Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 32 of 331

strate mix being treated. Minimization of hydrogen development occurs by the use of oxidation agents (Ox) (accelerators). Steel surface Fe + 2H+1 + 2Ox ➝Fe+2 Zinc-coated steel surface Aluminum surface Coating Reaction The second reaction that occurs is the coating reaction. Because of the con- sumption of the free acid in the liquid-metal interface, pH rises and the metal cations cannot stay soluble in the solution. They react with the phosphate in the solution and deposit on the metal surface as crystalline zinc phosphate. Depending on the process chemistry involved, several crystal structures are possible: 3Zn+2 2Zn+2 2Mn+2 2Zn+2 + 2H2 + Fe+2 + Zn+2 Sludge Reaction The metal ion (Fe+2 PO4 + 1H2 + 2H2 + Mn+2 + 2H2 PO4 PO4 PO4 -1 + 4H2O ➝Zn3 -1 + 4H2O ➝Zn2 -1 + 4H2O ➝Mn2 -1 + 4H2O ➝Zn2 (PO4 )2 Fe(PO4 .4H2 )2 Zn(PO4 Mn(PO4 O + 4H+ .4H2 )2 )2 . 4H2 . 4H2 O + 4H+ O + 4H+ O + 4H+ ing the accelerator (e.g., nitrite/nitrate, chlorate, or peroxide) and will precipitate out as sludge. In the case of zinc ions (Zn_2 ) that is dissolved from the pickling reaction is oxidized us- ) they are incorporated in the coating reaction and will not form any sludge. Aluminum ions (Al+3 Fe+3 + H2PO4 ➝FePO4 -1 AlF6 -3 + 3Na+➝Na3 Fe+2 + H+1 + Ox ➝HOx Fe+3 Al+3 + 6F-1➝AlF6 AlF6 ), by the use of flu- oride, react to form a fluoroaluminate complex. This complex is precipitated as kryolith. (sludge) + 2H+ (sludge) The created sludge in the process is normally filtered out from the solution uti- lizing some sort of filter media or equipment, such as lamella clarifiers or settling tanks, direct feed through a filter press, or hydromation filter technology. The sys- tem to be used is a matter of discussion and preference of customer or line manu- facturer. ZINC PHOSPHATING PRIOR TO PAINTING To increase the durability and service life of painted parts where maximum per- formance is essential, zinc phosphating is widely used. Today's zinc phosphate 31 + 2HOx Zn + 2H+1 + 2Ox ➝2HOx Al + 3H+1 + 3Ox ➝Al+3 + 3HOx

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