How to maintain black oxide solution concentration?

A. Make up your solution at concentration so that the level is 6 inches down from the top of the tank. Fix a horizontal pipe down 4" from the top with holes drills to squirt water at a 45 degree angle against the side of the tank. UP stream of this pipe place a NEEDLE VALVE, further upstream place a motorized valve (not a solenoid !). Turn the motorized valve off/on with a temperature controller set at 285 °F. When the solution boils off water and goes up to 286, then the motorized valve will come on an automatically add water, as the temp drops to 285, then 284, all the time while still boiling, then the motorized valve will close. Your concentration will be automatically controlled. As you drag out solution the level will drop to 7" down, then 8" down. Then the operator carefully shovels in more salt, still boiling, and the water automatically comes on and raises the level to match the amount of salt he added.

Robert Probert offers what appears to be sound advice based on long experience. The only thing I would add is that if pure water gets into a solution operating at 285 °F it will instantly flash to steam and erupt the solution onto the operators. People have been killed by black oxide solutions in this way. So you must exercise great care that there is no possibility of introducing water down into the bulk of the solution.

I have designed all kinds of metal finishing equipment, but I have not and will not design a black oxide tank because of concern over this possibility and always suggest that shops purchase black oxide tanks from specialized, experienced constructors.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2003

A. What I described is the safest and most widely acceptable way to control a hot black oxide, but It is difficult to explain unless I can stand in front of you and wave arms and sketch pictures. The water adding pipe is horizontal, 2 inches above the solution level, the water feeds slowly as the motorized valve opens, the water is choked down to a safe low rate by the needle valve (most of the time when I set the valve I remove the handle and throw it away, thus the operator cannot turn it up and hurt himself). As the water slowly feeds in, yes part of it steams right back into the air, most of it runs down the tank wall and goes into solution.

As drag-out occurs the level drops. Because the temperature controller maintains the boiling point it keeps the solution boiling and blackening. The operator has a chart on the wall which shows how much salt raises the level one inch. When he adds salt it has the effect of raising the boiling point at first, then the automatic water comes on and adjusts the level (all the time boiling and blackening is proceeding properly)

Call me on the phone, let's talk until I find the words to clear all questions.

"Salt" is the blended proprietary mixture from reputable black oxide house, OR roughly a blend of 80% caustic soda [affil links] plus 10% Sodium Nitrate plus 10% Sodium Nitrite. But if you are as new to black oxiding as you sound, please start with a proprietary blend - there are two very outstanding black oxide manufacturers and dozens of others with good products.

Mr. Probert:

You are right, I am actually a beginner in black oxide process (although I do have zinc barrel plating lines in operation). However, I think you have given me a very clear picture of how to set things up properly from the beginning, and I am very grateful for your advice. I did try your advice on a small scale pot, with various NaOH/NaNO2 composition and temperature readings as well, and I think I understand already on how to set it up properly. Please understand Mr. Probert, that I happen to live in Indonesia, and proprietary equipment maker that specialize in black oxide process, as far as I know, is non-existent. I might be able to import it somewhere else, but my quantity is small, I doubt that the investment might be profitable. There is one and only chemical supplier that is able to provide proprietary salt for black oxide process, but it is very expensive, and their engineer doesn't even know how to set up the equipment properly.

Again, thank you very much for your help Mr. Probert, I do have clear idea on how to do it properly now.

A. Hartarto:

Just to add something regarding safety. We use a system very similar to what Mr. Probert expertly recommends, the other thing that we use to avoid any hazards from the hot water, is that above the horizontal tube we have a steel shield, "u" shaped. this lowers the risk considerably.

Hi Neville. We appended your inquiry to a thread where it is already answered. Good luck.

Regards,