Phosphating on Ferrous and Aluminium Alloys

Reference : D. B. Freeman, Phosphating and Metal Pre-treatment, Industrial

Press Inc, New York, 1986.

Zinc Phosphate Processes for Paint Bonding : An Example of Applications

Steps in Immersion Processing

• Alkali Cleaning• Rinsing• Pickling (optional)• Rinsing• Phosphate coating• Rinsing• Chromate coating (post-treatment)• Drying

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.

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.

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)

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 : -

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

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

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)

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

Electropaint

• Anodic systems• Cathodic systems

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

• 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.

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.

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.

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

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.