Transcript Document
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 precleaning 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 prerinse. 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.