PHOSPHATE CONVERSION COATINGS

 

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ZINC PHOSPHATING PRIOR TO POWDER COATING

 

There are countless applications of powder coating coupled with phosphate pretreatment. They include the finishing of refrigerators, pressure vessels for fire extinguishers, garden furniture, electric panels, steel fencing, car wheels and other car accessories. The degree of success in the devolopment of powder coating is largely governed by the outcome of advances in other environmentally-friendly processes such as high solids containing, water based paints or two-pack systems in the market as a whole.

 

The principle of powder coating is to apply the organic coating, in finely-devided powder form to the surface of the work to be coated either at ambient or somewhat elevated temperatures. The film is  then formed by melting the powder on the surface of the substrate.

 

Powder coating is being increasingly used not least because of the absence of volatile solvents involved thus eleminating virtually all problems of airborne effluent discharge. The corrosion resistance of powder coating is many times better than of the equivalent thickness of conventional paints. Against this, it must be said that maintaining a given colour value is, by the nature of the process, more difficult for powder coating. In well-run plants using spray powder coating, the over-sprayed powder is collected and re-cycled which, if colour values are to be held, calls for high standarts of cleanliness.

 

Introduction of zinc phosphates and the manganese-modified zinc phosphates coupled with powder coating, allowed still further improvements as manifested in the atmospheric corrosion test, the cyclic salt spray water condensate test, and salt spray test or water condensate test. The beneficial effects of the manganese modified process are specially in evidence for galvanized steel. Even for these more recently introduced processes, the best results are found where the Cr (VI) post rinse has been used to passivate the phosphate. In certain cases, post-rinse solutions based Cr (III) or chromium-free solutions are used, the performance of which is intermediate between a simple water rinse and the Cr (VI) containin water rinse.

 

 

 

 

 

 

 

 

 

 
 
 


 

 

 

 

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