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Phosphate Coatings Technical Information


To view the properties of  phosphate coatings  click here.

Phosphate coatings are made up of thin crystalline layers of phosphate compounds that adhere to the surface of the metal substrate. The phosphate crystals are porous and can be formed from zinc, manganese, or iron phosphate solutions. Each of the three types offer a phosphate coating with slightly different properties. This allows a more specialized coating to be selected for the particular application required for the part.

These coatings are usually applied to carbon steel, low-alloy steel, and cast iron. The coating is formed by either spraying or immersing the substrate into a solution of dilute phosphoric acid combined with other chemicals that assist in the phosphate coating process. The phosphoric acid solution reacts with the surface of the metal to chemically form a mildly protective layer of insoluble crystalline phosphate. Phosphate coatings can also be applied to zinc, cadmium, aluminum, tin and galvanized steel, but are difficult to apply on material with high alloys which are often immune to the phosphoric acid.

Zinc and manganese phosphates produce more complex layers of crystals which absorb lubricants more readily. This makes these coatings effective in reducing galling, pick up and scuffing. Iron and zinc phosphates are used as under coatings for steel and other metals prior to painting to enhance paint adhesion and corrosion protection. Manganese phosphate is more resistant to alkalinity and also more suitable for wear applications or where heavy thicknesses are required. All phosphate coatings offer good corrosion protection during the manufacturing process.

Deposit Thickness of Phosphate Coatings

Phosphate coatings can range from less than 0.1 to over 2 mils in thickness. Industry standards usually express the coating requirements by weight per square meter or milligrams per square foot, rather than in actual thickness of the deposit. Coating weight requirements are generally dependant upon the type of phosphate process involved.

Typical Phosphate Coating Weights
  1. Iron phosphates typically yield lighter coating weights of 30 - 80 mg/SF
  2. Zinc phosphates usually yield heavier coatings of 150 - 2500 mg/SF
  3. Manganese phosphates are able to yield the heaviest coatings at well over 3500 mg/SF

Applications For Phosphate Coatings

Heavy zinc phosphate coatings are typically used to retain rust preventive oils and waxes. Zinc phosphate provides maximum corrosion protection and extended service life. The coatings provide good corrosion protection by bonding the final finish (stain, oil, wax or paint) to the treated material. The treatment produces a uniform coating and is particularly good for corrosion protection of threaded areas and parts that have small holes or recesses.

Manganese phosphate coatings are most commonly used because of their excellent break-in or wear resistance properties. They prevent metal-to-metal contact between moving parts and they have extremely good oil retentive properties, which increases both lubricity and corrosion protection. Because of this, Manganese phosphate is a common under coating called up by the military for equipment requiring oil or greased parts.

Another common use for Manganese Phosphate is as undercoating prior to dry film lubricants. The manganese phosphate coatings are very effective corrosion protection.

Iron Phosphate coatings are primarily used as base coatings for paint in order to increase adhesion to the iron or steel substrate. It can also be used for bonding fabrics, wood, and other materials to these surfaces as well. Iron Phosphate coatings are usually applied as part of the painting process. The coatings are so thin that they can corrode very quickly if a post treatment is not applied. To avoid the need for extra cleaning to remove the post treatment when painting is required, the phosphate coating is built right into the painting production line.

Some Applications for Phosphate Coatings

  • Bearings
  • Internal-combustion parts
  • Camshafts
  • Piston Rings
  • Cylinder Liners
  • Pulleys
  • Fasteners
  • Slides
  • Gears
  • Springs
  • Hydraulic components
  • Transmission Gears

To view the properties of phosphate coatings  click here.

Supplementary Post Treatments

Applying a supplementary post treatment after the phosphate is common practice and is even called for in many phosphate specifications. Phosphate has an excellent ability to absorb these supplementary coatings which improves appearance, corrosion resistance, and resistance to abrasion.

While phosphate coatings offer good corrosion resistance, a properly applied post treatment enhances the corrosion protection, while producing a deeper gray-black appearance.

Oil post treatments are one of the most common used after phosphate coating. The oil is absorbed into the crystal of the phosphate coating and also leaves a film of oil on the surface, offering increased corrosion protection and increased resistance to abrasion. The oil can be either water-soluble or solvent-based and can be controlled to allow a heavy oil film on the part or so that the part is virtually dry to the touch. This, however, will decrease the corrosion protection value of the oil post treatment. A special sealer may also be used as an alternative dry to the touch post treatment, where absolutely no oil film is desired.