Phosphophyllite

Figure 12. The crystal structures of phosphophyllite, veszelyite and kipushite (a) phosphophyllite showing (PO4) and tetrahedra at the vertices of a 4.8 net (b) phosphophyllite projected onto...

Hill RG (1977) The crystal stmctrue of phosphophyllite. Am Mineral 63 812-817 Hill RG, Jones JB (1976) The crystal stractrrre of hopeite. Am Mineral 61 987-995 Hoyos MA, Calderon T, Vergara I, Garcia-Sole J (1993) New stmctrrral and spectroscopic data for eosphorite. Mineral Mag 57 329-336... 

Fig. 2 Crystallography of the phosphate layer crystal structures of hopeite and phosphophyllite showing tetrahedral and octahedral metal ion sites within the crystal lattice. (Courtesy Jean Steinmetz, University Henri Poincare, Nancy I.)...

Dehydration Drying temperatures for cat-aphoretic paint are on the order of 150 to 200 °C. The dehydration of hopeite occurs in two steps [34] two water molecules are lost in the temperature range of 80 to 120 °C, and the two final water molecules are lost around 250 °C. The first dehydration step is reversible and in the presence of humidity, the tetrahydrate is reformed, but the crystals are finer and the orientations are different [35]. Van Ooij and Sabata [36] have proposed that the rehydration of the dihydrate is an important mechanism in the adherence failure of phosphate coatings. The presence of Mn and Ni substitutions in the hopeite phase increases the temperature of dehydration [37, 38]. In contrast to hopeite, the phosphophyllite phase is transformed into an amorphous phase, and the dehydration is irreversible. 

The phosphate layers are gray in color (weight 1.2-6.0 g/m ) and consist of Zn3(P04)2 4 H2O (hopeite), Zn2pe(P04)2 4 H2O (phosphophyllite), and Zn2Ca(P04)2 4 H2O (scholzite). Layer formation is complete when the metal is completely covered with a phosphate layer, and the pickling action initiating layer formation has stopped. The treatment baths contain 0.4-5 g/L of zinc and 6-25 g/L of phosphate, calculated as P2O5. 

At the cathodic sites, phosphophyllite Zn2Fe(P04)2-4Fl20 and hopeite, Zn3pe (P04)2-4H20 precipitate though reactions in Eqs.(13.4) and (13.5), respectively. 

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. 

With these reactions the phosphate film consists of phosphophyllite and hopeite. Phosphate solubilities are lowest in the pH range of 6-8. They are stable in neutral environments and are nonelectric conductive compounds. Phosphate film deposits on cathodic areas and anodic sites remain in the form of pinholes. Consequently the continuity of phosphate films is not as good as those of anodic oxide and chromate films. 

The specific phosphate compound also determines the solubility. For example, phosphated steel provides better adhesion for paint films than does phosphated zinc. Phosphates formed on steel are primarily phosphophyllite and those on zinc are hopeite. 

Phosphophyllite has a lower solubility than hopeite in low and high pH ranges. The solubility of hopeite increases sharply around pH 11, whereas the solubility of phosphophyllite remains low. 

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