Phosphate coatings are a crystalline conversion coating that is formed on a ferrous metal substrate. Phosphate coating is employed for the purpose of pretreatment 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 process relies on the basic pickling reaction that occurs on the metal substrate when the process solution comes in contact with the metal. The main benefits that phosphating provides is strong adhesion and corrosion protection. Typically, phosphate coatings used on steel parts but can also be used on aluminum.
Phosphate Coating & Phosphating
Manganese Phosphating
Metal Coatings offers manganese phosphate coatings (Type M) used for corrosion protection, anti-galling and lubricity. Of the numerous phosphate coating available, manganese phosphate coatings are the hardest, while providing unbeatable corrosion and abrasion protection. In comparison to zinc phosphate coatings, manganese phosphate coatings offer continued wear protection after the breaking in of components that are subject to wearing. These coatings are applied only by immersion. Uses for manganese phosphate applications include the production of bearings, bushings, fasteners and other common industrial products. Use of manganese phosphate is especially useful in projects that require sliding of parts, such as automotive engines and transmission systems
Zinc Phosphating
Zinc phosphate coatings (Type Z) are also available and are mainly used for rust proofing on ferrous metals. They can be applied by immersion or spraying. Zinc phosphate is a lighter alternative to manganese phosphate, while providing resistance to harsh elements that tend to wear products quickly.
| Phosphate Coating Statistics | |
| Colour | dark gray / black |
| Coating Weight | 2000-3000 mg / sq. ft |
| Top Coats | Oil or Paint |
Phosphate Specifications
MIL-DTL-16232G, TYPE M CL 1 | MIL-DTL-16232G, TYPE M CL 2 | MIL-DTL-16232G, TYPE M CL 3 | MIL-DTL-16232G, TYPE Z, CL 1 | MIL-DTL-16232G, TYPE Z, CL 2 | AMS 2481 | ASTM B633 TYPE II | CA-G102 PARA 8.1 CLASS A | GM 4435-M, CODE A | GMW 3179, CODE A | ISO9717 | MIL-DTL-16232G, TYPE Z, CL 3 | MIL-STD-171 | DOD-P-16232 | P 1292 G00 REV A | PPS 32.06 | CSP-16 | EEPS-27 | TT-C-490, TYPE I | WSS-M3P36-A2 | ZFN 2016 | 223.011-41
Uses
Phosphate coatings are often used to provide corrosion resistance, however, phosphate coatings on their own do not provide this because the coating is porous. Therefore, oil or other sealers are used to achieve corrosion resistance. This coating is called a phosphate and oil (P&O) coating. Zinc and manganese coatings are used to help break in components subject to wear and help prevent galling.
Most phosphate coatings serve as a surface preparation for further coating and/or painting, a function it performs effectively with excellent adhesion and electric isolation. The porosity allows the additional materials to seep into the phosphate coating and become mechanically interlocked after drying. The dielectric nature will electrically isolate anodic and cathodic areas on the surface of the part, minimizing underfilm corrosion that sometimes occurs at the interface of the paint/coating and the substrate.
Zinc phosphate coatings are frequently used in conjunction with sodium stearate (soap) to form a lubrication layer in cold and hot forging. The sodium stearate reacts with the phosphate crystal which in turn are strongly bonded to the metal surface. The reacted soap layer then forms a base for additional unreacted soap to be deposited on top so that a thick three part coating of zinc phosphate, reacted soap and unreacted soap is built up. The resulting coating remains adhered to the metal surface even under extreme deformation. The zinc phosphate is in fact abrasive and it is the soap which performs the actual lubrication. The soap layer must be thick enough to prevent substantial contact between the metal forming dies and phosphate crystal.
- Formation of the inter metallic layer due to the chemical reaction between iron in the steel and aluminium in the coating.
- The inter-layer is affected by the dipping time and aluminizing temperature.
- The carbon content of the steel substrate may also have a marked effect on the growth rate and morphology of the inter-metallic layer
- The thickness and morphology of the inter-metallic layer may be profoundly affected by additions to the molten bath, such as silicon.
- The viscosity of the molten metal may influence the wetting of the substrate.
- The roughness of the substrate surface determines the amount of layer of molten metal that adheres to the substrate during withdrawal from molten bath.
- Other factors are temperature of the metal and the time during which the coating layer is still liquid and free to draw off. During this time, the coating cools and solidifies and the intermetallic layer grows by consuming the outer layer of aluminum completely converting it to an alloy.