Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 398 of 707

buildup of approximately 20 millionths of an inch. Room temperature black oxides are applied most frequently by a simple seven- stage procedure as follows: Parts must be thoroughly cleaned in a heavy-duty alkaline soak cleaner. Cold water rinsed, pickled in dilute phosphoric acid solutions or solutions of a dry acid salt, overflowing cold water rinse, blacken in a 10% by vol- ume room temperature blackening solution, overflowing cold water rinse, and seal with a water-displacing oil, soluble oil, wax, or a clear acrylic finish. The black finish produced by room-temperature blackening solutions offers no enhanced corrosion resistance unless it is sealed with some type of supple- mental finish. When sealed with a moderately dry water-displacing oil, the room-temperature black oxide finish can yield 72 hours of 5% salt spray resistance per ASTM B 117. Like the true black oxide produced by the hot alkaline nitrate oxidizing solution, the room-temperature black oxide is more porous than the unblackened steel surface, thus it holds a supplemental topcoat to a greater extent than the unblackened surface yielding enhanced corrosion resistance. The room-temperature blackening solutions are more analogous to electroless plating solutions than to the traditional alkaline nitrate oxidizing solutions. Parts to be blackened in room-temperature blackening solutions must be thoroughly cleaned of grease, oil, rust, and carbon smut prior to blackening to yield uniform smut-free blackening. If rust, heat treat scale, or forging scale is pre- sent, it is best removed by mechanical means. In many cases, however, room-tem- perature black oxides have been successfully applied over light heat treat or forging scale although this is not the norm. As with plating solutions, good rinsing is extremely important between process steps to prevent solution cross-contamination. The room-temperature blackening solutions are mildly acid and slightly buffered and will be affected by either alkaline or acidic drag-in. As stated, the room temperature blackening solutions are mildly acid, i.e., hav- ing a pH of 2.0. The solutions are continuously replenishable and are maintained at working strength by means of a simple chemical titration. The working solu- tion will form an insoluble iron selenium phosphate compound that must be fil- tered out of the solution to maintain correct chemical balance. The use of a 50- micron bag filtration unit has been found to be appropriate. A room-temperature blackening solution should never be allowed to become depleted to less than 85% of its original working strength, otherwise the chem- ical balance of the bath will become altered; possibly yielding a nonadherent or smutty finish. There is a recommended limit on the number of square feet of work per gallon of room temperature blackening solution just as there is with phosphating solu- tions. Optimum results are normally achieved when tank volume is such that there are 0.6 to 0.8 ft2 of work per gallon of room temperature blackening solution. The system should also be run or worked at a rate that doesn't yield a turnover rate of greater than 50% per 8-hr shift. Example: If you have a 1,000 gallon working tank volume of concentration 10% by volume, it would be advisable to run at or below a rate that would require a replenishment of 50 gallons per 8-hr shift. Experience has shown that if a bath is worked at a rate greater than this, the rate of the naturally occurring precipitation reaction mentioned earlier can become dominant and the bath can actually start to consume itself. In this example this would be a production load of 20,000 ft2 8-hr shift. per 397

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