Metal Finishing Guide Book


Issue link:

Contents of this Issue


Page 132 of 903

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 minimum of 60 seconds and for a dip 90 seconds, but 120 seconds is preferred. Typical process examples are given in Table V. HEAVY ZINC PHOSPHATING FOR RUST PROTECTION Zinc phosphating or manganese-phosphate processes are also widely used for corrosion 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 components, 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 processes 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. Application of a sealant process can create salt-spray resistance of up to 24 hours. Application of a rust-preventive oil or wax impregnation can further increase the saltspray 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 coatings 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 technology has a limit in that the iron content has to be controlled because it can inhibit proper coating reaction and spontaneous sludge formation. The nitrate-accelerated processes cannot create the heavy coatings but they phosphate faster than the nitrate processes. Coating weights with the nitrite processes are also lower than the nitrate-accelerated processes. The crystal structure of the coatings is mainly hopeite and phosphophyllite for the zinc phosphating processes and if a manganese-phosphate process is used, a manganese-iron phosphate crystal is developed. The process specifics for the processes will vary but some guidelines are as follows: (1) Phosphate coatings with coating weights up to 4,500 mg/ft2 are deposited by processes with long immersion times and high temperatures. Tines can be as long as 60 minutes and temperatures are in the range of 180���205��F. (2) Phosphate processes containing accelerators, such as nitrates, produce zinc or manganese phosphate coatings with coating weights in the range of 1,000���3,500 mg/ft2 with treatment times in the area of 5 to 15 minutes. Temperature range is between 130��F and 200��F depending on the process. The coatings can also be treated with coloring agents so that the appearance of the parts can be varied to customer demands. (Fig. 3 shows a summary of the possible sequences.). COLD-FORMING PROCESSES FOR TUBE WIRE, COLD EXTRUSION The application of chemical processes for the purpose of assisting the cold forming of metals was already in general use at the beginning of this century. The in129

Articles in this issue

view archives of Metal Finishing Guide Book - 2012-2013