Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 40 of 331

improve paint adhesion and corrosion protection; however, nickel compounds are noxious and closely regulated in the effluent stream. Nickel-free processes, therefore, are desirable to satisfy health and environmental demands. Nickel-free processes are available and in use. Most processes replace the nickel with very low concentrations of copper. The corrosion protection of these copper-modified processes on steel is excellent; however, further progress is still needed if hot- dipped galvanized steel is part of the treatment mix. The paint adhesion under ca- thodic electrocoat as in use in the automotive industry is not sufficient with these processes to match the performance of nickel-containing processes on hot-dipped galvanized steel. PROCESS SETUP PRIOR TO PAINTING Several different processes can be utilized with zinc phosphating. Straight spray installations are popular as well as straight dip operations; combinations of both are also used to make them advantageous. When complex parts are being processed where a lot of recessed areas have to be treated and painted, a combi- nation of spray and/or straight dip operations is preferred. In a combination installation, spray can be utilized in areas such as first cleaner stages, rinse stages, and final rinse stages; however, if dip cleaning is used, then rinsing should be done in the dip as well as in the activation, phosphating, and sealing operations. Because the spray utilizes the kinetic energy of the spray pressure, concentrations and treatment times can be kept lower in spray operations compared to dip. As for the phosphate process, treatment time for a spray installation should be a mini- mum of 60 seconds and for a dip 90 seconds, but 120 seconds is preferred. Typi- cal process examples are given in Table V. HEAVY ZINC PHOSPHATING FOR RUST PROTECTION Zinc phosphating or manganese-phosphate processes are also widely used for cor- rosion protection purposes where the phosphate coating works as a carrier for the specific oil or wax film that is applied over it. Typical applications where the technology is used include screws, nuts, bolts, plain washers, brake compo- nents, clutch components, engine parts, and several others. The processes are applied in dip operations, either in bulk processes utilizing tumblers, or in a specific rack design. The coating weight produced by the process- es can vary in the range of 1,000–4,500 mg/ft2 . With no further after-treatment, these processes create a salt-spray resistance (SS DIN 50021) of 0.5 to 2 hours. Ap- plication of a sealant process can create salt-spray resistance of up to 24 hours. Ap- plication of a rust-preventive oil or wax impregnation can further increase the salt- spray resistance of the respective products in the range of 48 to 720 hours. The process technology is based on the same reaction as described earlier. Several accelerators can also be used in the process chemistry. The heaviest coat- ings are developed by the nitrate-accelerated processes, which are working on the "iron side," meaning that divalent iron is present in the process solution. This tech- nology has a limit in that the iron content has to be controlled because it can in- hibit proper coating reaction and spontaneous sludge formation. The nitrate-ac- celerated processes cannot create the heavy coatings but they phosphate faster than the nitrate processes. Coating weights with the nitrite processes are also low- er than the nitrate-accelerated processes. The crystal structure of the coatings is mainly hopeite and phosphophyllite 39

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