The Basics of Phosphate Conversion Coatings

The phosphate process is the treatment of a metal surface that produces a reasonably hard, electrically non-conducting surface coating of insoluble phosphate that adheres to the underlying metal. It is used to enhance corrosion resistance, lubricity or as a foundation for subsequent coatings or paint. Zinc and manganese phosphates are also used to help break in parts subject to wear and to prevent galling. Zinc phosphate is often used with sodium stearate for lubrication in forging processes. It may be applied to a variety of substrates.

Main Types of Phosphates

The main types of phosphates are iron, zinc and manganese. Manganese phosphates are used for corrosion resistance, wear resistance and lubricity. They are applied by an immersion process. Iron phosphate is typically used as a base for further coatings and may be applied via spray or immersion. Zinc phosphate is primarily used for corrosion resistance (Phos & Oil), wear resistance, lubricity or as a base for subsequent coatings. It may be applied by spray or immersion.

The phosphate coating, in itself, provides limited corrosion resistance and consequently is often used in conjunction with sealers, coatings, paints or oil.

The Phosphate Reaction

The application is the result of phosphate salts dissolved in a phosphoric acid solution chemically reacting with the surface of the part to form an insoluble, crystalline phosphate. When the iron, zinc or manganese salts dissolved in a phosphoric acid solution meet a metal surface, a metal/acid reaction takes place locally. This reaction depletes the hydronium ions, raising the pH of the solution. As phosphates have low solubility in medium or high pH solutions, the dissolved phosphate salts fall out of the solution and precipitate on the component’s surface. The acid / metal reaction also creates iron phosphate ( on ferrous substrates ) which may also be deposited. In the case of manganese and zinc phosphate, the iron phosphate may be considered undesirable. As the reaction takes place, it generates hydrogen gas bubbles which may adhere to the surface. This will slow the deposition process. To address this issue, an oxidizing agent may be added to react with the hydrogen to form water and prohibit a reduction in the speed of the process.

Performance of the phosphate coating is dependent on the crystalline structure and the coating weight. Micro crystalline structures are generally best for subsequent painting. A course grain structure is more desirable for applications that will see oil or requiring wear resistance.

Factors affecting the phosphate coating include concentration, temperature, time and additives.

Phosphate Coating Application

A standard application would be as follows:
1. Clean
2. Rinse
3. Surface activation (optional)
4. Phosphate
5. Rinse
6. Neutralizing rinse (optional)
7. Dry
8. Coating, painting, lubricant, sealer, oil

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