Industrial Surface Chemistry

Phosphate Coatings: Some Basic Points

The applications of paints and powder coatings have become more pronounced over the years.

Beginning in the 1980s into the 2000s, it was estimated these organic coatings had, in the time span, replaced plated finishes by approximately 65-80% on selected types of parts. A variety of parts that comprise products in consumer, automotive, aerospace, military, and construction abound, along with other end users. In everyday use, we are accustomed to seeing and using consumer items such as shopping carts, shelving, and especially the vehicles we ride in. Organic coatings continue to be in demand and popularly applied.

Iron phosphates are employed in multi-stage spray lines or in a cycle of dip tanks. Zinc and manganese phosphates are typically applied in immersion lines, although zinc phosphate treatments can also be sprayed.

Let us review some basic facts and guidelines for the common types of phosphate coatings.

What Are Phosphate Coatings?

After surface preparation (cleaning), metal parts are immersed, sprayed, or brushed on with an acidic solution, specially blended to react with the particular basis metal. This reaction forms a product that is a modified metal phosphate species. It can be referred to as a crystalline or amorphous material. The crystalline deposit that forms comprises insoluble salts that continually form on the metal part surface. Amorphous coatings are not crystalline but rather of a different formation. This deposit also forms on the metal parts’ surface. In both instances, there is a minor etching of the basis metal. Phosphate coatings improve the wear resistance, lubricity, corrosion protection, and service life of finished parts.

Crystalline phosphates absorb oils, thereby improving the corrosion resistance of processed coils and parts such as nuts and bolts. Iron, manganese, and zinc phosphates are typically used for most of these practical applications.

The coating forms a tight bond to the metal surface. It can be relatively thin, ranging from approx. 40 mg/ft2 to upwards of 4,000 mg/ft2, but uniform and dense. The coatings provide an excellent base for paints and powder coats, corrosion protection, accept lubricants and conditioners, enhance abrasion resistance, as well as accept waxes and rust-preventative oils. Because of their nature, phosphate coatings are quite resistant to chipping and abrasion.

The main types of phosphate coatings are iron, zinc (light to heavy), and manganese. Light to moderate zinc phosphates are crystalline, developing a range of coating weights from 500 to 3,000 mg/ft2. The coating tends to be smoother as the weight per square foot decreases. These coatings provide the best corrosion protection compared to the other phosphates. That is why they are ideal as a pretreatment before paints and powder coats.

In addition, zinc phosphates are excellent surface modifiers prior to cold forming. Having superior lubricant and drawing compound capacity holding capacity makes them ideal for metal extruding and forming dies, which last longer due to reduced friction. This superior capillary action readily absorbs and retains the fluids mentioned.

The application of phosphates is specific to the type of coating.

Read more: Phosphate Coatings: Some Basic Points

Contact Us

Contact Us

Interested in our services? Please leave your details and a represenative will assist you.

Contact Us ×
Contact Us