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Need to learn Hot Black Oxide

Q. I am in the process of being handed off an existing Black Oxide line in a facility I have been employed at for 2 months. I'm a 30-year machinist very familiar with lots of processes. Have done the cold process a lot through the years but vaguely familiar with the way the hot process works. Are any publications dedicated to Black Oxide process and techniques? Need to get myself educated in a hurry.

Mike Aemmer
Employee - Louisville, Kentucky
June 25, 2012

A. Hi Mike. Probably the first thing to learn is basic safety.

This is a very strongly alkaline process and very hot (approaching 300° F.). This means it will evaporate a lot of water which must be made good. But water can flash to steam at 212°. F, which means that one of the main dangers is a pocket of water flashing to steam and erupting the contents of the tank. Understand how water is added to your tank and what situations must be maintained for this to remain safe. Protective gear is especially important because an eruption can be fatal (I heard of a case where a tossed soda can fell into the tank, which caused an eruption, killing the operator).

A second point to understand is how temperature control works in a hot black oxide tank. You don't regulate the heat input to regulate temperature; you regulate the concentration so that the solution will boil at the temperature that you wish to maintain -- the temperature won't drop because you are inputting plenty of heat, and it won't rise because it's already boiling, so it can't. (Special thanks to Rod Henrickson who explained this to us in several threads).

You can get good instruction from the suppliers of proprietary hot black oxide processes, and you'll find a basic introduction in the Metal Finishing Guidebook.


Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey
June 25, 2012

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