Phosphate Coating is a type of crystalline conversion coating that will be formed in a ferrous metal substrate. The coating is used for the main purpose of pre-treatment that will be done right before painting or coating and for increasing corrosion protection as well as for improving the friction properties of the sliding components. In some instances, the Phosphate Coating is being applied on threaded parts and it is being top coated with a specific type of oil in order to add an anti-galling characteristic as well as rust inhibiting characteristics for the metal substrate.
Process of Phosphating
The process of Phosphating basically relies on the basic pickling reaction that will occur on the metal substrate if the process solution will come in contact with metal. The biggest benefit that Phosphating can give is corrosion protection and strong adhesion. Just like Phosphate Coating, the process of Phosphating is being used for steel parts although it is also sometimes being used for aluminum.
In some cases, companies that offer Phosphate Coating also offer Manganese Phosphating. This process is basically used as a corrosion protection and for giving a metal part a lubricity and anti-galling properties. Of the various Phosphating process available, it is the manganese Phosphating that is considered to be the hardest as it provides unbeatable abrasion and corrosion protection. Just like with the Zinc Phosphate Coating, the manganese Phosphating offers a continuous wear protection right after the breaking of components that are being subjected to wearing. The coatings can also be applied by means of immersion and the use of Manganese Phosphating will include the production of bushings, bearings, fasteners as well as other common industrial products. Manganese Phosphating is very useful for those projects that will require the sliding of parts, like the automotive engines as well as transmission systems.
As mentioned, another process of Phosphate Coating is called the Zinc Phosphating. This process is being used as a means to rustproof ferrous metals. They can be applied through a process called immersion also known as spraying. As you know, Zinc Phosphate is a lighter alternative to Manganese Phosphating and provides resistance to harsh elements that could wear products very quickly.
Process of Application
The application of Phosphate Coating will require the use of Phosphoric Acid while taking advantage of the low solubility of the phosphates in medium or high pH solutions. Zinc, Manganese or Iron phosphate sales are being dissolved to a solution of the Phosphoric acid. If the iron or steel parts are being placed on the Phosphoric acid, the Classic acid as well as metal reaction will take place that locally depletes the H3O+ or hydronium ions which increases the pH and cause the dissolved salt to fall off the solution and will be precipitated right on the surface. The acid as well as the metal reaction will also create iron phosphate locally that might also be deposited. In the event of depositing the Zinc or Manganese Phosphate, an additional Iron Phosphate could become an undesirable addition to the process of Phosphate Coating.