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Phosphate coating

Dr. Dmitri Kopeliovich

Phosphate coating (phosphating) is a conversion coating consisting of an insoluble crystalline metal-phosphate salt formed in a chemical reaction between the substrate metal and a phosphoric acid solution containing ions of metals (zinc, iron or magnesium).

Conversion coating is a film of a chemical compound formed in the reaction of the substrate substance with another substance. This reaction distinguishes conversion coating from a conventional coating applied on the substrate surface without changing its chemical state. Examples of conversion coating are Anodizing (electrochemical process of growing oxide film on the surface of anodically connected metal in an acidic electrolyte solution) and Black oxide (coating formed on the metal surface as a result of a chemical reaction of the metal atoms with an oxidizing agent).

Phosphating coatings are applied to steels, cast irons and aluminum alloys in order to increase their corrosion resistance, improve the anti-friction properties (break-in, wear resistance, ant-galling, coefficient of friction) and provide strong adhesion bonding for subsequent painting or other organic coating.

Chemistry of phosphating process

The main components of a phosphating solution are:

  • Phosphoric acid (H3PO4);
  • Ions (cations) of bivalent metals: Zn2+, Fe2+, Mn2+;
  • Accelerator - an oxidizing reagent (nitrate, nitrite, peroxide) increasing the coating process rate and reducing the grain size of the deposit.

When a metal part is immersed into a phosphating solution (for example zinc phosphate) the following chemical reactions start:

Iron dissolves in the phosphoric acid solution:
3Fe + 6H+ + 2PO43- = 3Fe2+ + 2PO43- + 3H2

Consumption of phosphoric acid for the reaction causes reduction of the acidity of the solution in the layer adjacent to the metal surface. Solubility of zinc phosphate in the neutralized solution is lowering resulting in prcipitation of the salt and its deposition on the substrate surface:
3Zn2+ + 2PO43- = Zn3(PO4)2

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Zinc phosphate

Zinc phosphate coating is applied when increased corrosion resistance is required. Zinc phosphate withstands 240 hours of neutral salt test.
A wide range of coating weights may be obtained: from very thin fine crystal films to heavy deposits with weight up to 4 g/ft2 (40 g/m2).
The coating color is gray of different tins: from light to dark. Finer zinc phosphate crystals produce darker color. Dark gray color is also characteristic for the high carbon steel substrates.
Zinc phosphate coatings may be applied by using immersion or spray technique.
Light and medium weight zinc coatings do not require substrate surface activation. The substrate surface should be acid activated prior to heavy coating deposition.
Zinc phosphate is used not only for non-coated Steels and cast irons but also for galvanized (zinc plated) steel parts.

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Iron phosphate

Iron phosphate coating is applied when strong adhesion of a subsequent painting is required.
In contrast to the solutions for Zinc phosphate and Manganese phosphate coatings, in which the metal ions are a constituent of the composition, to the iron phosphate solution iron ions are provided by the dissolving substrate.
Iron phosphate coatings have very fine Grain structure.
Iron phosphate is translucent therefore its color depends on the steel surface quality. The common color is blue or bluish-brown.
Iron phosphate is applied mostly by spray (three-stage or five-stage) method but immersion technique is also used.
The coating weight is typically in the range 20-100 mg/ft2 (0.2-1.0 g/m2).

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Manganese phosphate

Manganese phosphate coatings is applied when wear resistance and anti-galling properties are required. Manganese phosphate also possesses the ability to retain oil, which further improves anti-friction properties and imparts corrosion resistance to the coated parts.
Magnesium phosphate coatings typically have black color with a slight brown tint, intensity of which depends on the content of manganese oxide in the coating.
Manganese phosphate is applied by immersion method. The substrate surface should be acid activated prior to coating.
The coating weight is typically in the range 500-4000 mg/ft2 (5-40 g/m2). Because of their good anti-friction properties and corrosion resistance iron phosphate coatings are widely used for combustion engine parts (camshafts, piston rings, cylinder liners, gear parts), weapon mechanisms and other parts working with friction.

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Stages of phosphating process

  • Cleaning. The part is mechanically cleaned and degreased in an alkaline solution.
  • Hot water Rinsing at about 170ºF (77ºC).
  • Pickling (acid cleaning). Oxide films and rust stains are dissolved in acid.
  • Acid activation (if necessary).
  • Water rinsing.
  • Phosphating by immersion or spraying method. Typical operating temperature is about 150ºF (66ºC). Manganese phosphate coating is applied at 170-200ºF (77-93ºC). The treatment time is varying in the range 2-40 min.
  • Water rinsing.
  • Drying.

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phosphate_coating.txt · Last modified: 2013/12/14 by dmitri_kopeliovich
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