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The black oxide process

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Q. I have been asked by several customers to look into installing a black oxide coating system to coat their parts. Information gathered so far tells me the 8# per gallon of caustic at 295 degrees F. is a caustic blackening. Is this black oxiding or must I look into a furnace system which uses steam at 900-1100 degrees F.? This process is described as black oxiding in sources.

Dave P [last name deleted for privacy by Editor]

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

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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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July 24, 2011

A. Steam at 900-1100 degrees Fahrenheit? 600 degrees Centigrade?

Is this a boiler operating under pressure (like those old steam locomotives)? Or is this a steam superheater? Native black coatings on Iron are Ferric Oxide (Fe2O3) or Magnetite (Fe3O4). Neither of which will long withstand high temp steam. Automotive parts often use a Iron Silicate, great for dry heat, but that too will decompose in high temp steam. For STP (standard temperature and pressure) boilers it is important to avoid all alkaline boiler conditions (we already assume no air/oxygen so stainless steel is out) and usually Permanganate, or other Oxidizing Anion, is added to prevent Rust pit formation. Permanganate will leave a black coating of Manganese Dioxide on the Iron which is protective and adheres relatively well. In ship boilers and chemical plants it is more usual to use Copper-Nickel ("German Silver") or similar alloys (Cu-Al-Mn) for boiler tubes as they are more stable to high temperature steam.

Stephen Jacks
- Mount Vernon, Washington, USA

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Q. I also need help with black oxiding. I wish to black oxide a restored classic rifle. Many years ago I used this process to good effect. While I still have potassium nitrate, caustic soda, etc. I can not find my notes with the correct formula, can anyone give me the proportions to obtain the correct super saturated solution. many thanks. Maurice

MAURICE S [last name deleted for privacy by Editor]

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A. It seems to me this black oxide process is very interesting and have good commercial value. Is there any resource for a person like me to just read and learn about the technique? I do not intend to pry on any one's secret technology, just want to read something which is declassified.

Any info will help.

Thanks a bunch,

From the painted dessert of Arizona!

Gautam B [last name deleted for privacy by Editor]
- Arizona

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

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Q. I spend a lot of money annually outsourcing for black oxide finishing on low carbon steels. I wish to invest on a system that would allow us to do it internally. Could some one please assist us. What equipment is required, sources for chemicals, specs. Thanks

Michael W [last name deleted for privacy by Editor]
- Windsor, Ontario
  ^- Sorry! This RFQ is outdated -^

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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]. Good luck.

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

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Q. I want to know the whole chemistry of the blackening process of iron. I am also interested in knowing the different methods of blackening of iron. How is the blackening film adherent to the parent metal?

rajesh s [last name deleted for privacy by Editor]
- India

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A. Hi, Rajesh. The blackened surface is very thin and is just an oxide coating (sort of a rust), so adhesion is 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. finishing.com Brick, New Jersey

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Q. I'm wondering whether or not black oxidizing changes the coefficient of friction of a part? I'm looking into a coating that I could use on my inner Fork Tubes (motorcycle suspension)that would have a black/grey color and would either keep the coefficient of friction I have now or improve it. I looked at TiN and saw the coefficient of friction improves significantly, but was wondering if there was another material with a black color that does the same? Thanks.

Nic Burke
Hobbyist - Federal Way, Washington

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

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

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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.

Tim Wyatt
- Port Richey, Florida

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March 2, 2009

Q. I am working with corrosion resist manufacturing product company.i would like to know detail process of blackening . I want to know what is the process, which raw materials are used, and on which parts it can be applicable.

Sujit Patil
plating shop employee - sangli, maharastra, India

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March 2, 2009

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

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March 28, 2009

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, IE 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 [linked by editor to product info at Amazon] 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

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October 14, 2009

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 yrs of it and I've had my fare share of burns.

Michael Spencer
- Dayton, Ohio

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October 28, 2009

Q. Hello everyone.
I am a manufacturer of polished stainless steel (304) anyone can tell me if it's possible to give a black finish to this material without losing the bright appearance.

Best regards

Carlos Costa
- Portugal

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October 29, 2009

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. finishing.com Brick, New Jersey

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January 6, 2010

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.

B Rgds,

Anh Duc
- Ho Chi Minh City, Vietnam

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November 10, 2009

Q. Rust Preventive oil and wax has been recommended to protect black oxidised part from corrosion. Can anyone know how to apply the oil / wax just after black oxidizing when there is black oxidizing solution or droplets present on the metal surface?

Mabel Davidson
- Jamshedpur

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November 10, 2009

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. finishing.com Brick, New Jersey

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November 30, 2011

Q. Hi. I have just read the first few messages and am looking for help in blacking particular types of parts. I work with steel on a regular basis and am having problems with the black oxide process. it seems as though my tank only wants to blacken smaller type parts but I need it to work on much larger parts. Also I've noticed that GPS (ground and polished steel) does not take as well as cold rolled steel. My temp does not exceed 295F and does not drop below 291F. The parts that sit on the bottom of the basket do not want to blacken. What do I need to do?

Shane Brumlow
- Bartow, Georgia, U.S.A.

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December 2011

A. Hi, Shane.

Please carefully read Rod Henrickson's message about boiling. You must have your solution boiling; therefore you use concentration to control your temperature, not heat input. After you are adjusted so the solution boils at your 291 to 295 F, and it isn't possible to make it hotter via more heat input, then you are operating in the mode that Rod suggests. What is your heating mechanism -- gas burners below the tank?

Regards,

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

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December 1, 2011 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hi,
I have manufactured a part of machine (collet) of material EN-19 grade which has been black-oxide used for core cutting machine. For some reasons I had to rework on its part welded with S.S material. I performed the black-oxide process again on it but the welded s.s part remains colorless without showing black finish on it.
Please give me a good suggestion.

Thanks & Regards,

Amit Nikam
- Pune, Maharashtra, India

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December 2, 2011

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. finishing.com Brick, New Jersey

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January 24, 2012

Q. Hi,
I am wondering the way to thicker the black oxide layer for steel, normally, I can have 0.5 micro after treatment, under not high temperature and certain pressure. I am wondering if there any possibility to increase the thickness to 1 micro, by which process? the temperature is limited because I don't want to change the microstructure of the steel, it is pearlite, and I need to keep the strength at 1000mpa.

Thank you very much for this.

Chu Zhang
- Sweden