The process of Phosphate Coating the steel and aluminum parts is often considered as a type of conversion coating since the process will involve the removal of metal as being part of the reaction. However, the process is not similar to anodizing or black oxide where the phosphating is often due to a precipitation reaction. The final surface will be a layer of very fine phosphate crystals that adheres right to the surface of the metal.
When it comes to the paint as well as powder coatings, the Phosphate Coating will have two functions. First is that it provides an improved adhesion for the paint and power coating because the phosphate crystals will act as organic coating anchoring the sites. Second, the phosphate layer will act as corrosion barrier in case the organic coating will be scratched. When it comes to rust creep testing, the rust creep will be reduced if the phosphate is present right below the paint layer or the powder coat layer as compared to no conversion layer right below the organic coating. The Phosphate can be used as either a standalone coating for other purpose like lubricity on parts forming, but other functions are way beyond the scope of this article.
Most Common Phosphating Chemistries
The most common phosphating chemistries are zinc phosphate, iron phosphate, as well as manganese phosphate. There are also several other phosphating chemistries like Plaforizing, which is nontraditional when it comes to their chemistry as well as their application because they are a single step and are usually known as an organo phosphate that will react in both organic contaminants as well as the metal surface.
Lessen the Temperature Requirements
The main thrust in the recent years when it comes to improving the Phosphate Coating process is to lessen the temperature requirements used for the phosphate bath. Some other chemistry was developed in order to work well at a certain room temperature. Generally, there have been trend coming from high temperature, which is from 90 degree Fahrenheit to as much as 200 degree Fahrenheit, up to much lower temperatures, such as 70 degree Fahrenheit up to a maximum of 140 degree Fahrenheit that results to energy savings.
Most of the Phosphating lines are the single tank, which are a three step and five step processes. The three and five processes are the following.
Three Step Process
1. Clean or Phosphate
3. Rinse and Seal
Five Step Process
3. Activated Rinse
The single step process is more conservative on both energy and water as compared to the three and five stage processes. As for the Enviroprep of the Calvary Industries, the chemistry will be phosphate free, yet it still serves as surface preparation stage for powder and paint coating.
The three and five step processes of the Phosphate Coating can often be made with more water efficient to flow optimization of the rinse recirculation, rinses, rinse counter flow and in some instances, reactive rinses. Both of the three and five step processes can be made of energy efficient by changing to the lower temperature phosphating chemistry.