Phosphate coating is a crystalline conversion coating that is formed on a ferrous metal substrate. It is employed for the purpose of pre-treatment prior to coating or painting, increasing corrosion protection and improving friction properties of sliding components. In other instances, phosphate coatings are applied to threaded parts and top coated with oil (P&O) to add anti-galling and rust inhibiting characteristics. The phosphating sequence is normally broken down into the following stages– degreasing and cleaning, derusting and descalling, activation, phosphating, and post-treatment. The phosphoric acid solution reacts with the surface of the metal to chemically form a mildly protective layer of insoluble crystalline phosphate. Phosphate coatings can also be applied to zinc, cadmium, aluminum, tin and galvanized steel, but are difficult to apply on material with high alloys which are often immune to the phosphoric acid. Factors to be considered in this type of coating are nature of the metal to be coated, shape and surface condition of the metal, and number of parts and uniformity of their surface state.
Another type of coating is the Manganese Zinc which can also be called as Manganese Zinc coating. Zinc and manganese phosphate coatings are the treatment of iron or steel by immersion in a dilute solution of phosphoric acid and other additives. In the resulting chemical reactions, the surface of the metal is chemically converted to an integral protective layer of insoluble zinc and iron or manganese and iron phosphate crystals. Depending on the physical characteristics of the substrate and the pre-treatment methods used, the translucent crystals appear black to light grey in color for zinc phosphate and black to dark grey in color for manganese phosphates. Material to be coated is cleaned by immersion in a hot alkaline solution that removes most oils and loose soil. If surface oxides are present, the parts are then stripped in an acid-cleaning step that undercuts the rust or scale, exposing the bare metal beneath.
The work is then rinsed thoroughly and coated in a chemically balanced hot phosphoric acid solution via an autocatalytic reaction. The temperature, time and chemical composition of this bath must be carefully controlled to produce consistent results. After coating is completed, excess acid is neutralized and a supplementary treatment is applied if required. Material to be zinc or manganese phosphated may either be racked or bulk-processed in barrels, depending on the specifications of the end user. Normally threaded parts, soft alloys, or parts which weigh in excess of 6 ounces are racked to avoid nicks, distortion, and coating damage which result from bulk handling practices. The purposes of this type of coating are for lubrication during cold forming, corrosion resistance, torque-tension requirements, bonding organic coatings to metals and to reduce break-in wear on adjacent moving surfaces. This coating is commonly used by hydraulic system manufacturer, military armament suppliers ,rod-by-coil manufacturers, nuclear component fabricators, cold-forming industries, sports equipment manufacturers, automotive fastener companies, aerospace industries, motorcycle specialty groups, marine equipment vendors, various stamping applications, diesel engine manufacturers, and electrical connector manufacturers.