Specifications & Properties
Phosphating is a chemical conversion coating that transforms the surface of the basis metal into a non-metallic crystalline coating. Phosphate coatings consist of crystalline, non-reflective, water insoluble metal phosphates that are very adherent to the base metal. A phosphate coating converts the metallic surface into a poor conductor. This coating, which is only a few tenths of a micron thick, provides an insulating barrier to the flow of corrosion currents. Phosphate coatings are measured by coating weights such as mg/sqft and not thickness. Phosphates are available in a wide range of coating weights and crystal sizes. Depending on the type of crystal structure these coatings find the following uses:
1.) A base for paint
2.) A base for oil/lube to provide a corrosion resistance coating
3.) An aid to cold forming processes
The corrosion protection given to such metallic surfaces by phosphate coatings alone without any subsequent topcoat is limited. As such, the largest application for phosphate coatings is its use as an intermediate for bonding paints and other such finishes as rubber, vinyl, epoxies, Xylan®, plastisols and powder coatings. The phosphate coatings absorb the paint coatings uniformly resulting in improved paint appearance and paint film. Through its useful life, the phosphate coating acts as a flexible insulator and aids in the resistance of underpaint corrosion and improved paint adhesion if the paint surface is scratched, dented or impacted.
Two common applications may be associated in the following categories:
- Heavy zinc phosphate that has the ability to retain corrosion inhibiting oils, lubes, preservatives, or other specified finishes. MIL-DTL-16232G, Type Z
- Lighter zinc phosphate has a smoother crystal structure and is normally not oiled as it is used as a base for cosmetic paints.
- DOD-DTL-16232, Type Z
- TT-C-490, Various Types and Methods
- DOD-P-50002, Type Z
Coating weights per specification range anywhere between 1000 and 3500 mg/sqft, which may be oiled, waxed or given a coat of liquid organic top coat for outstanding corrosion resistance.
- Chosen for anti galling, wear resistance and the ability to retain oils.
- DOD-DTL-16232, Type M
- DOD-P-50002, Type M
Coating weights per specification range anywhere between 1500 to 4000 mg/ sqft depending upon the steel surface, and the type of surface preparation. Manganese Phosphates absorb and retain oils, and sealing waxes as well as can be coated with a liquid organic topcoat. They provide excellent corrosion resistance, anti-galling and break-in properties on mating wear surfaces.