The black oxide process and gun bluing

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Ed. note: We now have a FAQ on Black Oxide on line, Dave.

A. One of the most common "black oxide" process found in metal finishing operations is much like the hot caustic solution you described. Commercially available processes contain more than just caustic soda, i.e.: oxidizers and activators are also incorporated. They typically operate at 280 - 295 °F.

The action of the solution actually forms an iron oxide which is black in appearance. A well operated "hot oxide" will produce a beautiful black finish. To achieve a level of corrosion resistance, a rust preventative oil is applied after the black oxide coating has been applied ( and the part is thoroughly rinsed).

There are also room temperature black finishes for steel.

Trust this helps.

Ken Lemke Burlington, Ontario

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Ed. note: We now have a FAQ on Black Oxide on line, Gautam. There is a good chapter on Black Oxide in the Metal Finishing Guidebook if you have a copy.

A. There is a chapter on black oxiding in the Metal Finishing Guidebook. If you're unfamiliar with the process but think you might want to install the capacity to perform it in-house, you could retain a consultant to simultaneously design it and educate you. Or if you'd rather start the sequence by talking to salespeople, there are several suppliers including Electrochemical Products [New Berlin WI], and Heatbath [Springfield MA] who will get you up to speed. Good luck.

Ted Mooney, P.E. RET finishing.com Brick, New Jersey

A. Hi, Rajesh. The blackened surface is very thin and is just an oxide coating (sort of a black rust), so adhesion is quite good. Other black coatings include cold blackening (based on selenium deposits), black chrome plating, black zinc (zinc plating followed by black chromating), black nickel plating, and salt bath nitriding. Good luck.

Ted Mooney, P.E. RET finishing.com Brick, New Jersey

A. Hi, Nic. The blackened surface is very thin and probably not wear resistant enough for such an application. Black chromium plating or salt bath nitriding could probably satisfy your needs. Good luck.

Ted Mooney, P.E. RET finishing.com Brick, New Jersey

A. Nic, Dupont makes a product that is similar to Cookware coating. You paint this stuff on and bake it in an oven at about 400 Deg. If you go to their website you can find all the info you want.

A. You need a gunsmith. I happen to be a gunsmith of 25 years. I guess you are lucky I stumbled here by mistake. The simple formula is as follows. 1 gallon of water, 2 pounds of sodium nitrate (or common sodium nitrate fertilizer), 5 pounds of sodium hydroxide (common lye). Operating temperature between 280 deg and 310 deg Fahrenheit. Solution MUST be boiling! Too hot and not boiling, add water or parts will rust (red oxide). Too cold or too cold and boiling, no reaction (no black oxide). Boil off water until temperature comes up. Over 301^³¹⁰ deg Fahrenheit will burn the salts and kill the solution. Chemicals have to be added periodically to replace carry out. Solution dies after a certain amount of contaminants have been carried In. Certain chemicals like aluminum will KILL THE BATH IMMEDIATELY! Rod

Rod Henrickson     gunsmith Edmonton, Alberta, Canada

A. To further my last post there was a typo. Over 301 deg Fahrenheit will burn the salts and kill the solution. It should read that exceeding temperatures of 310 degrees Fahrenheit will kill the solution. In truth it is about 330 degrees but the solution at the bottom of the tank is often much hotter than the solution at what ever depth the thermometer happens to be sitting at. It is wise never to exceed 310 degrees just to be on the safe side.

In the firearms industry it costs about $100 to $200 just to mix a new batch of chemicals. That is only about 10 or 20 gallons. I would imagine tanks large enough to blue oilfield or automotive components would have to be much larger. Enough said, DON'T overheat the solution. Bath time is not the critical thing in caustic bluing. Temperature is! If the temperature is correct for the type of steel that you are bluing the parts can be fully blued in 3 minutes or less. Remember that the solution must be boiling or red oxide will form. Don't ask me why I'm not a chemist but after doing it for 25 years I know for a fact that it is no old wives tale. The amount of water regulates the temperature of the solution not the heat source. Too hot, add water. Too cold, boil off some water. Watch the temperature and keep the solution at a rolling boil.

Plain old leaded, cold roll steel will turn dark black blue at around 280 degrees. Chrome molybdenum steel will turn deep blue at 300 degrees. Nickel steel will turn a slightly lighter shade of blue at 305 to 315 degrees. Chrome vanadium steel will turn a deep orchid purple at 300 to 315 degrees. The amount of rust prevention is dependent on the type of finish the part has, careful maintenance, i.e., keeping oil on the blued surface and the steel type. The higher the polish on the part the more resistance it will have to rust. The oil used should be non detergent. WD-40 and other detergent oils will strip the blue over time. Blued chrome vanadium is the toughest. Nickel steel the next. Chrome molybdenum and lastly cold roll steel.

Make sure all parts are completely cleaned and degreased before putting them into the tanks. Certain chemicals will kill the bath as well as some metals. An ounce of aluminum will kill 10 gallons of solution. As well as heavily chlorinated water. If you find your baths dying try using lake or river water. Although it will not really harm the bath avoid lead or parts that have been soft soldered together. The bath loves to eat lead.

This solution will not blue stainless steel. It will with the addition of certain chemicals, one of which happens to be sodium cyanide. I know how to make a stainless bath but I will never do it and I won't give you the formula, so don't ask! Besides it is against the law to possess sodium cyanide outside of a laboratory in Canada and I imagine it is in the USA as well. If it is not, it should be. If you must turn stainless steel black have it black chromed, Teflon or powder coated. There are a dozen outlets in every city that offer this. --Rod

Rod Henrickson     gunsmith Edmonton, Alberta, Canada

A. Just an FYI, there are lots of companies around the country that are set up to do the work for you. Cost is very reasonable. For instance we charge a $10 minimum for the first 10 pounds, and 22 cents a pound there after. Don't be surprised if they charge a set rate for fire arms. Stainless is also done here. The mixture is commercially produced, so no need to handle cyanide or any other dangerous chemicals.

If you have any reservations about the safety of the process, take it to the professionals. Black oxide chemical mixtures will eat right through your cloths and skin in a matter of seconds. Trust me, 40 years of it and I've had my fare share of burns.

A. Hi, Carlos. A bright polished part should stay bright in the black oxide process, I think. Other bright finishing options include titanium coating it, black chrome plating it, or doing a special stainless steel anodizing process.

Regards,

Ted Mooney, P.E. RET finishing.com Brick, New Jersey

A. Dear Carlos,

Black chromium plating can give you black surface and more or less, brightness would remain.
But in some cases, chromium plated is forbidden, and you need to do by some different way.
Black Ruthenium plating may help you if the cost is still in your budget.
Hope this can help.

Best Regards,

A. Hi, Mabel. I may be misunderstanding the question, but it is common to rinse the work after black oxide, then dip it in a water soluble wax or water soluble oil.

If the finish is "bleeding" droplets, though, something is wrong with your process.

Regards,

Ted Mooney, P.E. RET finishing.com Brick, New Jersey

A. Hi, Amit.

Ron Henrickson suggests above that cyanide is needed to blacken stainless steel. I am not sure if this is the case, but proprietary blackening solutions for stainless steel are available from the major process suppliers, and you could inquire of them whether they contain cyanide. Good luck.

Regards,

Ted Mooney, P.E. RET finishing.com Brick, New Jersey

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.

Amit Nikam
There are baths made for stainless steel that might solve your problem. I believe they are non-cyanide or low cyanide and while certain precautions must be taken they are relatively safe to use. I have used some of them in the past and while they are fussy they do work. If no one here sells salts for stainless I believe the stuff I used to use was DU-LITE. It's a USA company but I believe they ship world wide.

Shane Brumlow
You are having heating issues. Large parts can take forever to warm up to the temperature of the bath. Contrary to what people think the bottom of the tank is actually the coldest area of the tank depending on your heating system. The tank works like a coffee percolator. Water on the bottom boils and shoots up to the top and often flows back down the sides to the bottom as it cools. It can some times do the opposite and boil up the sides leaving a cool spot of returning solution in the center. Make sure your tanks are at a rolling boil so you get good circulation of solution. KEEP PARTS ABOUT 1 INCH OFF THE BOTTOM. With large parts you can pre-heat them over a soft flame to about 250 before immersion and it will save you some tank time. Also, you may have to take your tanks over 300° F to get large parts to blue as the part never does get to bath temperature.

Mabel Davidson
For production work the best is to have 2 boil out tanks. One pure water which you allow the parts to boil in for 15 or 20 minutes to remove trapped salt and the second is a boiling tank with water soluble oil and water displacement oil. The water soluble oil will get into the nooks and cranny's and the displacement oil which will float on top will leave a film of oil on the parts when they are drawn out of the solution. The same procedure is used for wax's. If you are just doing a few items water displacement oil can be sprayed on after the parts have been taken out of the boil out tanks.

Rod Henrickson     gunsmith Edmonton, Alberta, Canada

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,

Ted Mooney, P.E. RET finishing.com Brick, New Jersey

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,

Ted Mooney, P.E. RET finishing.com Brick, New Jersey

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,

Ted Mooney, P.E. RET finishing.com Brick, New Jersey

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.

Rod Henrickson     gunsmith Edmonton, Alberta, Canada

A. It really does not matter whether you use sodium nitrate, 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.

Rod Henrickson     gunsmith Edmonton, Alberta, Canada