This coating is an adequate protection against corrosion and abrasion in its own right, but additionally acts as a sponge to absorb oil for greater corrosion resistance and is an excellent primer coat for paint. Most commonly, it is an immersion process, but it can also be applied in a spray. Phosphates have excellent anti-galling properties as well. You cannot phosphate stainless steel, aluminum, brass, or copper. Steels with high nickel content such as some older Winchester parts, may not take as well.
Manganese phosphate runs a medium to charcoal gray, as does iron phosphate.
Nevertheless, the color of manganese phosphate is most popular with our customers. Another benefit of phosphate coating is that the low operating temperature allows parts permanently bonded to plastics to be coated without melting as they might in baked-on coatings and hot bluing. HK cocking handles and retractable stocks, and StG-58 cocking handles are examples. One application of phosphate coatings that has been particularly useful in my business is to test questionable parts. There are some receivers and parts that are advertised as originals but are actually welded together from demilled components.