Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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of a dry acid salt or hydrochloric acid although it is best removed by mechan- ical means. If enhanced corrosion resistance is desired, or if you are black zinc phosphating in accordance to a military specification, a dip in a chromic acid/phosphoric acid sealer will be required. If required, the chromic acid/phos- phoric acid sealer is used very dilute and at 150O F. A rinse should not follow this . In many cases, however, customers are asking for a finer crystal structure than that produced by a heavy zinc phosphate, and a trend is toward sealing the black smutty finish with a fine-grained calcium- modified zinc phosphate to yield a coating weight of between 500 and 1,000 mg/ft2 . When using a calcium-mod- ified zinc phosphate, care must be taken to assure that dwell time in the black predip is kept to a minimum, otherwise the black layer will be too heavy and the phosphate may not have the capacity to totally seal the black resulting in some black rub-off. The zinc phosphates, whether heavy or calcium-modified, are typically used at a concentration of about 3 to 6% by volume and a temperature of between 170 and 190O F. Dwell times may be as short as 5 minutes or as long as 15 minutes. When properly applied, coating buildup is approximately 1 to 5 ten thou- sandths of an inch. No allowance is typically made for this buildup on close-fit- ting parts because the crystals of the zinc phosphate are soft and are easily bro- ken, allowing parts to return to their original uncoated dimensions. The bath must be filtered to remove insolubles that are a byproduct of the phosphate reaction. The black zinc phosphate coating is very porous and will absorb the final sealant to a much greater extent than the bare steel or black oxided steel surfaces. Black zinc phosphate conversion coatings, when properly applied, can yield salt spray resistance per ASTM B 117 at upwards of 260 hours. Black zinc phosphate conversion coatings have found their greatest use where a functional and attractive black finish yielding between 100 and 300 hours of salt spray resistance is required yet dimension change must be kept to a minimum. Black zinc phosphates have been used extensively by the fastener and tool industries. They also have had good application for protecting under-hood-type parts made from cast iron, malleable iron, and cast steel, such as master cylinders. Most of the problems encountered when applying a black phosphate conversion coating are the result of improper chemical balance in the zinc phosphate solution. The total to free acid ratio must be maintained within optimum range to insure proper phosphating. A free to total acid ratio of between 6.0:1 and 8:1 is typical. Too long a dwell time in the black predip as mentioned previously will result in a black smut that is too heavy to be thoroughly sealed by the phosphate solution. 399 step and it is beneficial to spin dry or oven dry the parts prior to immersion in the soluble or water-displacing oils to prevent the detrimental accumulation of hexavalent chromium in these solutions. The black predip solutions are primarily proprietary formulations, which are used at concentrations ranging from 5 to 15% by volume. As mentioned pre- viously, they are acidic, containing hydrochloric acid and should thus be vented. The black predip does not produce an adherent black finish but rather a smut- ty nonadherent deposit that is subsequently sealed in by the heavy zinc phosphate; therefore, if parts are to be processed in a tumbling barrel, the barrel should rotate at 1 to 2 rpm to minimize scratching of the smutty surface. The depth of work in the tumbling barrel should be kept at 6 inches or less. The black smut produced in the predip solution is typically sealed with a heavy zinc phosphate that will yield a coating weight of between 1,500 and 2,500 mg/ft2

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