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How to prevent danger of adding water to hot blackening solutions

During the hot blackening process water added is dangerous, how do we reduce that problem?

Karunanithi Shanmugam
M.O.G,tiruchengode - Salem, Tamilnadu, India
2006

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2006

Darned right it's dangerous. People have been killed and others very badly burned. The problem is that these solutions operate at 265 degrees F and up, because of their very high salt content, whereas the water you need to add to make up evaporation losses boils at 212 F. If a pocket of that relatively pure water finds itself under the solution level, and then is suddenly heated beyond 212, it can flash to steam and cause the contents of the tank to erupt and cover everyone and everything with 265 degree+ highly concentrated caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] .

For this reason I think black oxiding equipment should be purchased from specialty vendors who are well acquainted with the problem and who have accumulated experience with it. But my understanding is that vigorous agitation is needed during heat-up to prevent pockets, but that additions during operation should be made by floating the makeup water on top of the solution.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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MIL-HDBK-205A, pages 59-60, describes a horizontal diffuser pipe for water additions. A row of drilled holes in the pipe faces the rear inside wall of the tank above the solution level. Also, a needle valve is used to restrict the flow rate. Thus, water enters as a film flowing down the tank wall.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.
2006