Zinc Phosphate Coating
Zinc phosphate is a non-metallic, crystalline coating that chemically adheres to the substrate. Zinc coatings are extremely adherent, they provide a uniform coating with improved coating adhesion properties, better coating in recessed areas and better corrosion resistance. A typical zinc phosphate consists of phosphoric acid base accelerators and zinc salts. Zinc phosphate comes from the solution itself, not from the part surface like an iron phosphate coating. Crystals begin forming at anodic sites on the part surface and stop forming when they hit another crystal. The more origination sites the better the density of the coating. For powder coating, it is best to keep the and densely packed. Powder does not stay in the flow stage for very long. Larger phosphate crystals may not allow the powder material to completely wet the surface and a capillary layer may form under the coating. Moisture will penetrate the coating and cause corrosion that will lift the coating from the surface.
Many automobile manufacturers use zinc phosphate coatings as a primer during surface coating operations. In the factory, the application of the zinc phosphate coating is a multi-step, time consuming process. But zinc phosphate systems used as a primer in the refinishing industry are a simple, one step, process. A light coat of zinc phosphate is sprayed on the metal surface and allowed to dry for 30 to 60 minutes. The zinc phosphate etches the substrate and deposits a phosphate coating on the surface to provide protection from moisture. The result is a nonreactive roughened surface that is perfect for the application of a primer coat and needs no sanding prior to surface coating.
Activation -Prior to Zinc Phosphating
When zinc phosphating, the metal surface is activated by an additive
in the cleaner bath or in a conditioning rinse prior to phosphating.
Conditioners are mild alkaline suspensions of specialized active titanium
salts that adhere to steel, zinc and aluminum surfaces. The conditioner
will set up a network of uniform acceptor sites for zinc crystals
to deposit. This will increase the number of zinc phosphate crystals,
decrease the size of these crystals and generally improve the quality
of the zinc phosphate coating. The small crystal size will be more
uniform and lower weight, helping to promote adhesion, control the
cost of phosphating, and generating less sludge.
The crystal size of the phosphate coating has an impact on paint bonding capacity and corrosion resistance. A large crystal structure is more porous, has poorer corrosion resistance, and requires more paint to achieve a complete film. A fine-grained, tight, uniform coating will provide the best performance. The conditioner in the rinse preceding the phosphate stage can assist the development of this fine-grained phosphate coating.
A zinc phosphate solution will continually produce sludge through oxidation of soluble iron to an insoluble state that precipitates. A sludge removal system must be used to provide constant removal of this sludge.
The rinse stage following the phosphate should be ambient tap water. Phosphate salts are more soluble in cold water. The overflow volume should be sufficient to keep the rinse clean and reasonably cool.
Here you can see samples of zinc phosphated metal parts ;
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