When coating a metal part, phosphatizing can provide two big benefits: corrosion resistance and superior powder adherence.
When considering what type of phosphate process to use, you need to consider these things:
- What is your part made of?
- What elements (salt, UV rays, etc.) will the finish on the part need to resist?
- What is your budget?
Iron is affordable, very durable, extremely popular, and will suit most needs. Zinc is more complex and expensive, but in many cases offers advantages to the finished product. Chrome phosphate requires a complex process, but is used almost exclusively for aluminum parts.
How does it work? In a nutshell, when a cleaned part is washed in a certain type of acidic solution, it changes the surface of the metal. It seals it and deposits iron, zinc or chrome onto the surface. This will prevent corrosion by sealing the metal surface. It also provides a microscopically rough surface for coating media to firmly embed itself into.
Metal parts have numerous areas that retain tiny positive and negative charges. These areas are sometimes caused by stresses in the metal that occur during formation of the metal or the manufacture of the parts. They may also occur because of high concentrations of carbon in the steel that the parts are made from. When enough moisture is present (if the material is stored in a normal environment), oxygen in the air causes these areas to act somewhat like terminals on a battery, and galvanic corrosion begins to form. Phosphatizing during pretreatment results in a non-metallic coating being bonded to the surface of the metal parts. This relatively insoluble phosphate coating on the metal surface is non-conductive and insulates the positively and negatively charged areas from each other .As a result, the process of galvanic corrosion is substantially retarded, but is not completely eliminated.
The phosphate coating is relatively rough, so it also increases the surface area of a part because of its textured finish. This increases the mechanical adhesion of the film of powder that will be applied to the part.
These benefits make phosphatizing a very popular pretreatment process. It gives metal parts excellent corrosion protection and enhances the adhesion of powder coating media that will be applied later in the coating process.