Metal Finishing Guide Book

2012-2013

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Table III. Results of Corrosion Testing Phosphating Process Steel Salt spray test (SS DIN 50021) mm creepage from scribe after 1008 hr+Low zinc, phosphate Low zinc, manganese-modified Cycle test (VDA 621-415 paint creepage from scribe after 10 weeks in mm Normal Zinc Low zinc, phosphate Low zinc, manganese-modified Accelerated outdoor exposure test paint creepage from scribe after 12 months in mm Normal Zinc Low zinc, phosphate Low zinc, manganese-modified Cross hatch combined with humidity after 240 hr GT note and paint loss (%) Normal Zinc Low zinc, phosphate Low zinc, manganese-modified Electrogalvanized Steel 1���1.5 <1���1 1���7(480 hr) 1���12 (480 hr) 5���8 1���1.5 1 3���7 1���6 1���7 >13 2.5���3 1���2 2 1 1 5(80%0 1���2(5���15%) 0���1(0���5%) 5(80 to 100%) 2(15%) <1(<5%) Metal: Steel and zinc-coated steel sheet Phosphate process: Low zinc phosphating with and without manganese modifying Post rinse: Cr(VI) and Cr(III) Paint system: Cathodic electrocoat epoxy primer and topcoat try to handle mixed metal production. The traditionally used, so-called normal zinc processes have, in several cases, been replaced by the use of low zinc or low zinc-manganese-modified processes that create an outstanding performance in all corrosion testing environments (Table III). The differences between the processes used as pretreatment prior to painting involve the process chemistry. Whereas in normal zinc phosphating baths, approximately 2,000���4,000 ppm zinc and approximately 6,000���14,000 ppm phosphate are present, the corresponding concentrations in low-zinc baths are approximately 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 metal, 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 application 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)2.4H2O (hopeite) The low zinc technology shows a better coating performance because of sev125

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