The black oxide process and gun bluing

A. Chu Zhang
Bluing is not a plating. It actually changes the iron to Fe₃O₄. The process will only change iron which is exposed to the chemical bath. Once the exposed iron is converted to Fe3O4 the chemical change stops. There is no real way to increase the depth of the layer. You can leave the parts in the bath for an exaggerated length of time to make sure all exposed iron has been converted but you eventually reach a point of diminishing returns where you are simply doing no good.

A. Hi Jasmeet.

At room temperature HCl will be a much more powerful pickling acid than H₂SO₄. The latter must be heated to near boiling to be a powerful pickling acid. I am not familiar with a black oxide finish having red rust built in. I think you are not doing it right and should very carefully consider the advice from Rod and others about the need for the solution to be boiling. Good luck.

Regards,

Well, I have got better results with HCl. One because I don't require a heater anymore. Lesser fumes (I am not sure why, because I read that H2SO4 will give lesser fumes than HCl. But I have got opposite results). And no crystallization of iron as it used to happen with H2SO4.

Q. Now the only problem I am left with is deposition of salt on the top surface of the items. I am still searching for chemicals which will act as rectifiers or inhibitors. Any help on this is much appreciated.

A. Hi, James.

I'm not quite sure what you mean by "clean after blackening". The parts need to be rinsed and then oiled or waxed. A "clean", i.e., oil-free black oxide coating will flash rust almost immediately. It is the oil or wax, not the black oxide, that provides any corrosion resistance.

Regards,

A. To James R, - Guindy, Tamil Nadu, INDIA

The only way to get residual salts off parts is to boil them in water for about 20 minutes and then oil or wax the parts. If there are no nooks and cranny's for the salt to hide in you can short cut this by dipping and rinsing in boiling water. The solutions that would kill the salts instantly like acids also remove the bluing so there is no quick chemical way to deal with carried over salts. You just have to charge for the time in the boil out or you will get salt bleeding later on.

A. Hi Jim.

Black oxide is the finish on most rifle barrels, and must be treated "lovingly" like gun enthusiasts treat their firearms, and be carefully maintained. It doesn't sound like the right finish for your needs. You may need black chrome, black nickel, or black zinc plating, or salt bath nitriding. Good luck.

Regards,

A. Hi Sanjeev. We've referred you to our FAQ and the Metal Finishing Guidebook for introductory info, and black oxiding has already been the topic of the previous 30+ postings. Please try you best to phrase your questions in terms of what has already been said, and thus continue to build upon what is here, rather than asking that we start over :-)

Thanks for your understanding.

Regards,

A. The problem is common with stamped parts. Bending and stretching the steel causes different hardness and a difference in the actual surface polish of the metal and this is what is giving you the uneven color. You can anneal the material after stamping or try burnishing the parts in a revolving drum with small ceramic pyramids or steel balls.

There are a couple cheap tests you could preform just to test this theory. I would first try bead blasting the part with small steel balls to even the surface hardness and try bluing it. This failing heat one of the parts to a dull red with an even flame then remove and let it cool slowly and evenly in air and test blue.

I have had trouble bluing some Remington magazines after polishing and have resorted to these tricks. I suspect Remington beats this problem by blueing their magazines with a potassium bath which operates at 800-900 degrees although I have never asked.

A. It really does not matter whether you use Sodium Nitrate [affil links], potassium nitrate or ammonium nitrate. All that you are after is the niters. Sodium nitrate is always easier to get because of legal issues. Potassium nitrate can be converted to gunpowder quite simply and it can also be converted to Trinitrotoluene with less than high school chemistry. Ammonium nitrate can be combined with hydrocarbons to form an explosive called Anfo. Because of that and 911 there have been restrictions put on these two chemicals as of late.

A. Hi Abhijeet. In any process, you have a dozen or more parameters which must be right to get the right result, whereas any of the dozen being wrong will get you the wrong result. So it's almost never a question of "if the parts are red, the problem is surely this ..."

Still, few people understand the lesson that Rod Henrickson gives above (I know I didn't get it at first), so it is important to take the time to carefully study it ... The solution must be both boiling AND at the right temperature, which in turn indicates that it's at the right concentration as well as the right temperature. Good luck.

Regards,

A. Does the red rub off? If so, there is colloidal iron frozen on the surface. If not, there are several reasons the parts can turn red. What is the alloy?

A. Hi Grace,

Poor rinsing, leaving the parts wet, not using any sealers, not using any supplementary finishes, humid conditions, poor storage conditions...

More detail needed before anyone can give you a sensible answer.