Phosphating is surface modification of substrate materials before coating with others process. There are many application of Phosphating such as anti – corrosion, based for organic coating, anti-sizer for cold forming and wear resistance.
After surface was cleaning by suitable techniques, metal surface will be reacted with Phosphating chemical and generate an even crystal of the Phosphate compound film.
A typical phosphating reaction proceeds in a primary zinc phosphate/
phosphoric acid system at elevated temperature; the primary phosphate
is then converted into a tertiary zinc phosphate.
3Zn(H,P04), e Zn,(PO,), + 4H3P0, (1)
When a steel surface is dipped into the phosphating solution, it is
attacked by the free phosphoric acid which liberates hydrogen and thus
raises the pH of the solution, especially along the metal surfaceglO.
Fe + 2H3P04 –t Fe(H,PO,), + H, (2)
thus reaction (1) is driven to the right and the insoluble zinc phosphate
precipitated on the steel substrate surface.
The overall reactions can be simplified as follows:
Fe + 3Zn(H2P0,) -+ Zn,(PO,), + Fe(H,PO,), + 2H3P0, + H,
Zn,(PO,), . 4H,O
During the process of precipitation, the zinc phosphate is transformed
into an insoluble hopeite with 4 water molecules