Zinc Phosphate Coating is typically known as Phosphating. Phosphating Process is the crucial step before paint shop for every powder coating industry. It is popular because of its corrosion resistance property. But what are the parameters which made it so famous? Many people don’t know what reaction process takes place during the conversion coating. Here in this blog, we will try to discuss its chemistry with the metal substrate.
Zinc is more reactive than ferrous. When ferrous coated with Zinc, Ferrous become protected by Zinc and Zinc is exposed to open atmosphere. Because of Zinc’s inherent property to form corrosion byproduct that reduces the corrosion rate of ferrous metal. These byproducts of Zinc on the top surface of a metal considered as corrosion barriers.
Here is the pictorial representation of Zinc Phosphate Coating reaction with a metal substrate.
In this picture, a reaction is taking the place between ferrous metal and zinc phosphating solution. We will learn about this reaction step by step.
Zinc Phosphate solution is present here in the form of a complex of Zinc, Orthophosphoric Acids and other required ingredients. But for easy to understand only Zinc as Zn & Orthophosphoric acid as H3PO4 is showing.
First ZnH3PO4 complex dissociates into Zn2+ and H3PO4 & H3PO4 is dissociated in H+ and H2PO3-
The ferrous metal which is present in solid form releases it two electrons 2e. These 2e taken by Two H+ ions and will form H2 Hydrogen gas.
Then insoluble Zn2+ will fill the vacant place of Fe2+ and protect the ferreous outer surface with its inherent corrosion property.
Remaining Fe2+, few Zn and H2PO3- will form an insoluble precipitate. Its chemical formula will be Zn3PO4. It is also known as sludge.
So this is the reaction of ferrous and Zinc Phosphate in non-electrolytic solution. For the electrolytic solution, there will be some changes; we shall discuss it in our other blog.
This reaction is a continues reaction, means if the ferrous substrate presents in Zn bath solution, a coating thickness will increase with time. This coating thickness is also regulatory for better paint life. The standard range of coating thickness is 6-7 micron. Coating thickness higher than the given range can crack the coating itself.
There is also a coating weight parameter in Zinc Phosphate. The coating weight on a ferrous substrate should be 1.6–4.5 g/m2 for better paint life. A successful coating can also check by thumbnail mark.
The Zinc Phosphating process is a big subject, and it could not complete in a single blog. I will write more blogs on its subchapters. Now I am ending this blog with the hope that you have added more knowledge in the corrosion resistance area.