Metal Pretreatment

• Phosphating
• Iron phosphating
• Zinc phosphating
• Manganese phosphating

Iron phosphating :

• pH 4.0-5.0
• Primarily iron oxide
• Spraying is more used
• Iridescent blue to reddish blue color
• Coating weight 20-80 mg/ ft²
• Temperature 25-65 °C

Zinc phosphating:

• Coating weight for spraying 100-1000 mg/ft²
• Coating weight for dipping 150-4000 mg/ft²

Manganese phosphating :

• Immersion only
• Time period 5-30 minutes
• Coating weight 500-3000 mg/ft²
• Temperature 85-95 °C

Zinc phosphating :

• Total acidity: 25-27 points ( 10ml bath vs 0.1 N NaOH using phenolphthalein )
• Force acidity: using bromophenol blue or methyl orange
• Accelerator test:
  3 points for NaNO₂
  Dip a piece of Fe paper in the bath
  If it answers for Fe add NaNO₂ till it doesn’t answer for Fe
  

Iron phosphating :

Total acidity: 10 points
Iron formed in the bath is removed by HNO₂ which causes Fe to precipitate as ferric phosphate

Determination of coating weight of Zinc phosphate :

Phosphating a known area after cleaning with water, strip in 2½ % chronic acid solution at 71°C
immersing for 10 minutes, then clean with water and weigh it.

Coating voids :

Soak a piece of filter paper in a solution containing 7.5 g/L of potassium ferricyanide and 20 g/L
of NaCl. Allow each solution to drain and apply wet paper to the phosphates sample. Observe
for blue spots which are non-covered areas.
None to 3 spots ( up to 1 mm) – Excellent
Less than 10 spots – Good
Not more than 20 spots – Fine
3 large spots – Satisfactory

Formulations of Phosphate Coating Chemicals

Accelerators in phosphate coating chemical solution