Phosphating of Zinc or Galvanised Steel

Zinc phosphate layers of Zn3(P04)2 4H2O may be formed directly on zinc or galvanised steel surfaces merely by dipping them in phosphoric acid (conversion coatings). Alternatively, the zinc can be supplied from a solution of Zn( l2P0 2 in H3PO4 (pH 2.5 3.5).  

Various representative equations have been proposed for the different phosphating processes, which can be quite complex. The exact coating composition obtained in any process is dependent upon many factors. These include bath composition (including accelerators and other additives), temperature, pH, concentration, metal and its surface condition, and the degree of bath agitation. 

When zinc dihydrogen phosphate (obtained by dissolving zinc in H3PO4) is used, the overall coating produced on iron or steel can be represented approximately by an equation such as (12.38). Under some conditions the ferrous iron may substitute in the zinc phosphate, and such coatings have been shown to consist mainly of crystalline Phosphophyllite, Zn2Fe(P04)2 4H2O. Such processes may operate at pH 1.5-3.5 and at 25-90°C. 

In the presenee of oxidising aeeelerators (which are used in practice), ferric phosphate is deposited directly, together with zinc phosphate (12.39). The purpose of oxidising accelerators is not only to remove iron salts from the phosphating bath by precipitation, but also to reduce the formation of hydrogen gas bubbles which can slow down the coating process. 

The coating of mixed phosphates can be represented by an equation such as (12.39). Once this initial layer is formed, however, subsequent layers consist mainly of crystalline Zn3(P04)2 4H2O. A thickness of 1-5 g/m is obtained in practice. 

---