Cold bluing vs cold black oxide finish

Letter 14024

Cold bluing vs cold black oxide finish

Looking to restore the black oxide finish on some specialty German made metric fasteners for a vintage motorcycle project. Although I'd prefer to outsource such work, all the local jobshops aren't interested in running a small batch or used parts for that matter. However, I have found 2 commercially available hobbyist kits that may work. One is a cold bluing kit for firearms, which contains Selenium Dioxide. The other is a cold black oxide coating kit from a automotive restoration supply company, but they will not mention the active chemical.

The questions I have, is cold bluing the same process as cold black oxide coating? If not, will bluing provide the equivalent rust protection as a black oxide finish? Will either of these processes cause any changes in the strength of the fasteners such as Hydrogen embrittlement? Also how safe is it to work with Selenium Dioxide and are there any disposal issues?

Mark
- San Diego, CA , USA

Soluble selenium compounds are hideously toxic. There are no disposal issues, but wear disposable polyethylene plastic gloves and then wash your hands very thoroughly with soap and hot running water after handling the stuff and taking off the gloves, whether you think you got any on your hands or not. Selenium dioxide based "cold blues" produce a finish that can be patchy and blotchy, and does not wear especially well.

Alternately, you could get the phone book and look for gunsmiths, who, I'm sure, will be only too happy to do the job, and they'll surely use the hot caustic blue process--which wears a lot better than the selenium-based coloring and also tends to look better.

I considered going into the details of doing hot caustic bluing at home here, but the process is not without its dangers (the solution contains high concentrations of caustic strong alkali plus a strong oxidizing agent, and therefore it goes through flesh like a buzzsaw even when it's not at a full rolling boil), so I won't.

John Smith
I stand alone. - Ann Arbor, Michigan, USA

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