Pretreatment of aluminum surfaces

Abstract

A process for preparing the surface of aluminum products to receive colored coatings comprises a substantially pollution free, non-chromate pretreatment and aluminum conversion coating process. The process provides improved coating quality, increased process stability, increased throughput and extended operating life of the processing baths by controlling the electrical resistance of the deionized water used for washing the aluminum product being processed to at least about 50 k ohms and controlling the manner of washing between each step of the process.

1 . An improved process for the conversion coating of aluminum products comprising the steps of: 
 a) immersing an aluminum product in a solution for removal of dirt and oil, followed by    b) immersion in an oxidizing solution, and then followed by    c) immersion in a solution of a permanganate based conversion composition, the improvement comprising, following each of step a), b) and c) above, subjecting the aluminum product, upon removal from immersion in said solution, to at least one spray wash with deionized water and at least one immersion in deionized water, the deionized water being processed or continuously reprocessed through a series of filters and ion beds to maintain an electrical resistance of at least about 50 k ohms, said deionized water maintained at a temperature suitable to not form boehmite.    
     
     
         2 . The process of  claim 1  wherein the aluminum product, after each of steps a), b) and c) is exposed to 
 i. a first spray wash with deionized water,    ii. followed by immersion in deionized water, followed    iii. by a second spray wash with deionized water.    
     
     
         3 . The process of  claim 1  wherein the deionized water is maintained at an electrical resistance of at least about 50 k ohms by passing it through at least a 0.5 micron filter, a carbon filter, a cation bed, an anion bed and a DI mixed bed.  
     
     
         4 . The process of  claim 1  wherein the deionized water is processed serial through a 0.5μ filter, a carbon filter, a cation bed, two anion beds, a DI mixed bed and a 0.5μ filter.  
     
     
         5 . The process of  claim 1 , wherein at least about 270,000 ft 2  of exposed surface of aluminum parts is processed before streaking, spotting, peeling or discoloration of the surface appears.  
     
     
         6 . The process of  claim 1 , wherein aluminum parts can be processed for at least 480 hours before unacceptable streaking, spotting, peeling or discoloration of the surface appears.  
     
     
         7 . An improved method of applying a conversion coating to aluminum parts, said aluminum parts being exposed to treatment baths for a cleaning the parts, b) oxidizing the exposed surface of the parts and c) conversion coating the parts, 
 wherein the parts are spray washed and immersed in deionized water between each treatment bath exposure, the deionized water provided at a temperature sufficiently low so as to not form boehmite and having an electrical resistance equal to or greater than about 50 k ohms.    
     
     
         8 . The improved method of  claim 7  wherein the deionized water is maintained at a resistance of at least about 50 k ohms by continuously recirculating the water through at least a 0.5μ filter, a carbon filter, a cation bed, an anion bed and a mixed bed at a flow rate from about 3 gal/min to about 10 gal/min.  
     
     
         9 . The improved process of  claim 1  further including a deoxidizing step between step a) and step b) using a solution of nitric acid and a fluoride compound.