Phosphate coating is the process of chemically converting the surface of a material referred to as the substrate (typically a metallic element) into a layer of a non-metallic, crystalline conversion coating. Phosphate coating is an important process in many industrial contexts.
There are four main reasons that phosphate coating services are utilized: one, in order to enhance the lifetime of organic coatings; two, in order to provide superior paint or solid film bonding; three, to increase the corrosion-resistance of the material; and four, to provide an excellent lubricant and drawing compound base. Due to the multiple purposes of phosphate coatings, there are a correspondingly vast amount of applications and industries that they are used in, including: automotive, for use in the coating of various power transmission components; industrial manufacturing, to coat castings, iron and steel-based industrial parts; military, for the coating of various defensive moving and threaded parts according to military specifications (MIL-Spec); and aerospace, for adhesive and defensive coatings of various aviation components. Also used in the nuclear energy industry, there are many advantages that come from utilizing phosphate coating services including: improves friction, increases corrosion-resistance and provides both rust-inhibiting and anti-galling characteristics.
Phosphates refer to a salt (also called an ester) of phosphoric acid and typically include three main types: manganese, zinc and iron. In the general process of phosphate coating, phosphate salts (any of the three) are dissolved into a solution of phosphoric acid. Next, the substrate is immersed within the solution, causing a reaction to take place between the chemicals and the metal. This chemical reaction accomplishes four things: one, it locally depletes the hydroxonium ions; two, it raises the pH; three, it causes the dissolved salt to fall out of the solution; and four, it causes the dissolved salt to be precipitated on the surface of the substrate. While the general process remains basically the same, there are some differences between phosphate coatings depending on the type of phosphate salt that is used in the process. In manganese phosphating, the resulting coat specifically exhibits exceptional oil absorptive characteristics and is often achieved in tumbling barrels. In zinc phosphating, heavy zinc phosphates are commonly used in order to drastically improve the corrosion-resistance of the substrate. Lastly, in iron phosphating a lot depends upon the receptiveness of the substrate's surface, which can affect whether the coating is very fine, or it is coarse. This type of coating is different from metal finishing because it does not focus on the aesthetic appearance of the finished piece.