Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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dependent on local energy costs and part configuration and thus isn't always easy to determine. It would be safe to say that the total chemical cost to blacken by means of the alkaline nitrate method would be within the range of 0.5 to 1.0 cents/ft2 . Energy costs are constant as long as the tank is at operating temperature; therefore, it is advisable to run as much work in a given period of time as possible. The energy costs per square foot of work processed decrease as the amount of work increases. The hot alkaline nitrate oxidizing solutions have found their greatest appli- cation in job shops and captive shops where a simple economical means of applying a moderately corrosion-resistant finish to a variety of steel alloys with minimal or no dimensional change is desired. The hot alkaline nitrate oxidizing method offers the following benefits: 1. It is simpleā€”one process will blacken a great variety of steel alloys with varying surface hardness (cast and malleable iron will require a bath oper- ating at 255O F, however). 2. The solution is very forgiving of inadequate cleaning due to its high alka- linity and the slight agitation due to boiling action. 3. As long as the boiling point is maintained within the specified range, 285- 295O F, there is little that can go wrong with the bath. 4. Pollution control problems are minimized. In most cases neutralization is all that is required. 5. Parts may be racked, processed in baskets, or processed in tumbling barrels. ROOM TEMPERATURE BLACK OXIDE Room temperature blackening formulations for steel have been around for a good number of years. These solutions were initially developed as touch-up solutions to blacken scratches on parts that had been blackened in the hot alkaline nitrate blackening solutions. With the increasing cost of energy during the early and mid 1970s the room- temperature touch-up blackening solutions became attractive alternatives to the energy intensive alkaline nitrate oxidizing solutions that were/are com- monly used. These room-temperature touch-up blackening solutions have been refined and improved to yield the present-day proprietary formulations that have found wide-spread acceptance. Unlike the black oxide produced by the hot alkaline nitrate oxidizing solutions, the black oxide produced by the room-temperature oxidizing formulations is not a true conversion coating. The black finish produced in the room-temperature blackening solution is the result of an autocatalytic self-perpetuating deposition of a black amorphous selenium copper iron compound, the exact chemical nature of which is hard to determine. The present-day room-temperature blackening solutions are generally aque- ous solutions of phosphoric acid, which contain selenium and copper com- pounds. The liquid concentrates are diluted with water to yield working solution with concentrations of between 10 and 15% by volume and are normally used at a temperature of between 70 and 80O F. Like the true black oxide produced by hot alkaline nitrate oxidizing solutions, the black finish produced by room-temperature blackening solutions are normally assumed not to effect the tolerances of parts blackened although there is a minimal 396

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