Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 37 of 331

involve the process chemistry. Whereas in normal zinc phosphating baths, ap- proximately 2,000–4,000 ppm zinc and approximately 6,000–14,000 ppm phos- phate are present, the corresponding concentrations in low-zinc baths are ap- proximately 400–1,700 ppm zinc and 16,000–22,000 ppm phosphate. The low zinc phosphate technology has further been developed by incorporating a third met- al, namely manganese, in the process bath. These processes are known as manganese-modified, low-zinc phosphate processes and are characterized by their outstanding performance. A comparison of zinc phosphating technologies is given in Table IV. Coating weights that are recommended prior to painting depend on the ap- plication and the subsequent paint process. As a rule of thumb, the coating weight is in the range of 100–500 mg/ft2 . CRYSTAL STRUCTURE Depending on the process and the substrate, different crystal structures are possible. For the "normal zinc" technology the crystal structure is the same on all substrates. Zn3(PO4 )2 .4H2 O (hopeite) The low zinc technology shows a better coating performance because of sev- eral factors: A longer pickling reaction and thereby a better chemical cleaning of the metal surface; slower deposition reaction and thereby a denser phosphate struc- ture; and an increased amount of zinc-iron phosphate (phosphophyllite) on steel surfaces. The following are the crystal structure of various substrates. Steel surfaces Zn3 Zinc-coated steel Zn3 Aluminum Zn3 (PO4 )2 .4H2 O With the development of manganese-modified, low-zinc phosphate processes a further step was taken to increase corrosion protection as well as paint adhesion. 36 (PO4 )2 .4H2 O (PO4 )2 .4H2 O (phosphophyllite)

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