Powder Coating Pretreatment
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Phosphating, or conversion coating, is the application of an iron or

zinc phosphate coating to the substrate. Conversion coating can be a

very critical part of the pretreatment process, adding significantly to

the performance of the finished coating.

A phosphate coating converts the metal substrate to a uniform, inert surface, which improves bonding, minimizes the spread of oxidation if the coating is scratchedand improves the overall corrosion resistance

of the final part.

A conversion coating can be iron, zinc, polycrystalline, chromate, or

manganese phosphate film. They are developed on both ferrous (iron

based) and non-ferrous surfaces (zinc, aluminum, terne and manganese).

Parts are subjected to an acidic bath and a chemical conversion

forms a complete film on the part surface, changing the chemical and

physical nature of the metal surface.

Iron Phosphate

Iron phosphate is the thinnest of phosphate films. In the application

process, an iron oxide base is developed, followed by a flat or amorphous

metal phosphate topcoat. The treated metal surface will typically

have a gray to blue iridescent or blue-gold iridescent color, depending

on the coating weight and the base metal. A typical iron

phosphate consists of:

  • phosphate acid base
  • accelerators/oxidizers
  • surfactant package (optional)

pretreatment powder coating

In an iron phosphate solution, the metal surface is etched, releasing

some iron into the bath. When metal ions are etched from the part

surface, the surface becomes positively charged. The metal ions in

the bath are converted to iron phosphate, negatively charged. A pH

rise occurs at the interface of the solution and the part, causing the

iron phosphate ions to deposit an amorphous coating on the metal


The acid salt content, type and amount of accelerator, and the type

and amount of acid etchants varies from one compound to another.

These compositions are all moderately acidic. Although crystal site

activators are not typically required prior to application of iron phosphate

coatings, formulations commonly contain oxidizers and/or accelerators.

The oxidizers, such as nitrite or chlorate, act to initiate

attack on ferrous parts, providing the iron for the iron phosphate

coating. Accelerators, such as molybdate or vanadate, provide active

sites for iron phosphate deposition. Choice of oxidizer or accelerator

in a particular product may affect the performance or appearance of

the final coating.

In a three-stage iron phosphate treatment process, the cleaning and

coating are combined by incorporation of a detergent surfactant package

in the iron phosphate solution. A source of fluoride ions may be

added if aluminum is also being processed to increase the etching

effect on the oxide surface of the aluminum.

Iron phosphate coatings can be applied by hand wiping, with a handheld

spray wand, immersion, or a spray washer. The number and type

of process stages is directly dependent on finished part requirements.

A cleaner/coater combination followed by a rinse is the typical minimum

chemical cleaning and phosphating process used. The addition

of stages in the process can provide enhanced performance.

The most effective and commonly used method is a multi-stage spray

washer. Spray washers are built with as few as two stages and as

many as eight.

  • Two Stage: clean/coat, rinse
  • Three Stage: clean/coat, rinse, rinse/seal
  • Four Stage: clean/coat, rinse, rinse/seal, DI rinse*
  • Five Stage: clean, rinse, phosphate, rinse, rinse/seal
  • Six Stage: clean, rinse, phosphate, rinse, rinse/seal, DI rinse
  • Seven Stage: clean, clean, rinse, phosphate, rinse, rinse/seal,
  • DI rinse
  • Eight Stage: clean, rinse, clean, rinse, phosphate, rinse, rinse/seal,
  • DI rinse
  • * Deionized water; water that has been filtered to remove negative and positive ions.


Iron Phosphate Controls

In addition to the number of process stages, the factors that will affect

the weight of an iron phosphate coating are time, temperature, concentration, acid consumed (pH), the condition of the substrate and the

spray pressure.

  • Time in Process - The more time that the chemistry has to work, the more work it will do. The process must be long enough to
  • allow the chemistry to form to a uniform coating on the surface.
  • Temperature of the Solution - Soils become more reactive in a heated solution and the chemicals become more aggressive.
  • Concentration - A higher concentration of chemical will provide more total acid, more accelerators and it can provide more coating weight.

Zinc Phosphate

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
  • 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.

Unlike the iron phosphate, a zinc phosphate can not clean and coat

simultaneously in a three-stage process, a separate cleaning stage is


See also: Phosphating for Spray and Immersion Processes -SurTec

See also: Parkerizing (also called phosphating & phosphitizing) - Wikipedia

Interesting article: Nanoceramic coating to Replace iron Phosphating -Henkel Locite Adhesives


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June 14, 2012