Black Oxide process problems

A. Hi Alex. This may not be totally responsive to your question, but is at least tangential ...

I had a friend/plater who worked with me on several consulting projects a couple of decades back and then went to work at a shop and eventually opened a shop himself. Wherever we/he worked, he seemed fond of trying to implement the concept of casual semi-cleanroom environments. While electronic grade cleanrooms are very expensive with their downdraft floor-sweeping ventilation, HEPA filtration, and rigid protocols, simply reducing dirt and dust even by as much as an order of magnitude usually isn't really all that hard. It just requires asking yourself where all that dust & dirt is coming from, looking around, and addressing targets which look both simple and cheap to address :-)

Luck & Regards,

A. Hi Sam. Although it was on a hot black line, not a cold blackening line, and for steel not stainless, I have seen mirror polished parts come out jewel blue while rougher parts come out dark black from the same processing line at the same time. If the fading to blue seems otherwise random, you might consider whether the parts which turn blue are more highly polished.

Luck & Regards,

A. The caustic black oxide solution is not actually corrosive to steel as far as I know, and I suppose as you do that the white stuff is improperly rinsed caustic. Corrosion products would typically be red rust.

The black oxide & oil parts should exhibit some resistance to corrosion, so to me the question is why remove the oil?

P.S. March 28, 2009: Rod Henrickson's answer below is much better than mine was.

A. First, it may be whatever is being utilized to remove the soluble oil from the black oxide, leaving a residual film before mineral spirits is applied. Second, why not have the black oxider apply this material instead of removing the oil?

A. Curtis, you could confirm the source of the white residue by carefully scraping some of it off the parts and analyzing it. If the residue is sodium or potassium salts, it is likely that it came from the black oxide process.

Another possibility is it may be from the rust preventative oil applied over the black oxide. Many rust preventative oils contain calcium based materials. If you are using solvent to remove the oil, the calcium based materials will be left on the surface of the parts. Without the other ingredients that were present in the original oil, the calcium materials migrate to the surface and form the white residue.

A third possible source is from your cleaning step. In the above scenario, I have assume you are using solvent to clean the original oil off the parts. If you are using an aqueous cleaner, the white residue could be salts from the cleaner.

From the outside looking in, you may be able to solve the residue problem and simplify your process by getting the shop that applies the black oxide to apply the oil you use to the parts instead of the oil they currently use. That would safe you the cleaning and reoil steps. Hopefully, I have given you some things to think about.

A. Very simple answer to your question. I have the same problem when I am bluing firearms and it is close to quitting time. Like cockroaches, bluing salts literally find their way into every nook & cranny. Dang stuff is insidious and will hide in the darndest places and wait until the customer has picked up his part or gun and will then wait another three weeks before quietly creeping out over the metal surface. When the guy brings it back in he is generally freaking out as he has already found that no amount of oil will stop it.

Whose fault is it? Well it's the fault of the guy that blued the parts. All parts that are caustic blued MUST be boiled out for at least a half hour after they come out of the bluing solution. If the parts have nooks and crannies the solution will hide there. If the parts have been sand blasted the danged stuff will hide in the bottoms of the tiny craters the sand blasting creates. Forget oil! Bluing salts are made from sodium nitrate ⇦this on Amazon [affil link] or ammonium nitrate if you can stand the smell and sodium hydroxide. None of these chemicals are oil soluble. Bluing salts will crawl out of a hole and walk over an oiled surface like a baby crawling across the living room rug.

Crawling salts are simple to spot. They can appear as a light snow on the surface or an ugly white mould spot that just keeps growing. It can also take on a light bluish tinge if the chemicals happen to be new. The only way to avoid it is to boil the parts in fresh water for thirty minutes after bluing and then oil or wax everything down. Waxing is the best that I have found. Keep a separate hot water tank and toss a cup of any paste floor wax in it. The wax will melt and float on top. Dip the part in and out several times and then rinse with cold water and dry. The wax will fill the nooks and crannies. After this apply some light gun oil. Don't worry about the part rusting in the boil out tank. As long as it is under water it won't rust in the thirty minutes it is in there. When you take it out of the tank then you can start to worry as rust will form in less than 15 minutes. Go straight from the boil out tank to the wax tank or get oil on it right away.
Some people add water soluble oil to their boil out tanks. Does that really help? Well, I have tried 101 different hangover cures and short of getting drunk again they never seemed to work either. I suppose if you believe water soluble oil helps, far be it from me to tell you anything different. Rod

Q. Hello Rod, would this process (post process boiling and waxing) be the same for the black oxide process, and if so what wax do you recommend as I am having a hard time finding a water soluble floor wax? Thank you for your time.

John Taylor
- Evansville, Indiana
September 6, 2022

A. Hi John. Rod's posting was from 13 years ago so I don't know if he'll return to answer you, but suppliers to the metal finishing industry like Heatbath (DuBois) offer waxes designed/formulated for this specific purpose.

Luck & Regards,

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

A. Hi, Jay. In my estimation it will not work. I have been to firearms factories where they remove the oil from blued (black oxided) guns for certain rework, and often suffer flash rusting within a few hours. It is only my opinion but I believe the idea of an oil and wax-free black oxide finish is doomed to failure.

Regards,

A. Dear Friends:
To Remove any black oxide coating, as they applied oil on them, you should first degrease the parts in "Hot Degreasing" materials or use some solvent base degreasing material, then after rinsing, dip the parts in HCl base mild acid solution (20% for e.g.) plus corrosion inhibitors at room temp. And then you can see that all the black coating will be vanished in a second. But if the coating did not change, then the parts have been blackened by Cold Black Oxide material, (Selenium salts base)and the only safe way to remove the coating is mechanical rubbing or blasting (sand or glass). To prevent rusting before the second process you can use anti-oxidants in your last rinsing process at the rate of 3-5% (check with the producer).
Good Luck

A. Hi Liza. We appended your question to a similar earlier thread where Rod Henrickson has offered his knowledgable answer. However, black oxide is not corrosion resistant (although the oil or wax might have been sufficient if it hadn't got wet). Once it has been in the rain for several hours, I don't think you'll get it back to right, and your plan of dissembling and painting it is probably the most practical approach.

Regards,

A. Dear Liza:
Sometimes the oils used as topcoat are water soluble and the color is kind of milky color. I did not see you parts but Sodium or Potash residue is colorless and can`t be seen as white liquid. Anyway I suggest you to repair the blackening in a simple way: First cover all the part with plastics and tape. Just leave the damaged places. Check for Cold Black Oxide Chemicals in your area and then degrease the part (By the offered products by companies), remove the damaged blackening by 15-20% HydroChloric acid (HCl) by spray, then rinse well and then go for Blackening instruction. Please bear in mind that you should wear safety gloves, goggles and mask. Use the blackening material in spray form through the revealed area, then wash and dry it. After finishing the process use protective oil on it and leave it for 1 or 2 days for better results.
Good Luck

A. Hi Peter. It sounds normal to me. Black oxide is simply a form of iron oxide rust, and I'd expect mild acids like fruit juices to easily dissolve iron oxide. The waxes or oils on the black oxide might provide a tiny bit of temporary rust resistance, but apparently not enough to resist the fruit juice in this case.

Regards,

A. Hi cousin Murali. Although I'm no expert in black oxide, such questions are so perennial that I've seen them repeatedly :-)

1. The white deposit is most likely failure to boil out, as Rod explains above.
2. The red patches are perhaps due to controlling temperature by heat addition instead of concentration. The tank must be BOTH at a rolling boil AND at the right temperature; that is accomplished by holding the concentration as Rod explains on some other threads. Search for his name, "red" and "black oxide" to find those threads.
3. Yes, oiling or waxing is required.
4. Black oxide is one form of "rust" in a desirable color and form. If not protected from the elements by oil or wax it will convert to the undesirable color and form.

Good luck.

Regards,

A. Seriously, nobody can say whether some unknown facility is appropriately designed and in good enough condition for such an operation. But what would you use a chiller for in a hot bluing operation?

Regards,

A. Hi again. I think we are having trouble understanding each other. A chiller is used to make a process tank colder. What are you planning to do with the chiller in a hot black oxide process?

Regards,

Brownells Gun-Kote


(affil links)

Hi Kabz. Where did you hear about that word and in what context please? My guess might be KG Gun-Kote offered by Brownells. It's an epoxy low friction bake-on coating available in a number of colors. Good luck.

Regards,

A. Hi Nathan. Look to Rod Henrickson's explanation earlier in this thread. It surely seems that failure to "boil out" is the cause of that growth. Good luck.

Regards,

A. White "growth" after bluing are the salts crystallizing and bleeding out of the hidden places. We control this by placing hot parts directly into very hot water into which we add a very small amount of vinegar ⇦in bulk on eBay or Amazon [affil links] The vinegar will naturalize the base or caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] . Let it remain for 10 plus minutes then place in oil or WD-40 ⇦this on eBay or Amazon [affil links] and let it rest until cool. Leave overnight to cure.

A. Hi Tim. I appended your inquiry to this thread because I thought Rod Henrickson's posting was so applicable and informative. Please have your shop try the full half-hour boil-out that Rod suggests; Robert Legg's suggestion of a little vinegar might help (I don't know).

Please return their favor by letting us know what happens :-)

Regards,

A. Hi Julie. At the risk of being Captain Obvious, I think you need to either jiggle the baskets or re-think the basket design. I would not attempt any temperature or concentration change for those basket marks.

Regards,

Thank you so much Sir. We did just that! It has definitely corrected the problem. Again, thank you so much for your feedback!

A. Black Oxide works best when it is free to move around and under a part unobstructed. You can prop up the parts to allow for this in the basket or simply put a surface on the bottom such as rebar as a base material to prevent the flat contact by the parts to the basket. Make a birds nest of rebar on the bottom of the baskets and you should be fine.

A. Hi Marc. We added your question to a thread where it looks to me like gunsmith Rod Henrickson answered it. But feel free to expound if his answer doesn't suffice.

Regards,

A. Hi again. Wax vs. oil didn't strike me as the heart of Rod's suggestion; to me it seemed the half-hour boil-out was the heart of the matter.

Regards,

Hi Chatrush. Hopefully you'll get some immediate feedback from someone who has obtained a gray finish, but questions really should be longer than the answer you are seeking -- so it wouldn't hurt to flesh out the inquiry a bit with a few paragraphs about: concentration, temperature, salt boil-out done or not done, whether it's a problem that popped up after years of satisfactory operation or whether you are just starting and have never successfully blackened, whether it affects every piece you are blackening or only a few, etc.

I hate to nitpick, but the actual case is that you did not maintain all the correct parameters or the work would have come out correctly :-)
... there is obviously one or more important parameters that are either not being monitored, or where you interpretation of their correct values is in error, or where you have been given incorrect information regarding those values. One of those parameters may, of course, be the material composition or surface characteristics of the component you are trying to black oxide.

Luck & Regards,

A. Hi Elias. Are you a production plating and metal finishing shop or a gunsmith?
Usually a production processing line for black oxide will not be a single black oxide tank, but will be a whole series of tanks, including cleaners and rinses; the last tank in the process will often contain a water soluble wax purchased from your metal finishing process supplier. Sorry, I don't know the actual chemistry of them.
A gunsmith is usually more or less a bespoke manufacturer who offers a tailored service to the buyer, and he might well do manual waxing or oiling after the black oxide process.

Luck & Regards,

A. Hi Armando. May we assume that you are in fact encountering a surface cracking problem? Please tell us when it was noted, how prevalent the problem was, etc.

I spent 20 minutes or so on google unsuccessfully looking for an answer to the abstract question of whether black oxide ever can cause cracking of dual phase steel, but I did read that it dual phase steel should not be carbonitrided.

Luck & Regards,