Hot black oxide (blackodize) vs. cold blackening

A. Room temperature blackening with proprietary solutions is more expensive and less durable, Thomas, and doesn't look as good. Whether it will prove "durable enough" would depend on the application, but the cold blackening processes that I know of deposit selenium compounds on the surface, which do tend to rub off as a smut. Hot black oxiding is a nasty, dangerous, process though. So, if the durability of room temperature blackening is sufficient for you, and the cost is affordable, it may be worthwhile :-)

You may be interested in our FAQ on Black Oxide & Cold Blackening, and further info can be found in The Metal Finishing Guidebook. Best of luck!

A. Tom,

Ditto on what Ted said. A well done hot process looks a lot better and my gut feeling is that it is ten times as durable as a well done cold process. The cold process is adequate for most of the tool room uses. Have done both and unless you absolutely need the better protection of hot, use the cold. Hot is going to cause you some problem with EPA more so than the cold. Personal feelings.

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Ed. note: You can try EPI / Electrochemical Products Inc. [a finishing.com supporting advertiser] for cold blackening processes; they are well known for them.

A. If you do, that will make you a Hazardous waste generator. Suggest that you check with your region of the state EPA and with your local sewer folks before you go any further. You probably will change your mind. Minimum fine is substantial if you are caught. If an angry customer turns you in, they will visit you to clear the complaint.

Response to John G. If you really have the proper background to install a mini plating shop in your garage, then you should know you will be exposing children, pets and neighbors to dangerous chemicals. You should also be aware that you will have chemicals and waste water to dispose of, and that you need a city permit, a state permit and EPA clearance to do all of this.

Are you prepared to put in a separate sewer line, pay the business fees and license fees to all the proper agencies, or take the livelihood from an honest, reputable plating shop that does all these things.

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A. The principal advantage of cold blackening is that it is far safer to operate than the hot process, which involves a boiling hot highly concentrated aqueous solution of caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] which, if not handled properly, can erupt and burn or kill people. The operating temperature of hot black oxide tanks is much higher than the boiling point of water, such that when water is added to make good the evaporation losses, it has the potential of instantly flashing to steam and blowing the contents of the tank all over the place.

Cold blacking chemicals are more expensive, do not have equivalent corrosion resistance or the shiny uniform appearance of hot black oxide, and can rub off as an abrasive grit which can be a problem in certain manufacturing processes.

We have a FAQ on Black Oxide & Cold Blackening which may be helpful, and if you can find a copy of the Metal Finishing Guidebook, it will have a chapter on the subject. Good luck.

The EN31 (400) series material can be treated with low temperature chemi-black systems that operates at ambient temperature.

A. Those steps will be the same, Ben. Scale must be removed, then the parts thoroughly alkaline cleaned before putting them in the hot black oxide tank. Usually there will be a post-dip involving a wax or oil.

The biggest difference is that hot black oxide is quite dangerous. The problem is that the process operates at about 290 °F so there are evaporation losses, and water must be constantly added. But water flashes to steam at 212 °F. So, if a good slug of make-up water finds itself surrounded by this 290 ° solution, it can flash to steam, propelling this hot concentrated caustic onto an operator. Some people have been killed and many severely burned from these eruptions, so extreme care must be used to prevent it from happening, and protective gear worn in case it happens anyway. Good luck!

A. Hi, Jimmy. You can have a different oil applied that is less smutty, or a wax instead. But a black oxide finish derives almost all of its limited corrosion resistance from the oil or wax. If you remove it, it will flash rust.

Regards,

A. Hi Tom.

My understanding is that all cold blackening processes rely on a deposit of a selenium sulfide compound on the steel, not an actual black oxide. My limited personal experience is that even the best of these are "smutty" (rub off fairly easily, leaving hands or cloth dark). Hot black oxide is, in my opinion, a nicer looking and superior finish. But the high temperature does present the safety issue I mentioned.

In one case that I know of, someone tossed an unopened can of soda to a co-worker, but it missed and landed in a black oxide tank where it flashed to steam then exploded that hot caustic onto the operator, killing him. Everyone treats a pot of boiling water with respect, and these tanks are much bigger, much hotter, and filled with concentrated caustic rather than water. So face shield ⇦ this on eBay or Amazon [affil links] , apron ⇦ this on eBay or Amazon [affil links] , gloves, and boots is not at all excessive in my opinion ... and they would be a requirement if I were the supervisor. Good luck.

Regards,

A. Hi Kabz. Both are widely done because there is no such thing as "better way" ... it all depends on which parameters are most important to you. However, there is little question that hot black oxiding produces a nicer looking and more corrosion resistant finish. As you see, we appended your question to a thread which already addresses it. Good luck!

Regards,

Q. Hi Ted,
Thanks for giving me an idea.
Can I ask more questions: I'm working now in a government company and I don't have any idea what's the better process. I will do the hot bluing or the spray-on, bake-on finish process? and I don't have also the idea of the man-hours of both processes because this is my first time I will do this kind of work. And we're planning a small arm and a big arm that will undergo bluing or coating. Can you give some advice what's good to do, where the man-hours also are short and can produce more finished products.
Thank you so much.

Regards, ave

A. Hi again Kabz. The reason so many different finishes are available is because none of them is all-around superior to the others. Manufacturers with decades of experience have difficulty choosing, but Step 1 is to have sample parts finished and tested before you consider building a factory to mass produce a possible mistake.

Still, here's one thing for you to consider: bluing, even hot bluing, doesn't offer much corrosion resistance. It's a beautiful finish for those who will lovingly maintain their favorite hunting weapon, cleaning and oiling it after each outing. But as a robust finish for mass produced and mass distributed arms? ... I don't think so. The spray-on and bake coating sounds more applicable. But there are many other choices including electroless nickel plating and salt bath nitriding.

As for man-hours, I think you would need to hire a consultant to work through all of the issues with you. Estimating the man-hours involved in a whole manufacturing and inspection sequence is a very big job. I am personally not interested in cheapening stuff anyway. I'm for sustainability, whereas perpetually re-manufacturing stuff that doesn't last due to cheap, poor finishes represents wastefulness that the planet can't afford :-)

Regards and good luck,

Hi Hemanth; Hi Salah.
People are very welcome to try to help you, but it looks like they're not doing so this time :-)

This forum is good for directing people to sources of information, which I did in my first response on this page, and the forum is also good for specific answers to highly specific queries. But it's probably unrealistic to hope that a reader will design a process line for you for free via this forum. Good luck, but after reviewing the recommended resources, please try to ask highly specific questions. Thanks.

Regards,

A. Hi Mitchell. That sounds like an interesting finish! Unfortunately I'm not personally familiar with it, and the only cold blackening that I know is actually black. You can try contacting EPI / Electrochemical Products Inc. [a finishing.com supporting advertiser], who specialize in cold blackening, or hopefully a reader will be familiar with your need.

Regards,

A. Hi Darin. Try a small and simple one first and let us know how it works out because I have no experience with it. A "dry" hot black oxide would probably be highly subject to rusting, but if you keep it waxed with butcher's wax or oiled with mineral oil, my guess is that it will be okay. Remember that petroleum oils (which would be the most commonly applied oils) are poisonous!

Regards,

A. Hi Paul. These are proprietary processes, so I think the suppliers' material safety data sheet may offer good info.

Regards,