Zinc 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 and phosphate salts is applied via immersion and chemically reacts with the surface of the part being coated to form a layer of insoluble, crystalline phosphates. Phosphate conversion coatings can also be used on aluminium, zinc, cadmium, silver and tin.

Zinc phosphates are used for rust proofing, a lubricant base layer, and as a paint/coating base

Process

The application of phosphate coatings makes use of phosphoric acid and takes advantage of the low solubility of phosphates in medium or high pH solutions. Zinc 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 hydroxonium (H3O+) ions, raising the pH, and causing the dissolved salt to fall out of solution and be precipitated on the surface. 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. To overcome this sodium nitrite is frequently added to act as an oxidizing agent that reacts with the hydrogen to form water. This chemistry 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
7. drying
8. application of supplemental coatings: lubricants, sealers, oil, etc.

The performance of the phosphate coating is significantly dependent on the crystal structure as well as the weight. FMF’s processes are deisgned to produce a microcrystalline structure that is usually optimal for corrosion resistance or subsequent painting.

Uses

Phosphate coatings are often used to provide corrosion resistance, however, phosphate coatings on their own do not provide this because the coating is porous. Therefore, oil or other sealers are used to achieve corrosion resistance. This coating is called a phosphate and oil (P&O) coating.

Zinc phosphate coatings serve as a surface preparation for further coating and/or painting, a function it performs effectively with excellent adhesion and electric isolation.