volatile organic compounds. 
     
     
         16 . The method of  claim 1 , wherein the top coating has a thickness of about 10 μm to about 100 μm. 
     
     
         17 . The method of  claim 1 , wherein the method further comprises coloring the top coating. 
     
     
         18 . The method of  claim 17 , wherein coloring comprises adding a pigment dispersion to the top coating. 
     
     
         19 . The method of  claim 1 , wherein the top coating does not require a heat or chemical cure.