Metal finishing Q&As since 1989

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How do 'Safer hot black oxides' perform?

2003

There is a process of hot black oxide that claims equal results as the true hot process but is done at 190 F and claims to be much safer. It also claims it eliminates the problem of salt buildup in holes and is suitable for castings and powder metal parts.I require the results of hot black oxide but would prefer a safer process.

Have you had any experience with it and if so what is your opinion.

Thanks,

Robert G. Oltean
- Windsor , Ontario, Canada

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We went through the same search a few years back. We tried many sample products in the lab and ended up with the 190° bath that you have mentioned. In our shop, the bath does not see a great deal of work, but we have been pleased with it. You should follow the manufacturer's pretreatment steps precisely. The only change I would suggest is to operate at closer to 200° instead of 190°. Still, this is much safer than the hot black oxides that are more common.

Jon Barrows, MSF, EHSSC
GOAD Company

Independence, Missouri

2003

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I have posted a question on Information on Steel and Stainless Steel finishes and my biggest problem is finding the bath solutions (chemicals) used in Black Oxide finishing on steel (both Cold and Hot). I haven't been able to find a specific list of chemicals used. All I have found so far is that Hot Blackening uses caustic solution and Cold Blackening uses acidic solution. Could you please help me with this? I see you are experts in this process. Cheers,

Damineh Akhavan
- Vancouver, British Columbia, Canada
2003

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You can find some general info in the Metal Finishing Guidebook, but most blackening is done from proprietary processes. Ask the vendors for a "technical data sheet" and it will contain all the information they are willing to disclose about the chemical composition. As a general rule, the further back you go in the literature, the more detail you will find; if you can find plating journals from the 50s, 60s and 70s you will get a lot of composition information that is no longer generally available.

It's generally known that the hot black oxide process oxidizes steel with caustic and nitrates whereas the room temperature blackening applies a selenium-based coating. I don't know which is the principal mechanism of the mid-temperature process. Again, a technical data sheet from the manufacturer, and an MSDS, will reveal a little more about the solutions but probably not much more. If you want detail, you need to go back to early research such as you will find through a literature search of very old metal finishing journals. Good luck!

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2003