Metal Finishing Guide Book


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Page 129 of 903

Table IV. Comparison of Zinc Phosphating Technologies Property Normal Zinc Low Zinc Low-Zinc Manganese Phosphating speed Coating weight Crystal size Chemical consumption Corrosion protection with cathodic E-coat Chip corrosionresistance Wet adhesion of cathodic E-coat Higher Higher/same Higher/same Same Poor Poor Poor Lower Same/lower Same/lower Same Good Good Good Modified higher Same/lower Lower Same Excellent Excellent Excellent eral factors: A longer pickling reaction and thereby a better chemical cleaning of the metal surface; slower deposition reaction and thereby a denser phosphate structure; and an increased amount of zinc-iron phosphate (phosphophyllite) on steel surfaces. The following are the crystal structure of various substrates. Steel surfaces Zn3(PO4)2.4H2O (phosphophyllite) Zinc-coated steel Zn3(PO4)2.4H2O Aluminum Zn3(PO4)2.4H2O With the development of manganese-modified, low-zinc phosphate processes a further step was taken to increase corrosion protection as well as paint adhesion. Zinc is partially replaced by nickel and/or manganese in trication phosphate processes. Some of the advantages are lower coating weights with better thermal stability that provide improved adhesion and corrosion protection under paint. The following represents the crystal structure on various substrates: Steel surfaces Zn2(Fe or MN)(PO4)2.4H2O Mn2Zn(PO4)2.4H2O Zn3(PO4)2.4H2O Zinc-coated steel Mn2Zn(PO4)2.4H2O Zn3(PO4)2.4H2O Aluminum Mn2Zn(PO4)2.4H2O Zn3(PO4)2.4H2O Another process technology that has been widely used in the appliance in126

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