Phosphate Process – Phosphate coatings use phosphoric acid and take advantage of the low solubility of phosphates in medium or high pH solutions. Iron, zinc or manganese phosphate salts are dissolved in a solution of phosphoric acid. When steel or iron parts are placed in the phosphoric acid, a classic acid and metal reaction takes place which locally depletes the hydronium (H₃O⁺) ions, raising the pH, and causing the dissolved salt to fall out of solution and precipitate on the surface. The acid and metal reaction also creates iron phosphate locally which may also be deposited. In the case of depositing zinc phosphate or manganese phosphate, the additional iron phosphate is frequently an undesirable addition to the coating.

The acid and metal reaction also generates hydrogen gas in the form of tiny bubbles that adhere to the surface of the metal. These prevent the acid from reaching the metal surface and slows down the reaction. Therefore, sodium nitrite is frequently added acting as an oxidizing agent that reacts with the hydrogen to form water. This is known as a nitrate accelerated solution. Hydrogen is prevented from forming a passivating layer on the surface by the oxidant additive.

The following is a typical phosphating procedure:

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

Phosphate coating is significantly dependent on the crystal structure and 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. A common additive is used to seed the metal surface with tiny particles of titanium salts by adding these to the rinse bath preceding the phosphating. This is known as activation.

Uses For Phosphate Coating – Phosphate coating is often used for corrosion resistance. However, on its own, phosphate coating does not provide corrosion resistance because the coating is porous. Therefore, oil or other sealers are used to achieve corrosion resistance. Zinc and manganese coatings are used to prevent galling and help break in components subject to deterioration.

With excellent adhesion and electric isolation, phosphate coatings serve as surface preparation for further coating and/or painting. The porosity allows the additional materials 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 which occurs at the interface of the paint/coating and the substrate.^[3]

Zinc phosphate coatings are frequently used in conjunction with sodium stearate (soap) to form a lubrication layer in cold and hot forging. The sodium stearate reacts with the phosphate crystal which bond to the metal surface.  The reacted soap layer then forms a base for additional unreacted soap to be deposited on top so that a thick three part coating of zinc phosphate, reacted soap and unreacted soap is formed. The coating remains bonded to the metal surface even under extreme deformation. The zinc phosphate is abrasive and the soap performs the lubrication. The soap layer must be thick enough to prevent substantial contact between the metal forming dies and phosphate crystal.