Granodizing metal is a commercial name for phosphating metal. This is a method of protecting a metal surface from corrosion and increasing its resistance to wear through the application of an electrochemical phosphate conversion coating. It produces "pickling" effect and chemical conversion, through spraying or phosphate and phosphoric acid bath immersion.
Granodizing metal is a transformation of metal surfaces into new surfaces having non-metallic and non-conductive properties. It is widely used in the manufacture of metal products for the following principal reasons:
- Precondition metal surfaces
- Improve corrosion resistance
- Prevent scuffing during bedding in sliding parts
Granodizing is also known as Parkerizing, bonderizing, phosphatizing and Asfocoat.
Granodizing metal produces a uniform layer of phosphate crystals of various thickness and shape, chemically bonded to the substrate, conferring new properties to the processed metal:
- Excellent wear resistance
- Friction reduction
- Anchorage for plastic material application
- Perfect painting adherence
The granodizing coating on metal is formed as a result of a topochemical reaction, which causes the surface of the base metal to integrate itself as a part of the corrosion-resistant film. It improves the corrosion resistance of a steel surface.
It is a process of chemical conversion on a metal surface to produce thin adherent phosphate compound coatings. Phosphate coatings are usually applied to:
- Carbon steel
- Low-alloy steel
- Cast iron
Phosphate processes are difficult to apply on high alloys because these alloys are likely immune to the phosphoric acid. Therefore, phosphating is one of the most useful non-metallic coatings.
Phosphate coatings for corrosion protection are usually given a final rinse with aqueous solutions containing chromium, or certain organic compounds, including tannins.
This process consists of immersing the metal articles into a hot solution containing ferrous phosphate with some free phosphoric acid, and with additions of zinc and/or manganese phosphates. As the phosphoric acid in the solution attacks the steel, the local acidity is reduced and crystalline films of iron phosphate precipitate onto metal, which gives corrosion protection.