Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 399 of 707

The room-temperature blackening solutions have found their greatest effec- tive use in captive blackening operations where parts manufactured from one or two alloys are processed. The use of room-temperature blackening solutions affords the customer an eco- nomical, safe, and highly productive means of producing a moderately corrosion- resistant, esthetically attractive finish on his product. The chemical cost to apply the room-temperature black oxide is typically between 3 and 5 cents per square foot. Most production problems to date have been due to any one of the following: 1.Overworking of a production bath. 2.Poor or improper preparation of the parts prior to blackening. Remember, room- temperature blackening is more analogous to electroless nickel plat- ing than any other metal-finishing operation. 3.Poor solution maintenance, i.e., run the bath below 85% working con- centration. 4.Poor product application, initially. If these problems are avoided, room-temperature black oxidizing becomes a safe, convenient, economical alternative to zinc plating or painting where mod- erate corrosion resistance is required. The benefits afforded by using room-temperature black oxide that have resulted in its wide acceptance are as follows: 1. Little or no heating required. 2.Long lasting equipment. 3. Blackening solution is operated at room temperature and in mildly acid conditions and is thus safe to use. 4.Venting of the blackening tank is unnecessary. 5.You can blacken steel, cast iron, malleable iron, powdered metal, and mild steel in the same solution. With the hot alkaline nitrate oxidizing, a sep- arate solution is required to process cast and malleable iron. 6.Dimensional changes are minimal. 7. Parts can be bulk processed via tumbling barrels. Rotation should be 2 rpm. 8.High productivity due to short cycle times. BLACK ZINC PHOSPHATE Black zinc phosphate may be the most overlooked functional metal-finishing process around today. Black zinc phosphating is a specialized heavy zinc phosphating procedure in which the steel, cast iron, or malleable iron parts are blackened prior to immer- sion in the zinc phosphating solution. The process used to produce the black zinc phosphate conversion coating in its simplest form consists of seven process steps. Assuming that the parts are free from rust and heat treat scale, the parts should be thoroughly cleaned in a heavy-duty alkaline soak cleaner, rinsed in over- flowing cold water, immersed in the hydrochloric-acid-based black predip solu- tion, rinsed in overflowing cold water, phosphated in the heavy zinc phosphate, rinsed in overflowing cold water, and immersed in a soluble oil or a water-dis- placing oil. Should rust or scale be present it might be removed by going into a solution 398

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