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 spraying or 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 aluminum, zinc, cadmium, silver and tin.
The main types of phosphate coatings are manganese, iron and zinc. Manganese phosphates are used both for corrosion resistance and lubricity and are applied only by immersion. Iron phosphates are typically used as a base for further coatings or painting and are applied by immersion or by spraying. Zinc phosphates are used for corrosion resistance (phosphate and oil), a lubricant base layer, and as a paint/coating base and can also be applied by immersion or spraying.
The immersion method is used for the processing of Manganese phosphate coatings. The steps of immersion include degreasing/cleaning, water rinses, pickling in mineral acid, activation, manganese phosphating, an optional final drying, and lubrication using oils or emulsion. The first step of this method is to degrease and clean, typically with strong alkaline cleaners. Pickling in mineral acid is a useful process described by the removal of oxide, resulting in a clean surface. The water pre-rinse activation allows the cleaning and pickling to take place while avoiding the formation of coarse-crystalline phosphate. Manganese phosphating is performed, typically in a bath of dilute phosphoric acid that is at approximately 95˚C for about 5–20 minutes (the time varies with the state of the surface being coated). The elements that were coated are then allowed to dry, possibly in an oven. The final step of manganese phosphate coating is to lubricate the materials with oil by immersing them in the oil bath and then letting them drain. This results in different thicknesses of oil when different concentrations and types of oil are used. Manganese phosphate can also be used for coatings (which are processed as described above). These coatings are useful for protection against corrosion and wear over time, due to their toughness. Manganese phosphate coating can often be found in the oil and gas industry, firearms and ordnance, aerospace, gears and bearings, and in marine equipment. They are also common in engines, screws, nuts and bolts, washers, and the list continues.
In addition to corrosion resistance, the zinc phosphate coating process can improve the appearance of finished goods through the creation of a uniform gray or black appearance. The process is also a cost-effective alternative to plating or painting, particularly on hidden metal parts.
Zinc phosphate coatings are used to improve corrosion resistance. The zinc phosphate coating process converts the metal surface to which it is applied into a nonmetallic, polycrystalline coating that contains iron, manganese, nickel and zinc phosphates. This process can be used to treat metals individually or in mixed production.
Heavy Manganese Phosphate
FPW offers a dense, fine grained heavy Manganese Phosphate coating to provide a superior deposit for break-in barrier coating for mating parts in final assemblies. The process provides excellent corrosion resistance for long term storage of both oiled and non-oiled parts. The highly developed product produces excellent uniform coverage over difficult to process heat treated and welded parts.
FPW offers a highly functional zinc phosphate system with built in accelerators and stabilizers which deposit a very fine grained zinc phosphate coating on ferrous surfaces. The coating inhibits corrosion and increases the adhesion and durability of subsequent finishes.
Heavy Zinc Phosphate
Heavy coating weights may be produced which comply with DOD-P-16232. The macro crystalline structure provides superior protection with or without oil preservative coatings. This process produces an excellent base for PVC coating of phosphated components. The tight macro crystal structure provides a superior break-in barrier for mating parts in assembled components. The macro crystalline structure provides superior oil preservative retention over the entire part.
Micro Crystalline Zinc Phosphate
This process is available to produce a variety of coating weights suitable for paint adhesion (TTC-490), e-coat, paint or rubber bonding. Accelerated, or calcium modified coatings provide a tight microcrystalline structure. The process can be done by spray application or traditional immersion. A dense microcrystal deposit provides the needed corrosion protection for extended transfer time prior to final paint or rubber/adhesive bond coat.