This document discusses different methods of phosphating ferrous and aluminum alloys as a pre-treatment for paint bonding, including immersion zinc phosphate processes, spray zinc phosphate processes, and electropaint processes. It provides details on the chemistry, operating conditions, and advantages/disadvantages of orthodox immersion processing, calcium-modified immersion processing, and various spray phosphate processes. It also compares anodic and cathodic electropaint systems and their performance. Phosphate coatings formed by immersion are generally superior to sprays as a pre-treatment for electropaint.

1. Phosphating on Ferrous and
Aluminium Alloys
Reference : D. B. Freeman, Phosphating
and Metal Pre-treatment, Industrial
Press Inc, New York, 1986.

2. Zinc Phosphate Processes
for Paint Bonding :
An Example of Applications

3. Steps in Immersion Processing
• Alkali Cleaning
• Rinsing
• Pickling (optional)
• Rinsing
• Phosphate coating
• Rinsing
• Chromate coating (post-treatment)
• Drying

4. Orthodox Immersion Processing
• Nitrite/nitrate-accelerated processes are most commonly employed.
• Condition
– Accelerator : nitrite
– Coating weight : 3.0-7.0 g/m2
– Coating type : Zinc phosphate
– Operating conditions : Dip, 5-15 min., 60-70 oC
– Control parameters
• Total acid : 15-20 ml
• Free acid : 6-10 ml
• Accelerator : 2-3 ml
– Tank materials : mild steel
– Remarks : coating weight and crystal structure are very much affected by pre-
cleaning method -> tends to give coarse coatings after strong alkali cleaning or
acid pickle without refining pre-dip, requires regular nitrite additions.

5. Calcium-modified Immersion
Processing
• Coat-refining agents
– Organic refining agents : limited effect
– Calcium -> mixed zinc calcium phosphate (Scholzite, Zn2Ca(PO4)2.2H2O
• Smoother than normal zinc phosphate coating
• Unaffected by pre-cleaning
• Condition
– Accelerator : nitrite
– Coating weight : 2.0-4.5 g/m2
– Coating type : Calcium modified zinc phosphate
– Operating conditions : Dip, 2-5 min., 60-70 oC
– Control parameters
• Total acid : 18-22 ml
• Accelerator : 1.0-2.5 ml
– Tank materials : mild steel
– Remarks : refined coatings after alkali or acid cleaning without refining pre-
rinse. Particulary suitable for one-coat finishes. Requires regular nitrite
additions.

7. Spray Processing
• Typical Spray Zinc Phosphate for Paint Bonding
• Conditions
– Accelerator : nitrite
– Coating weight : 1.6-2.4 g/m2
– Coating type : Zinc phosphate
– Operating conditions : Spray, 1-2 min., 45-60 oC
– Control parameters
• Total acid : 15-20 ml
• Free acid : 0.4-0.6 ml
• Accelerator : (no information)
– Tank materials : mild steel
– Remarks : (no information)

8. Spray Processing
• Multimetal Spray Zinc Phosphate for Paint Bonding
• Nickel and simple and/or complex fluorides are added.
• used in automotive industry, where significant amounts of galvanized steel
are employed.
• widespread in USA
• Conditions
– Accelerator : nitrite
– Coating weight : 2.0-3.5 g/m2
– Coating type : Zinc phosphate
– Operating conditions : Spray, 1-3 min., 55-70 oC
– Control parameters
• Total acid : 15-20 ml
• Free acid : (no information)
• Accelerator : 0.5-2.5 ml
– Tank materials : mild steel, stainless steel for longer life
– Remarks : -

9. Spray Processing
• Low-temperature Spray Zinc Phosphate for Paint Bonding
• Manganese is added.
• used in UK and Italy
• Conditions
– Accelerator : nitrite
– Coating weight : 1.8-2.4 g/m2
– Coating type : Zinc manganese phosphate
– Operating conditions : Spray, 1-2 min., 25-35 oC
– Control parameters
• Total acid : 15-25 ml
• Free acid : 0.2-1.0 ml
• Accelerator : 3-4 ml
– Tank materials : mild steel
– Remarks : low temperature, low stain

10. Spray Processing
• Spray Zinc Phosphate for Paint Bonding for Closed-loop Operation
• NaOH used as neutralising agent for hydrogen peroxide system leads to
accumulation of Na ions in the bath. Zinc carbonate is used instead.
• Conditions
– Accelerator : hydrogen peroxide
– Coating weight : 1.4-2.0 g/m2
– Coating type : Zinc phosphate
– Operating conditions : Spray, 1-2 min., 55-60 oC
– Control parameters
• Total acid : 14-16 ml
• Free acid : 0.7-1.2 ml
• Accelerator : 3-4 ml
– Tank materials : mild steel, stainless steel for longer life
– Remarks : can be incorperated in closed-loop ststem to give total recycling of
rinse water and no liquid effluent

12. Spray Processing
• Spray Zinc Phosphate for Paint Bonding
• Low stability of nitrite -> Chlorate + Sodium metanitrobenzene
sulphonate
• Conditions
– Accelerator : Chlorate/metanitro-benzene sulphonate
– Coating weight : 1.4-2.0 g/m2
– Coating type : Zinc phosphate
– Operating conditions : Spray, 40-120 s, 45-50 oC
– Control parameters
• Total acid : 10-12 ml
• Free acid : 0.8-1.0 ml
• Accelerator : (no information)
– Tank materials : mild steel, stainless steel for longer life
– Remarks : (no information)

13. Spray Processing
• Low temperature Spray Zinc Phosphate for Paint Bonding
• Conditions
• Accelerator : Chlorate/metanitro-benzene sulphonate
• Coating weight : 1.4-2.0 g/m2
• Coating type : Zinc phosphate
• Operating conditions : Spray, 80-180 s, 25-35 oC
• Control parameters
• Total acid : 24-26 ml
• Free acid : 0.7-1.0 ml
• Accelerator : (no information)
• Tank materials : mild steel, stainless steel for longer life
• Remarks : may contain fluoride and nickel for treatment of galvanized
steel and limited quantities of aluminium

14. Electropaint
• Anodic systems
• Cathodic systems

15. Anodic electropaint
• fluoride-containing phosphate process helped to
smooth out the finish
• After 1970, anodic electropaints based on epoxy resins
• For steel as anode :
– Primary reaction : Electrolysis of water
• H2O -> H+ + OH-
• 2(OH)- -> H2O + O2-
• 2O2- -> O2 + 2e-
– Secondary reaction : Coagulation of resin and dissolution
of substrate
• (R-COO)- (soluble) + H+ -> R-COOH (insoluble)
• Fe -> Fe2+ + 2e-
• 2(R-COO)- + Fe2+ -> (R-COO)2-Fe

18. • Scab blistering and filiform corrosion are
particular forms of cosmetic corrosion in anodic
electropaint.
• Phosphate coating (1.1-3.9 g/m2) can achieve the
standard salt spray test for 240 to 360 hours
exposure.
• Use of chromate post-treatment and dry-off
temperature above 100 oC tends to have a
favourable effect on salt spray performance, but
less marked for polybutadiene-based pains.

20. Cathodic electropaint
• Japan and Europe led the change to cathodic
primers for higher standards of corrosion
protection.
• Primary reaction : Electrolysis of water
– H2O -> H+ + OH-
– 2H+ + 2e- -> H2
• Secondary reaction : Coagulation of resin
– Some dissolution of substrate does occur.

21. Advantages of cathodic over anodic
deposition
• Better throwing power
• Better salt spray performance (up-graded
from 250-400 hours with anodic systems to
500-1000 hours)
• Reduced susceptibility to cosmetic corrosion
• Better performance on steel that has not been
pre-treated.

22. Disadvantages of cathodic over anodic
deposition
• Higher stoving temperatures
• Increased need for corrosion-resistant plant as
a result of lower operating pH
• Problems with low rupture voltage on certain
zinc-coated steels
• More stringent pre-treatment requirement

23. Phosphate coating as pre-treatment
for electropaint
• Trend to full dip
• Zinc phosphate coatings formed by immersion are superior
that those obtained by spray methods.
• Immersion leads to higher content of phosphophyllite than
hopeite.
• Amorphous iron compounds may also present leading to
favourable characteristics, not only the presence of
phosphophyllite.
• Ideal coating weight for cathodic paint lies between 1.4-2.4
g/m2.
• Nickel and zinc phosphating with a high ratio Ni:Zn results
in a mixed nickel zinc phosphate, Zn2Ni(PO4)2.4H2O was also
developed.