Phosphate Coating Process
The Phosphate Coating are used on various steel parts for the purpose of converting it to become corrosion resistant. It is also used for the lubricity of the steel parts, as well as for the foundation of subsequent coatings and paintings. Furthermore, the Phosphate Coating are also used as a conversion coating wherein a dilute solution of the phosphoric acid along with phosphate salts is applied by means of spraying or by immersion until it chemically reacts with that area of the part that is being coated in order to come up with a layer of insoluble and crystalline phosphates. The coating is sometimes used for zinc and manganese, and in this case, it will be called the Manganese Zinc Coatings.
The main varieties of the Phosphate Coating are the iron, manganese and zinc. The iron phosphates are usually used as a base for further painting and coatings and are often applied through spraying or by immersion. On the other hand, the Manganese Zinc Coatings are used for both lubricity resistance as well as corrosion resistance and are usually applied through the immersion process. The zinc phosphate is used as a corrosion resistance as well, ideal for both the oil and the phosphate. Sometimes, it is used as a paint or coating base and a lubricant base layer.
The process of application of the Phosphate Coating would involve the use of phosphoric acid. It also makes use of the low solubility of the phosphates with a medium or high pH solution. The iron, manganese or zinc salts are then dissolved in a phosphoric acid solution. When the steel or iron components are placed in the acid, a metal reaction will take place. This reaction will locally deplete the hydronium ions and will then raise the pH level which will cause the dissolution of the salt to fall out of the solution and then it will precipitate on the surface. The acid as well as the metal reaction will also create an iron phosphate locally which might also be deposited. When it comes to the Manganese Zinc Coatings, the additional iron phosphate will become an undesirable addition to the coatings.
A typical phosphating procedure would often start by cleaning the surface. After that, rinsing will take place and then the surface activation will soon follow. After this, the Phosphate Coating will have to take place. Rinsing will be done again and then a neutralizing rinse will be done, which is optional. The final stage would require drying the material and then the application of supplemental coatings which is often in the form of sealers, oil, lubricants, etc.
It should be noted that the performance of the Phosphate Coating will depend mainly on the crystal structure as well as the weight. A microcrystalline structure is often optimal for corrosion resistance and subsequent painting. The coarse grain structure will then be filled with oil although it might be the most desirable in order for it to become wear resistance.