Phosphating is the most widely used metal pretreatment process for the surface treatment and finishing of ferrous and non-ferrous metals. Zinc phosphates have become the preferred undercoat treatment in the metal finishing industry. The reason for this is that they afford excellent corrosion resistance, wear resistance, adhesion, and lubricative properties. Since phosphating can be either by spray or immersion, phosphating is a preferred ferrous metal undercoat pretreatment process in automobile and appliance industries. More importantly, phosphating provides an inexpensive, non-toxic, reasonably hard, highly adherent, and electronically non-conducting phosphate coating.
The phosphating solution often contains three components, zinc orthophosphate tetrahydrate [Zn3(PO4)2·4H2O], H3PO4, and water. Exemplary zinc phosphating liquid recipes may consist of 0.9 to 2.6 wt% zinc orthophosphate tetrahydrate, 1.8 to 5.1 wt% H3PO4, and 97.3 to 92.3 wt% water. The recipes may be tuned for improved substrate adhesion by incorporating a water-soluble polymer at concentrations ranging from 0 to 5.0% by weight of the total solution. In addition, the PH of the phosphating recipe can be adjusted to be alkaline, neutral, or acidic, as needed.
