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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)
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
surface.
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
required.
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