The two main types of zinc phosphate crystals formed during zinc phosphating on iron components are:
1. Hopeite (Zn₃(PO₄)₂·4H₂O)
Appearance: Plate-like or blocky crystals
Conditions: Formed in neutral to mildly acidic baths, especially when iron dissolution is minimal
Properties:
Dense, uniform coating
Good paint adhesion
Corrosion resistance
2. Phosphophyllite (Zn₂Fe(PO₄)₂·4H₂O)
Appearance: Needle-like or prismatic crystals
Conditions: Formed when iron dissolves into the bath (from substrate), providing Fe²⁺ ions
Properties:
Less dense but enhances coating adhesion
Often mixed with hopeite in practical baths
Credits: Dr E. Ramanathan, PhD Thesis, University of Madras
Ratio of Hopeite to Phosphophyllite depends on:
Substrate (steel vs. galvanized)
Bath pH
Iron content in bath
Accelerator concentration
Balanced formation of both enhances coating performance.
How are they formed?
H:P Ratio
The H:P ratio refers to the Hopeite to Phosphophyllite ratio in a zinc phosphate coating.
Typical H:P Ratio Range:
Hopeite-rich coating: H:P ≈ 3:1 to 5:1
Balanced coating: H:P ≈ 1:1 to 2:1
Phosphophyllite-rich coating: H:P < 1:1
Influence on Coating Properties:
H:P Ratio
Coating Type
Properties
High (>3)
Hopeite-dominant
Dense, uniform crystals; good corrosion resistance
Balanced
Mixed (Hopeite & Phosphophyllite)
Optimal paint adhesion and durability
Low (<1)
Phosphophyllite-dominant
Thinner, less dense; may reduce corrosion resistance but aid adhesion
Bath control (pH, Fe²⁺, Zn²⁺, accelerator level) is key to maintaining desired H:P ratio for performance-specific applications.
The P Ratio
The physical characteristics of phosphate coating is investigated by examining the physical appearance by naked eyes, determining coating thickness or coating weight, acid resistance, testing hygroscopicity, estimating porosity by number of Prussian blue spots developed on a filter paper soaked in the mixture of potassium ferricyanide, sodium chloride and gelatin, cyclic voltammetric studies on porosity of the phosphate layer, determining thermal stability and chemical stability, evaluating surface morphology by SEM, AFM and XRD. SEM and AFM studies show that surface conditioning prior to phosphating refines the grain size of the phosphate coating. Finer the particle size of the phosphate coating, greater will be the adhesion of paint film and corrosion resistance. Bubert et al [34] used XRD as an effective tool to characterize the phosphate coating in terms of ‘P ratio’. It is an important parameter to measure the forms of phosphate crystals obtained in the zinc phosphating process.
Where P is the intensity of the characteristic planes of phosphophyllite and H is the intensity of characteristic plane of hopeite. There are number of factors affecting the p ratio viz., temperature, agitation method of application. Bubert found that the spray phosphating panel contains phosphophyllite crystals predominantly while hot immersion phosphating panel contains a mixture of hopeite and phosphophyllite. The first coat formed on the metal panel is predominantly hopeite and prolonged immersion of panels for more than 60 s does not increase the coating thickness. The symmetry of FeO2 (H2O)4 octahedron in phosphophyllite is different from that of the ZnO2 (H2O)4 tetrahedron in hopeite.
