Phosphate conversion coating

Phosphate coatings are used on steel parts for corrosion resistance, lubricity, or as a foundation for subsequent coatings or painting. It serves as a conversion coating in which a dilute solution of phosphoric acid, which is applied via spraying or immersion, chemically reacts with the surface of the part being coated to form a layer of insoluble, crystalline phosphates.^[1] Phosphate conversion coatings can also be used on aluminium, zinc, cadmium, silver and tin.^[2]

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Phosphate and oil coatings

Phosphate coatings are porous, so they must be treated with oils or other sealers in order to provide corrosion resistance. P&O (phosphate and oil) coatings are frequently used for this purpose, for lubricity and to prevent galling.^[1]

Base for painting and coating

Most phosphate coatings serve as a surface preparation for further coating and/or painting, a function it performs effectively with excellent adhesion and electric isolation. The porosity allows the additional materials to to seep into the phosphate coating and become mechanically interlocked after drying. The dielectric nature will electrically isolate anodic and cathodic areas on the surface of the part, minimizing underfilm corrosion that sometimes occurs at the interface of the paint/coating and the substrate.^[1]

Types

The main types of phosphate coatings are manganese, iron and zinc. Manganese phosphates are used both for corrosion resistance and lubricity and are applied only by immersion. Iron phosphates are typically used as a base for further coatings or painting and are applied by immersion or by spraying. Zinc phosphates are used for rust proofing, lubricity (P&O), and as a paint/coating base and can also be applied by immersion or spraying.^[1]

Performance

The performance of the phosphate coating is significantly dependent on the crystal structure as well as the weight. For example, a microcrystalline structure is usually optimal for corrosion resistance or subsequent painting. A coarse grain structure impregnated with oil, however, may be the most desirable for wear resistance. These factors are controlled by selecting the appropriate phosphate solution, using various additives, and controlling bath temperature, concentration, and phosphating time.^[1]

Chemistry

Phosphating solutions are based on phosphoric acid, which attacks the surface of the substrate and caused some of the metal atoms to go into solution. This neutralizes the acid solution in a thin zone along the metal-bath interface, resulting in a lower solubility of the metal phosphates and the subsequent precipitation of them onto the surface of the workpiece by electrostatic forces.

In addition to phosphoric acid, phosphating solutions contain divalent metal phosphates and chemicals that serve to increase the rate of coating buildup by removing hydrogen from the surface of the substrate. Hydrogen is a by-product of the chemical reaction and can stall the process by aggregating to preventing the bath from coming into contact with the metal.^[1]

Process

A typical phosphating procedure consists of performing the following on the substrate:^[1]

1. cleaning the surface
2. rinsing
3. surface activation (in some cases)
4. phosphating
5. rinsing
6. neutralizing rinse (optional)
7. drying
8. application of supplemental sealers, oil, etc.

ASTM B633 Type IV specifies zinc plating plus phosphate conversion on iron and steel parts.

See also

• Metallurgy
• Phosphates

Notes and references

1. ^ ^{a b c d e f g} Dufour, Jim (2006). An Introduction to Metallurgy, 5th ed. Cameron, IX 11-12. 
2. ^ Edwards, Joseph (1997). Coating and Surface Treatment Systems for Metals. Finishing Publications Ltd. and ASM International, pp. 214-217. ISBN 0-904477-16-9.