Powder coat plant with spray pretreatment system


Crystals of zinc phosphate coatings

Click here to view mind map of zinc phosphate crystals

Given below the structures of three types of zinc phosphate crystals:

1) Hopeite

Source: http://rruff.info/hopeite/display=default/R050254

2) Parahopeite

Source: http://rruff.info/hopeite/display=default/R050463

3) Phosphophyllite

Source: http://rruff.info/Phosphophyllite

Trouble shooting in phosphating plant

Trouble shooting in a phosphating plant


S. No.





Coarse crystal Low accelerator Add accelerator


Patchy coating Low concentration of phosphate solution Add phosphate solution


Dark brown colour of both solution and substrate Excess iron Add accelerator


Yellowing after passivation Poor water rinse, excess passivation solution, improper draining of bath solution, insufficient drying Good water rinse, dilute passivation solution, proper draining, thorough dry off


Effect of metal cleaning on phosphating

  1. Sand blasted surface needs no phosphating. Epoxy finishes can be done directly on the freshly sand blasted surface.
  2. Sand blasted surface gives coarse phosphate coating
  3. Mineral acid pickling results large crystals
  4. Organic acid pickling results medium crystals
  5. Solvent vapour degreasing gives fine crystals
  6. Surface conditioning by colloidal titanium base gives fine crystals and thin coating
  7. Tool marks on the metal surface create nuclear sites for phosphate crystals.

Types of chemical conversion coatings

Basic Types of phosphate coating:

  1. Iron phosphating – 30 – 100 mg/sq.ft
  2. Zinc phosphating
    • Light – 100 – 400 mg/sq.ft
    • Medium – 450 – 1000 mg/sq.ft
    • Heavy – 1000 – 3000 mg/sq.ft
  3. Manganese phosphating – 1000 – 3000 mg/sq.ft

New trends in chemical conversion coatings:

  1. Low zinc, managanges phosphating
  2. Trication phosphating
  3. Electrophosphating
  4. Calcium modified phosphating
  5. Insitu phosphatizing
  6. Galvanic coupled phosphating
  7. Nanoceramic coating

Technical know-hows are available on request to email: eramanath@gmail.com