Phosphate Coatings: Some Basic Points

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

Stephen Rudy, CEFBeginning 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. 

Iron Phosphate

Iron phosphate part.Iron phosphate may be applied to the surface in an immersion or spray application. Parts must first be cleaned to remove surface oils, grease, and shop dirt. This may be accomplished in a separate cleaning step or in a combination of cleaning and phosphate in a single process tank or station. Most iron phosphate coatings serve as a base prior to applying an organic coating, such as a paint or powder coat. Typical coating weights of 25 to 75 (+) milligrams/ft2 can be obtained. These coatings may be a deep blue to a light golden blue in color. Since these coatings are amorphous, they are less absorbent than the crystalline coatings. Because they are non-crystalline, iron phosphates are not absorbents for oils, lubricants, and waxes. Iron phosphate is applied to steel, zinc (plated zinc), and aluminum before the application of organic paint and powder coat. 

That is why iron phosphates are normally just a pre-paint treatment. They are simpler to apply, less sludging, and operate at lower temperatures than manganese and zinc phosphates. 

Zinc Phosphate

Zinc phosphate coating.Zinc phosphate may be applied as an immersion or spray. Five or up to seven-stage units may comprise a zinc phosphate line. Process solutions provide very little cleaning ability. Therefore, a soak or spray cleaner should precede the zinc phosphate. Zinc phosphates provide the best corrosion protection of the types described. In addition, they are very good for cold forming, extruding, drawing, and bending wire, along with paint bonding. Zinc phosphate crystals may range in coating weight from 500 to 3,000 milligrams/ft2. Because they are absorbent, the crystals hold drawing compounds and lubricants on the surface. This gives treated parts exceptional characteristics with regard to drawing and extrusion operations. The lower coating weights of 500 to 750 milligrams/ft2 provide an excellent base for adhesion of paints. The heavy zinc phosphate coatings are typically 1,000 to 3,000 milligrams/ft2. This range supports the absorption and retention of rust preventatives, lubricants, oils, and waxes. Zinc phosphate on steel can be applied for paint adhesion, adhesion of organic coatings, rust proofing, oil retention, and cold forming. Heavy zinc phosphate on steel is preferred for oil retention and rust preventatives. 

Manganese Phosphate

Manganese phosphate coating.Manganese phosphate is applied in an immersion process. It comprises the heaviest range of coating weights for the three processes discussed, 1,000 to 4,000 milligrams/ft2. The structure is coarse and quite crystallized. This permits the crystals to retain more rust-preventative oils and lubricants. Mild steel alloys are treated to improve wear resistance and provide exceptional lubrication. Gears, bearings, and internal engine parts so coated improve break-in and prevent galling. Manganese phosphate is applied on steel and steel alloys for improved wear resistance. Specifically, engine parts, such as camshafts, gears, and piston rings, benefit from the coating’s protection, especially at performance break-ins. This protection is to prevent galling and control wear of the moving parts. 

Manganese phosphates are typically the heavier ones, with a thickness range of 1,000 to 4,000 mg/ft². The crystalline structure is quite coarse and black in color. The unique crystalline structure improves the retention of rust-preventative oils and lubricants. Subsequent corrosion protection is very good, along with minimizing the galling of moving parts.

Zirconium Based Coating

ASTM B117 salt spray comparison of Iron Phosphate (I) and Zirconium Treatment (Z) of powder coated panels.Over the last decade, zirconium-based pretreatment of steel has become very popular and environmentally compliant. Zirconium coatings have effectively increased corrosion protection per ASTM testing on paint and powder coats over treated steel, galvanized steel, and aluminum. Concerns of phosphates discharged into water are eliminated as most Zirconium processes are 100% phosphate-free. The reaction contains no accelerators (as iron and zinc phosphates require). It is an ambient temperature process. The coating weight range is 5-25 mg/ft². Low temperature is a significant energy cost savings. To date, the only cautions are to maintain proper in-range operating parameters to avoid flash rusting and filtration to remove precipitated iron. Filtering in itself can significantly extend the operating bath service life. 

Applications of Phosphate Coating Systems 

Typical phosphate bath operating parameters

Applications of Phosphate Coating Systems 

Phosphated parts exhibit significant corrosion protection as per ASTM B-117 salt spray. It is not surprising, therefore, that phosphate panels that have been painted or powder coated may improve salt spray resistance ten times versus the same panels without the underlying phosphate coating.

Some Processing Applications

• MIL-C-46487, Iron Phosphate 
• MIL -DTL-16232G, Heavy Manganese or Zinc phosphate
• MIL-HDBK-205A. Phosphate Coating on black oxidized steel

Process Control

Of the three phosphates, iron phosphate by analysis is the simpler control procedure. Titration and pH measurement. Zinc and manganese phosphates require three separate titrations: total acid, free acid, and iron. The ratio of total to free acid is critical.

Testing Phosphate Coatings

Sample phosphate-coated panels are accurately weighed on an analytical balance. Then, the coating is 100% stripped in a special no-etching solution, dried, and re-weighed. The difference in weight is reported as the coating in milligrams/ft². 

Testing for coated parts: 100% Humidity test according to ASTM D2247. Salt fog corrosion according to ASTM B117. Water Immersion per ASTM D870.

In addition, there are other specific testing procedures that include adhesion and chip resistance.

Stephen F. Rudy, CEF, is president of Chem Analytic and has written extensively about the finishing industry. Visit www.chemanalytic.com or call him at 917-604-5001.