Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 38 of 331

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 Mn2 Zn3 Zinc-coated steel Mn2 Aluminum Mn2Zn(PO4 Zn3 (PO4 )2 )2 .4H2 .4H2 O O Another process technology that has been widely used in the appliance in- dustry is the calcium-modified normal zinc technology. This technology has an advantage in that the process does not need a grain refiner or activation compound prior to phosphating, but because of its disadvantages with high process tem- perature as well as heavy sludge development, the process has been replaced by the use of the manganese-modified, low-zinc phosphate processes. ACTIVATION OF PHOSPHATE COATING FORMATION The phosphating rate, thickness, and crystal size of the zinc phosphate coating depend not only on the composition and form of the phosphating bath, but also on the pretreatment of the metal surface prior to phosphating. Strong al- kaline cleaner and acid pickling processes frequently produce a coarsening of the coating texture and prolong the time required for the formation of a phosphate coating with uniform coverage. Mechanical treatments, for example, grinding or polishing, brushing, and even simply wiping, produce marked refinement of the subsequent zinc phosphate coating. The reason for this phenomenon lies in the different number of nuclei on the metal surface at which crystallization of the phosphate coatings can begin. Special prerinses applied to the metal surface prior to phosphating provide a considerable increase in the number of nuclei for phosphate crystallization. This is termed activation of the phosphate coating formation. It is manifested in the subsequent phosphating process by an increase in the number of phosphate crystals per unit surface area; a decrease in the dimensions of the individual crystals of the coating; a reduction in coating weight; and a reduction in the minimum phosphating time. Titanium phosphate produced according to a special process has been found to be a highly effective nucleating agent. In industrial practice, the activating agent is added either directly to the rinse bath prior to phosphating or to preliminary al- kaline cleaning baths. Since the lifetime of the activation agents is affected by sev- eral factors, such as pH, water hardness, and temperature, they are normally added 37 Zn3 Zn(PO4 (PO4 )2 )2 .4H2 .4H2 O O Zn(PO4 (PO4 )2 )2 )2 .4H2 .4H2 O .4H2 O O

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